Activated Sludge Processes

The basic activated-sludge process (Fig. 11.4) consists of aerating and agitating the effluent in the presence of a flocculated suspension of micro-organisms on particulate organic matter — the activated sludge. This process was first reported by Arden and Lockett (1914)

and is now the most widely used biological treatment process for both domestic and industrial wastewaters. The raw effluent enters a primary sedimentation tank where coarse solids are removed. The partially clarified effluent passes to a second vessel, which can be of a variety of designs, into which air or oxygen is injected by bubble diffusers, paddles, stirrers, surface aerators etc. Vigorous agitation is used to ensure that the effluent and oxygen are in contact with the activated sludge. After a predetermined residence time of several hours, the effluent passes to a second sedimentation tank to remove the flocculated solids. Part of the sludge from the settlement tank is recycled to the aeration tank to maintain the biological activity. The overflow obtained from the settlement tank should be of a 20:30 standard or better and be suitable for discharge to inland waters. The excess sludge is dewa-tered and dried, to be sold as a fertilizer, incinerated or landfilled. In conventional activated sludge processes, organic loading rates are 0.5-1.5 kg BOD m 3 day"1 with hydraulic retention times of 5-14 hours depending on the nature of the wastewater, giving BOD reductions of 90-95%. High-rate activated-sludge processes can be used as a partial treatment for strong wastes prior to further treatment or discharge to a sewer and are widely used in the food processing and dairy industries. The organic loading rate is 1.5-3.5 kg BOD m ' day"1, and with hydraulic retention times of only 1-2 hours, BOD reductions of 60-70% are possible (Gray, 1989).

A number of modifications of the basic process can be used to improve treatment efficiency, or for a more specific purpose such as denitrification (Winkler and Thomas, 1978; Gray, 1989). Tapered aeration and stepped feed aeration are used to balance oxygen demand (which is greatest at the point of wastewater entry to the aeration basin) with the amount of oxygen supplied. Contact stabilization exploits biosorption processes and thereby allows considerable reduction in basin capacity (~ 50%) for a given wastewater throughput. Denitrification (the biological reduction of nitrate to nitrite and on to nitrogen gas under anoxic conditions) can be accomplished in an activated-sludge plant when the first part of the basin is not aerated.

In advanced activated-sludge systems the amount of dissolved oxygen available for biological activity is increased to improve treatment rate. One vessel of this type is the 'Deep Shaft' (Hemming et al„ 1977), which is quite distinctive from the other aeration tanks and has been developed from the ICI pic SCP process (Taylor and Senior, 1978; Chapter 7). The 'Deep Shaft' (Fig. 11.5) consists of a shaft 50 to 150 m deep, separated into a down-flow section (down-comer) and an up-flow section (riser). The shaft may be 0.5 to 10 m diameter, depending on capacity. Fresh effluent is fed in at the top of the 'Deep Shaft' and air is injected into the down-flow section at a suffficient depth to make the liquid circulate at 1 to 2 m s"1. The driving force for circulation is created by the difference in density (due to air bubble volume) between the riser and down-flow sections. For starting up, circulation of liquid is stimulated by injecting air at the same depth in the riser. Air injection is then gradually all transferred to the air injection point in the down-comer. Because of the pressure created in the down-comer, oxygen-transfer rates of 10 kg 02 m"3 h"1 can be achieved and bubble contact times of 3 to 5 minutes are possible instead of 15 seconds in diffused air systems. BOD removal rates of 90% are achievable at organic loadings of 3.7-6.6 kg BOD m"3 day"1 at hydraulic retention times of 1.17-1.75 hours (Gray, 1989). Sludge production was found to be much less than that for conventional sewage-treatment processes.

Two types of pure oxygen systems have also been developed to increase the rate of oxygen transfer:

(i) closed systems which operate in oxygen-rich atmospheres and,

(ii) open systems employing fine bubble diffusers.

Influent

Influent

Tapered Aeration Activated Sludge

Treated effluent

Fig. 11.4. Simplified cross-section of an activated sludge process.

Treated effluent

Waste sludge

Fig. 11.4. Simplified cross-section of an activated sludge process.

Air-

Compressor Start-up air -

Inlet -

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