环境工程概论（二）：Chapter8

8 Attached Growth Biological Treatment Processes 8-1 Background Evolution of Attached Growth Processes Attached growth processes can be grouped into three general classes: (1) nonsubmerged attached growth processes, (2) suspended growth processes with fixed-film packing, and (3) submerged attached growth aerobic processes. Nonsubmerged Attached Growth Processes. Trickling filters with rock packing have been a common, simple, and low-energy process used for secondary treatment since the early 1900s. A trickling filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continously. Treatment occurs as the liquid flows over the attached biofilm. The concept of a trickling filter grew from the use of contact filters in England in the late 1890s. Originally they were watertight basins filled with broken stones and were operated in a cyclic mode. The bed was filled with wastewater from the top, and the wastewater was allowed to contact the packing for a short time. The bed was then drained and allowed to rest before the cycle was repeated. A typical cycle required 12 h (6 h for operation and 6h of resting). The limitations of the contact filter included a relatively high incidence of clogging, the long rest period required, headloss, and the relatively low loading that could be used. Because of the clogging problems, larger packing was used until a rock size of 50 to 100 mm was reached. In the 1950s, plastic packing began to replace rock in the United States. The use of plastic packing allowed the use of higher loading rates and taller filters (also known as biotowers) with less land area, improved process efficiency, and reduced clogging. In the 1960s, practical designs were developed for rotating biological contactors (RBCs), which provided an alternative attached growth process where the packing is rotated in the wastewater treatment tank, versus pumping and applying the wastewater over a static packing. Both trickling filters and RBCs have been used as aerobic attached growth processes for BUD removal only, combined BOD removal and nitrification, and for tertiary nitrification after secondary treatment by suspended growth or attached growth processes. The principal advantages claimed for these aerobic attached growth processes over the activated-sludge process are as follows: . Less energy required . Simpler operation with no issues of mixed liquor inventory control and sludge wasting . No problems of bulking sludge in secondary clarifiers . Better sludge thickening properties . Less equipment maintenance needs . Better recovery from shock toxic loads In comparison to the activated-sludge process, disadvantages encountered for trickling filters are a poorer effluent quality in terms of BOD and TSS concentrations, greater sensitivity to lower temperatures, odor production, and uncontrolled solids sloughing events. In general, the actual limitations of the processes (1) make it difficult to accomplish biological nitrogen and phosphorus removal compared to single-sludge biological nutrient removal suspended growth designs, and (2) result in an effluent with a higher turbidity than activated-sludge treatment. Trickling filters and RBCs have also been used in combined processes with activated sludge to utilize the benefits of both processes, in terms of energy savings and effluent quality. Suspended Growth Processes with Fixed-Film Packing. The placement of packing material in the aeration tank of the activated-sludge process dates back to the 1940s with the Hays and Griffith processes. Present-day designs use more engineered packings and include the use of packing materials that are suspended in the aeration tank with the mixed liquor, fixed packing material placed in portions of the aeration tank, as well as submerged RBCs. The advantages claimed for these activated-sludge process enhancements are as follows: . Increased treatment capacity . Greater process stability . Reduced sludge production . Enhanced sludge settleability . Reduced solids loadings on the secondary clarifier . No increase in operation and maintenance costs Submerged Attached Growth Processes. Beginning in the 1970s and extending into the 1980s, a new class of aerobic attached growth processes became established alternatives for biological wastewater treatment. These are upflow and downflow packed-bed reactors and fluidized-bed reactors that do not use secondary clarification. Their unique advantage is the small footprint with an area requirement that is a fraction (one-fifth to one-third) of that needed for activated-sludge treatment. Though they are more compact, their capital costs are generally higher than that for activated-sludge treatment. In addition to BOD removal, submerged attached growth processes have also been used for tertiary nitrification and denitrification following suspended or attached growth nitrification. Downflow and upflow packed-bed reactors, fluidized-bed reactors, and submerged RBCs can be used for postanoxic denitrification. Trickling filters and upflow packed-bed reactors are also used for preanoxic denitrification. Mass Transfer Limitations A significant process feature of attached growth processes in contrast to activated-sludge treatment is the fact that the performance of biofilm processes is often diffusion-limited. Substrate removal and electron donor utilization occur within the depth of the attached growth biofilm and subsequently the overall removal rates are a function of diffusion rates and the electron donor and electron acceptor concentrations at various locations in the biofilm. By comparison, the process kinetics for the activated-sludge process are generally characterized by the bulk liquid concentrations. The diffusion-limited concept is especially important when considering the measurable bulk liquid DO concentrations on attached growth process biological reaction rates. Where a DO concentration of 2 to 3 mg/L is generally considered satisfactory for most suspended growth aerobic processes, such low DO concentrations can be limiting for attached growth processes. For uninhibited nitrification in the biofilm, a much higher DO concentration may be required depending on the ammonia concentration. The concept of diffusion limitations on nitrification rates and the ability to develop anaerobic layers within the biofilm may be exploited to accomplish both nitrification and denitrification in attached growth processes with positive bulk liquid DO concentrations. 8-2 Trickling Filters Trickling filters have been used to provide biological wastewater treatment of municipal and industrial wastewaters for nearly 100 years. As noted above, the tricklings filter is a nonsubmerged fixed-film biological reactor using rock or plastic packing over which wastewater is distributed continuously. Treatment occurs as the liquid flows over the attached biofilm. The depth of the rock packing ranges from 0.9 to 2.5 m and averages 1.8 m. Rock filter beds are usually circular, and the liquid waste Water is distributed over the top of the bed by a rotary distributor. Many conventional trickling filters using rock as the packing material have been converted to plastic packing to increase treatment capacity. Virtually all new trickling filters are now constructed with plastic packing. Trickling filters that use plastic packing have been built in round, square, and other shapes with depths varying from 4 to 12 m. In addition to the packing, other components of the trickling filter include a wastewater dosing or application system, an underdrain, and a structure to contain the packing. The underdrain system is important both for collecting the trickling filter effluent liquid and as a porous structure through which air can circulate. The collected liquid is passed to a sedimentation tank where the solids are separated from the treated wastewater. In practice, a portion of the liquid collected in the underdrain system or the settled effluent is recycled to the trickling filter feed flow, usually to dilute the strength of the incoming wastewater and to maintain enough wetting to keep the biological slime layer moist. Influent wastewater is normally applied at the top of the packing through distributor arms that extend across the trickling filter inner diameter and have variable openings to provide a uniform application rate per unit area. The distributor arms are rotated by the force of the water exiting through their opening or by the use of electric drives. The electric drive designs provide more control flexibility and a wider range of distributor rotational speeds than possible by the simple hydraulic designs. In some cases, especially for square or rectangular filters, fixed flat-spray nozzles have been used. Primary clarification is necessary before rock trickling filters, and generally used also before trickling filters with plastic packing, though fine screens (smaller than 3mm openings) have been used successfully with plastic packing. With increases in plastic and rubber floatable materials in wastewater, screening of these materials is important to reduce fouling of the packing. In some installations a wire-mesh screen is placed over the top of plastic packing to collect debris that can be vacuumed off periodically. A slime layer develops on the rock or plastic packing in the trickling filters and contains the microorganisms for biodegradation of the substrates to be removed from the liquid flowing over the packing. The biological community in the filter includes aerobic and facultative bacteria, fungi, algae, and protozoans. Higher animals, such as worms, insect larvae, and snails, are also present. Facultative bacteria are the predominating organisms in trickling filters, and decompose the organic material in the wastewater along with aerobic and anaerobic bacteria Achromobacter, Flavobacterium, Pseudomonas, and Alcaligenes are among the bacterial species commonly associated with the trickling filter. Within the slime layer, where adverse conditions prevail with respect to growth, the filamentous forms Sphaerotilus natans and Beggiatoa will be found. In the lower reaches of the filter, the nitrifying bacteria will be present. The fungi present are also responsible for waste stabilization, but their role is usually important only under low-pH conditions or with certain industrial wastes. At times, fungi growth can be so rapid that the filter clogs and ventilation becomes restricted. Among the fungi species that have been identified are Fusarium, Mucor, Penicillium, Geotrichum, Sporatichum, and various yeasts. Algae can grow only in the upper reaches of the filter where sunlight is available Phormidiun, Chlorella and Ulothrix are among the algae species commonly found in trickling filters . Generally, algae do not take a direct part in waste degradation, but during the daylight hours they add oxygen to the percolating wastewater. From an operational standpoint, the algae may be troublesome because they can cause clogging of the filter surface. The protozoa in the filter are predominantly of the ciliate group, including Vorticella, Opercularia, and Epistylis. Their function is to feed on the biological films and, as a result, effluent turbidity decreases and the biofilm is maintained in a higher growth state. The higher animals, such as worms, snails, and insects, feed on the biological film. Snails are especially troublesome in trickling filters used mainly for nitrification, where they have been known to consume enough of the nitrifying bacteria to significantly reduce treatment efficiency. The slime layer thickness can reach depths as much as 10 mm. Organic material from the liquid is adsorbed onto the biological film or slime layer. In the outer portions of the biological slime layer (0.1 to 0.2 mm), the organic material is degraded by aerobic microorganisms. As the microorganisms grow and the slime layer thickness increases, oxygen is consumed before it can penetrate the full depth, and an anaerobic environment is established near the surface of the packing. As the slime layer increases in thickness, the substrate in the wastewater is used before it can penetrate the inner depths of the biofilm. Bacteria in the slime layer enter an endogenous respiration state and lose their ability to cling to the packing surface. The liquid then washes the slime off the packing, and a new slime layer starts to grow. The phenomenon of losing the slime layer is called sloughing and is primarily a function of the organic and hydraulic loading on the filter. The hydraulic loading accounts for shear velocities, and the organic loading accounts for the rate of metabolism in the slime layer. Hydraulic loading and trickling filter sloughing can be controlled by using a wastewater distributor with an electric motor drive to vary rotational speed. The mechanisms of biological film loss in plastic and rock packing are different. Continuous, small-scale sloughing of the film occurs in high-rate plastic filters due to hydraulic shear, while large-scale, spring-time sloughing occurs in rock filters located in temperate zones. Sloughing is due to the activity of insect larvae, which become active in the warmer spring temperatures and consume and mechanically dislodge thick biofilms that accumulate over the winter. When a rock filter sloughs, the effluent before settling will contain higher amounts of BOD and TSS than the applied wastewater. Trickling Filter Classification and Applications Trickling filter applications and loadings, based on historical terminology developed originally for rock filter designs, are summarized in Table 8-1. Trickling filter designs are classified by hydraulic or organic loading rates. Rock filter designs have been classified as low- or standard-rate, intermediate-rate, and high-rate. Plastic packing is used typically for high-rate designs; however, plastic packing has also been used at lower organic loadings, near the high end of those used for intermediate-rate rock filters. Much higher organic loadings have been used for rock or plastic packing designs in "roughing" applications where only partial BOD removal occurs. Low-Rate Filters. A low-rate filter is a relatively simple, highly dependable device that produces an effluent of consistent quality with an influent of varying strength. The filters may be circular or rectangular in shape. Generally, feed flow from a dosing tank is maintained by suction level controlled pumps or a dosing siphon. Dosing tanks are small, usually with only a 2-min detention time based on twice the average design flow, so that intermittent dosing is minimized. Even so, at small plants, low nighttime flows may result in intermittent dosing and recirculation may be necessary to keep the packing moist. If the interval between dosing is longer than 1 or 2 h, the efficiency of the process deteriorates because the character of the biological slime is altered by a lack of moisture. In most low-rate filters, only the top 0.6 to 1.2 m of the filter packing will have appreciable biological slime. As a result, the lower portions of the filter may be populated by autotrophic nitrifying bacteria, which oxidize ammonia nitrogen to nitrite and nitrate forms. If the nitrifying population is sufficiently well established, and if climatic conditions and wastewater characteristics are favorable, a well-operated low rate filter can provide good BOD removal and a highly nitrified effluent. With a favorable hydraulic gradient, the ability to use gravity flow is a distinct advantage. If the site is too flat to permit gravity flow, pumping will be required. Odors are a common problem, especially if the wastewater is stale or septic, or if the weather is warm. Filters should not be located where the odors would create a nuisance. Filter flies (Psychoda) may breed in the filters unless effective control measures are used. Intermediate- and High-Rate Filters. High-rate filters use either a rock or plastic packing. The filters are usually circular and flow is usually continuous. Recirculation of the filter effluent or final effluent permits higher organic loadings, provides higher dosing rates on the filter to improve the liquid distribution and better control of the slime layer thickness, provides more oxygen in the influent wastewater flow, and returns viable organisms. Recirculation also helps to prevent ponding in the filter and to reduce the nuisance from odors and flies. Intermediate-and high rate trickling filters may be designed as single- or two-stage processes. Flow diagrams for various trickling filter configurations are shown on Fig. 8-2. Two filters in series operating at the same hydraulic application rate (m3/m2.h) will typically perform as if they were one unit with the same total depth. Roughing Filters. Roughing filters are high-rate-type filters that treat an organic load of more than 1.6 kg/m3·d and hydraulic loadings up to 190 m3/m2·d. In most cases, roughing filters are used to treat wastewater prior to secondary treatment. Most roughing filters are designed using plastic packing. One of the advantages of roughing filters is the low energy requirement for BOD removal of higher strength wastewaters as compared to activated-sludge aeration. Because the energy required is only for pumping the influent waste-water and recirculation flows, the amount of BOD removal per unit of energy input can increase as the wastewater strength increases until more recirculation is needed to dilute the influent wastewater concentration or to increase wetting efficiency. The energy requirement for a roughing application may range from 2 to 4 kg BOD applied/kWh versus 1.2 to 2.4 kg BOD/kWh for activated-sludge treatment. Two-Stage Filters. A two-stage filter system, with an intermediate clarifier to remove solids generated by the first filter, is most often used with high-strength waste-water (Fig. 8-2b). Two-stage systems are also used where nitrification is required. The first-stage filter and intermediate clarifier reduce carbonaceous BOD, and nitrification takes place in the second stage. Nitrification. Both BOD removal and nitrification can be accomplished in rock or plastic packing trickling filters operated at low organic loadings. Heterotrophic bacteria, with higher yield coefficients and faster growth rates, are more competitive than nitrifying bacteria for space on the fixed-film packing. Thus, significant nitrification occurs only after the BOD concentration is appreciably reduced. Bruce et al.(1975) demonstrated that the effluent BOD had to be less than 30 mg/L to initiate nitrification and less than 15 mg/L for complete nitrification. Harrem es (1982) considered the soluble BOD, and concluded that a concentration less than 20 mg/L is needed to initiate nitrification. Nitrification can also be accomplished in separate trickling filters following secondary treatment. Design of Physical Facilities Factors that must be considered in the design of trickling filters include (1) type and physical characteristics of filter packing to be used; (2) dosing rate; (3) type and dosing characteristics of the distribution system; (4) configuration of the underdrain system; (5) provision for adequate airflow (i.e., ventilation), either natural or forced air; and (6) sealing tank design. Filter Packing. The ideal filter packing is a material that has a high surface area per unit of volume, is low in cost, has a high durability, and has a high enough porosity so that clogging is minimized and good air circulation can occur. Typical trickling filter packing materials are shown on Fig. 8-3. The physical characteristics of commonly used filter packings, including those shown on Fig. 8-3, are reported in Table 8-2. Until the mid-1960s, the material used was either high-quality granite or blast-furnace slag. Since the 1960s, plastic packing material, either cross-flow or vertical- flow, has become the packing of choice in the United States. Where locally available, rock has the advantage of low cost. The most suitable material is rounded fiver rock or crashed stone, graded to a uniform size so that 95 percent is within the range of 75 to 100 mm. The specification of size uniformity is a way of ensuring adequate pore space for wastewater flow and air circulation. Other important characteristics of filter packing materials are strength and durability. Durability may be determined by the sodium sulfate test, which is used to test the soundness of concrete aggregates. Because of the weight of the packing, the depth of rock filters is usually on the order of 2 m. The low void volume of rock limits the space available for airflow and increases the potential for plugging and flow short circuiting. Because of plugging, the organic loadings to rock filters are more commonly in the range of 0.3 to 1.0 kg BOD/m3·d. Various forms of plastic packings are shown on Fig. 8-3. Molded plastic packing materials have the appearance of a honeycomb. Flat and corrugated sheets of polyvinyl chloride are bonded together in rectangular modules. The sheets usually have a corrugated surface for enhancing slime growth and retention time. Each layer of modules is turned at right angles to the previous layer to further improve wastewater distribution. The two basic types of corrugated plastic sheet packing are vertical and cross flow (see Fig. 8-3b, c, and d). Both types of packing are reported to be effective in BOD and TSS removal over a wide range of loadings. Biotowers as deep as 12 m have been constructed using plastic packing, with depths in the range of 6 m being more common. In biotowers with vertical plastic packing, cross-flow packing can be used for the uppermost layers to enhance the distribution across the top of the filter. The high hydraulic capacity, high void ratio, and resistance to plugging offered by these types of packing can best be used in a high-rate-type filter. Redwood or other wood packings have been used in the past, but with the limited availability of redwood, wood packing is seldom used currently. Plastic packing has the advantage of requiring less land area for the filter structure than rock due to the ability to use higher loading rates and taller trickling filters. Grady et al. (1999) noted that when loaded at the similar low organic loadings rates (less than 1.0 kg BOD/m3·d), the performance of rock filters compared to filters with plastic packing is similar. At higher organic loading rates, however, the performance of filters with plastic packing is superior. The higher porosity, which provides for better air circulation and biofilm sloughing, is a likely explanation for the improved performance. Dosing Rate. The dosing rate on a trickling filter is the depth of liquid discharged on top of the packing for each pass of the distributor. For higher distributor rotational speeds, the dosing rate is lower. In the past, typical rotational speeds for distributors were about 0.5 to 2 min per revolution. With two to four arms, the trickling filter is dosed every 10 to 60 s. Results from various investigators have indicated that reducing the distributor speed results in better filter performance. Hawkes (1963) showed that rock trickling filters dosed every 30 to 55 min/rev outperformed a more conventional operation of 1 to 5 min/rev. Besides improved BOD removal, there were dramatic reductions in the Psychoda and Anisopus fly population, biofilm thickness ,and odors. Albertson and Davies (1984) showed similar advantages from an investigation of reduced distributor speed. At a higher dosing rate, the larger water volume applied per revolution (1) provides greater wetting efficiency, (2) results in greater agitation, which causes more solids to flush out of the packing, (3) results in a thinner biofilm, and (4) helps to wash away fly eggs. The thinner biofilm creates more surface area and results in a more aerobic biofilm. If the high dosing rate is sustained to control the biofilm thickness, the treatment efficiency may be decreased because the liquid contact time in the filter is less. A daily intermittent high dose, referred to as a flushing dose, is used to control the biofilm thickness and solids inventory. A combination of a once-per-day high flushing rate and a lower daily sustained dosing rate is recommended as a function of the BOD loading as shown in Table 8-3. The data in Table 8-3 are guidelines to establish a dosing range. Optimization of the dosing rate and flushing rate and frequency is best determined from field operation. Flexibility in the distributor design is needed to provide a range of dosing rates to optimize the trickling filter performance. Distribution Systems. A distributor consists of two or more arms that are mounted on a pivot in the center of the filter and revolve in a horizontal plane (see Fig. 8-4). The arms are hollow and contain nozzles through which the wastewater is discharged over the filter bed. The distributor assembly may be driven either by the dynamic reaction of the wastewater discharging from the nozzles or by an electric motor. The flow-driven rotary distributor for trickling filtration has been used traditionally for the process because it is reliable and easy to maintain. Motor drives are used in more recent designs. Clearance of 150 to 225 mm should be allowed between the bottom of the distributor arm and the top of the bed. The clearance permits the wastewater streams from the nozzles to spread out and cover the bed uniformly, and it prevents ice accumulations from interfering with the distributor motion during freezing weather. Distributors are manufactured for trickling filters with diameters up to 60 m. Distributor arms may be of constant cross section for small units, or they may be tapered to maintain minimum transport velocity. Nozzles are spaced unevenly so that greater flow per unit of length is achieved near the periphery of the filter than at the center. For uniform distribution over the area of the filter, the flowrate per unit of length should be proportional to the radius from the center. Headloss through the distributor is in the range of 0.6 to 1.5 m. Important features that should be considered in selecting a distributor are the ruggedness of construction, ease of cleaning, ability to handle large variations in flowrate while maintaining adequate rotational speed, and corrosion resistance of the material and its coating system. In the past, fixed nozzle distribution systems were used for shallow rock filters (see Fig. 8-4b). Fixed nozzle distribution systems consist of a series of spray nozzles located at the points of equilateral triangles covering the filter bed. A system of pipes placed in the filter is used to distribute the wastewater uniformly to the nozzles. Special nozzles having a flat spray pattern are used, and the pressure is varied systematically so that the spray falls first at a maximum distance from the nozzle and then at a decreasing distance as the head slowly drops. In this way, a uniform dose is applied over the whole area of the bed. Half-spray nozzles are used along the sides of the filter. In current practice, fixed nozzle systems are seldom used. Underdrains. The wastewater collection system in a trickling filter consists of underdrains that catch the filtered wastewater and solids discharged from the filter packing for conveyance to the final sedimentation tank. The underdrain system for a rock filter usually has precast blocks of vitrified clay or fiberglass grating laid on a reinforced-concrete subfloor (see Fig. 8-5). The floor and underdrains must have sufficient strength to support the packing, slime growth, and the wastewater. The floor and underdrain block slope to a central or peripheral drainage channel at a 1 to 5 percent grade. The effluent channels are sized to produce a minimum velocity of 0.6 m/s at the average daily flowrate. Underdrains may be open at both ends, so that they may be inspected easily and flushed out if they become plugged. The underdrains also allow ventilation of the filter, providing the air for the microorganisms that live in the filter slime. The underdrains should be open to a circumferential channel for ventilation at the wall as well as to the central collection channel. The underdrain and support system for plastic packing consists of either a beam and column or a grating. A typical underdrain system for a tower filter is shown on Fig. 8-6. The beam and column system typically has precast-concrete beams supported by columns or posts. The plastic packing is placed over the beams, which have channels in their tops to ensure free flow of wastewater and air. All underdrain systems should be designed so that forced-air ventilation can be added at a later date if filter operating conditions should change. Airflow. An adequate flow of air is of fundamental importance to the successful operation of a trickling filter to provide efficient treatment and to prevent odors. Natural draft has historically been the primary means of providing airflow, but it is not always adequate and forced ventilation using low-pressure fans provides more reliable and controlled airflow. In the case of natural draft, the driving force for airflow is the temperature difference between the ambient air and the air inside the pores. If the wastewater is colder than the ambient air, the pore air will be cold and the direction of flow will be downward. If the ambient air is colder than the wastewater, the flow will be upward. The latter is less desirable from a mass transfer point of view because the partial pressure of oxygen (and thus the oxygen transfer rate) is lowest in the region of highest oxygen demand. In many areas of the country, there are periods, especially during the summer, when essentially no airflow occurs through the trickling filter because temperature differentials are negligible. The volumetric air flowrate may be estimated by setting the draft equal to the sum of the head losses that result from the passage of air through the filter and underdrain system. Where natural draft is used, the following needs to be included in the design: 1. Underdrains and collecting channels should be designed to flow no more than half full to provide a passageway for the air. 2. Ventilating access ports with open grating types of covers should be installed at both ends of the central collection channel. 3. Large-diameter filters should have branch collecting channels with ventilating manholes or vent stacks installed at the filter periphery. 4. The open area of the slots in the top of the underdrain blocks should not be less than 15 percent of the area of the filter. 5. One square meter gross area of open grating in ventilating manholes and vent stacks should be provided for each 23 m2 of filter area. The use of forced- or induced-draft fans is recommended for trickling filter designs to provide a reliable supply of oxygen. The costs for a forced-draft air supply are minimal compared to the benefits. For a 3800 m3/d wastewater treatment flow the estimated power requirement is only about 0.15 kW. As an approximation, an airflow of 0.3 m3/m2·min of filter area in either direction is recommended. A downflow direction has some advantage by providing contact time for treating odorous compounds released at the top of the filter and by providing a richer air supply where the oxygen demand is highest. Forced-air designs should provide multiple air distribution points by the use of fans around the periphery of the tower or the use of air headers below the packing material, as there is very little headloss through the filter packing to promote air distribution. For applications with extremely low air temperature, it may be necessary to restrict the flow of air through the filter to keep it from freezing. Settling Tanks. The function of sealing tanks that follow trickling filters is to produce a clarified effluent. They differ from activated-sludge settling tanks in that the clarifier has a much lower suspended solids content and sludge recirculation is not necessary. All the sludge from trickling filter settling tanks is sent to sludge-processing facilities or returned to the primary clarifiers to be settled with primary solids. Trickling filter performance has historically suffered from poor clarifier designs. The use of shallow clarifiers for trickling filter applications, with relatively high overflow rates, was recommended in previous versions of the "Ten States Standards". Unfortunately, the use of shallow clarifiers typically resulted in poor clarification efficiency. Clarifier overflow rates recommended currently in the “Ten States Standards” are more in line with those used for the activated-sludge process. Clarifier designs for trickling filters should be similar to designs used for activated-sludge process clarifiers, with appropriate feedwell size and depth, increased sidewater depth, and similar hydraulic overflow rates. With proper clarification designs, single-stage trickling filters can achieve a less than 20 mg/L concentration of BOD and TSS. Process Design Considerations The trickling filter process appears simple, consisting of a bed of packing material through which wastewater flows and an external clarifier. In reality, a trickling filter is a very complex system in terms of the characteristics of the attached growth and internal hydrodynamics. In view of these complexities, trickling filter designs are based mainly on empirical relationships derived from pilot-plant and full-scale plant experience. In this section trickling filter performance for BOD removal and nitrification, features that affect performance, and commonly used process design approaches are reviewed. Effluent Characteristics. Historically, trickling filters have been considered to have major advantages of using less energy than activated-sludge treatment and being easier to operate, but have disadvantages of more potential for odors and lower-quality effluent. Some of these shortcomings, however, have been due more to inadequate ventilation, poor clarifier design, inadequate protection from cold temperatures, and the dosing operation. With proper design, trickling filters have been used successfully in a number of applications. Typical applications, process loadings, and effluent quality are summarized in Table 8-4. Loading Criteria. In the activated-sludge process, biodegradation efficiency was shown to be related to the average SRT for the biomass or the F/M ratio. For both of these parameters, the solids or biomass can be sampled and reasonably well quantified. However, for trickling filters quantifying the biomass in the system is not possible, and only recently has progress been made to control the solids inventory to some degree by the dosing operation. The attached growth is not uniformly distributed in the trickling filter, the biofilm thickness can vary, the biofilm solids concentration may range from 40 to 100 g/L, and the liquid does not uniformly flow over the entire packing surface area, which is referred to as the wetting efficiency. With the inability to quantify the biological and hydrodynamic properties of field trickling filter systems, broader parameters such as volumetric organic loading, unit area loadings, and hydraulic application rates have been used as design and operating parameters to relate to treatment efficiency. For BOD removal, the volumetric BOD loading has been correlated well with treatment performance for both BOD removal and nitrification in combined BOD and nitrification trickling filter designs. The original design model for rock trickling filters was developed by the National Research Council (NRC) in the early 1940s at military installations. The NRC formulations were based on field data for BOD removal efficiency and the organic loading rate. The NRC design model was used even though there was a significant amount of data scatter. Bruce and Merkens (1970 and 1973) found that the organic loading rate controlled trickling filter performance and not the hydraulic application rate. For combined BOD removal and nitrification systems, nitrification efficiency has been related to the volumetric BOD loading. For tertiary nitrification applications, very little BOD is applied to the trickling filter and a thin biofilm develops on the packing that consists of a high proportion of nitrifying bacteria. The nitrification removal efficiency is related to the packing surface area and correlated with the specific nitrogen loading rate in terms of g NH4-N removed/m2 packing surface area·d. BOD Removal Design. The first empirical design equations for BOD removal were developed for rock trickling filters from an analysis of trickling filter performance at 34 plants at military installations treating domestic wastewater. The effect of volumetric BOD loading and recirculation ratio on treatment performance was accounted for in the equations. The equations given below should only be used as an estimate of performance as they are based on a limited data base and the influent BOD values at the installations sampled were relatively high compared to most municipal primary effluents. The BOD removal includes the effect of the secondary clarifier, so that if the equation overpredicts treatment performance, improved and deeper secondary clarifier designs used today may help in meeting expected treatment performance. Recirculatlon. The minimum hydraulic application rate recommended by Dow Chemical is 0.5 L/m2.s to provide maximum efficiency. Shallow tower designs require recirculation to provide minimum wetting rates. When above the minimum hydraulic application rate, recirculation was reported to have little benefit. For filters with low hydraulic application rates and higher organic loadings, recirculation may improve efficiency. For design systems such as rock filters with low hydraulic application rates, recirculation provides a higher flow to improve wetting and flushing of the filter packing. Solids Production. Solids production from trickling filter processes will depend on the wastewater characteristics and the trickling filter loading. At lower organic loading rates, a greater amount of the particulate BOD is degraded, the biomass has a longer SRT, and, as a result, less biomass is produced. Mass Transfer Limitations. One of the concerns in the process design for trickling filters is at what organic loading the filter performance becomes limited by oxygen transfer. When this condition occurs, treatment efficiency at the higher organic load is limited and odors may be produced due to anaerobic activity in the biofilm. Based on an evaluation of the data in the literature, for influent BOD concentrations in the range of 400 to 500 mg/L, oxygen transfer may become limiting. Hinton and Stensel (1994) reported that oxygen availability controlled organic substrate removal rates at soluble biodegradable COD loadings above 3.3 kg/m3.d. Nitrification Design Two types of process design approaches have been used to accomplish biological nitrification in trickling filters, either in a combined system along with BOD removal or in a tertiary application following secondary treatment and clarification for BOD removal. The secondary treatment process may be a suspended growth or fixed-film process. Empirical design approaches based on pilot-plant and full-scale plant results are again used to guide nitrification designs in view of the difficulty in predicting the actual biofilm coverage area, wetting efficiency, and biofilm thickness and density. Major impacts on nitrification performance are the influent BOD concentration and dissolved oxygen concentration within the trickling filter bulk liquid. As the BOD to TKN ratio of the influent wastewater increases, a greater proportion of the trickling filter packing area is covered by heterotrophic bacteria and the apparent nitrification rate (kg/m3.d) based on the total trickling filter volume is decreased. A number of investigations have shown that BOD, if at high enough concentration, inhibits nitrification. Studies by Harrem es (1982) showed that nitrification (1) could occur at a maximum rate at soluble BOD (sBOD) concentrations below 5 mg/L, (2) was inhibited in proportion to the sBOD concentration above 5 mg/L, and (3) was insignificant, in proportion to the sBOD concentration of 30 mg/L or more. In a study with a flat plat experimental design, Huang and Hopson (1974) demonstrated a steady inhibition of nitrification rates occurred as the sBOD concentration was increased from 1.0 to 8.0 mg/L. Figueroa and Silverstein (1991) found that nitrification rates in fixed-film processes are inhibited at BOD concentrations above 10 mg/L, which finding is in agreement with observations by others. Design Basis for Combined BOD Removal and Nitrification. Nitrification efficiency has been correlated with the volumetric BOD loading for rock trickling filters. For 90 percent nitrification efficiency, a BOD loading of less than 0.08 kg BOD/m3.d is recommended. At a loading of about 0.22 kg BOD/m3.d, about 50 percent nitrification efficiency could be expected. It was noted that increased recirculation rates improved nitrification performance. Instead of volumetric BOD loading values, the nitrification efficiency has been related to the BOD loading based on the packing surface area. In comparing nitrification performance for both rock and plastic packing, Parker and Richards (1986) found that the nitrification efficiency was similar at similar BOD surface loading rates (g BOD/m2.d) for both packings. A surface loading rate as low as 2.4 g BOD/m2.d is necessary for ≥90 percent NH4-N removal. The DO concentration had a greater effect on the nitrification rates than temperature. The effect of DO concentration is supported by fundamental mass transfer considerations in which it can be shown that a bulk liquid DO concentration of 2.8 mg/L is required for nitrification without oxygen diffusion limitations, at a liquid NH4-N concentration of 1.0 mg/L. Tertiary Nitrification. A number of facilities exist where trickling filters with plastic packing are used after secondary treatment for nitrification. The influent BOD concentration is relatively low at <10 mg/L and in some cases less than 5 mg/L. Nitrification trickling filter performance will depend on the ammonia loading rate, oxygen availability, temperature, and packing design. Effluent NH4-N concentrations will vary with summer and winter operation and can range from < 1.0 mg/L at warm temperatures and from < l to 4 mg/L at cold temperatures. Hydraulic application rates may range from 0.40 to 1.0 L/m2.s. The higher nitrification rates represent the results obtained at higher hydraulic loading rates, with effluent NH4-N concentrations above 5 mg/L. Other investigators have observed minimal temperature effects for tertiary nitrification, and have attributed the minimal observed rate change more to the effect of dissolved oxygen concentration and hydraulics. It is generally well accepted that in the upper portion of the trickling filter the nitrification rate is limited by oxygen availability and diffusion into the biofilm. To overcome the oxygen limitation, forced-draft air is generally used to assure maximum oxygen availability. Higher hydraulic rates that provide better wetting efficiency and agitation of the biofilm surface generally produce better performance. Because plugging is less of an issue with the exception of snails, a medium-density packing material is preferred (i.e., specific surface area of about 100 m2/m3) to provide more area as a function of the percent of the reactor volume. In a large fraction of the nitrification tower, the NH4-N concentration is high enough so that the nitrification rate is oxygen-limited and thus zero-order with respect to nitrogen. Farther down in the packing as the NH4-N concentration decreases, the nitrification rate is limited by the NH4-N concentration and thus decreases. The decline in nitrification rate is further affected by a lower growth of nitrifying bacteria due to the low amount of NH4-N available. The use of nitrification trickling filters in series with operational modifications has been shown to compensate for this limitation. The order of operation of the towers is reversed every few days so that a higher nitrifying bacteria population can be developed and be available where the NH4-N concentration is low. Anderson et al. (1994) showed a 20 percent improvement in nitrification efficiency with this method. Predator Problems. A significant problem for nitrifying filters is the development of a snail population, which may graze on the biofilm to reduce the nitrifying bacteria population and nitrification performance. In addition, snails can cause problems with plugging of channels and pumps, accumulating in digesters, and causing wear and tear on equipment. A sump can be provided in an effluent collection chamber upstream of the secondary clarifiers to facilitate removal of snails from the effluent. Methods proposed to control snails are periodic flooding of the trickling filter, reducing the distributor speed to create higher flushing rates, high pH dosing, chlorination, saline water dosing, and dosing with copper sulfate at 0.4 g/L. Periodic flooding does not appear to control snails but does eliminate filter flies. Some success has been claimed for reduced distributor speed and high pH treatment. An alkaline backwash to control the snail population and increase nitrification rates was demonstrated successfully at the Littleton/Englewood, CO, wastewater treatment plant. The nitrification tower trickling filters were flooded on three separate occasions, from October to November 1993, after nitrification efficiency had declined due to predator growth. For the first backwash, the pH was 10, but a pH of 9.0 was used for the last two backwashes to minimize loss of nitrification activity. After the high-pH backwash, nitrification efficiency improved and was maintained long-term. 8-3 Rotating Biological Contactors Rotating biological contactors (RBCs) were first installed in West Germany in 1960 and later introduced in the United States. Hundreds of RBC installations were installed in the 1970s and the process has been reviewed in a number of reports. An RBC consists of a series of closely spaced circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated through it (see Fig. 8-7). The cylindrical plastic disks are attached to a horizontal shaft and are provided at standard unit sizes of approximately 3.5 m in diameter and 7.5 m in length. The surface area of the disks for a standard unit is about 9300 m2, and a unit with a higher density of disks is also available with approximately 13,900 m2 of surface area. The RBC unit is partially submerged (typically 40 percent) in a tank containing the wastewater, and the disks rotate slowly at about 1.0 to 1.6 revolutions per minute (see Fig. 8-11a). Mechanical drives are normally used to rotate the units, but air-driven units have also been installed. In the air-driven units, an array of cups (see Fig. 8-1lc) is fixed to the periphery of the disks and diffused aeration is used to direct air to the cups to cause rotation. As the RBC disks rotate out of the wastewater, aeration is accomplished by exposure to the atmosphere. Wastewater flows down through the disks, and solids sloughing occurs. Similar to a trickling filter, RBC systems require pretreatment of primary clarification or fine screens and secondary clarification for liquid/solids separation. A submerged RBC design was also introduced in the early 1980s but has seen limited applications. The submergence is 70 to 90 percent and air-drive units are used to provide oxygen and rotation. The advantages claimed for the submerged unit are reduced loadings on the shaft and bearings, improved biomass control by air agitation, the ability to use larger bundles of disks, and ease of retrofit into existing aeration tanks. However, because of the comparatively low levels of dissolved oxygen in the liquid, biological degradation activity by the submerged units may be oxygen-limited. To prevent algae growth, protect the plastic disks from the effects of ultraviolet exposure, and to prevent excessive heat loss in cold weather, RBC units are covered (see Fig. 8-7b). The history of RBC installations has been troublesome due to inadequate mechanical design and lack of full understanding of the biological process. Structural failure of shafts, disks, and disk support systems has occurred. Development of excessive biofilm growth and sloughing problems has also led to mechanical shaft, bearing, and disk failures. Many of these problems were related to a lack of conservatism in design and scale-up issues from pilot-plant to full-scale units. Many of the problems associated with earlier installations have been solved and numerous RBC installations are operating successfully. Process Design Considerations There are many similarities between RBC design considerations and those described for trickling filters. Both systems develop a large biofilm surface area and rely on mass transfer of oxygen and substrates from the bulk liquid to the biofilm. The complexity in the physical and hydrodynamic characteristics requires that the design of the RBC process be based on fundamental information from pilot-plant and field installations. As for trickling filters, the organic loading affects BOD removal efficiency and the nitrogen loading after a minimal BOD concentration is reached affects the nitrification efficiency. In contrast to the trickling filter where the wastewater flow approaches a plug flow hydraulic regime, the RBC units are rotated in a basin containing the wastewater, so that separate baffled basins are needed to develop the benefits of a staged biological reactor design. The design of an RBC system must include the following considerations: (1) staging of the RBC units, (2) loading criteria, (3) effluent characteristics, and (4) secondary clarifier design. Typical design information for RBCs is presented in Table 8-5. Staging of RBC Units. Staging is the compartmentalization of the RBC disks to form a series of independent cells. Based on mass transfer and biological kinetic fundamentals, higher specific substrate removal rates will occur in RBC biofilms at higher bulk liquid substrate concentrations. Because a low effluent substrate concentration and high specific substrate removal rates are generally the ultimate treatment goal, reduced disk area requirements can be realized only by using staged-RBC units. The RBC process application typically consists of a number of units operated in series. The number of stages depends on the treatment goals, with two to four stages for BOD removal and six or more stages for nitrification. Stages can be accomplished by using baffles in a single tank or by use of separate tanks in series. Staging promotes a variety of conditions where different organisms can flourish in varying degrees from stage to stage. The degree of development in any stage depends primarily on the soluble organic concentration in the stage bulk liquid. As the wastewater flows through the system, each subsequent stage receives an influent with a lower organic concentration than the previous stage. Typical RBC staging arrangements are illustrated on Fig. 8-8. For small plants, RBC drive shafts are oriented parallel to the direction of row with disk clusters separated by baffles (see Fig. 8-8a). In larger installations, shafts are mounted perpendicular to flow with several stages in series to form a process train (see Fig. 8-8b). To handle the loading on the initial units, step feed (see Fig. 8-8d) or a tapered system (see Fig. 8-8e) may be used. Two or more parallel flow trains should be installed so the units can be isolated for turndown or repairs. Tank construction may be reinforced concrete or steel, with steel preferred at smaller plants. Treatment systems employing RBCs have been used for BOD removal, pretreatment of industrial wastewater, combined BOD removal and nitrification, tertiary nitrification, and denitrification. The principal advantages of the RBC process are simplicity of operation and relatively low energy costs. The History of RBC Loading Criteria. Based on experience, the performance of an RBC system is related to the specific surface loading rate of total and soluble BOD for BOD removal and NH4-N for nitrification. For successful treatment, the loading rates must be within the oxygen transfer capability of the system. Poor performance, odors, and biofilm sloughing problems have occurred when the oxygen demand due to the BOD loading has exceeded the oxygen transfer capability. A characteristic of this problem is the development of Beggiatoa, a reduced-sulfur oxidizing bacteria, on the outer portion of the biofilm, which prevents sloughing. A thick biofilm can develop to create enough weight to stress the structural strength of the plastic disks and shaft. Under overloaded conditions, anaerobic conditions develop deep in the attached film. Sulfate is reduced to H2S, which diffuses to the outer layer of the biofilm, where oxygen is available. Beggiatoa, a filamentous bacteria, which is able to oxidize the H2S and other reduced sulfur compounds, forms a tenacious whitish biofilm that does not slough under the normal RBC rotational sheer conditions. In designing RBC units, it is important to select a low enough BOD loading for the initial units in the staged design to prevent overloading. Odor problems are most frequently caused by excessive organic loadings, particularly in the first stage. Because the soluble BOD is used more rapidly in the first stage of an RBC system, most manufacturers of RBC equipment specify a specific soluble BOD loading in the range of 12 to 20 g sBOD/m2.d for the first stage. Assuming a 50 percent soluble BOD fraction, the total BOD loading ranges from 24 to 30 g BOD/m2.d. For some designs that involve higher-strength wastewaters, the loading criteria are met by splitting the flow to multiple RBC units in the first stage or using a step feeding approach as shown on Fig. 8-8d. For nitrification, the design approach for RBC systems can be very similar to that shown for tertiary nitrification trickling filters after the sBOD concentration is depleted in RBC units preceding nitrification. An sBOD concentration of less than 15 mg/L must be met before a significant nitrifying population can be developed on the RBC disks. The maximum nitrogen surface removal rate has been observed to be about 1.5 g N/m2.d, which is quite similar to values observed for trickling filters. Effluent Characteristics. Treatment systems with RBCs can be designed to provide secondary or advanced levels of treatment. Effluent BOD characteristics for secondary treatment are comparable to well-operated activated-sludge processes. Where a nitrified effluent is required, RBCs can be used to provide combined treatment for BOD and ammonia nitrogen, or to provide separate nitrification of secondary effluent. Typical ranges of effluent characteristics are indicated in Table 8-5. An RBC process modification in which the disk support shaft is totally submerged has been used for denitrification of wastewater. Physical Facilities for RBC Process The suppliers of RBC equipment differ in their disk designs, shafts, and packing support, and configuration designs. The principal elements of an RBC system design am the shaft, disk materials and configuration, drive system, enclosures, and settling tanks. Shafts. The RBC shafts are used to support and rotate the plastic disks. Maximum shaft length is presently limited to 8.23 m with 7.62 m occupied by disks. Shorter shaft lengths ranging from 1.52 to 7.62 m are also available. Shaft shapes include square, round, and octagonal, depending on the manufacturer. Steel shafts are coated to protect against corrosion and thickness ranges from 13 to 30 mm. Structural details and the life expectancy of the disk shaft are important design considerations. Disk Materials. High-density polyethylene is the material used most commonly for the manufacture of RBC disks, which are available in different configurations or corrugation patterns. Corrugations increase the available surface area and enhance structural stability. The types of RBC disks, classified based on the total area of disks on the shaft, are commonly termed low- (or standard) density, medium-density, and high-density. Standard-density disks, defined as disks with a surface area of 9300 m2 per 8.23 m shaft, have larger spaces between disks and are normally used in the lead stages of an RBC process flow diagram. Medium- and high-density disk assemblies have surface areas of 11,000 to 16,700 m2 per 8.23-m shaft, and are used typically in the middle and final stages of an RBC system where thinner biological growths occur. Drive Systems. Most RBC units are rotated by direct mechanical drive units attached directly to the central shaft. Motors are typically rated at 3.7 or 5.6 kW per shaft. Air-drive units are also available. The air-drive assembly consists of deep plastic cups attached to the perimeter of the disks, an air header located beneath the disks, and an air compressor. Airflows necessary to achieve design rotational speeds are about 5.3 m3/min for a standard-density shaft and 7.6 m3/min for a high-density shaft. The release of air into the cups creates a buoyant force that causes the shaft to run. Both systems have proved to be mechanically reliable. Variable -speed features can be provided to regulate the speed of rotation of the shaft. Tankage. Tankage for RBC systems has been optimized at 0.0049 m3/m2 of disk area, resulting in a stage volume of 45 m3 for a shaft with a disk area of 9300 m2. Based on this volume, a detention time of 1.44 h is provided for a hydraulic loading of 0.08 m3/m2.d. A typical sidewater depth is 1.5 m to accommodate a 40 percent submergence of the disks. Enclosures. Segmented fiberglass reinforced plastic covers are usually provided over each shaft. In some cases, units have been housed in a building for protection against cold weather, to improve access, or for aesthetic reasons. RBCs are enclosed to (1) protect the plastic disks from deterioration due to ultraviolet light, (2) protect the process from low temperatures, (3) protect the disks and equipment from damage, and (4) control the buildup of algae in the process. Settling Tanks. Settling tanks for RBCs are similar to trickling filter settling tanks in that all of the sludge from the settling tanks is removed to the sludge processing facilities. Typical design overflow rates for settling tanks used with RBCs are similar to that described for trickling filters with plastic packing. RBC Process Design Empirical design approaches have been developed for RBC systems based on pilot-plant and full-scale plant data and that consider such fundamental factors as the disk surface area and specific loadings in terms of g/m2 disk area.d. Approaches for designing staged RBC systems for BOD removal and nitrification are presented in this section. BOD Removal. In a design comparison, the models generally resulted in lower recommended BOD loadings than that determined from manufacturer's literature and were, in some cases, similar for BOD removals below 90 percent. Nitrification. Treatment systems employing RBC units can be used to develop nitrifying biofilms for nitrification of secondary effluents or at low sBOD loadings where nitrification can occur in BOD removal systems. For tertiary nitrification the same procedure used for the design of trickling filters can be followed. A rn,max value of 1.5 g N/m2.d is recommended based on field test results. For combined BOD removal and nitrification, nitrification will be prevented or inhibited by the addition of sBOD to the RBC unit. The nitrifying bacteria can compete for space on the RBC disk once the sBOD concentration is reduced to 10 to 15 mg/L. The sBOD concentration remaining in an RBC tank will be related to the sBOD loading. 8-4 Combined Aerobic Treatment Processes Several treatment process combinations have been developed that couple trickling filters with the activated-sludge process. The combined biological processes are known as dual processes or coupled trickling filter/activated-sludge systems. Combined processes have resulted as part of plant upgrading where either a trickling filter or activated-sludge process is added; they have also been incorporated into new treatment plant designs. Combined processes have the advantages of the two individual processes, which can include (1) the stability and resistance to shock loads of the attached growth process, (2) the volumetric efficiency and low energy requirement of attached growth process for partial BOD removal, (3) the role of attached growth pretreatment as a biological selector to improve activated-sludge settling characteristics, and (4) the high-quality effluent possible with activated-sludge treatment. The three principal types of combined processes are described below. Trickling Filter/Solids Contact and Trickling Filter/Activated-Sludge Processes The first group Of the combined treatment processes, as illustrated on Fig. 8-9, is commonly referred to as the trickling filter/solids contact (TF/SC) or trickling filter/activated-sludge (TF/AS) process. The principal difference between these processes is the shorter aeration period in the TF/SC process of minutes versus hours for the TF/AS process. Both processes use a trickling filter (with either rock or plastic packing), an activated-sludge aeration tank, and a final clarifier. In both processes, the trickling filter effluent is fed directly to the activated-sludge process without clarification and the return activated sludge from the secondary clarifier is fed to the activated-sludge aeration basin. A process modification that has been incorporated only into the TF/SC process (see Fig. 8-9a, c, and d) is a return-sludge aeration tank and flocculating center-feed well for the clarifier. The TF/AS process is illustrated on Fig. 8-9b. The most common application for the TF/AS process is where the trickling filter is designed as a roughing filter for 40 to 70 percent BOD removal and may be referred to as a roughingfilter/activated-sludge (RF/AS) process. The trickling filter loading is about 4 times that used for the TF/SC process. The aeration basin hydraulic retention time may be 50 to 70 percent of that used in the conventional activated-sludge process. The RF/AS process is attractive for treating higher-strength industrial waste-water because of the relatively low energy use per quantity of BOD removed on the trickling filter. The use of the trickling filter also results in good SVI values for the activated-sludge mixed liquor, as it acts as a biological selector in removing soluble BOD. The main differences between the TF/SC and RF/AS processes are in their trickling filter loadings and activated-sludge SRT values, which are summarized in Table 8-7. A relatively low organic load for the trickling filter is used for the TF/SC process, and the purpose of the aeration tank is to remove remaining soluble BOD and to develop a flocculent activated-sludge mass that incorporates dispersed solids from trickling filter sloughing. The process is able to produce an advanced treatment effluent quality, which is low in TSS and BOD concentrations, near 10 mg/L and typically less than 15 mg/L. The detention time and SRT of the TF/SC aeration contact basin is quite low, ranging from 0.15 to 1.0 h and 0.5 to 2.0 d, respectively. In some designs an aerated channel is used to provide the aerobic contact time. The TF/SC process has also been demonstrated for nitrification applications. At a low enough organic loading, nitrification can occur in the trickling filter. At BOD loadings ranging from 0.40 to 0.70 kg/m3.d, nitrification occurred in a high-density trickling filter tower at Windsor, Ontario, at temperatures ranging from 11 to 20。C. The trickling filter effluent NH4-N concentration averaged 2.8 mg/L, and the final effluent concentration averaged 1.9 mg/L with an aeration contact zone detention time of 15 rain and 2 to 4 d SRT. Chemical treatment was used with primary clarification to result in low enough BOD (30 to 70 g/m3) and TSS concentrations to allow use of the high-density plastic packing without plugging problems. Daigger et al.(1994) also showed similar performance characteristics for low-loaded trickling filters in the TF/SC process and noted that nitrifying bacteria sloughed from the trickling filter result in nitrification in the contact activated-sludge process at lower than theoretical SRTs due to the seed population. Activated Biofilter and Biofilter Activated-Sludge Processes The second group of combined processes is similar to the first, described above, with the exception that the PAS is returned directly to the trickling filter as illustrated on Fig. 8-10 and an aeration basin may or may not be used. The two combined processes are termed the activated biofilter (ABF) and the biofilter/activated-sludge (BF/AS) process. The ABF and BF/AS processes are not used much today, in part because the original designs relied on redwood filter packing, which is available only at prohibitive cost. In both cases, rock cannot be used because of the potential plugging and oxygen availability problems created by feeding return activated sludge to the trickling filter. High-rate plastic packing can be used in lieu of redwood packing. The ABF process can produce a secondary effluent quality at low organic loads to the trickling filter. At higher organic loading rates, an acceptable secondary effluent quality with the ABF process has been difficult to produce. To improve effluent quality, the ABF process is followed by a short HRT aeration basin, which fits the description of the BF/AS process (see Fig. 8-10b). In essence the BF/AS process is very similar to the RF/AS process with the exception of RAS return to the trickling filter instead of to the aeration basin. Harrison et al.(1984) showed similar performance for BF/AS and RF/AS processes for full-scale plants surveyed. Series Trickling Filter-Activated-Sludge Process In the third approach employing combined processes, a trickling filter and an activated-sludge process are operated in series, with an intermediate clarifier between the trickling filter and activated-sludge process (see Fig. 8-11). The combination of a trickling filter process followed by an activated-sludge process is often used (1) to upgrade an existing activated-sludge system, (2) to reduce the strength of wastewater where industrial and domestic wastewater is treated in common treatment facilities, and (3) to protect a nitrification activated-sludge process from toxic and inhibitory substances. In systems treating high-strength wastes, intermediate clarifiers are used between the trickling filters and the activated-sludge units to reduce the solids load to the activated-sludge system and to minimize the aeration volume required. Typical process design parameters for the trickling filter activated-sludge process are given in Table 8-8. Design Considerations for Combined Trickling Filter Activated-Sludge Systems For the combined processes without intermediate clarification, the activated-sludge design is affected by the trickling filter design loading and performance. The amount of oxygen required in the activated-sludge aeration basin depends on how much BOD is removed in the trickling filter. All of the solids produced, which are the biological solids plus the influent nonbiodegradable solids, will end up in the activated-sludge aeration tank, but the amount of biomass is affected by how much BOD is removed in the trickling filter. An estimate of the SRT in the trickling filter as a function of the BOD loading is presented on Fig. 8-12. At higher BOD loadings the concentration of biomass produced in the trickling filter can be determined based on the estimated SRT and amount of BOD removed in the trickling filter. The amount of BOD removed in the trickling filter is difficult to predict. Both particulate and soluble BOD are removed by biomass in the trickling filter, and current empirical design models are generally based on influent and final settled BOD, and thus do not distinguish between particulate (pBOD) and soluble BOD (sBOD) removal rates. These models may be used to estimate the sBOD removal as the final suspended solids concentration after settling and their BOD contribution can be estimated. However, pBOD not degraded in the trickling filter will most likely be degraded in the activated-sludge process, therefore affecting the oxygen demand. Thus, to determine the oxygen required for the activated-sludge process, the amount of pBOD degraded in the trickling filter is critical. The removal of pBOD was studied in a combined trickling filter activated-sludge pilot plant over a wide range of trickling filter BOD loadings. Intensive sampling with COD and BOD solids balances on the trickling filter were used to determine the amount of pBOD degradation. The amount of pBOD degraded increased as the BOD loading to the trickling filter was decreased. With an estimate of the amount of pBOD and sBOD removal in the trickling filter and the trickling filter SRT, the amount of biomass produced can be calculated. With that information the amount of oxygen demand satisfied in the trickling filter can be estimated. The trickling filter biomass and nondegraded pBOD and sBOD concentrations can then be used to estimate the activated-sludge aeration basin oxygen demand. The sBOD/ BOD ratio in the effluent from the trickling filter will typically vary from 0.25 to 0.50. A solids balance is also done to determine the basin volume as a function of the design SRT and MLSS concentration. The design procedure to determine the oxygen requirements, sludge production, and aeration volume of the activated-sludge basin for a combined trickling filter activated-sludge process including RF/AS and TF/SC processes is summarized in Table 8-9. 8-5 Activated Sludge With Fixed-Film Packing Several types of synthetic packing materials have been developed for use in activated-sludge processes. These packing materials may be suspended in the activated-sludge mixed liquor or fixed in the aeration tank. A term used to describe these types of processes is an integrated fixed-film activated-sludge process. These processes are intended to enhance the activated-sludge process by providing a greater biomass concentration in the aeration tank and thus offer the potential to reduce the basin size requirements. They have also been used to improve volumetric nitrification rates and to accomplish denitrification in aeration tanks by having anoxic zones within the biofilm depth. Because of the complexity of the process and issues related to understanding the biofilm area and activity, the process designs are empirical and based on prior pilot-plant or limited full-scale results. In this section these processes are introduced and described and some design considerations and parameters are presented. Processes with Internal Suspended Packing for Attached Growth There are now more than 10, and counting, different variations of processes in which a packing material of various types is suspended in the aeration tank of the activated-sludge process (see Fig. 8-13). Captor and Linpot. In the Captor and Linpor processes foam pads with a specific density of about 0.95 g/cm3 are placed in the bioreactor in a free-floating fashion and retained by an effluent screen (see Fig. 8-14). The pad volume can account for 20 to 30 percent of the reactor volume. Dimensions of the various packing materials are presented in Table 8-10. Mixing from the diffused aeration system circulates the foam pads in the system, but without additional mixing methods, they may tend to accumulate at the effluent end of the aeration basin and float at the surface. An air knife has been installed to continuously clean the screen and a pump is used to return the packing material to the influent end of the reactor. Solids are removed from a conventional secondary clarifier and wasting is from the return line as in the activated-sludge process. The principal advantage for the sponge packing systems is the ability to increase the loading on an existing plant without increasing the solids load on existing secondary clarifiers, as most of the biomass is retained in the aeration basin. Loading rates for BOD of 1.5 to 4.0 kg/m3.d with equivalent MLSS concentrations of 5000 to 9000 mg/L have been achieved with these processes. Based on the results with full-scale and pilot-scale tests with the sponge packing installed it appears that nitrification can occur at apparent lower SRT values, based on the suspended growth mixed liquor, than those for activated sludge without internal packing. Kaldnes. A moving-bed biofilm reactor (MBBR) has been developed by a Norwegian company, Kaldnes Miljoteknologi. The process consists of adding small cylindrical-shaped polyethylene carrier elements (specific density of 0.96 g/cm3) in aerated or nonaerated basins to support biofilm growth (see Fig. 8-14). The small cylinders are about 10 mm in diameter and 7 mm in height with a cross inside the cylinder and longitudinal fins on the outside. The biofilm carriers are maintained in the reactor by the use of a perforated plate (5～25 mm slots) at the tank outlet. Air agitation or mixers are applied in a manner to continuously circulate the packing. The packing may fill 25 to 50 percent of the tank volume. The specific surface area of the packing is about 500 m2/m3 of hulk packing volume. The MBBR does not require any return activated-sludge flow or backwashing. A final clarifier is used to settle sloughed solids. The MBBR process provides an advantage for plant upgrading by reducing the solids loading on existing clarifiers. The presence of packing material discourages the use of more efficient fine bubble aeration equipment, which would require periodic drainage of the aeration and removal of the packing for diffuser cleaning. Two different applications in which the MBBR has been applied are illustrated on Fig. 8-15. The first is a more common design application for BOD removal, nitrification, and denitrification. For the anoxic-aerobic treatment mode (see Fig. 8-15a), a 6-stage reactor design is used. Chemical addition for phosphorus removal is done after the MBBR. The Kaldnes packing is added to provide a specific packing surface area of 200 to 400 m2/m3 in the reactors. In the second application the MBBR is used in place of the trickling filter in the solids contact process (see Fig. 8-15b). Typical process design parameters for the moving-bed biofilm reactor and the moving-bed biofilm reactor/solids contact (MBBR/SC) processes are reported in Tables 8-11 and 12, respectively. Processes with Internal Fixed Packing for Attached Growth There are now more than half a dozen, and counting, different variations of processes in which a fixed packing material is placed in the aeration tank of the activated-sludge process. Three typical examples of fixed packing processes include the Ringlace and BioMatrix processes, Bio-2-Sludge process, and submerged RBCs. Ringlace. Ringlace packing is a looped polyvinyl chloride material that is about 5 mm in diameter. It is placed in about 25 to 35 percent of the activated-sludge basin volume in modules with individual strands at 40 to 100 mm apart (see Fig. 8-16). The specific surface area provided ranges from 120 to 500 m2/m3 of tank volume. The packing placement location in the aeration tank is important. To provide efficient contact with the wastewater the packing should be placed along one side of the aeration vessel with the aeration equipment providing a spiral roll pattern for flow through the packing. Spiral roll aeration is usually less efficient than full floor coverage aeration with fine bubble diffusers. The location along the length of the tank is also important for nitrification and denitrification system operations. Randall and Sen (1996) recommend a location where sufficient BOD remains to develop a biofilm growth, but where the BOD demand is low enough so that ammonia oxidation can occur in the film. However, they noted that the optimal rate can be difficult to achieve as variations in BOD loading can vary the biofilm growth on the packing and the competition between heterotrophic and autotrophic bacteria for surface area. In some applications, the advantages of using a ftxed internal packing were negated due to the growth of bristle worms in the biofilm. Bio-2-Sludge Process. A schematic of the Bio-2-Sludge process is illustrated on Fig. 8-17. The PVC packing bundle with a surface area of 90 to 165 m2/m3 and a minimum opening of 20 ×20 mm to prevent clogging is positioned along the walls of the aeration tank. The air diffusion system is designed to create a mixed liquid recirculation flow through the packing. Submerged Rotating Biological Contactors. Rotating biological contactor units have been installed in activated sludge. The submerged rotating biological contactor (SRBC) is operated at approximately 85 percent submergence. The SRBC units can be as large as 5.5 m diameter with a surface area of 28,800 m2.The rotation is driven by aeration and may be mechanically assisted. The submerged operation reduces the load on the packing shaft. 8-6 Submerged Attached Growth Processes Aerobic submerged fixed-film processes consist of three phases: a packing, biofilm, and liquid. The BOD and/or NH4-N removed from the liquid flowing past the biofilm is oxidized. Oxygen is supplied by diffused aeration into the packing or by being predissolved into the influent wastewater. Aerobic fixed-film processes include downflow packed-bed reactors, upflow packed-bed reactors, and upflow fluidized-bed reactors. The type and size of packing is a major factor that affects the performance and operating characteristics of submerged attached growth processes. Designs differ by their packing configuration and inlet and outlet flow distribution and collection. No clarification is used with aerobic submerged attached growth processes, and excess solids from biomass growth and influent suspended solids are trapped in the system and must be periodically removed. Most designs require a backwashing system much like that used in a water filtration plant to flush out accumulated solids, usually on a daily basis. The major advantages of submerged attached growth processes are their relatively small space requirement, the ability to effectively treat dilute wastewaters, no sludge settling issues as in activated-sludge process, and aesthetics. Also for many processes solids filtration occurs to produce a high-quality effluent. Such fixed-film systems have equivalent hydraulic retention times of less than 1 to 1.5 h, based on their empty tank volumes. Their disadvantages include a more complex system in terms of instrumentation and controls, limitations of economies of scale for application to larger facilities, and generally a higher capital cost than activated-sludge treatment. A wide variety of submerged attached growth processes have been used. The purpose of this section is to describe the more common processes used and their design loadings and performance capability. The processes described are the downflow Biocarbone process, the upflow Biofor and Biostyr process, and the aerobic upflow fluidized-bed reactor. Downflow Submerged Attached Growth Processes The Biocarbone process is a typical example of a downflow submerged attached growth process. Over 100 facilities have been constructed worldwide since the development of the process in France in the early 1980s. The process has also been termed the biological aerated filter (BAF). Although activated carbon packing was used in the original design, a 3- to 5-mm fired clay material is used in current designs. Plant installations range in size from 2000 to 80,000 m3/d. A schematic of the Biocarbone process is shown on Fig. 8-18. The system is designed much like a water filter with the addition of an air header about 300 mm above the underdrain nozzles to sparge air through the packing. The air header is uniformly arranged across the bed to assure that oxygen is provided throughout the entire bed of packing. The actual oxygen transfer efficiency is in the range of 5 to 6 percent, which is comparable to free bubble diffused aeration performance for that depth. Backwashing is normally done once per day or when the headloss increases to about 1.8 m. The design must consider both organic loading and hydraulic application rate. Hydraulic application rates in the range of 2.4 to 4.8 m3/m2.h are recommended to prevent excessive headloss. The Biocarbone process has been used in aerobic applications for BOD removal only, combined BOD removal and nitrification, and tertiary nitrification. Typical design loadings for Biocarbone applications are presented in Table 8-13. In combined BOD removal and nitrification systems, the nitrification rate is about 0.45 kg/m3.d. Higher DO concentrations in the range of 3 to 5 mg/L are recommended for efficient nitrification. For BOD removal applications, effluent BOD and TSS concentrations are generally < 10 mg/L and for nitrification, effluent NH4-N concentrations may range from 1 to 4 mg/L. A similar process, but using an expanded shale packing, has been developed and installed in England by the Thames Water Utilities. Upflow Submerged Attached Growth Processes Two upflow submerged attached growth processes that have been applied with success include the Bifor and Biostyr processes. The key features of these processes are described below. B|ofo Process. The Biofor process, an upflow submerged aerobic attached growth process (see Fig. 8-19), is being used at more than 100 installations in Europe and North America. The upflow reactor has a typical bed depth of 3 m but designs in the range of 2- to 4-m bed depth have been used. The packing, termed Biolite, is an expanded clay material with a density greater than 1.0 and a 2- to 4-mm size range. Inlet nozzles distribute the influent wastewater up through the bed, and an air header (Oxazur system) provides process air across the bed area. Backwashing is typically done once per day with a water flush rate of 10 to 30 m/h to expand the bed. Fine screening of the wastewater is needed to protect the inlet nozzles. The Biofor process has been applied for BOD removal and nitrification, tertiary nitrification, and denitrification. Recommended operating limits for the Biofor process are shown in Table 8-14. The loadings are similar to that used for the Biocarbone process. A study of 12 plants by Canler and Perret (1994) showed similar treatment performance results for COD removal as a function of the hydraulic application rate and COD loading for both the Biocarbone and Biofor systems. Biostyr Process. The Biostyr process is an upflow process, developed in Denmark, and uses 2- to 4-mm (specific area about 1000 m2/m3) polystyrene beads that have a specific density less than waten The installed packing porosity is about 40 per. cent, providing an effective area of about 400 m2/m3 for biofilm growth. Packing depths range from 1.5 to 3 m. A schematic of the Biostyr process is shown on Fig. 8-20. The bed can be operated entirely aerobic by providing air at the bottom or as an anoxic/ aerobic bed by providing air at an intermediate level. Nitrified effluent is recycled for the anoxic/aerobic operation. The floating packing is retained by nozzle plates and is compressed as the wastewater flows upward to provide filtration. Backwash water is stored above the treatment bed. During the backwash cycle treated water flows down through the packing at a very high rate, which results in a downward expansion of the originally compressed packing. The solids retained in the lower portion of the reactor and excess biomass produced on the packing are flushed out into a backwash collection tank. The normal backwash procedure consists of repeated rinse (water flushing) and air scouring steps. Typically four water phases and three air phases are used. The Biostyr process has been used for BOD removal only, combined B0D removal and nitrification, tertiary nitrification, and postdenitrification. The organic loadings are in the same range as that for the Biocarbonev and Biofor processes. Nitrification testing work by Payraudeau et al. (2000) suggests that a higher nitrification load is possible for tertiary nitrification. At loadings of 1.5 to 1.8 kg N/m3.d, 85 to 90 percent nitrogen oxidation was shown. Effluent performance for a number of full-scale facilities was given by Borregaard (1997), but the process loadings were not defined. Average effluent BOD, TSS, and NH4-N concentrations of 7, 11, and 1.8 mg/L, respectively, were shown for long-term operation. Fluidized-Bed Bioreactors (FBBR) In fluidized-bed bioreactors the wastewater is fed upward to a bed of 0.4- to 0.5-mm sand or activated carbon. Bed depths are in the range of 3 to 4 m. The specific surface area is about 1000 m2/m3 of reactor volume, which is greater than any of the other fixed-film packing. Upflow velocities are 30 to 36 m/h. Effluent recirculation is necessary to provide the fluid velocity within the necessary treatment detention times. Hydraulic retention times in FBBRs range from 5 to 20 min. As the biofilm increases in size, the packing becomes lighter and accumulates at the top of the bed where it can be removed and agitated periodically to remove excess solids. A schematic of an FBBR system is shown on Fig. 8-21. For aerobic applications reciculated effluent is passed through an oxygenation tank to predissolve oxygen. Adding air to the fluidized-bed reactor would discharge packing to the effluent. For municipal wastewater treatment, FBBRs have been used mainly for post-denitrification. Anaerobic applications of the FBBRs are described in Chap. 10. Aerobic FBBRs are frequently used to treat groundwater contaminated with hazardous substances. In these applications activated carbon is used for the packing to provide both carbon adsorption and biological degradation. The main advantages for the FBBR technology in this application are (1) it provides an extraordinarily long SRT for microorganisms necessary to degrade the xenobiotic and toxic compounds; (2) shock loads or nonbiodegradable toxic compounds can be absorbed onto the activated carbon; (3) high-quality effluent is produced low in TSS and COD concentration; (4) the oxygenation method prevents stripping and emission of toxic organic compounds to the atmosphere; and (5) the system operation is simple and reliable. 8-7 Attached Growth Denitrification Processes Biological denitfification processes have been applied following secondary treatment nitrification processes to reduce the nitrate/nitrite produced. Typically, an exogenous carbon source is added to provide an electron donor that can be oxidized biologically using nitrate or nitrite. Both attached growth and suspended growth processes have been used for postanoxic denitrification with success. The different types of attached growth postanoxic processes, illustrated on Fig. 8-22, include (1) downflow and upflow packed-bed reactors, (2) upflow fluidized-bed reactors, and (3) submerged rotating biological contactors. All of the processes require carbon addition in the form of methanol to the influent. Methanol to NO3-N dose ratios are in the range of 3.0 to 3.5 kg methanol/kg NO3-N. Downflow Packed-Bed Postanoxic Denitrification Processes Downflow packed-bed denitrification filters (see Fig. 8-22a) are deep bed filters(1.2 to 2.0 m) that have been used in many installations for postanoxic nitrate removal. The denitrification filter used most commonly is a proprietary process of TETRA Technologies. The filters provide both suspended solids removal and denitrifieation by microbial growth on the filter packing. Typical design criteria are reported in Table 8-15. Sand is the filter packing and the size selected is small enough to provide effective filtration and sufficient surface area for microbial growth but large enough to accommodate solids capture and microbial growth without excessive headloss. At hydraulic application rates of 80 to 100 m3/m2.d (Table 8-15), effluent TSS concentrations of < 5.0 mg/L are commonly achieved. With proper control of the methanol dose, total effluent nitrogen concentrations of 1 to 3 mg/L are possible. Overdosing of the methanol can lead to odor production, due to biological sulfate reduction in the filter. During operation of the denitrification filter, headloss gradually increases because of solids accumulation (filtration), biomass growth, and accumulation of nitrogen gas due to denitrification. The falter is "bumped" periodically by a hydraulic surge to remove nitrogen gas and backwashed for solids removal. A water-only flush is used at a rated 12 m/h for about 3 to 5 min for a "bump" to release the accumulated nitrogen gas. The bump frequency may vary from once every 2 to 4 h. An air and water backwash is required every 24 to 48 h, depending on the solids accumulation and headloss. The solids storage capacity is estimated to be about 4.0 kg TSS/m3 before high headloss occurs. The backwash typically consists of an air scour followed by an air and water backwash. Upflow Packed-Bed Postonoxic Denitrification Reactors As illustrated on Fig. 8-23b, upflow packed-bed reactors have been demonstrated for postanoxic denitrification for both the Biostyr and Biofor processes. As described previously, the packing used in these processes is a 2.0-m depth of 2- to 5-mm poly-styrene beads for the Biostyr and a 2-m depth of a 2- to 4-mm expanded clay (Biolite) for the Biofor process. Reported NO3-N loading rates for these processes are in the range from 3.0 to 4.0 kg NO3-N/m3.d to achieve effluent NO3-N concentrations below 5.0 mg/L, assuming a sufficient amount of electron donor is supplied. Higher hydraulic application rates of up to 330 m3/m2.d have been claimed for these processes. No special backwash "bumping" is provided for nitrogen gas release during backwash as compared to the proprietary Tetra filter. Fluldized-Bed Reactors for Postonoxic Denitrifieation In the fluidized-bed reactor (FBR), the upward flow to the reactor containing sand or other suitable packing material is at a sufficient velocity to expand and fluidize the sand particles coated with biofilms (see Fig. 8-23c). The intense mixing provides good mass transfer and the dense biofilm that develops results in an equivalent reactor biomass concentration of 20 to 30 g/L for postdenitrification applications (U.S. EPA, 1993). Design criteria for fluidized-bed reactors for postanoxic denitrification are summarized in Table 8-16. Effluent recirculation ratios of 2:1 to 5:1 are used to maintain the high upflow liquid velocity to fluidize the sand. Volumetric NO3-N loadings of 2.0 to 6.0 kg/m3-d have been shown to be feasible for denitrification applications, depending on the temperature. Empty-bed liquid retention times are only 10 to 20 min, and effluent NO3-N concentrations of 2 to 4 mg/L have been achieved when treating municipal nitrified wastewaters. The FBR systems are operated without the need of effluent filtration or clarification. The sand packing is removed at the top of the reactor and passed through a high shear pump to separate biomass from the sand. The cleaned packing material is returned to the FBR. Packing with thicker biofilms migrates to the top of the bed due to the lower overall particle density caused by the biofilm growth. The excess biomass is removed from the top of the bed to control the bed packing depth. Various separation techniques including screens with spray washers, external cyclones, and hydraulic separators are used to separate the sheared biomass from the packing material. In a properly designed and operated system the FBR effluent TSS concentration can be in the range of 15 to 20 mg/L. The influent manifold system is also a critical physical design element of the FBR. The manifold must be designed to distribute the infiuent flow uniformly across the fluidized bed. The Reno Sparks, NV, wastewater-treatment plant was the first full-scale installation of a FBR for postanoxic denitrification, with operation beginning in the early 1980s. Attached Growth Preanoxic Denitrification Processes Preanoxic denitrification can be used with attached growth processes. In the first case, an attached growth process is used for denitrification using organic material in the influent wastewater for the electron donor to reduce NO3-N. Nitrate is provided in the recycle stream, which is pumped at a rate that is 3 to 4 times the influent flowrate. Attached growth preanoxic processes that have been used for preanoxic denitrification include: (1) trickling filters, (2) the Biofor process at pilot scale, and (3) a moving bed fixed film reactor. An important advantage of the preanoxic denitrification treatment scheme is that influent BOD is used for nitrate reduction, and thus the cost of methanol addition is eliminated. A disadvantage is the effect of the nitrate recycle flows on the design and operating cost. Where trickling filters are used for nitrification, the energy required for pumping the recycle and influent flow to the trickling filter increases significantly. The nitrified trickling filter effluent is recycled to provide nitrate for a suspended growth preanoxic reactor. An intermediate clarifier is used to separate the denitrifying mixed liquor and to provide return activated sludge to the anoxic tank. The recycle flow needed to provide nitrate to the preanoxic zone has a significant effect on the clarifier size and pumping requirements to the trickling filter, and thus the overall system economics. Part of a full-scale trickling filter facility at Salisbury, MD, was modified to evaluate biological nitrogen removal by converting an existing trickling filter to a preanoxic filter followed by a nitrification trickling filter. Both filters were converted from rock to plastic packing with an increase in height and packing depth. The preanoxic filter was operated as a submerged reactor, and the internal recycle ratio from the effluent of the nitrification trickling filter to the preanoxic filter was 5:1 based on the plant influent flowrate. With most of the BOD removal occurring in the preanoxic filter, a higher DO concentration (3 to 6 mg/L) was obtained in the nitrification filter effluent. The trickling filter nitrification rates were on the order of 1.5 to 2.0 gN/m2.d and controlled the treatment process flowrate. An effluent total nitrogen concentration of less than 8 mg/L was achieved at temperatures as low as 13。C. Long-term operating data were not sufficient to assess solids inventory and control issues in the preanoxic trickling filter.. The high DO concentration in the nitrification trickling filter effluent and its effect on nitrate removal in the preanoxic filter was of concern. In another design using trickling filters, Dorias and Baumenn (1994) used a nonsubmerged preanoxic trickling filter with the recycle and influent feed flows supplied by the trickling filter distributor. A gastight cover over the trickling filter was necessary to develop sustained anoxic conditions, and the preanoxic trickling filter effluent NO3-N concentrations ranged from 2.0 to 4.0 mg/L with specific NO3-N removal rates of 70 to 100 g/m3·d. The Biofor attached growth process for preanoxic denitrification and nitrification in series was demonstrated in a pilot plant with municipal wastewater. An internal recycle ratio of 2.5, based on the effluent flowrate, was used and nitrogen removal efficiency was affected by the preanoxic reactor feed bCOD/NO3-N ratio and internal recycle ratio. A COD/N value greater than 10 was needed to maximize performance.