8 Attached Growth Biological Treatment Processes
8-1 Background
Nonsubmerged Attached Growth Processes
A trickling filter is a nonsubmerged
fixed-film biological reactor using rock
or plastic packing over which
wastewater is distributed continously,
The limitations of the trickling filter
included a relatively high incidence of
clogging,the long rest period required,
headloss,and the relatively low
loading that could be used,
In the 1950s,plastic packing began to
replace rock,which 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),where the packing is rotated in the
wastewater treatment tank,
Both trickling filters and RBCs have been
used as aerobic attached growth processes
for BOD removal only,combined BOD
removal and nitrification,
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,
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,
8-2 Trickling Filters
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
wastewater is distributed over the top of the
bed by a rotary distributor,
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,
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,
Distributor arms 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,
In some cases,especially for square or
rectangular filters,fixed flat-spray nozzles
have been used,
Primary clarification is necessary before
rock trickling filters,
In some installations,a wire-mesh screen is
placed over the top of plastic packing to
collect debris that can be vacuumed off
periodically,
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,
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,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,Generally,algae do
not take a direct part in waste degradation,but
during the daylight hours they add oxygen to the
percolating wastewater,
The function of protozoa 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,
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,
Trickling Filter Classification and Applications
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,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
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,
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,
Odors are a common problem,especially if
the wastewater is stale or septic,or if the
weather is warm,Filter flies (Psychoda)
may breed in the filters unless effective
control measures are used,
Intermediate- and High-Rate Filters
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,
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,
Two-Stage Filters
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,
Design of Physical Facilities
(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,
Distribution Systems
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,
Nozzles are spaced unevenly so that greater flow per unit
of length is achieved near the periphery of the filter than at
the center,Headloss through the distributor is in the range
of 0.6 to 1.5 m,
Underdrains
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,
Fig,8-6 Typical under-drain system for Tower filter
Airflow
Natural ventilation 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 ventilation,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,
8-3 Rotating Biological Contactors
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 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,Mechanical drives are
normally used to rotate the units,but air-driven
units have also been installed,
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,
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),
