TM 5-814-8
mance of trickling filters is dependent upon
will depend somewhat on effluent criteria. Where
several other factors including: wastewater char-
suspended solids are critical, a columnar unit may
acteristics, filter depth, recirculation, hydraulic
also serve as a filter. In other instances, the
and organic loading, ventilation and temperature.
.
suspended growth system will usually be most
W h i l e all of these factors are important,
appropriate.
wastewater temperature is the one which is most
c. Fixed film processes.
responsible for secondary effluent criteria not
(1) Trickling filters. This type of treatment
being met during winter operating conditions.
method has proven very popular over numerous
Based on data from several high rate filters in
years in the U.S. In 1968, more than 3,700
Michigan, filter performance was observed to
trickling filter installations existed in this coun-
vary 21 percent between summer and winter
try. In the past, the use of the trickling filter has
months. Covering trickling filters or providing an
been considered as the ideal method for popula-
additional stage should be considered for improv-
tions of 2,500 to 10,000. The principal reasons for
ing and maintaining performance.
its past popularity have been cost, economics and
(2) Rotating biological contractors. Another
operational simplicity as compared to the acti-
type of biological secondary treatment system is
vated sludge process.
the rotating biological contactor. This system has
(a) Types. The trickling filter process is
been used in Europe, particularly West Germany,
well documented in TM 5-814-3 and will not be
France and Switzerland. Manufacturers indicate
repeated herein. The types of trickling filters used
1000 installations in Europe treat wastewaters
and their basic design criteria are set forth in
ranging in size from single residences to 100,000
table 6-3. BOD and hydraulic loadings are based
population equivalent. Domestic, industrial and
on average influent values. Filters at military
mixtures of domestic and industrial wastewaters
installations have either been low or high rate
have been treated. In the process, the large
single stage facilities. One advantage of most low
rate filters is that the longer solids retention time
diameter corrugated plastic discs are mounted on
a horizontal shaft and placed in a tank. The
(SRT) in the unit allows for production of a
medium is slowly rotated with about 40 percent
highly nitrified effluent, provided the climatic
of the surface area always submerged in the
conditions are favorable. By comparison, interme-
flowing wastewater. The process is similar in
diate and high rate filters, which are loaded at
function to trickling filters since both operate as
higher organic and hydraulic loadings, do not
fixed film biological reactors. One difference is
achieve as good an overall BOD removal effi-
t h a t the biomass is passed through the
ciency and preclude the development of vitrifying
wastewater in the biological contactor system
bacteria. The other classification of filters are
rather than the wastewater over the biomass as
those termed as super rate. These employ syn-
in a trickling filter unit. No sludge or effluent
thetic media and have been shown to be able to
recycle is employed. The system has several
sustain much higher loadings than a stone me-
advantages, including:
dium unit. As a result, the super rate filters, in
Low energy requirements compared with
addition to normal applications for domestic and
activated sludge.
industrial wastewaters, have found applications
Small land area requirement compared
as roughing filters prior to subsequent treatment
with trickling filters.
facilities. The large surface area per unit volume
A high degree of vitrification can be
(specific surface area) and high percent voids of
achieved.
synthetic media allow higher organic and hydrau-
-- A more constant efficiency can be
lic loadings. The greater surface area permits a
achieved during cold weather than with
larger mass of biological slimes per unit volume.
trickling filters since the units are easily
The increased void space allows for higher hy-
covered. The covers allow sufficient venti-
draulic loadings and enhanced oxygen transfer
lation, but minimize the effect of low
due to increased air flow.
ambient air temperatures.
(b) Performance. Most existing trickling fil-
While the system has achieved high BOD re-
ter installations must be upgraded to meet the
moval efficiencies on domestic wastewaters in the
new secondary treatment requirements. Decreas-
U. S., pilot testing should be performed for any
ing hydraulic or organic loading at existing facili-
industrial application. A recent U.S. EPA study
ties will not produce a significant increase in
(42) on an industrial waste showed the biological
BOD removal above original design values; in-
contractors could not perform at the anticipated
stead, additional treatment operations will be
loading rate and achieve required removal efficien-
needed to achieve greater BOD removals. Perfor-
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