The composition of the ...

The composition of the wastewater is not the deciding factor. The choice of treatment process depends upon the volume to be treated, available space, costs of construction and quality of the discharge required.

Trickling filters or RBCs are generally the first choice for small municipal works; they are cheaper to operate and maintain than activated sludge but occupy a larger area relative to the flow treated. Activated sludge is typically used for populations larger than about 50,000 but this is highly variable - it depends on the price/availability of land. Activated sludge is also better if there is a requirement to remove nitrogen from the wastewater, since trickling filters can't provide the anoxic conditions needed to remove nitrates. Activated sludge is also preferred in cold regions because trickling filters are susceptible to freezing.

If there is insufficient land for activated sludge, or if the discharge quality requirements are so tight that it makes economic sense, processes like membrane bioreactors are used, but at the expense of higher capital and operating costs.

Industrial effluent is more highly variable and the treatment process can depend on the characteristics, for example trickling filters can be used to cool hot effluent.