2 Constituents in Wastewater
An understanding of the nature of wastewater is essential in the design and operation of collection, treatment, and reuse facilities, and in the engineering management of environmental quality. To promote this understanding, the information in this chapter is presented in eight sections dealing with (1) an introduction to the constituents found in wastewater, (2) sampling and analytical procedures, (3) physical characteristics, (4) inorganic nonmetallic constituents, (5) metallic constituents, (6) aggregate organic constituents, (7) individual organic constituents and compounds,and (8) biological characteristics.
2-1 Wastewater Constituents
Constituents Found in Wastewater
The principal physical properties and the chemical and biological constituents of wastewater, and their sources, are reported in Tab 2-1. It should be noted that many of the physical properties and chemical and biological characteristics are interrelated. For example, temperature, a physical property, affects both the amounts of gases dissolved in the wastewater and the biological activity in the wastewater.
Constituents of Concern in Wastewater Treatment
The important constituents of concern in wastewater treatment are listed in Tab 2-2.Secondary treatment standards for wastewater are concerned with the removal of biodegradable organics, total suspended solids, and pathogens. Many of the more stringent standards that have been developed recently deal with the removal of nutrients, heavy metals, and priority pollutants. When wastewater is to be reused, standards normally include additional requirements for the removal of refractory organics, heavy metals, and in some cases, dissolved inorganic solids.
2-2 Sampling and Analytical Procedures
Proper sampling and analytical techniques are of fundamental importance in the characterization of wastewater. Sampling techniques, the methods of analysis, the units of measurement for chemical constituents, and some useful concepts from chemistry are considered below.
Sampling
Sampling programs are undertaken for a variety of reasons such as to obtain (1) routine operating data on overall plant performance, (2) data that can be used to document the performance of a given treatment operation or process, (3) data that can be used to implement proposed new programs, and (4) data needed for reporting regulatory compliance. To meet the goals of the sampling program, the data collected must be:
1. Representative. The data must represent the wastewater or environment being sampled.
2. Reproducible. The data obtained must be reproducible by others following the same sampling and analytical protocols.
3. Defensible. Documentation must be available to validate the sampling procedures. The data must have a known degree of accuracy and precision.
4. Useful. The data can be used to meet the objectives of the monitoring plan. Because the data from the analysis of the samples will ultimately serve as a basis for implementing wastewater management facilities and programs, the techniques used in a wastewater sampling program must be such that representative samples are obtained.
Table 2-1 Common analyses used to assess the constituents found in wastewater
Test
Abbreviation/
definition
Use or significance of test results
Physical characteristics
To assess the potential of reuse a wastewater and to determine the most suitable type of operations and processes for its treatment
Total solids
TS
Total volatile solids
TVS
Total fixed solids
TFS
Total suspended solids
TSS
Volatile suspended solids
VSS
Fixed suspended solids
FSS
Total dissolved solids
TDS(TS-TSS)
Volatile dissolved solids
VDS
Total fixed dissolved solids
FDS
Settleable solids
To determine those solids that will settle by gravity in a specified time period
Particle size distribution
PSD
To assess the performance of treatment processes
Turbidity
NTU
Used to assess the quality of treated wastewater
Color
Light brown,gray,black
To assess the condition of wastewater(fresh or septic)
Transmittance
%T
Used to assess the suitability of treated effluent for UV disinfection
Odor
TON
To determine if odors will be a problem
Temperature
℃
Important in the design and operation of biological processes in treatment facilities
Density
ρ
Conductivity
EC
Used to assess the suitability of treated effluent for agricultural applications
Inorganic chemical characteristics
Free ammonia
NH4+
Used as a measure of the nutrients present and the degree of decomposition in the wastewater;the oxidized forms can be taken as a measure of the degree of oxidation
Organic nitrogen
Org N
Total Kjeldahl nitrogen
TKN
Nitrates
NO2-
Total nitrogen
TN
Inorganic phosphorus
Inorg P
Total phosphorus
TP
Organic phosphorus
Org P
pH
pH
A measure of the acidity or basicity of an aqueous solution
Alkalinity
∑HCO3-+CO32-+OH--H+
A measure of the buffering capacity of the wastewater
Chloride
Cl-
To assess the suitability of the wastewater for agricultural reuse
Sulfate
SO42-
To assess the potential for the formation of odors and may impact the treatability of the waste sludge
Metals
As,Cd,Ca,Cr,Co,Cu,Pb,Mg,Hg,Mo,Ni,Se,Na,Zn
To assess the suitability of the wastewater for reuse and for toxicity effects in treatment.Trace amounts of metals are important in biological treatment.
Specific inorganic elements and compounds
To assess presence or absence of specific constituents
Various gases
O2,CO2,NH3,H2S,CH4
To assess presence or absence of specific gases
Organic chemical characteristics
Five-day carbonaceous biochemical oxygen demand
CBOD5
A measure of the amount of oxygen required to stabilize a waste biologically
Ultimate carbonaceous biochemical oxygen demand
UBOD(BODu,BODL)
A measure of the amount of oxygen required to stabilize a waste biologically
Nitrogenous oxygen demand
NOD
A measure of the amount of oxygen required to oxidize biologically the nitrogen in the wastewater to nitrate
Chemical oxygen demand
COD
Often used as a substitute for the BOD test
Total organic carbon
TOC
Often used as a substitute for the BOD test
Specific organic compounds and classes of compounds
MBAS,CTAS
To determine presence of specific organic compounds and to assess whether special design measures will be needed for removal
Biological characteristics
Coliform organisms
MNP
To assess presence of pathogenic bacteria and effectiveness of disinfection process
Specific organisms
Bairusescteria,protozoa,helminthes,v
To assess presence of specific organisms in connection with plant operation and for reuse
Toxicity
TUa,TUc
Toxic unit acute,toxic unit chronic
Tab 2-2 Principal constituents of concern in wastewater treatment
Constituents
Reason for importance
Suspended solids
Suspended solids can lead to the development of sludge deposition and anaerobic conditions when untreated wastewater is discharged in the aquatic environment.
Biodegradable organics
Composed principally of proteins, carbohydrates, and fats. Biodegradable organics are measured most commonly in terms of BOD and COD. If discharged untreated to the environment their biological stabilization can lead to the depletion of natural oxygen resources and to the development of septic conditions.
Pathogens
Communicable diseases can be transmitted by the pathogenic organisms that may be present in wastewater.
Nutrients
Both nitrogen and phosphorus, along with carbon, are essential nutrients for growth. When discharged to the aquatic environment, the nutrients can lead to the growth of undesirable aquatic life. When discharged in excessive amount on land, they can also lead to the pollution of groundwater.
Priority pollutants
Organic and inorganic compounds selected on the basis of their known or suspected carcinogenicity, metogenicity, teratogenicity or high acute toxicity. Many of these compounds are found in wastewater.
Refractory organics
These organics tend to resist conditional methods of wastewater treatment. Typical examples include surfactants, phenols, and agricultural pesticides.
Heavy metals
Heavy metals are usually added to wastewater from commercial and industrial activities and may have to be removed if the wastewater is to be reused.
Dissolved inorganics
Inorganic constituents such as calcium, sodium, and sulfate are added to the original domestic water supply as a result of water use and may have to be removed if the wastewater is to be reused.
There are no universal procedures for sampling; sampling programs must be tailored individually to fit each situation. Special procedures are necessary to handle sampling problems that arise when wastes vary considerably in composition.
Before a sampling program is undertaken, a detailed sampling protocol must be developed along with a quality assurance project plan (QAPP) (known previously quality assurance/quality control, QA/QC). As a minimum, the following items must be specified in the QAPP. Additional details on the subject of sampling may be found in Standard Methods.
1.Sampling plan. Number of sampling locations, number and type of samples, time intervals (e.g., real-time and/or time-delayed samples).
2.Sample types and size, Catch or grab samples, composite samples, or integrated samples, separate samples for different analyses (e.g., for metals). Sample size (i.e., volume) required.
3.Sample labeling and chain of custody. Sample labels, sample seals, field log book, chain of custody record, sample analysis request sheets, sample delivery to the laboratory, receipt and logging of sample, and assignment of sample for analysis.
4.Sampling methods. Specific techniques and equipment to be used (e.g., manual, automatic, or sorbent sampling).
5.Sampling storage and preservation. Type of containers (e.g., glass or plastic), preservation methods, maximum allowable holding times.
6.Sample constituents. A list of the parameters to be measured.
7.Analytical methods. A list of the field and laboratory test methods and procedures to be used, and the detection limits for the individual methods.
If the physical, chemical, and/or biological integrity of the samples is not maintained during interim periods between sample collection and sample analysis, a carefully performed sampling program will become worthless. Considerable research on the problem of sample preservation has failed to perfect a universal treatment or method, or to formulate a set of fixed rules applicable to samples of all types. Prompt analysis is undoubtedly the most positive assurance against error due to sample deterioration. When analytical and testing conditions dictate a lag between collection and analysis, such as when a 24 h composite sample is collected, provisions must be made for preserving samples. Current methods of sample preservation for the analysis of properties subject to deterioration must be used. Probable errors due to deterioration of the sample should be noted in reporting analytical data.
Methods of Analysis
The analyses used to characterize wastewater vary from precise quantitative chemical determinations to the more qualitative biological and physical determinations. The quantitative methods of analysis are either gravimetric, volumetric, or physicochemical.
In the physicochemical methods, properties other than mass or volume are measured. Instrumental methods of analysis such as turbidimetry, colorimetry, potentiometry, polarography, adsorption spectrometry, fluorometry, and nuclear radiation are representative of the physicochemical analyses. Details concerning the various analyses may be found in Standard Methods , the accepted reference that details the conduct of water and wastewater analyses.
Regardless of the method of analysis used, the detection level must be specified Several detection limits are defined and are listed below in order of increasing level.
Units of Measurement for Physical and Chemical Parameters
The results of the analysis of wastewater samples are expressed in terms of physical and chemical terms of measurement. The most common units for these measurements are, for example, kg/m3 ,%(by volume or by mass), pg/L, ng/L, μg/L, mg/L, g/L, ppb, ppm, mol/L, eq/L, meq/L and so on. The concentration of trace constituents is usually expressed as micrograms per liter (μg/L) or nanograms per liter (ng/L).
For dilute systems, such as those encountered in natural waters and wastewater, in which one liter of sample weighs approximately one kilogram, the units of mg/L or g/m3 are interchangeable with ppm. The terms "parts per billion" (ppb) and "parts per trillion"(ppt) are used interchangeably with μg/L and ng/L, respectively.
2-3 Physical Characteristics
The most important physical characteristic of wastewater is its total solids content, which is composed of floating matter, settleable matter, colloidal matter, and matter in solution. Other important physical characteristics include particle size distribution, turbidity, color, transmittance, temperature, conductivity, and density, specific gravity and specific weight. Odor, sometimes considered a physical factor, is discussed in the following section.
Solids
Wastewater contains a variety of solid materials varying from rags to colloidal material. In the characterization of wastewater, coarse materials are usually removed before the sample is analyzed for solids. The various solids classifications are identified in Tab 2-3. The interrelationship between the various solids fractions found in wastewater is illustrated graphically on Fig. 2-1. The standard test for settleable solids consists of placing a wastewater sample in a 1-liter Imhoff cone and noting the volume of solids in millimeters that settle after a specified time period (1 h). Typically, about 60 percent of the suspended solids in a municipal wastewater are settleable. Total solids (TS) are obtained by evaporating a sample of wastewarer to dryness and measuring the mass of the residue. As shown on Fig. 2-1, a filtration step is used to separate the total suspended solids (TSS) from the total dissolved solids (TDS). Filters with nominal pore sizes varying from 0.45 μm to about 2.0 μm have been used for the TSS test (see Fig. 2-2).
Fig. 2-1 Interrelationships of solids found in water and wastewater. In much of the water quality literature, the solids passing through the filter are called dissolved solids.
Tab 2-3 Definitions of solids found in wastewater
Test
Description
Total solids(TS)
The residue remaining after a wastewater sample has been evaporated and dried at a specified temperature(103 to 105 ℃)
Total volatile solids(TVS)
Those solids that can be volatiled and burned off when the TS are ignited(500±50℃)
Total fixed solids(TFS)
The residue that remains after TS are ignited(500±50℃)
Total suspended solids(TSS)
Portion of the TS retained on a filter with a specified pore size, measured after being dried at a specified temperature(105℃).The filter used most commonly for the determination of TSS is the Whatman glass fiber filter ,which has a nominal pore size of about 1.58 μm.
Volatile suspended solids(VSS)
Those solids that can be volatilized and burned off when the TSS are ignited (500±50℃)
Fixed suspended solids(FSS)
The residue that remains after TSS are ignited (500±50℃)
Total dissolved solids(TDS)
Those solids that pass through the filter, and are then evaporated and dried at specified temperature. It comprised of colloidal and dissolved solids. Colloids are typically in the size range from 0.001 to 1μm
Total volatile dissolved solids(VDS)
Those solids that can be volatilized and burned off when the TDS are ignited (500±50℃)
Fixed dissolved solids(FDS)
The residue that remains after TDS are ignited (500±50℃)
Settlable solids
Suspended solids, expressed as milliliters per liter, that will settle out of suspension within a specified period of time
Fig. 2-2 Size ranges of organic contaminants in wastewater and size separation and measurement techniques used for their quantification
Particle Size Distribution
As noted above, TSS is a lumped parameter. In an effort to understand more nature of the particles that comprise the TSS in wastewater, measurement of particle size is undertaken and an analysis of the distribution of particle sizes is conducted.
Information on particle size is of importance in assessing the effectiveness of treatment processes (e.g., secondary sedimentation, effluent filtration and effluent disinfection).
Because the effectiveness of both chlorine and disinfection is dependent on particle size, the determination of particle size has become more important, especially with the move toward greater effluent reuse.
Information on the size of the biodegradable organic particles is significant from a treatment standpoint, as the biological conversion rate of these particles is dependent on size. The methods can be divided into two general categories: (1)methods based on observation and measurement and (2) methods based on separation analysis techniques. The methods used most commonly to study and quantify the paticles in wastewater are: (1) serial filtration, (2) electronic particle counting, and microscopic observation(See Tab 2-4).
Tab 2-4 Analytical techniques application to particle size analysis of wastewater contaminants
Technique
Typical size range,μm
Observation and measurement
Microscopy
Light
Transmissoin electron
Scanning electron
Image analysis
Particle counters
Conductivity difference
Equivalent light scattering
Light blockage
0.2- >100
0.2- 100
0.002- 50
0.2- >100
0.2- >100
0.005- >100
0.2- >100
Separation and analysis
Centrifugation
Field flow fractionation
Gel filtration chromatography
Sedimentation
Membrane filtration
0.08- >100
0.09->100
<0.0001- >100
0.05- >100
0.0001-1
Serial Filtration. In the serial filtration method, a wastewater sample is passed sequentially through a series of membrane filters (see Fig. 2-3) with circular openings of known diameter (typically 12, 8, 5, 3, 1. and 0. l μm), and the amount of solids retained in each filter is measured. What is interesting to note is the amount of colloidal material found between 0.1 and 1.0 μm. If a 0. l-μm filter had been used to determine TSS for the treated effluent instead of a filter with a nominal pore size equal to or greater than 1.0 μm (2.0 μgm as specified in Standard Methods for the TSS test), more than 20 mg/L of additional TSS would have been measured.
Fig. 2-3 Definition sketch for the determination of the particle size distribution using serial filtration with membrane filters
Although some information is gained on the size and distribution of the particles in the wastewater sample, little information is gained on the nature of the individual particles. This method is useful in assessing the effectiveness of treatment methods (e.g., microfiltration) for the removal of residual TSS.
Electronic Particle Size Counting. In electronic particle size counting, particles in wastewater are counted by diluting a sample and then passing the diluted sample through a calibrated orifice or past laser beams. As the particles pass through the orifice, the conductivity of the fluid changes, owing to the presence of the particle. The conductivity is correlated to the size of an equivalent sphere. In a similar fashion, as a particle passes by a laser beam, it reduces the intensity of the laser because of light scattering . The reduced intensity is correlated to the diameter of the particle. The particles that are counted are grouped into particle size ranges (e.g., 0.5 to 2, 2 to 5, 5 to 20 μm, etc). In turn, the volume fraction corresponding to each particle size range can be computed.
Typical effluent volume fraction data from two activated sludge treatment plant are reported on Fig. 2-5. As shown, the particle size data for small particles are the same for both treatment plants. However, the particle size data for the large particles are quite different, owing primarily to the design and operation of the secondary clarification . Particle size information, such as that shown on Fig. 2-5 , is useful in assessing the performance of secondary sedimentation facilities, effluent filtration, and the potential for chlorine and ultraviolet radiation disinfection.
Fig. 2-4 Imhoff cone used to determine settleable solids in wastewater.Solids that accumulate in the bottom of the cone after 60 min are reported as mL/L
Microscopic Observation. Particles in wastewater can also be enumerated microscopically by placing a small sample in a particle counting chamber and counting the individual particles. To aid in differentiating different types of particles, various types of stains can be used. In general, microscopic counting of particles is impractical on a routine basis. Nevertheless, this method can be used to qualitatively assess the nature and size of the particles in wastewater. A quantitative assessment of wastewater particles can be obtained with a microscope by means of a process called optical imaging.
A small sample of wastewater is placed on a microscope slide. The images of the wastewater particles are collected with a video camera attached to a microscope and transmitted to a computer where various measurements of the wastewater particles can be assessed.
Fig. 2-5 Volume fraction of particle sizes found in the effluent from two activated-sludge plants with clarifiers having different side water depths
The types of measurements that can be obtained are dependent on the computer software but typically include the mean, minimum, and maximum diameter, the aspect ratio (length to width ratio), the circumference, the surface area, the volume, and the centroid of various particles. Particle imaging greatly reduces the time required to measure various characteristics of wastewater particles, but the cost of the software and equipment is often prohibitive for many small laboratories.
Turbidity
Turbidity, a measure of the light-transmitting properties of water, is another test used to indicate the quality of waste discharges and natural waters with respect to colloidal and residual suspended matter. The measurement of turbidity is based on comparison of the intensity of light scattered by a sample to the light scattered by a reference suspension under the same conditions. Formazin suspensions are used as the primary reference standard. The results of turbidity measurements are reported as nephelometric turbidity units (NTU). Colloidal matter will scatter or absorb light and thus prevent its transmission. It should be noted that the presence of air bubbles in the fluid will cause erroneous turbidity readings. In general, there is no relationship between turbidity and the concentration of total suspended solids in untreated wastewater. There is, however, a reasonable relationship between turbidity and total suspended solids for the settled and filtered secondary effluent from the activated sludge process. The specific value of the conversion factor will vary for each treatment, depending primarily on the operation of the biological treatment process. The conservation factors for settled secondary effluent and for secondary effluent filtered with a granular-medium depth filter will typically vary from 2.3 to 2.4 and 1.3 to 1.6, respectively.
One of the problems with the measurement of turbidity (especially low values in filtered effluent) is the high degree of variability observed, depending on the light source (incandescent light versus light-emitting diodes) and the method of measurement (reflected versus transmitted light). Another problem often encountered is the light-absorbing properties of the suspended material. However, turbidity readings at a given facility can be used for process control. Some on line turbidity meters used to monitor the performance of microfiltration units are affected by the air used to clean the membranes.
Color
Historically, the term "condition" was used along with composition and concentration to describe wastewater. Condition refers to the age of the wastewater, which is determined qualitatively by its color and odor. Fresh wastewater is usually a brownish-gray color. However, as the travel time in the collection system increases, and more anaerobic conditions develop, the color of the wastewater changes sequentially from gray to dark gray, and ultimately to black. When the color of the wastewater is black, the wastewater is often described as septic. Some industrial wastewaters may also add color to domestic wastewater. In most cases, the gray, dark gray, and black color of the wastewater is due to the formation of metallic sulfides, which form as the sulfide produced under anaerobic conditions reacts with the metals in the wastewater.
Absorption/Transmittance
The absorbance of a solution is a measure of the amount of light, of a specified wave-length, that is absorbed by the constituents in a solution. Absorbance, measured using a spectrophotometer and a fixed path length (usually 1.0 cm), is given by following relationship:
A = log(I0/I)
Where A-absorbence, absorbence unit, a.u./cm
I0-Initial detector reading for the blank (i.e. distilled water) after passing through a solution of known depth
I- Final detector reading for the blank (i.e. distilled water) after passing through
solution containing constituents of interest
Absorbance is measured with a spectrophotometer using a specified wavelength, typically 254 nm. Typical absorbance values for various wastewater at 254 nm are:
1. Primary:0.5 to 0.8/cm
2. Secondary:0.3 to 0.5/cm
3. Nitrified secondary:0.25 to 0.45/cm
4. Filtered secondary:0.02 to 0.40/cm
Transmitttance T, % = (I/Io)×100
Fig. 2-6 Transmittance measured at various wavelengths for activated-sludge effluents and lagoon effluents
The principal wastewater characteristics that affect the percent transmission include selected inorganic compounds (e.g., copper; iron, etc.), organic compounds (e.g., organic dyes, humic substances, and conjugated ring compounds such as benzene and toluene), and TSS. Of the inorganic compounds which affect transmittance, iron is considered to be the most important with respect to UV absorbance because dissolved iron can absorb UV light directly and because iron will adsorb onto suspended solids, bacterial clumps and other organic compounds. The sorbed iron can prevent the UV light from penetrating the particle and inactivating organisms that may be embedded within the particle. Where iron salts are added in the treatment process, dosage control is extremely important when UV disinfection is to be used. Organic constituents, identified as being absorbers of UV light, are compounds with six conjugated carbons or a five- or six-member conjugated ring. The reduction in transmittance observed during storm events is often ascribed to the presence of humic substances from stormwater flows. Typical transmittance values for treated wastewater from several activated-sludge biological treatment plants and two lagoon systems are presented on Fig. 2-6. Percent transmittance is affected by all substances in wastewater that can absorb or scatter light. Unfiltered and filtered transmittance are mearsured in wastewater in connection with the evalution and design of UV disinfection systems.
Temperature
The temperature of wastewater is commonly higher than that of the local water supply, because of the addition of warm water from households and industrial activities. As the specifc heat of water is much greater than that of air, the observed wastewater temperatures are higher than the local air temperatures during most of the year and are lower only during the hottest summer months. Depending on the geographic location, the mean annual temperature of wastewater in the United States varies from about 3 to 27℃; 15.6℃ is a representative value. Temperatures as high as 30 to 50 ℃ have been reported for countries in Africa and the Middle East. Depending on the location and time of year, the effluent temperatures can be either higher or lower than the corresponding influent values.
Effects of Temperature. The temperature of water is a very important parameter because of its effect on chemical reactions and reaction rates, aquatic life, and the suitability of the water for beneficial uses. Increased temperature, for example, can cause a change in the species of fish that can exist in the receiving water body. Industrial establishments that use surface water for cooling water purposes are particulally concerned with the temperature of the intake water.
In addition, oxygen is less soluble in warm water than in cold water. The the rate of biochemical reactions that accompanies an increase in temperature, combined with the decrease in the quantity of oxygen present in surface waters, can often cause serious depletions in dissolved oxygen concentrations in the summer months. When significantly large quantities of heated water are discharged to natural receiving waters,these effects are magnified. It should also be realized that a sudden change in temperature can result in a high rate of mortality of aquatic life. Moreover, abnormally high temperatures can foster the growth of undesirable water plants and wastewater fungus.
Optimum Temperatures for Biological Activity. Optimum temperatures for bacterial activity are in the range from 25 to 35℃. Aerobic digestion and nitrification stops when the temperature rises to 50℃. When the temperature drops to about 15℃, methane-producing bacteria become quite inactive, and at about 5℃, the autotrophic-nitrifying bacteria practically cease functioning. At 2℃,even the chemoheterotrophic bacteria acting on carbonaceous material become essentially dormant. The effects of temperature on the performance of biological treatment processes are considered in greater detail in latter chapters.
Conductivity
The electrical conductivity (EC) of a water is a measure of the ability of a solution to conduct an electrical current. Because the electrical current is transported by the ions in solution, the conductivity increases as the concentration of ions increases. In effect, the measured EC value is used as a surrogate measure of total dissolved solid(TDS)concentration. At present, the EC of a water is one of the important parameter used to determine the suitability of a water for irrigation. The salinity of treated wastewater to be used for irrigation is estimated by measuring its electrical conductivity.
2-4 Inorganic Nonmetallic Constituents
The chemical constituents of wastewater are typically classified as inorganic and organic. Inorganic chemical constituents of concern include nutrients, nonmetallic constituents, metals, and gases. Organic constituents of interest in wastewater are classified as aggregate and individual. Aggregate organic constituents are comprised of a number of individual compounds that cannot be distinguished separately. Both aggregate and individual organic constituents are of great significance in the treatment, disposal, and reuse of wastewater.
The sources of inorganic nonmetallic and metallic constituents in wastewater derive from the background levels in the water supply and from the additions resulting from domestic use, from the addition of highly mineralized water from private wells and groundwater, and from industrial use. Domestic and industrial water softeners also contribute significantly to the increase in mineral content and, in some areas, may represent the major source. Occasionally, water added from private wells and groundwater infiltration will serve to dilute the mineral concentrauon in the wastewater. Inorganic nonmetallic constituents considered in this section include pH, nitrogen, phosphorus, alkalinity, chlorides, sulfur, other inorganic constituents, gases, and odors.
Chlorides
Chloride is a constituent of concern in wastewater as it can impact the final reuse applications of treated wastewater. Chlorides in natural water result from the leaching of chloride-containing rocks and soils with which the water comes in contact, and in coastal areas from saltwater intrusion. In addition, agricultural, industrial, and domestic wastewaters discharged to surface waters are a source of chlorides.
Human excreta, for example, contain about 6 g of chlorides per person per day. In areas where the hardness of water is high, home regeneration type water softeners will also add large quantities of chlorides. Because conventional methods of waste treatment do not remove chloride to any significant extent, higher than usual chloride concentrations can be taken as an indication that a body of water is being used for waste disposal.
Infiltration of groundwater into sewers adjacent to saltwater is also a potential source of high chlorides as well as sulfates.
Alkalinity
Alkalinity in wastewater results from the presence of the hydroxides [OH-], carbonates [CO32-], and bicarbonates [HCO3- ] of elements such as calcium, magnesium, sodium, potassium, and ammonia. Of these, calcium and magnesium bicarbonates are most common. Borates, silicates, phosphates, and similar compounds can also contribute to the alkalinity. The alkalinity in wastewater helps to resist changes in pH caused by the addition of acids. In some cases, wastewater may be alkaline, receiving its alkalinity from the water supply, the groundwater, and the materials added during domestic use. The concentration of alkalinily in wastewater is important where chemical and biological treatment is to be used, in biological nutrient removal, and where ammonia is to be removed by air stripping.
Alkalinity is determined by titrating against a standard acid; the results are expressed in terms of calcium carbonate, mg/L as CaCO3. For most practical proposes alkalinity can be defined in terms of molar quantities, as:
Alk, eq/m3 = meq/L = [HCO3-] + 2 [CO32- ] + [OH-] –[H+]
The corresponding expression in terms of equivalents is
Alk, eq/m3 = (HCO3-) + (CO32-) + (OH-) - (H+)
In practice, alkalinity is expressed in terms of calcium carbonate. To convert from meq/L to mg/L as CaCO3, it is helpful to remember that
Milliequivalent mass of CaCO3 = [100 (mg/mmole)]/[2 meq/mmole]=50 mg/meq
Thus 3 meq/L of alkalinity would be expressed as 150 mg/L as CaCO3.
Alkalinity, Alk as CaCO3 =3.0meq/L×50mg/meq CaCO3 = 150 mg/L as CaCO3
Nitrogen
The elements nitrogen and phosphorus, essential to the growth of microorganisms, plants, and animals, are known as nutrients or biostimulants. Trace quantities of other elements, such as iron, are also needed for biological growth, but nitrogen and phosphorus are, in most cases, the major nutrients of importance. Because nitrogen is an essential building block in the synthesis of protein, nitrogen data will be required to evaluate the treatability of wastewater by biological processes. Insufficient nitrogen can necessitate the addition of nitrogen to make the waste treatable. Nutrient requirements for biological waste treatment are discussed in the later chapters. Where control of algal growths in the receiving water is necessary, removal or reduction of nitrogen in wastewater prior to discharge may be desirable.
Sources of Nitrogen. The principal sources of nitrogen compouns are (1)the nitrogenous compounds of plant and animal origin, (2) sodium nitrate, and (3)atomspheric nitrogen. Ammonia derived from the distillation of bituminous coal is an example of nitrogen obtained from decayed plant material. Sodium nitrate (NaNO3) is found principally in mineral deposits in Chile and in the manure found in seabird rookeries. The production of nitrogen from the atmosphere is termed fixation. Because fixation is a biologically mediated process and because NaNO3 deposits are relatively scarce, most sources of nitrogen in soil/groundwater are of biological origin.
Forms of Nitrogen. The chemistry of nitrogen is complex, because of the several oxidation states that nitrogen can assume and the fact that changes in the oxidation state can be brought about by living organisms. To complicate matters furthur, the oxidation state changes brought about by bacteria can be either positive or negative depending upon whether aerobic or anaerobic conditions prevail. The oxidation states of nitrogen are summarized below.
NH3(-III) → N2(0) → N20(I) → NO(II) → N2O3(III) → NO2(IV) → N2O5(V)
The most common and important forms of nitrogen in wastewater and their corresponding oxidation state in the water/soil enviromnent are ammonia (NH3. -III), ammonium(NH4+, -III), nitrogen gas (N2, 0), nitrite ion (NO2-. +III), and nitrate ion (NO3-. +V). The oxidation state of nitrogen in most organic compounds is -III.
Total nitrogen, as reported in Tab 2-5, is comprised of organic nitrogen, ammonia, nitrite, and nitrate. The organic fraction consists of a complex mixture of compounds including amino acids, amino sugars, and proteins (polymers of amino acids).
The compounds that comprise the organic fraction can be soluble or particulate. The nitrogen in these compounds is readily converted to ammonium through the action of microorganisms in the aquatic or soil enviromnent. Urea. readily conveted to ammonium carbonate, is seldom found in untreated municipal wastewaters. Organic nitrogen is determined analytically using the Kjeldald method. The aqueous sample is first boiled to drive off the ammonia, and then it is digested. During digestion the organic nitrogen is converted to ammonium through the action of heat and acid. Total Kjeldabl nitrogen (TKN) is determined in the same manner as organic nitrogen except that the ammonia is not driven off before the digestion step. Total Kjeldabl nitrogen is therefore the total of the organic and ammonia nitrogen.
Tab 2-5 Definition of the various terms used to define various nitrogen species
Form of nitrogen
Abbrev.
Definition
Ammonia gas
NH3
NH3
Ammonium ion
NH4+
NH4+
Total ammonia nitrogen
TAN
NH3+ NH4+
Nitrite
NO2-
NO2-
Nitrate
NO3-
NO3-
Total inorganic nitrogen
TIN
NH3+ NH4++ NO2-+ NO3-
Total Kjeldabl nitrogen
TKN
Organic N + NH3+ NH4+
Organic Nitrogen
Organic N
TKN - (NH3+ NH4+)
Total nitrogen
TN
Organic N + NH3+ NH4+ + NO2-+ NO3-
As biological nutrient removal has become more common, information on the various organic nitrogen fractions has become important. The principal fractions are particulate and soluble. In biological treatment studies, the particulate and soluble fractions of organic nitrogen are fractionated further to assess wastewater treatability. Fractions that have been used include (1) free ammonia, (2) biodegradable soluble organic nitrogen, (3) biodegradable particulate organic carbon, (4) nonbiodegradable soluble organic nitrogen, and (5) nonbiodegradable particulate organic nitrogen. Unfortunately, there is little standardization on the definition of soluble versus particulate organic nitrogen. Where filtration is the technique used to fractionate the sample, the relative distribution between soluble and particulate organic nitrogen will vary depending on the pore size of the filter used. In many cases, colloidal organic nitrogen has been classified as soluble or dissolved. The lack of standardized definition will also affect other aggregate constituents (i.e., chemical oxygen demand and total organic carbon).
Ammonia nitrogen exists in aqueous solution as either the ammonium ion (NH4+) or ammonia gas (NH3), depending on the pH of the solution, in accordance with the following equilibrium reaction:NH4+ <……> NH3 + H+
Nitrite nitrogen, determined colorimetrically, is relatively unstable oxidized to the nitrate form. It is an indicator of past pollution in the process of stabilization and seldom exceeds 1 mg/L in wastewater or 0.1 mg/L in surface waters or groundwaters. Although present in low concentrations, nitrite can be very important in wastewater or water pollution studies because it is extremely toxic to most fish and other aquatic species. Nitrites present in wastewater effluents are oxidized by chlorine and thus increase the chlorine dosage requirements and the cost of disinfection.
Nitrate nitrogen is the most oxidized form of nitrogen found in wastewaters. Where secondary effluent is to be reclaimed for groundwater recharge, the nitrate concentration is important. The U.S. EPA primary drinking water standards limit nitrogen to 45 mg/L as NO3-, because of its serious and occasionally fatal effects on infants. Nitrates may vary in concentration from 0 to 20 mg/L as N in wastewater effluents. Assuming complete nitrification has taken place, the typical range found in treated effluents is from 15 to 20 mg/L as N. The nitrate concentration is typically determined by colorimetric methods or with specific-ion electrodes.
Nitrogen Pathways in Nature. The various forms of nitrogen that are preent in nature and the pathways by which the forms are changed in an aquatic environment are depicted on Fig. 2-7. The nitrogen present in fresh wastewater is primarily combined in proteinaceous matter and urea. Decomposition by bacteria readily changes the organic form to ammonia. The age of wastewater is indicated by the relative amount of ammonia that is present. In an aerobic environment, bacteria can oxidize the ammonia nitrogen to nitrites and nitrates. The predominance of nitrate nitrogen in wastewater indicates that the waste has been stabilized with respect to oxygen demand. Nitrates, however, can be used by plants and animals to form protein. Death and decomposition of the plant and animal protein by bacteria again yields ammonia. Thus, if nitrogen in the form of nitrates can be reused to make protein by algae and other plants, it may be necessary to remove or reduce the nitrogen that is present to prevent these growths.
Fig. 2-7 Generalized nitrogen cycle in the aquatic and soil environment
Phosphorus
Phosphorus is also essential to the growth of algae and other biological organisms. Because of noxious algal blooms that occur in surface waters, there is presently much interest in controlling the amount of phosphorus compounds that enter surface waters in domestic and industrial waste discharges and natural runoff. Municipal wastewaters, for example, may contain from 4 to 16 mg/L of phosphorus as P.
The usual forms of phosphorus that are found in aqueous solutions include the orthophosphate, polyphosphate, and organic phosphate. The orthophosphates, for example, PO43-, HPO42- , H2PO4-, H3PO4, are available for biological metabolism without further breakdown. The polyphosphates include those molecules with two or more phosphorus atoms, oxygen atoms, and, in some cases, hydrogen atoms combined in complex molecule. Polyphosphates undergo hydrolysis in aqueous solutions and revert to the orthophosphate forms; however, this hydrolysis is usually quite slow. The organically bound phosphorus is usually of minor importance in most domestic wastes, but it can be an important constituent of industrial wastes and wastewater sludges.
Orthophosphate can be determined by directly adding a substance such as ammonium molybdate which will form a colored complex with the phosphate. The polyphosphates and organic phosphates must be converted to orthophosphates using an acid digestion step before they can be determined in a similar manner.
Sulfur
The sulfate ion occurs naturally in most water supplies and is present in wastewater as well. Sulfur is required in the synthesis of proteins and is released in their degradation. Sulfate is reduced biologically under anaerobic conditions to sulfide which, in turn, can combine with hydrogen to form hydrogen sulfide (H2S). The following generalized reactions are typical:
Organic matter + SO42-→H2S + H2O + CO2
S2- + 2H+ → H2S
If lactic acid is used as the precursor organic compound, the reduction of sulfate to sulfide occurs as follows:
2CH3CH(OH)COOH + SO42- →2CH3COOH + S2- + 2H2O + 2CO2
Hydrogen sulfide gas, which will diffuse into the headspace above the wastewater sewers that are not flowing full, tends to collect at the crown of the pipe. The accumulated H2S can then be oxidized biologically to sulfuric acid, which is corrosive to concrete sewer pipes. This corrosive effect, known as "crown rot," can seriously threat the structural integrity of the sewer pipe.
Sulfates are reduced to sulfides in sludge digesters and may upset the biological process if the sulfide concentration exceeds 200 mg/L. Fortunately, such concentrations are rare. The H2S gas, which is evolved and mixed with the wastewater gas (CH4 + CO2), is corrosive to the gas piping and, if borned in gas engines, the products of combustion can damage the engine and severely corrode exhaust gas heat recovery equipment especially if allowed to cool below the dew point.
Gases
Gases commonly found in untreated wastewater include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), hydrogen sulfide (H2S), ammonia (NH3), and methane(CH4).
The first three are common gases of the atmosphere and will be found in all waters exposed to air. The latter three are derived from the decomposition of the organic matter present in wastewater and are of concern with respect to worker health and safety. Although not found in untreated wastewater, other gases with which the environmental engineer must be familiar include chlorine (Cl2) and ozone (O3) (for disinfection and odor control), and the oxides of sulfur and nitrogen (in combustion processes).
Under most circumstances, the ammonia in untreated wastewater will be present as the ammonium ion. However, before discussing the individual gases it will be useful to review the ideal gas law and to consider the solubility of gases in water and Henry's law as applied to the gases of interest.
Solubility of Gases in Water. The actual quantity of a gas that can be present in solution is governed by :(1) the solubility of the gas as defined by Henry's law, (2) the partial pressure of the gas in the atmosphere, (3) the temperature, and (4) the concentration of the impurities in the water (e.g., salinity, suspended solids, etc.).
The Ideal Gas Law. The ideal gas law, derived from a consideration of Boyle's law (volume of a gas is inversely proportional to pressure at constant temperature) and Charles' law (volume of a gas is directly proportional to temperature at constant pressure) is
PV = nRT
where P - absolute pressure, atm
V - volume occupied by the gas, L, m3
n - moles of gas, mole
R - universal gas law constant, 0.082057 at.·L/mole. K
= 0.000082057 atm.m3/mole. K
T - temperature, K (273.15 + ℃)
Using the universal gas law, it can be shown that the volume of gas occupied by one mole of a gas at standard temperature [0℃] and pressure ( 1.0 atm ) is equal to 22.414 L.
Tab 2-6 Henry’s law constants at 20 ℃
Parameter
Henry’s constant,atm
Henry’s constant
unitless
Temperature coefficients
Air
66,400
49.68
557.60
6.724
Ammonia
0.75
5.61×10-4
1887.12
6.315
Carbon dioxide
1420
1.06
1012.40
6.606
Carbon monoxide
53,600
40.11
554.52
6.621
Chlorine
579
0.43
875.69
5.75
Chlorine dioxide
1500
1.12
1041.77
6.73
Hydrogen sulfide
483
0.36
884.94
5.703
Methane
37,600
28.13
675.74
6.880
Nitrogen
80,400
60.16
537.62
6.739
Oxygen
41,100
30.75
595.27
6.644
Sulfide dioxide
36
2.69×10-2
1207.85
5.68
Dissolved Oxygen. Dissolved oxygen is required for the respiration of aerobic microorganisms as well as all other aerobic life forms. However, oxygen is only slightly soluble in water.
Because the rate of biochemical reactions that use oxygen increases with increasing temperature, dissolved oxygen levels tend to be more critical in the summer months.
The problem is compounded in summer months because stream flows are usually lower, and thus the total quantity of oxygen available is also lower. The presence of dissolved oxygen in wastewater is desirable because it prevents the formation of noxious odors.
Hydrogen Sulfide. Hydrogen sulfide is formed, as mentioned previously, from the anaerobic decomposition of organic matter containing sulfur or from the reduction of mineral sulfites and sulfates, it is not formed in the presence of an abundant supply of oxygen. This gas is a colorless, inflammable compound having the characteristic odor of rotten eggs. Hydrogen sulfide is also toxic, and great care must be taken in its presence. High concentrations can overwhelm olfactory glands, resulting in a loss of smell. This loss of smell can lead to a false sense of security that is very dangerous. The blackening of wastewater and sludge usually results from the formation of hydrogen sulfide that has combined with the iron present to form ferrous sulfide(FeS). Various other metallic sulfides are also formed. Although hydrogen sulfide is the most important gas formed from the standpoint of odors, other volatile compound such as indol, skatol, and mercaptans, which may also be formed during anaerobic decomposition, may cause odors far more offensive than that of hydrogen sulfide.
Methane. The principal byproduct from the anaerobic decomposition of the organic matter in wastewater is methane gas. Methane is a colorless, odorless, combustible hydrocarbon of high fuel value. Normally, large quantities are not encountered in untreated wastewater because even small amounts of oxygen tend to be toxic to the organisms responsible for the production of methane. Occasionally, however, as a result of anaerobic decay in accumulated bottom deposits, methane has been produced. Because methane is highly combustible and the expolosion hazard is high, access ports (manholes) and sewer junctions or junction chambers where there is an opportunity for gas to collect should be ventilated with a portable blower during and before the time required for operating personnel to work in them for inspection renewals, or repairs. In treatment plants, methane is produced from the the anaerobic treatment process used to stabilize wastewater sludges. In treatment plants where methane is produced, notices should be posted about the plant warning of explosion hazards, and plant employees should be instructed in safety measures to be maintained while working in and about the structures where gas may be present.
Odors
Odors in domestic wastewater usually are caused by gases produced by the decomposition of organic matter or by substances added to the wastewater. Fresh wastewater has a distinctive, somewhat disagreeable odor, which is less objectionable than the odor of wastewater which has undergone anaerobic (devoid of oxygen) decomposition. The most characteristic odor of stale or septic wastewater is that of hydrogen sulfide, which as discussed previously, is produced by anaerobic microorganisms that reduce sulfate to sulfide. Industrial wastewater may contain either odorous compounds or compounds that produce odors during the process of wastewater treatment.
Odors have been rated as the foremost concern of the public relative to the implementation of wastewater treatment facilities. Within the past few years, the control of odors has become a major consideration in the design and operation of wastewater collection, treatment, and disposal facilities, especially with respect to the public acceptance of these facilities. In many areas, projects have been rejected because of the concern over the potential for odors. In view of the importance of odors in the field of wastewater management, it is appropriate to consider the effects they produce, how they are detected, and their characterization and measurement.
Effects of Odors. The importance of odors at low concentrations in human terms is related primarily to the psychological stress they produce rather than to the harm they do to the body. Offensive odors can cause poor appetite for food, lowered water consumption, impaired respiration, nausea and vomiting, and mental perturbation. In extreme situations, offensive odors call lead to the deterioration of personal and community pride, interfere with human relations, discourage capital investment, lower socioeconomic status, and deter growth. Also, some odorous compounds (e.g., H2S) are toxic at elevated concentrations. These problems can result in a decline in market and rental property values, tax revenues, payrolls, and sales.
Detection of Odors. The malodorous compounds responsible for producing psychological stress in humans are detected by the olfactory system, but the precise mechanism involved is at present not well undersfood. Since 1870, more than 30 theories have been proposed to explain olfaction. One of the difficulties in developing a universal theory has been the inadequate explanation of why compounds with similar structures may have different odors and why compounds with very different structures may have similar odors. At present, there appears to be some general agreement that the odor of a molecule must be related to the molecule as a whole. Over the years, a number of attempts have been made to classify odors in a systematic fashion. The major categories of offensive odors and the compounds involved are listed in Table 2-7. All these compounds may be found or may develop in domestic wastewater, depending on local conditions.
Tab 2-7 Major categories of odorous compounds associated with untreated wastewater
Odorous compound
Chemical formula
Odor quality
Amines
CH3NH2, (CH3)3H
Fishy
Ammonia
NH3
Ammoniacal
Diamines
NH2(CH2)4NH2, NH2(CH2)5NH2
Decayed flesh
Hydrogen sulfide
H2S
Rotten eggs
Mercaptans (methyl and ethyl)
CH3CSH, CH3(CH2)3SH
Skunk
Organic sulfides
(CH3)2S, (C6H5)2S
Rotten cabbage
Skatole
C9H9N
Fecal matter
Odor Characterization and Measurement. It has been suggested that four independent factors are required for the complete characterization of all odor: intensity, character, hedonics, and detectability. To date, detectability is the only factor that has been used in the development of statutory regulations for nuisance odors.
Odor can be measured by sensory methods, and specific odorant concentrations can be measured by instrumental methods. It has been shown that, under carefully controlled conditions, the sensory (organoleptic) measurement of odors by the human olfactory system can provide measurement and reliable information. Therefore, the sensory method is often used to measure the odors emanating from wastewater-treattment facilities. The availability of a direct reading meter for hydrogen sulfide which can be used to detect concentrations as low as 1 ppb is a significant development.
In the sensory method, human subjects (often a panel of subjects) are exposed to odors that have been diluted with odor-free air, and the number of dilutions required to reduce an odor to its minimum detectable threshold odor concentration (MDTOC) noted. The detectable odor concentration is reported as the dilutions to the MDTOC commonly called D/T (dilutions to threshold). Thus, if four volumes of dilute air must be added to one unit volume of sampled air to reduce the odorant to its MDTOC, the odor concentration would be reported as four dilutions to MDTOC. Other terminology commonly used to measure odor strength is ED50. The ED50 value represents the number of times all odorous air sample must be diluted before the average person(50 percentile) can barely detect an odor in the diluted sample. Details of the test procedure are provided in ASTM (1979). However, the sensory determination of this minimum threshold concentration can be subject to a number of errors. Adaptation and cross adaptation, synergism, subjectivity, and sample modification are the principal errors. To avoid errors in sample modification during storage in samples collection containers, direct-reading olfactometers have been developed to measure odors at their source without using sampling containers.
The threshold odor of a water or wastewater sample is determined by diluting the sample with odor-free water. The "threshold odor number" (TON) corresponds to the greatest dilution of the sample with odor-free water at which an odor is just perceptible. The recommended sample size is 200 mL. The numerical value of the TON is determined as follows:
TON = (A + B)/A
Where TON=threshold odor number, A=mL of sample, B=mL of odor-free water
It is often desirable to know the specific compounds responsible for odor. Although gas chromatography has been used successfully for this purpose, it has not been used as successfully in the detection and quantification of odors derived from wastewater collection, treatment, and disposal facilities. Equipment developed and found useful in the chemical analysis of odors is the triple-stage quadrupole mass spectrometer. The spectrometer can be used as a conventional mass spectrometer to produce simple mass spectra or as a triple stage quadrupole to produce colletionally activated disassociation spectra. The former operating mode provides the masses of molecular or parent ions present in samples, while the latter provides positive identification of compounds.
Types of compounds that can be identified include ammonia, ammo acids, and volatile organic compounds.
2-5 Metallic Constituents
Trace quantities of many metals, such as cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni), and zinc (Zn) are important constituents of most waters. Many of these metals are also classified as priority pollutants. However, most of these metals are necessary for growth of biological life, and absence of sufficient quantities of them could limit growth of algae, for example. The presence of any of these metals in excessive quantities will interfere with many beneficial uses of the water because of their toxicity; therefore, it is frequently desirable to measure and control the concentrations of these substances.
Importance of Metals
Metals of importance in the treatment, reuse, and disposal of treated effluents and biosolids are summarized in Tab 2 -8. All living organisms require varying amounts (macro and micro) of metallic elements, such as iron, chromium, copper, zinc, and cobalt, for proper growth. Although macro and micro amounts of metals are required for proper growth, the same metals can be toxic when present in elevated concentrations. As more use is made of treated wastewater effluent for irrigation and landscape watering, a variety of metals must be detemtinad to assess any adverse effects that may occur. Calcium, magnesium, and sodium are of importance in determining the sodium adsorption ratio (SAR), which is used to assess the suitability of treated effluent for agricultural use. Where composted sludge is applied in agricultural applications, arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc must be determined.
Sources af Metals
The sources of trace metals in wastewater include the discharges from residential dwellings, groundwater infiltration, and commercial and industrial discharges. Many of the sources of heavy metals are identified in Tab 2-9. For example, cadmium, chromates, lead, and mercury are often present in industrial wastes. These are found particularly in metal-plating wastes and should be removed by pretreatment at the site of the industry rather than be mixed with the municipal wastewater. Fluoride, a toxic anion, is found commonly in wastewater from electronics manufacturing facilities.
Sampling and Methods of Analysis
Methods for determining the concentrations of these substances vary in complexity according to the interfering substances that may be present. Metals are determined typically by flame atomic absorption, electrothermal atomic absorption, inductively coupled plasma, or IPC/mass spectrometry. Various classes of metals are defined as: (1) dissolved metals are those metals present in unacidified samples that pass through a 0.45-μm membrane filter, (2) suspended metals are those metals present in unacidified samples that are retained on a 0.45-μm menthrane filter, (3) total metals is the total of the dissolved and suspended metals or the concentration of metals determined on an unfiltered sample after digestion, (4) acid extractable metals are those metals in solution after an unfiltered sample is treated with a hot dilute mineral acid.
Tab 2-8 Metals of importance in wastewater management
Metal
Nutrients necessary for biological growth
Concentration threshold of inhibitory effect on heterotrophic organisms, mg/L
Used to determine SAR for land application of effluent
Used to determine if biosolids are suitable for land applications
macro
micro
As
0.05
√
Cd
1.0
√
Ca
√
√
Cr
√
10
Co
√
Cu
√
1
Fe
√
√
Pb
√
0.1
Mg
√
√
√
√
Mn
√
Hg
0.1
√
Mo
√
√
Ni
√
1
√
K
√
Se
√
√
Na
√
√
W
√
V
√
Zn
√
1
√
Tab 2-9 Typical waste compounds produced by commercial, industrial, and agricultural acitivities that have been classified as priority pollutants
Name
Formula
Concern
Arsenic
As
Carcinogen and mutagen. Long-term sometimes can cause fatigue and loss of energy; dermatitis
Barium
Ba
Flammable at room temperature in powder form. Long-term increased blood pressure and nerve block
Cadium
Cd
Flammable in powder form. Toxic by inhalation of dust or fume. A carcinogen. Soluble compounds of cadmium are highly toxic. Long-term concentrates in the liver, kidneys, pancreas, and thyroid; hypertension suspected effect
Chromium
Cr
Hexavalent chromium compounds are carcinogenic and corrosive on tissue. Long-term skin sensitization and kidney damage
Lead
Pb
Toxic by ingestion or inhalation of dust or fumes. Long-term brain and kidney damage; birth defects
Mercury
Hg
Highly toxic by skin adsorption and inhalation of fume or vapor. Long-term toxic to central nervous system,may cause birth defects
Selenium
Se
Long-term red staining of fingers,teeth,and hair;general weakness;
depression;irratation of nose and mouth
Silver
Ag
Toxic metal. Long-term permanent gray discoloration of skin, eyes, and mucous membranes
2-6 Aggregate Organic Constituents
Organic compounds are normally composed of a combination of carbon, hydrogen, and oxygen, together with nitrogen in some cases. The organic matter in wastewater typically consists of proteins (40 to 60 percent), carbohydrates (25 to 50 per cent), and oils and fats (8 to 12 percent). Urea, the major constituent of urine, is another important organic compound contributing to fresh wastewater. Because urea decomposes rapidly it is seldom found in other than very fresh wastewater. Along with the proteins, carbohydrates, fats and oils, and urea, wastewater typically contains small quantities of a very large number of different synthetic organic molecules, with structures ranging from simple to extremely complex.
Over the years, a number of different analyses have been developed to determine the organic content of wastewaters. In general, the analyses may be classified into those used to measure aggregate organic matter comprising a number of organic constituents with similar characteristics that cannot be distinguished separately, and those analyses used to quantify individual organic compounds.
Measurement of Organic Content
In general, the analyses used to measure aggregate organic material may be divided into those used to measure gross concentrations of organic matter greater than about 1.0 mg/L and those used to measure trace concentrations in the range of l0 –12 to 100 mg/L. Laboratory methods commonly used today to measure gross amounts of organic matter (typically greater than 1 mg/L) in wastewater include: (1) biochemical oxygen demand(BOD), (2) chemical oxygen demand (COD), and (3) total organic carbon (TOC). Complementing these laboratory tests is the theoretical oxygen demand (ThOD), which is determined from the chemical formula of the organic matter.
Other methods used in the past included: ( l ) total, albuminoid, organic, and ammonia nitrogen, and (2) oxygen consumed. These determinations, with the exception of albuminoid nitrogen and oxygen consumed, are still included in complete wastewater analyses. Their significance, however, has changed. Whereas formerly they were used almost exclusively to indicate organic matter, they are now used to determine the availability of nitrogen to sustain biological activity in industrial waste treatment processes and to determine whether undesirable algal growths will occur in receiving waters.
Trace organics in the range of 10–12 to 10-13 mg/L are determined using instrumental methods including gas chromotography and mass spectroscopy. Within the past 10 years, the sensitivity of the methods used for the detection of trace organic compounds has improved sufficiantly and detection of concentrations in the range of 10-9 mg/L is now almost a routine matter.
Biochemical Oxygen Demand (BOD)
The most widely used parameter of organic pollution applied to both wastewater and surface water is the 5 day BOD (BOD). This determination involves the measurement of the dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter. Despite the widespread use of the BOD test, it has a number of limitations, as discussed later in this section. It is hoped that, through the continued efforts of workers in the field, one of the other measures of organic content, or perhaps a new measure, will ultimately be used in its place. Why, then, if the test suffers from serious limitations, is further space devoted to it in this text? The reason is that BOD test results are now used (1) to determine the approximate quantity of oxygen that will be required to biologically stabilize the organic matter present, (2) to determine the size of waste treatment facilities, (3) to measure the efficiency of some treatment processes, and (4) to determine compliance with wastewater discharge permits. Because it is likely that the BOD test will continue to be used for some time, it is important to know the details of the test and its limitations.
Basis for BOD Test. If sufficient oxygen is available, the aerobic biological decomposition of an organic waste will continue until all of the waste is consumed.
Three more or less distinct activities occur. First, a portion of the waste is oxidized to end products to obtain energy for cell maintenance and the synthesis of new cell tissue. Simultaneously, some of the waste is converted into new cell tissue using part of energy released during oxidation. Finally, when the organic matter is used up, the new cells begin to consume their own cell tissue to obtain energy for cell maintenance. This third process is called endogenous respiration. Using the term COHNS(which represents the elements carbon, oxygen, hydrogen, nitrogen, and sulfur) to represent the organic waste and the term C5H7NO2 (first proposed by Hoover and Porges) to represent cell tissue, the three processes are defined by the following generalized chemical reactions:
Fig. 2-8 Procedure for settling up BOD test bottles: (a)with unseeded dilution water and (b) with seeded dilution water
Oxidation:
COHNS + O2 + bacteria --> CO2 + H2O + NH3 + other end products + energy
Synthesis:
COHNS + O2 + bacteria + energy --> C5H7NO2
New cell tissue
Endogenous respiration:
C5H7NO2 + 5O2 --> 5CO2 + NH3 + 2H2O
If only the oxidation of the organic carbon that is present in the waste is considered, the ultimate BOD is the oxygen required to complete the three reactions given above. This oxygen demand is known as the ultimate carbonaceous or first-stage BOD, and is usually denoted as UBOD.
BOD Test Procedure. In the standard BOD test (see Fig. 2-8),a small sample of the wastewater to be tested is placed in a BOD bottle (volume = 300mL).The bottle is then filled with dilution water saturated in oxygen and containing the nutrients required for biological growth. To ensure that meaningful results are obtained, the samples must be suitably diluted with a specially prepared dilution water so that adequate nutrients and oxygen will be available during the incubation period. Normally, several dilutions are prepared to cover the complete range of possible values.
BOD that can be measured with various dilutions based on percentage mixtures and direct pipetting are reported in Tab 2-10. Before the bottle is stoppered, the oxygen concentration in the bottle is measured (see Fig. 2-9). After the bottle is incubated for 5 days at 20℃, the dissolved oxygen concentration is measured again. The BOD of the sample is the difference in the dissolved oxygen concentration values, expressed in milligrams per liter, divided by the decimal fraction of sample used. The computed BOD value is known as the 5-day, 20℃ biochemical oxygen demand. When testing waters with low concentrations of microorganisms, a seeded BOD test is conducted (see Fig. 2-8). The organisms contained in the effluent from primary sedimentation facilities are used commonly as the seed for the BOD test. Seed organisms can also be obtained commercially. When the sample contains a large population of microorganisms (e.g., untreated wastewater), seeding is not necessary.
Fig. 2-9 Functional analysis of the BOD test: (a)Interrelationship of organic waste, bacterial mass and (b) idealized representation of the BOD test
Tab 10 Measurable of BOD using various dilutions of samples
By using percent mixture
By direct pipetting into 300-mL bottles
% mixture Range of BOD, mg/L
mL Range of BOD, mg/L
0.01
20,000-70,000
0.02
30,000-105,000
0.02
10,000-35,000
0.05
12,000-42,000
0.05
4,000-14,000
0.1
6,000-21,000
0.1
2,000-7,000
0.2
3,000-10,500
0.2
1,000-3,500
0.5
1,200-4,200
0.5
400-1,400
1
600-2,100
1
200-700
2
300-1,050
2
100-350
5
120-420
5
40-140
10
60-210
10
20-70
20
30-105
20
10-35
50
12-42
50
4-14
100
6-21
100
0-7
300
0-7
The standard incubation period is usually 5 days at 20℃, but other lengths of time and temperatures can be used. Longer time periods (typically 7 days), which correspond to work schedules, are often used, especially in small plants where the laboratory staff is not available on the weekends. The temperature, however, should be constant throughout the test. The 20℃ temperature used is an average value for slow-moving streams in temperate climates and is easily duplicated in an incubator. Different results would be obtained at different temperatures, because biochemical reaction rates are temperature-dependent. After incubation, the dissolved oxygen of the sample is measured and the BOD is calculated using the following Eq.:
BOD,mg/L = (D1-D2)/P
When diluted water is seeded:
BOD,mg/L = [(D1-D2)-(B1-B2)]f/P
Where D1=dissolved oxygen of diluted sample immediately after preparation,mg/L
D2=dissolved oxygen of diluted sample after 5-day incubation at 20 ℃,mg/L
B1=dissolved oxygen of seed control before incubation,mg/L
B2=dissolved oxygen of seed control after incubation,mg/L
f=fraction of seeded dilution water volume in sample to volume of seeded dilution water in seed control
P=fraction of wastewater sample volume to total combined volume
Biochemical oxidation theoretically takes an infinite time to go to completion because the rate of oxidation is assumed to be proportional to the amount of organic matter remaining. Within a 20-day period, the oxidation of the carbonaceous organic matter is about 95 to 99 percent complete, and in the 5-day period used for the BOD test, oxidation is from 60 to 70 percent complete.
Effect of particle size on BOD Reaction Rates. If a separation and analysis technique, such as membrane filtration, is used to quantify the size distribution of the solids in the influent wastewater, the various size fraction can be correlated to observed oxygen(BOD) uptake rates, determined using a respirometer. The observed BOD reaction rate coefficients are affected significantly by the size of the particles in wastewater. It is clear that the treatment of a wastewater can be effected by modifying the particle size distributions will respond differently.
Limitations in the BOD test. The limitations of the BOD test are as followings(1)a high concentration of active, acclimated seed bacteria is required; (2)pretreatment is needed when dealing with toxic wastes, and the effects of nitrifying organisms must be reduced; (3)only the biodegradable organics are measured; (4)the test does not have stoichiometric validity after the soluble organics matter present in solution has been used;(5)the relatively long period of time required to obtain test results. Of the above, perhaps the most serious limitation is that the 5-day period may or may not correspond to the point where the soluble organic matter that is present has been used. The lack of stoichiometric validity at all times reduces the usefulness of the test results.
Total and soluble chemical oxygen demand(COD and SCOD)
The COD test is used to measure the oxygen equivalent of the organic material in wastewater that can be oxidized chemically using dichromate in an acid solution, as illustrated in the following equtation,when the organic nitrogen is in the reduced state:
CnHaObNc + dCr2O72- + (8d+c)H+ → NCO2 + [(a+8d-3c)/2] H2O + cNH4+ + 2dCr3+
Although it would be expected that the value of the ultimate carbonaceous BOD would be as high as the COD, this is seldom the case. Some of the reasons for the observed difference are as follows:(1)many organic substances which are difficult to oxidize biologically, such as lignin,can be oxidized chemically,(2)inorganic substances that are oxidized by the dichromate increase the apparent organic content of the sample,(3)certain organic substances may be toxic to the microorganisms used in the BOD test, and (4)high COD values may occur because of the presence of inorganic substances with which the dichromate can react. From an operational standpoint, one of the main advantages of the COD test is that it can be completed in about 2.5 h, compared to 5 or more days for the BOD test. To reduce the time further, a rapid COD test that takes only about 15 min has been developed.
A new methods of biological treatment have been developed, especially with respect to biological nutrient removal, it has become more important to fractionate the COD. The principal fractions are particulate and soluble COD. In biological treatment studies, the particulate and soluble fractions are fractionated further to assess wastewater treatability. Fractions that have been used include:(1)readily biodegradable soluble COD,(2)slowly biodegradable colloidal and particulate COD, (3)nonbiodegradable soluble COD,and (4)nonbiodegradable colloidal and particulate COD. The readily biodegradable soluble COD is often fractionated further into complex COD that can be fermented to volatile fatty acids(VFAs) and short chain VFAs. Unfortunately, as noted previously, there is little standardization on the definition of soluble versus particulate COD. An alternative method used to determine the soluble COD involves precipitation of the suspended solids and a portion of the colloidal material. The COD of the clarified liquid corresponds to the soluble COD.
Oil and Grease
The term oil and grease, as commonly used, includes the fats, oils, waxes, and other related constituents found in wastewater The fats, oil, and grease (FOG) used previously in the literature has been replaced by the term oil and grease. The oil and grease content of a wastewater is determined by extraction of the waste sample with trichlorotrifluoroethane (oil and grease are soluble in trichlorotrifluoroethane).
Other extractable substances include mineral oils, such as kerosene and lubricating and road oils. Oil and grease are quite similar chemically; they are compounds (esters) of alcohol or glycerol(glycerin) with fatty acids. The glycerides of fatty acids that are liquid at ordinary temperatures are called oils, and those that are solids are called grease (or fats).
If grease is not removed before discharge of treated wastewater, it can interfere with the biological life in the surface waters and create films. The thickness of oil required to form a translucent film on the surface of a water body is about 0.0003048 mm (0.0000120 in).
Fats and oils are contributed to domestic wastewater in butter, lard, margarine,and vegetable fats and oils. Fats are also commonly found in meats, in the germinal area of cereals, in seeds, in nuts, and in certain fruits. The low solubility of fats and oils reduces their rate of microbial degradation. Mineral acids attack them, however, resulting in formation of glycerin and fatty acid. In the presence of alkalis, such as sodium hydroxide, glycerin is liberated, and alkali salts of the fatty acids are formed. These alkali salts are known as soaps. Common soaps are made by saponification of fats with sodium hydroxide. They are soluble in water, but in the presence of hardness constituents,the sodium salts are changed to calcium and magnesium salts of the fatty acids, or so-called mineral soaps. These are insoluble and are precipitated.
Kerosene, lubricating, and road oils are derived from petroleum and coal tars and contain essentially carbon and hydrogen. These oils sometimes reach the sewers in considerable volume from shops, garages, and streets. For the most part, they float on the wastewater, although a portion is carried into the sludge on settling solids. To an even greater extent than fats, oils, and soaps, the mineral oils tend to coat surfaces. The particles interfere with biological action and cause maintenance problems.
Surfactants
Surfactants, or surface-active agents, are large organic molecules that are slightly soluble in water and cause foaming in wastewater treatment plants and in the surface waters into which the waste effluent is discharged. Surfactants are most commonly composed of a strongly hydrophobic group combined with a strongly hydrophilic group. Typically, the hydrophobic group is a hydrocarbon radical (R) made up of 10 to 20 carbon atoms. Two types of hydrophobic groups are used: those that will and those that will not ionize in water. Anionic surfactants are negatively charged [e.g., (RSO3N)-Na+]; whereas cationic surfactants are positively charged [e.g., (RMe3N)+C1-]. Nonionizing (nonionic) surfactants commonly contain a polyoxyethylene hydrophilic group(ROCH2CH2OCH2CH2……OCH2CH2OH, often abbreviated Ren, where n is the average number of --OCH2CH2- units in the hydrophtic group). Hybrids of these types also exist. In the United States, ionic surfactants amount to about two-thirds of the total surfactants used and nonionics to about one-third (Standard Methods, 1998).
Surfactants tend to collect at the air-water interface with the hydrophilic in the water and the hydrophobic group in the air. During aeration of wastewater, these compounds collect on the surface of the air bubbles and thus create a very stable foam. Before 1965,the type of surfactant present in synthetic detergents, called alcyl-benzene-sulfonate(ABS), was especially troublesome because it resisted breakdown by biological means.As a result of legislation in 1965, ABS has been replaced in detergents by linear-alkyl-sulfonate (LAS), which is biodegradable. Because surfactants come primarily from synthetic detergents, the foaming problem has been greatly reduced. It should be noted that so-called "hard" synthetic detergents are still used extensively in many foreign countries.
Two tests are now used to determine the presence of surfactants in water and wastewater. The MBAS (methylene blue active substances) test is used for anionic surfactans. The determination of surfactants is accomplished by measuring the color change in a standard solution of methylene blue dye. Nonionic surfactants are measured using the CTAS (cobalt thiocyanate active substances) test. Nonionic surfactants will react with the CTAS to produce a cobalt containing product which can be extracted into a organic liquid and then measured. It should be noted that the CTAS method requires sublimation to remove nonionic surfactants and ion exchange to remove the cationic and anionic surfactants (Standard Methods, 1998).
2-7 Individual Organic Compounds
Individual organic compounds are determined to assess the presence of priority pollutants identified by the U.S. Environmental Protection Agency (U.S. EPA) and a number of new emerging compounds of concern. Priority pollutants (both inorganic and organic) have been and are continuing to be selected on the basis of their known or suspected carcinogenicity, mutagenicity, teratogenicity, or high acute toxicity. As the techniques used to identify specific compounds continue to improve, a number of other organic compounds have been detected in public water supplies and in treated wastewater effluents.
Priority Pollutants
The Environmental Protection Agency has identified approximately 129 priority pollutants in 65 classes to be regulated by categorical discharge standards (Federal Register,1981).
Two types of standard are used to control pollutant discharges to publicly owned treatment works (POTWs). The first, "prohibited discharge standards," applies to all commercial and industrial establishments which discharge to POTWs. Prohibited standards restrict the discharge of pollutants that may create a fire or explosion hazard in sewers or treatment works, are corrosive (pH < 5.0), obstruct flow, upset treatment processes, or increase the temperature of the wastewater entering the plant to above 40℃. “Categorical standards" apply to industrial and commercial discharges in 25 industrial categories("categorical industries"), and are intended to restrict the discharge of the 129 priority pollutants. It is anticipated that this list will continue to be expanded in the future.
Disinfection Byproducts
It has been found that when chlorine is added to water containing organic matter a variety of organic compounds containing chlorine are formed. Collectively, these compounds, along with others, are known as disinfection byproducts (DBPs). Although general present in low concentrations, they are of concern because many of them are known as suspected potential human carcinogens. Typical classes of compounds include trihalomethanes (THMs), haloacetic acids (HAAs), trichlorophenol, and aldehydes.
More recently, N-nitrosodimethylamidne (NDMA) has been found in the effluent from wastewater-treatment plants. The reason for concern over this compound is that as a group of compounds nitrosamines are among the most powerful carcinogens known. These compounds are also known to be strongly carcinogenic to various fish species at low concentrations. The U.S. EPA action limit for NDMA is 2 parts per trillion. Based on the result of recent studies, NDMA appears to be formed during the chlorination process, in treated effluent, the nitrite ion can react with hydrochloric acid, present as a result of the use of chlorine for disinfection, to form nitrous acid. In turn, nitrous acid can react with dimethylamine to form NDMA .The compound dimethylamine is common in wastewater and surface waters, and is found in urine, feces, algae, and plant tissues. Dimethylamine is also part of polymers used for water treatment (such as polydiallyl dimethylamine) and for ion-exchange resins. The formation of NDMA under basic and alkaline conditions has been reported by Wainwright (1986).
Because of the concern over the formation of DBPs and NDMA, considerable attention has been focused over the past five years on the use of ultraviolet disinfection as a possible replacement for chlorine. In addition, considerable attention has been focused on the modifications to conventional treatment processes to improve the treatment of these compounds and to advanced treatment processes for the removal of these substances. The use of UV radiation for disinfection and the destruction of NDMA is considered in the later chapters.
2-8 Biological Characteristics
The biological characteristics of wastewater are of fundamental importance in the control of diseases caused by pathogenic organisms of human origin, and because of the extensive and fundamental role played by bacteria and other microorganisms in the decomposition and stabilization of organic matter, both in nature and in wastewater treatment plants.
Microorganisms Found in Surface Waters and Wastewater
Organisms found in surface water and wastewater include bacteria, fungi, algae, protozoa, plants and animals, and viruses. Bacteria, fungi, algae, protozoa, and viruses can only be observed microscopically. The general classification of these organisms and a general description of the organisms found in wastewater are considered in the following discussion. The growth and metabolic and environmental requirements of microorganisms are considered in detail in latter chapters. Living single-cell microorganisms that can only be seen with a microscope are responsible for the activity in biological wastewater treatment. The basic functional and structural unit of all living matter is the cell. Living organisms are divided into either prokaryote or eukaryote cells as a function of their genetic information and cell complexity. The prokaryotes have the simplest cell structure and include bacteria, blue green algae (cyanobacter), and archaea. The archaea are separated from bacteria due to their DNA composition and unique cellular chemistry, such as differences in the cell wall and ribosome structure. Many archaea are bacteria that can grow under extreme conditions of temperature and salinity, and also include methanogenic methane-producing bacteria, important in anaerobic treatment processes.
In contrast to the prokaryotes, the eukaryotes are much more complex and contain plants and animals and single-celled organisms of importance in wastewater treatment including protozoa, fungi, and green algae.
The prokaryote organisms are generally much smaller compared to eukaryote organisms. The absence of a nuclear membrane to contain the cell DNA is also a distinguishing feature of the prokaryota organisms. The eukaryoric organisms have much more complex internal structures. These include the endoplasmic reticulum, which is a distinct organaelle that contains the sites of ribosomes with internal membranes. The golgi bodies are also distinct membrane structures and contain sites for the secretion of enzymes and other macromolecules. The mitochondrion is a complex internal membrane structure where respiration occurs for eukaryotic cells, and is lacking in prokaryotic ceils. While the prokaryotes can have photosynthetic pigments, they do not contain chloroplasts, which are used in photosynthesis by green algae.
Viruses are obligate intracellular parasites that require the machinery of a host cell to support their growth. Although viruses contain the genetic information (either DNA or RNA) needed to replicate themselves, they are unable to reproduce outside of a host cell. Viruses are composed of a nucleic acid core (RNA or DNA) surrounded by an outer coat of protein and glycoprotein. Viruses are classified separately according to the host infected. Bacteriophage, as the name implies, are viruses that infect bacteria.
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