732 MUNICIPAL WASTEWATER
not contaminated. Odors are a problem and removals decline
markedly in cold weather.
Intermittent sand filters are much like the slow sand fil-
ters used for potable water production. The sewage is applied
to a sandy area and allowed to percolate downward. Raw
sewage may be applied at rates as high as 80,000 gal/acre ×
day and secondary effluents at rates as high as 800,000 gal/
acre × day. Application of the secondary effluent would be
considered tertiary treatment. Biological films that form on
the sand grains undergo continuous stabilization. It is neces-
sary to rest the bed between dosings so that objectional con-
ditions do not develop. Surface accumulations of solids must
be periodically removed. This method is not recommended
for areas underlain by fissured limestone.
Fill and draw beds operate as the name indicates. A tank,
packed with coarse material, is filled with sewage and allowed
to stand full. It is then drained and allowed to rest. Air is
drawn into the bed during filling and emptying. Loadings are
about 200,000 gal/acre × ft × day. There is little application
of this method today in treatment of municipal wastewater
but it does find use in industrial waste treatment. Fill and
draw beds are a batch operation and the trickling filter is a
continuous operation.
Sewage is distributed over a trickling filter by slowly
revolving arms equipped with nozzles and deflectors. Some
earlier plants had fixed nozzles but this is no longer done.
Revolving arms are driven by hydraulic head. Sewage dis-
charged is allowed to flow slowly downward through the
bed. Air is down into the bed by temperature differential,
thus maintaining a supply of oxygen for the process. Filter
media is usually stone. Sizes are in the range of 1 to 4 inches.
Packing of this size permits air to be drawn into the bed and
the bed is not clogged by biological slime. There appears
to be a trend toward more use of plastic filter media. Filter
depths range from 3 to 14 feet. A common depth is 6 feet.
After passage through the filter the sewage is collected
in tile underdrains. These underdrains serve two purposes:
(1) collection of filter effluent and (2) circulation of air
into the filter. The underdrains discharge to a main collec-
tion channel which, in turn, discharges to the final settling
(humus) tank. The importance of the function of the final
settling tank can be seen by an examination of what occurs
in the filter itself.
A new filter is “broken in” by applying sewage as in
normal operation. After a time the microbial (zoogleal) mass
establishes itself on the filter media and carries on the work
of waste stabilization. Waste material in the flowing sewage
(food) is first absorbed into the zoogleal mass and then
assimilated by the microorganisms. Much of the organic
waste material has, at this point, been utilized for cell syn-
thesis and energy. There must be continuous removal of
filter slime or the process becomes sluggish due to a lower
feeding rate of old organisms. Since waste material is now
a part of the filter slime there must be a means provided for
removal of sloughed off organisms or the waste material,
now in different form, would still appear in the plant efflu-
ent and little constructive would have been accomplished.
The required removal is carried out in the secondary settlingtank. A rate of application lower than that of the primary
tank is necessary here because of the different character
of the material to be removed. Rates in this portion of the
system are in the range of 600 gal/ft^2 × day.
A portion of the effluent is recirculated, as shown in Figure- This is done in order to (1) smooth out flow, (2) keep the
food concentration more constant, (3) lower the film thickness
and, thus, control the psychoda fly, and (4) reseed the applied
sewage with acclimatized organisms. The psychoda, or filter,
fly is a very small insect which breeds in trickling filter slime.
It does not bite but can be extremely bothersome because it
does get into the nose and mouth. The range of flight is short
but the creature can be carried great distances by the wind.
Control of the fly in its developmental phase can be achieved
by flooding the filter periodically or through chlorination of
influent.
Trickling filters can be classified on the basis of (1) hydrau-
lic loading per unit area and (2) applied pounds of BOD per
1000 ft^3 of filter volume.
The low rate trickling filter, with hydraulic loadings
of 2 to 4 million gallons per acre per day (mgad) and 10
to 20 pounds BOD per 1000 ft^3 , is usually found in use in
smaller plants. With proper operation, BOD removals of
80 to 85% can be routinely expected. Raising the applied
sewage to 10 mgad produced greater BOD removals per
unit filter volume but the effluent organic concentration was
found to be high. Influent organic concentration was reduced
by greater effluent recirculation and lower effluent organic
concentration was realized. Units that operate in the 10 to 40
mgad range are called high rate trickling filters. BOD load-
ings are up to 90 pounds per 1000 ft^3 , but removals to be
expected are in the range of 65 to 75%.
In the 4 to 10 mgad range operational difficulties were
frequently encountered and this range was avoided for many
years. It appears that, in this range, the hydraulic application
was inadequate to keep the filter slime from attaining exces-
sive thickness. Many plants had operated well in this range,
but other plants had many problems. The solution seems to
have been reached with use of relatively large, 2 to 4 inches,
filter stones.
Experimental plants using plastics media have recently
achieved very high removal efficiencies (97%) at hydraulic
loading rates of 100 mgad. Much of the microbial mass is
in the recirculated effluent and these plants are, in effect,
modifications of the activated sludge process. Organic
influent
Headworks PrimarySed. TreatmentBiological SecondarySed. outrecirculation
excess to
digesterto digesterPrimary TreatmentCI 2FIGURE 4C013_007_r03.indd 732C013_007_r03.indd 732 11/18/2005 10:43:41 AM11/18/2005 10:43:41 AM