Cold Weather Operation in Aerated Lagoons
Treating Pulp and Paper Mill Wastes
P. L. TIMPANY, Process Specialist
L. E. HARRIS, Process Engineer
L. C. WHITE, Process Engineer
K. L. MURPHY, Special Consultant
T. W. Beak Consultants, Limited
Vancouver, British Columbia
INTRODUCTION
Biological oxidation of pulp and paper mill wastes in aerated lagoons has
recently replaced stabilization basins as the most widely used form of secondary
waste treatment. Rising land costs and increasingly stringent effluent quality
standards have necessitated a more efficient means of waste treatment. The aerated
lagoon has been widely accepted due both to its simplicity and design flexibility as
compared to other processes. It is one of the most economical methods for treating
the large volumes of waste from a pulp and paper mill.
The aerated lagoon is basically a biological treatment process and as such is
similar in principle to an activated sludge system. Unlike the activated sludge system,
there is no recycle of microorganisms, consequently the lagoon must be of sufficient
volume so that the growth rate of microorganisms exceeds the loss of microorganisms
in the effluent. The large volume associated with this requirement allows the lagoon
to withstand shock loadings and produce a consistent effluent quality even when
subjected to severe changes in the waste characteristics.
At the same time the lack of recycle limits the ability of the operator to
control the effluent quality—the only parameters subject to operational control being
the input horsepower required for oxygen transfer and mixing and the addition of
biological nutrients. With this limited control it is essential that the system design
incorporate the necessary features to enable a suitable effluent to be discharged. Of
particular significance to the designer is the possible reduction in treatment
efficiency caused by the substantial annual temperature variations, as experienced in
most parts of Canada and in the northern United States.
As part of an overall program to evaluate the use of aerated lagoons for the
treatment of pulp and paper mill wastes, temperature studies were carried out on
three five-day retention aerated lagoons. The effect of both climatic conditions and
input horsepower have been related both to heat loss and treatment efficiency. The
studies were conducted during 1970 and 1971 at three mills; two are coastal mills,
situated in southern and central British Columbia, the other is located in central
Alberta, Canada.
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