MODIFIED EXTENDED AERATION PLANT
FOR MINIMIZING SLUDGE PRODUCTION
Alan F. Rozich, Associate Scientist
Anthony F. Gaudy, Jr., Professor
Civil Engineering Department
University of Delaware
Newark, Delaware 19716
INTRODUCTION
The economical disposal of sludges generated in wastewater treatment plants is a problem which
has gained national attention. This concerns not only in-plant strategies with regard to solids handling
processes but also achieving the ultimate disposal of sludges generated in wastewater treatment plants.
It should be pointed out that, as an ultimate goal, the aim of wastewater treatment should be the
removal of organics from waste streams via respiration and conversion into microbial biomass and
the subsequent conversion of microbial biomass into carbon dioxide and water, which ultimately
result in completing the carbon cycle. Conventional treatment plant schemes may eventually achieve
this goal with a series of solids handling processes such as thickening, dewatering, etc., which are
required to precede an ultimate disposal process such as incineration. In any event, a number of
physical processes are inserted between the conversion of soluble organics into biomass and the oxidation of biomass to C02 and water.
An alternate conceptual approach to the sludge disposal problem consists of converting soluble
organics into biomass and simultaneously oxidizing the biomass to CO, and water. This is the total
oxidation approach, and it is operationally attempted in extended aeration activated sludge processes.
In these systems, the engineering controls are designed to realize an operating condition wherein
microbial growth is offset by cellular decay, which results in zero secondary sludge production. It
also has been demonstrated that it is feasible to digest primary sludge aerobically; consequently, these
facts suggest that an activated sludge system could be modified for use as both a wet stream process
and a sludge disposal process. In this case, the goal is total oxidation of the sludges (primary and
secondary), which means that, if this system is successful, solids handling facilities would not be
required.
The purpose of this report is to describe a modification of the extended aeration process which
can be utilized as a method for significantly reducing or possibly eliminating both the primary and
secondary sludge streams in wastewater treatment plants. The crux of this process modification rests
on utilizing an aerobic digester as a source of recycle cells for the aeration tank in an activated sludge
process; both primary and secondary sludges are digested in this reactor and no sludge is wasted from
this system. Because of the relatively lengthy hydraulic detention time in the aerobic digester, the
concentration of recycle sludge, XR, becomes relatively constant; consequently, the aeration tank
benefits from the constant X„ control technique. In this report, a preliminary modelling approach
is given for this process modification, and it is compared and contrasted with a conventional activated
sludge treatment plant scheme. In a cost analysis which compares capital and operating/maintenance
costs for a 10 MGD flow rate, the modified process is shown to be significantly more cost-effective
than the conventional treatment plant scheme.
PROCESS DESCRIPTION
A typical activated sludge treatment plant process is shown in Figure 1 while the recommended
activated sludge modification is depicted in Figure 2. Figure 1 is a familiar flow sheet, whereas Figure
2 incorporates the total oxidation concepts of treatment and sludge disposal which have been shown
to be promising in bench-scale research with synthetic wastes; the results of these studies are described
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