Package/Mechanical Sewage Treatment Plants
Commercially available prefabricated treatment plants known as "package plants" are often used to serve small communities up to population equivalent (PE) of 5,000.
Package plants require little design work and can be installed quickly although they require the same operational and maintenance care as conventional treatment plants. Claims that package plants produce no sludge is incorrect.
Care must be taken in using package plants where large variations in flow (hydraulic shock are experienced), in addition adequate provision must be made for sludge removal, scum and grease removal and the proper control of air supply.
The most common types of package plants use Extended Aeration, Contact Stabilisation, Bio-Filter, Sequenced Batch Reactors and Rotating Biological Contactor processes.
The performance of package plants can be improved by sizing the components conservatively. In general, the careful selection of the right process should lead to an adequate plant for small isolated communities requiring sewage treatment. A plant such as the one using an "Extended Aeration Activated Sludge" process should produce a good effluent quality, have low sludge yield and be easy to operate.
There are now a large number of package plants in the market using a variety of equipment. Uncontrolled selection of plants can lead to problems with operational knowledge and supply of spare parts. Hence, some form of regulation for the use of these plants will be employed to ensure overall industry efficiency.
Further, market forces have driven the manufactures to make optimistic claims for the treatment capacity of small package plants, leading to plants designed in the high rate activated sludge mode in order to reduce the capital cost of construction. This often leads to high operational, maintenance and operator costs due to the need for high operator involvement to keep the plants running within the design parameters.