Environmental Microbiology of Aquatic and Waste Systems

10.3 Systems for the Treatment of Sewage 259

90–95% of the BOD is removed. When the contact

time is less (in the high-rate treatment), BOD removal

is 60–70% and the sludge produced is more. With lon-

ger contact time, say several days, BOD reduction is

over 95% and sludge extremely low.

With systems where oxygen is introduced as in the

closed tank system, or where there is great oxygen

solubility as in the deep shaft system, contact time

could be as short as 1 h but with up to 90% BOD reduc-

tion along with substantially reduced sludge.

10.3.1.2 The Trickling Filter...........................................

The trickling filter consists of round rocks 1–4 in. diam-

eter arranged in a bed 6–10 ft deep (Fig. 10.9). Sewage

Fig. 10.7 The deep shaft aeration system (From Okafor 2007. With permission)

Fig. 10.8 Compact activated sludge system (From Okafor
2007. With permission)