Experience with Low Rate Biological
Treatment Processes
K. R. DEVONES, Technical Director
Kamloops Pulp & Paper Co.
Kamloops, B. C.
D. R. FISHER, Senior Biologist
Weyerhaeuser Company
Cosmopolis, Washington
O. P. MORGAN, Technical Director
Weyerhaeuser Company
Springfield, Oregon
INTRODUCTION
Biological treatment of pulp mill wastes has been found to be quite versatile
and adaptable to a range of circumstances and needs. Treatment practices in the
industry range from low rate processes such as stabilization ponds and extended
aeration ponds to various high rate activated sludge systems. The underlying
mechanism of the biological reaction is essentially similar in all of the principal
methods, and all are capable of attaining high levels of BOD reduction. Stabilization ponds generally have a retention time of 30 days or more and depend on
natural atmospheric reaeration rather than on the use of mechanical aerators. In
view of the natural reaeration features, these ponds are relatively shallow, having
average depths of six ft or less. Because of this and because of the long retention
periods, sizable land areas are required for stabilization ponds.
Extended aeration ponds differ from stabilization ponds in that they are
aerated by mechanical devices. The ponds are also deeper, averaging 10-12
ft in depth. Retention times are much shorter, usually around 4-10 days. Due to
these differences the land area requirements for extended aeration ponds are considerably less than for stabilization ponds.
The activated sludge treatment system is considered a high rate system in
that desired levels of BOD reduction are achieved in terms of hrs as compared to
days in the case of extended aeration, or to weeks with stabilization ponds. In
view of the very short retention times and very high rates of mechanical reaeration, land area requirements are therefore least.
Capital costs and operational costs are found to be much greater in the high
rate activated sludge treatment process than in the low rate systems, such as stabilization ponds and extended aeration. It has also been general experience that
the higher the reaction rate of treatment the greater the operational difficulties
encountered. Thus, whereas a stabilization pond requires little or no nutrients
(N and P), a minimum of pH control, and can withstand considerable stress due
to abnormal loadings, the activated sludge process is more sensitive to changes in
such variables.
Due to its moderate land area requirement, operational adaptability, and
overall economy, the extended aeration process has become widely used in the
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