Developments to Upgrade Settlement Tank Performance,
Screening, and Sludge Dewatering Associated with
Industrial Wastewater Treatment
JOHN B. CROCKFORD, SR., Engineer
Crockford Company
Spencerville, Ohio
VAUGHN SPARHAM, Development Engineer
N. Greening, Ltd.
Warrington, England
INTRODUCTION
Consideration is given in this paper to various techniques used in Europe and being
introduced in the United States dealing with: 1) Polishing clarification equipment; 2) Static
continuous solids/ liquids separation equipment; 3) Combination of (1) and (2); and 4)
Static sludge dewatering equipment. Reference is made to other equipment and processes
and some operating theory is presented.
POLISHING CLARIFICATION
Treatment to a superior level may require different supplementary biological
oxidation or effluent polishing technique and genuine effluent polishing can only be
effectively carried out when the effluent quality approaches 30 mg/1 suspended solids. To
polish treated industrial effluent, an unusual type of upward flow clarifier containing a
wedgewater decking and, on occasion, a plastic matrix can be incorporated in a final
settlement tank. Effluents with no more than 5 mg/1 suspended solids at 7 - 10 mg/1 BOD
can be attained without recourse to filtration.
Figure I -    Simple Eddyflow polishing
clarifier.
In this type of clarifier, the wedgewater deck is suspended horizontally between 9" and
18" below the type water level, according to conditions of operation (See Figure 1). Treated
water in a sedimentation tank thus fitted must first pass upwards through the wedgewater
clement (and, for certain requirements, a plastic matrix), (See Figure 2), before it flows to
the outlet weir in the normal way. As in the comparatively well known "Banks" upward flow
pebble bed clarifier, suspended particles are flocculated by this "Eddyflow" device and
come to rest on the surface of the stabilizing media (the wedgewater).
Fine particles come to rest on the downstream, but upward facing side, of the
wedgewater and appear to form a fragile fluidized blanket. On small installations, after
some days of operation this blanket needs to be removed by temporarily arresting the flow
to the tank and dropping the top water level to just below the wedgewater deck. The blanket
is thus flushed back through the deck. On larger installations a blanket removal device that
does not require interruption of flow may be used.
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