Aerobic Digestion of Thickened
Waste Activated Sludge
TOM D. REYNOLDS, Associate Professor
Civil Engineering Department
Texas A & M University
College Station, Texas 77843
INTRODUCTION
The activated sludge process and its various modifications have been the most
successful treatments for medium-to-large sized waste treatment installations. The
processes, although they have a high operating cost, do have a lower capital cost than do
trickling filter plants. The activated sludge process, when operated properly, has a high
efficiency of BOD removal and produces an effluent that is highly nitrified and stable. One
drawback to the activated sludge process is the disposal of waste activated sludge. For the
treatment of domestic wastes the waste sludge flow is normally 1.0 to 3.0 percent of the
incoming plant flow and has a suspended solids concentration from 5,000 to 12,000 mg/1.
The solids in the waste sludge flow represent the net growth of biological cells that have been
synthesized in the utilization of the incoming organic material to the plant.
Three unit processes have generally been successful in the treatment of waste activated
sludge (1). These are: I) Anaerobic digestion; 2) Oxidation ponds; and 3) Aerobic digestion.
Anaerobic digestion is the oldest of the three methods and has proved to be highly
successful in the decomposition of the organic or volatile material. Thedigester is very large
in relation to the plant size if the sludge is not thickened prior to digestion. The digester
volume for a plant using unthickened sludge is approximately twice the digester volume
required for a trickling filter plant. Oxidation ponds may be used for waste activated sludge;
however, the area required is quite large if the ponds are to be designed as aerobic systems. If
space is available the oxidation pond is a good facility to use. Aerobic digestion is the newest
method for treating waste activated sludge. It is based on the principle that biological cells
will use their own cell material and dead cells present as food if there is no external source of
food in their environment (2).
Advantages of aerobic digestion as compared to anaerobic digestion include: 1)
volatile-solids reduction approximately equal to that obtained anaerobically; 2) lower BOD
concentrations in supernatant liquor; 3) production of an odorless, humus-like, biologically
stable end product that can be disposed of easily; 4) production of a sludge with excellent
dewatering characteristics; 5) recovery of more of the basic fertilizer values in the sludge; 6)
fewer operational problems; and 7) lower capital cost. In comparing aerobic digestion to
anaerobic digestion the main disadvantages are that higher energy costs are required to
furnish the oxygen and that the useful by-product methane is not produced.
This research was devoted to determining criteria for the design of aerobic digestion
that treat thickened waste activated sludge. Pertinent parameters studied include: 1) Rates
of digestion; 2) Required digestion times; 3) Percent solids destruction; 4) Oxygen
requirements; 5) Organic loadings; 6) Hydraulic detention times; 7) Solids retention times;
and 8) Mixing requirements.
The research consisted of both laboratory studies and field studies on the aerobic
digester for the biosorption (contact stabilization) wastewater treatment plant located at
Texas A & M University, College Station, Texas.
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