Environmental Microbiology of Aquatic and Waste Systems

260 10 Waste Disposal in the Aqueous Medium: Sewage Disposal

is uniformly spread on the bed by a rotating distributor
which is powered by an electric motor or driven by
hydraulic impulse where a hydraulic head is available.
The sewage percolates by gravity over the rocks and
through the spaces between the rocks and the effluent is
collected in an under-drain. From the under-drain, the
liquid is allowed to settle in a sedimentation tank which
is an integral part of the system.
The sludge consisting of microorganisms and unde-
composed matter is removed from time to time and
may be used as manure. Various modifications may be
made to this basic design. In one modification (see

Fig. 10.10), the sewage may be pre-sedimented before

filtration; in another, two filters may operate; and in yet

others, the effluent may be re-cycled.

Trickling filters may be low-rate or high-rate. Low

rate filters handle 2–4 million gallons per acre per day

(mgad). They are very efficient in BOD reduction and

may attain 85–90% BOD. The effluent is usually still

rich in oxygen; hence, nitrates are produced in abun-

dance by nitrifying bacteria. High rate filters handle

10–40 mgad and BOD reduction is only 65–75%. The

effluent is usually low in nitrates since much of the O 2

is used in breaking down the organic matter load.

Fig. 10.9 Section through trickling filter bed (From Okafor 2007. With permission)

Raw

sewage

Pump

S

Sedimentation

tank

Effluent

Sludge

Removed

Convential

(Single stage filtration

without pre-sedimen-

tation)

Filter

Raw

sewage

Pump

S

Sedimentation

tank

Effluent

Sludge

Removed

Convential

(Single stage filtration

without pre-sedimen-

tation)

Filter

Fig. 10.10 Scheme
illustrating two
arrangements of trickling
filter: conventional and
single stage (From Okafor
2007. With permission)