COMPARISON OF PARALLEL CLARIFIERS
WITH LEVEL AND NONLEVEL WEIRS
R. W. Sackellares, Control Strategy Engineer
W. A. Barkley, Environmental Engineer
Weyerhaeuser Company
Tacoma, Washington 98477
R. D. Simmons, Senior Process Engineer
Weyerhaeuser Company
Longview, Washington 98632
INTRODUCTION
Secondary clarifiers are designed to serve the dual function of clarifying the effluent and thickening the settled solids. Studies by Keinath [1] have shown that batch flux thickening theory adequately describes clarifier failure Within pilot clarifiers. When the solids flux to the clarifier exceeded
the sedimentation rate, the sludge blanket rose to the surface and massive amounts of solids were
transported over the effluent weir. Recent recognition of the relationship between sludge blanket
depth and effluent quality has lead to the design of deeper clarifiers [2]. However, clarifier performance under more normal conditions is a function of hydraulics which is addressed by largely empirical design parameters such as surface overflow rate and weir loadings. The rationale for these
criteria are based upon batch settling tests and the ideal particle pathways which would result in a
perfectly quiescent tank with plug flow characteristics.
As early as 1945, Anderson recognized that a particle's path within a full-scale clarifier did not
conform to theory [3]. More recent studies by Crosby [4] and Mazurczyck [5] confirm these findings
and describe clarifier assessment methodology, much of which is borrowed from the field of chemical
engineering reactor design [6].
BACKGROUND
In 1982, a Weyerhaeuser research team had the unique experience of applying these test methods
to three parallel secondary clarifiers receiving the same mixed liquor solids. The physical characteristics of the clarifiers were similar except for the levelness of their effluent weirs. One clarifier had
an extremely level weir while another's weir was so uneven that a portion of the weir was flooded
and a portion was dry. This paper will present the results from this study and describe the effect of
hydraulic non-idealities upon performance.
Facility Description
Weyerhaeuser's Longview, Washington facility is a large, integrated wood products manufacturing complex. The pulp and paper operations, which includes bleached kraft pulp, paper and pa-
perboard, semichemical corrugating medium and thermomechanical newsprint, generate a wastewater
load of 50,000 kg/d (110,000 lb/d) of BOD and 109,000 kg/d (240,000 Ib/d) of TSS at a ftowrate of
227,000 cubic meters per day (60 MGD). The wastewater treatment plant consists of a primary clarifier (diameter = 90 m (295 ft)), followed by two nine-meter (30 ft) deep, aeration tanks, which are
connected through a common splitter box to three secondary clarifiers, each having a diameter of
64 m (210 ft) (Figure 1). Typically, 9,000 kg/d (20,000 lb/d) of BOD and 11,000 kg/d (24,000 Ib/d)
of TSS are discharged to the Columbia River.
Forty % of the influent flow bypasses primary treatment and mixes with the remainder of the
flow and the returned activated sludge in a splitter tank. From the splitter tank, the flow is distributed
to two aeration tanks each having a hydraulic residence time of five and a half hours. Flows from
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