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ACTIVATED SLUDGE
Activated sludge may be defined as the flocculant microbial biomass that is produced when wastewater is continuously aerated. It consists mainly of organisms that are able to metabolise and break down the principal contaminants of the wastewater. The conventional activated sludge process, first developed by Ardern and Lockett in Manchester in 1913, consists of mixing and aerating settled biodegradable wastewater with a flocculent suspension of micro-organisms comprising bacteria in different stages of development aggregated with organic debris, protozoa and occasionally fungi. During aeration the removal and stabilisation of organic matter in wastewater depend upon the processes of adsorption, absorption, synthesis and respiration.
Essential plant requirements for the activated sludge process are:
- An aeration tank in which the activated sludge floccs and the incoming wastewater stream are mixed and aerated for a period of time.
- An adequate supply of dissolved oxygen.
- A secondary settlement tank or other means of separating the activated sludge floccs from the treated effluent.
- Facilities for returning the activated sludge floccs from the settlement tank to the aeration tank at a controlled rate.
- Facilities for the handling, treatment and disposal of the surplus activated sludge.
There are several variations on the basic activated sludge process:
- Completely Mixed System - An aerobic system where the wastewater stream entering the aeration tank is rapidly distributed throughout the mixed liquor so that no pollution concentration gradient exists within the tank.
- Plug Flow - An aerobic system where the wastewater stream and the return activated sludge (RAS) are mixed at the inlet of a long aeration tank or channel and the treated mixed liquor is withdrawn at the opposite end theoretically creating a 'plug' moving along the tank .
- Extended Aeration - An aerobic system where there is usually no primary settlement though screening/comminution/de-gritting should be considered essential to ensure effective operation of the plant.
- Contact Stabilisation - The contact stabilisation (biosorption) process is an aerobic process that comprises treatment of the wastewater stream in three distinct stages. There is often a fourth stage for treatment of the surplus sludge from the plant.
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