8    WASTEWATER TREATMENT BY A
BIOLOGICAL-PHYSICOCHEMICAL TWO-STAGE
PROCESS SYSTEM
Milos Krofta, President
Krofta Engineering Corporation
Lenox, Massachusetts 01240
Lawrence K. Wang, Director
Lenox Institute for Research, Inc.
Lenox, Massachusetts 01240
PROJECT DESCRIPTION
The effectiveness of a Krofta Sandfloat Sedifloat clarifier (Type SASF-5) on treating the secondary
effluent at the Norwalk Wastewater Treatment Plant, Ohio, has been investigated.
Norwalk is located in northern Ohio, approximately 50 miles southwest of Cleveland and supports
a population of 14,500 people. The existing Norwalk Wastewater Treatment Plant consists of one
primary sedimentation, one aeration basin, one primary trickling filter, two secondary trickling filters
and one chlorination- sedimentation basin. The plant treats combined domestic sewage and food-
processing waste.
At present the Norwalk Wastewater Treatment Plant discharges a secondary effluent containing an
average of 100.4 mg/L of total suspended solids (TSS), 7.64 mg/L of phosphate, and 40.57 mg/L of
5-day biochemical oxygen demand (BOD).
The primary objective of this investigation was to further treat the secondary effluent by a tertiary
Sandfloat Sedifloat clarifier in order to meet the US. Environmental Protection Agency established
tertiary effluent standards:
Total suspended solids: 10 mg/L
Biochemical oxygen demand, 5-day: 10 mg/L
Phosphate: 1 mg/L
PROCESS CONCEPT
Many multiple stage wastewater treatment systems have been developed based on the following
hypotheses:
a. Wastewater generally contains both biodegradable and non-biodegradable dissolved
organic pollutants. If a biological process system is used first for removal of biodegradable organics (i.e., BOD), then a supplemental physicochemical process system consisting of mixing, flocculation and sedimentation can be used to remove the remaining nonbiodegradable organics hoping that most of the non-biodegradable organics can be
flocculated.
b. Wastewater generally contains both flocculatable and non-flocculatable dissolved
organics. If a physicochemical process system is used first for removal of flocculatable
organics, then a biological process system (i.e., activated sludge, trickling filter, lagoon,
biological tower, rotating biological contactor, etc.) can be used to remove the remaining non-flocculatable organics hoping that most of the non-flocculatable organics are
biodegradable.
c. Non-biodegradable and non-flocculatable dissolved organics can usually be removed by
granular activated carbon, ion exchange and/or membrane processes.
Several important chemical process facts have been overlooked by environmental engineers and
scientists for many decades until now. Some residual non-flocculatable dissolved organics in the
clarified effluent of the first-stage physicochemical system (consisting of flocculation and clarifica-
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