Design and Performance of
Aerated Lagoons for Pulp and Paper
Waste Treatment
W. WESLEY ECKENFELDER, JR.
Associate Professor of Civil Engineering
Manhattan College
New York, New York
The aerated lagoon as discussed in this paper is a basin of significant
depth (six - 12 ft) in which oxygenation is accomplished by mechanical or
diffused aeration units and from induced surface aeration. The turbulence
level maintained in the basin insures distribution of oxygen throughout the
basin but is usually insufficient to maintain solids in suspension. As a result,
the major portion of inert suspended solids and nonoxidized biological solids
settle to the bottom of the basin where they undergo anaerobic decomposition.
This type of basin can be modified to include a separate sedimentation area
to produce a more highly clarified effluent. If the turbulence level in the
basin is increased to maintain solids in suspension the system becomes
analogous to the activated sludge process.
BOD REMOVAL CHARACTERISTICS
Studies have been conducted to determine the BOD removal characteristics from mixed pulp and paper mill wastes and from board mill wastes. Because of the heterogeneous nature of these wastes the removal reaction is usually of a retardant nature in time of concentration. This means that the rate
of removal progressively decreases as the more readily oxidizable constituents
are removed. For example, BOD removal from a board mill waste without
the addition of nutrients has been formulated according to the retardant relationship:
L/L0 = (1 + 0. 55 t) "°- 78    Equation 1
In which
L0 = Initial BOD
L   = BOD remaining after time t
t    = Lagoon detention period
In some cases a small fraction of the BOD is not removed even after long periods of aeration. This has been attributed to an equilibrium between synthesis and the release back to solution of cellular constituents released through
auto-oxidation of the bacterial cells.
The rate of BOD removal is markedly influenced by the presence of
available nutrients. Figure 1 shows the difference in BOD removal rate from
the batch aeration of a board mill waste with and without nutrient addition.
The addition of nutrients modifies the BOD removal relationship from Equation 1 to the retardant form:
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