Use of Oxygen in Sludge Stabilization
J. H. BRUEMMER, Research Chemist
Linde Division
Union Carbide Corporation
Tonawanda, New York
INTRODUCTION
In the conventional sewage treatment plant primary and secondary sludges are
digested anaerobically to a non-putrescible residue and a biologically unstable supernatant liquor. The residue is biologically stable for further processing such as
sand-bed drying, and eventual disposal. The liquor, on the other hand, is potentially active, and must be further treated. This is accomplished by recycle to
secondary treatment which removes much of the metabolically useful organic
compounds. However, considerable amounts of the inorganic plant nutrients are
not removed in secondary treatment, and they find their way eventually into the
streams and lakes. These nutrients are readily utilized by the marine biota to
produce "algae blooms" and other life surges. In this way secondary effluents
contribute to eutrophication of the lakes, which is recognized as one of the more
serious problems in water pollution
Various modifications of the secondary treatment process have been proposed
to remove the more abundant nutrients, nitrogen and phosphorus. Ludzack and
Ettinger (1) controlled the partial pressure of oxygen (Po2) in the activated sludge
process to effect significant removal of nitrogen through denitrification. Levin
(2) demonstrated Pq -dependent removal of phosphorus at "luxury" levels in activated sludge. He proposed a leaching step and sludge recirculation in the patented process (3). Tertiary treatment of secondary effluents using adsorption and
dialysis techniques has shown promise in removing all contaminants for water reuse (4). While chemical precipitation of phosphate with alum and ferric salts is
effective it is also expensive (5).
These modifications and additional treatment steps are essentially remedial
measures necessitated by the ineffectiveness of the anaerobic sludge stabilization
system. The anaerobic digestion process stabilizes the sludge but in doing so as
much as 59 per cent of the nitrogen (6), 70 to 80 per cent of the phosphorus (7),
and significant amounts of other nutrients are released from the sludge into the
supernatant liquor. Masselli (8) listed 413 mg/1 NH3-N and 211 mg/1 phosphorus
as average values for filtrates from anaerobic-digested sludge from 23 sewage
treatment plants in New England. Thus, in spite of the fact that anaerobic digestion is effective in solids reduction and in producing a stable residue for disposal, the anaerobic process must be recognized as one source of plant nutrients
that increase eutrophication in lakes.
Aerobic treatment of sludge, in contrast to the anaerobic process, stabilizes
both the residue and the liquor for disposal. Irgens and Halvorson (9) reported
less than 15 mg/1 of biodegradable substances (five day BOD) in filtrates of aero-
bically stabilized sludge. They showed that nitrogen and phosphorus compounds
in primary sludge were incorporated very effectively into cell mass leaving less
than one mg/1 ammonia-N and less than 10 mg/1 each of phosphate and total
nitrogen in the filtrates. These concentrations contrast sharply with the high
average values for these substances in filtrates from anaerobic-digested sludge.
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