BENTHAL INTERACTIONS IN A KRAFT MILL WASTEWATER
LAGOON
M. R. Collins, Graduate Student
Z. K. Chowdhury, Graduate Student
C. W. Bryant, Assistant Professor
Department of Civil Engineering and Engineering Mechanics
University of Arizona
Tucson, Arizona 85721
INTRODUCTION
The performance of an aerated stabilization basin (ASB) is affected by the influent properties,
the kinetics within the liquid layer, and the physical and biological processes in the benthal zone.
This paper describes the impact of the later processes within an operational Kraft mill lagoon.
The focus of this study was the first cell of a three-cell lagoon system treating a daily flow of
about 100,000 m'/day from a Kraft mill. The treatment system is shown in Figure 1. Prior to input to
the lagoon system, the alkaline waste stream was passed through a clarifier to remove cellulose fibers.
The first cell to treat the entire waste stream was a partially suspended lagoon with a theoretical detention time of 7.7 days and an applied power level of 1.13 W/m'. The second partially suspended cell had
a detention time of 6.5 days and an applied power level of 0.55 W/m'. The final cell provided a detention time of 12.6 days and no mechanical aeration. By florescent dye studies [1] the first aerated cell was
found to have primary flow patterns as shown in Figure 2. The cell behaved as a series of three mixed
reactor zones that interact with three corresponding dead zones, as shown in Figure 3. The total dead
zone volume was 59% of the lagoon volume, and the effective detention time was only 3.3 days. After
the hydraulics of the cell had been defined, il was of interest to define whether there was a discernible
variation in benthal activity from zone to zone.
Processes within the benthal zone result in the solubilization of organic solids and the subsequent
stabilization or feedback of the soluble products. The total oxygen demand of the benthal solids may
be: 1) exerted as a direct demand in the aerobic zone; 2) transported to the overlying water column
Figure 1. Complete Kraft mill lagoon
treatment system. Circles indicate the location of surface aerators.
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