 Recent developments in sewage treatment
technology include the combination of various aerobic treatment
processes to obtain the best performance and most economical
treatment of sewage.
Combining the various aerobic processes available can derive
a large number of treatment systems. The main reason for combining
these processes is that they provide the stability and resistance
to shock loads of attached growth processes and the high-quality
effluent of suspended growth systems.
Various standard combinations exist such as Activated
Bio-Filter Process, Roughing - Filter Activated Sludge Process,
Bio-Filter Activated Sludge Process and Series Trickling Filter
Activated Sludge Process.
In particular, the combination of a trickling filter and
solids-contact process produces a high quality effluent with
complete reliability. The trickling-filter solids-contact
(TF/SC) process consists of a trickling filter, an aerobic
contact tank, and a final clarifier. Modifications to this
system include a return-sludge aeration tank and flocculating
center-well clarifiers.
The trickling filters are sized to remove the major portion
of the BOD, typically 60 to 85 percent. The biological solids
formed on the trickling filter are sloughed off and concentrated
through sludge recirculation in the contact tank. In the contact
tank, the suspended growth is aerated for less than one hour,
causing the flocculation of the suspended solids and further
removal of soluble BOD. When short solids contact times are
used, a sludge re-aeration tank is usually required. Because
of the high level of dispersed solids in contact tank effluent,
flocculating center-well clarifiers have been found to be
effective in maximising solids capture.
Combined Processes can be used to upgrade both existing trickling
filter or extended aeration plants. It is particularly suitable
where there are problems of high suspended solids in the final
effluent. This can occur where solids in the effluent do not
settle out sufficiently in the final clarifier.
Typical figures for Combines Processes are as follows:
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