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: