Low Pressure Wet Air Oxidation
of Sewage Sludge
G.H. TELETZKE, Deputy General Manager
Zimpro Division of Sterling Drug, Inc.
Rothschild, Wisconsin
INTRODUCTION
The objective of Zimpro sludge treatment is to produce a sterile, nonputres-
cible solid residue, unidentifiable as a sewage origin and easily concentrated to
the minimum volume consistent with the lowest overall treatment and disposal
cost.
In Zimpro wet air oxidation sewage sludge is heated with air to an initiating
reaction temperature usually between 300 and 400 F. The mixture then enters a
reactor where the desired oxidation is accomplished and a temperature rise occurs. The degree of oxidation achieved depends on the temperature, pressure,
holding time and concentration of the sludge entering the process. Operating
pressures may be 150 to over 3,000 psi, depending upon the results desired. If the
process is not thermally self-sustaining, steam may be injected to sustain the reaction temperature. Heat residing in the oxidized products is recovered by heat
exchange with the entering sludge and air and the gaseous effluent separated from
the liquid carrying the solids. The exhaust gases may be scrubbed and discharged
to the atmosphere or, where economic conditions make it attractive, may be expanded in a power recovery unit to recover the energy residing therein. Tne solids
may be separated from the carrying liquid and concentrated by settling, sand bed
drainage, vacuum filtration or centrifugation, depending upon the situation. A
general flow sheet is shown in Figure 1.
A wide range of end products and results is possible with the wet air oxidation
process, depending upon the degree of oxidation accomplished. Figure 2 shows
results of several levels of oxidation of a typical sludge, and the resulting volumes
of settled and drained solid residue. The drained residue is sterile and completely
unobjectionable for final disposal after as little as 10 per cent COD reduction.
The advantage of lower degrees of oxidation is that with usual existing cost
relationships they can be accomplished for lower investment and operating costs
than the higher degrees of oxidation. Figure 3 shows typical cost relationships for
various levels of COD reduction.
The one disadvantage of a low degree of COD reduction is that there is a
greater volume of residual solids remaining for ultimate disposal than from a high
degree of oxidation. While this may be a disadvantage of major importance in
very large cities, we find that in the great majority of situations existing in the
United States at the present time the cost savings acnieved by utilizing a low degree of oxidation are of much more importance than the small additional volume
of unobjectionable solids produced.
DRAINAGE CHARACTERISTICS
The production of a readily drainable solid residue is a major advantage of
wet air oxidation. A small amount of oxidation converts the water-holding sludge
structure to an easily drained material.
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