NPDES IMPACT ON SOLIDS SEPARATION IN ACTIVATED
SLUDGE SYSTEM DESIGN AND OPERATION
A. W. Busch, Environmental Engineering Consultant
Dallas, Texas 75230
R. L. Irvine, Associate Professor
Department of Civil Engineering
Notre Dame University
Notre Dame, Indiana 46556
INTRODUCTION
Public Law 92-500 sets a national goal of fishable and swimmable public waters by
1983.   A major regulatory mechanism for attaining this goal is the NPDES permit
system for point source discharges.   The "second round" permits due generally for
issuance in 1979 will require lower limits on most or all parameters.   This paper considers the issue of effluent suspended solids concentration for activated sludge systems.
Generally, requirements for lower effluent suspended solids concentrations have been
met by adding tertiary waste treatment components to secondary systems.   These tertiary components may range from granular, deep bed, filters to large "polishing lagoons".
Filter performance is highly dependent on the nature and concentration of solids to be
removed.    Lagoons or ponds are uncontrolled appendages whose performance varies
seasonally and which, dependent on other effluent constituents such as nitrogen and
phosphorous, may cause an increase in suspended solids from algal growth.
The add-on philosophy of treatment is a conceptual convenience which does not
necessarily result in the most cost effective system.   In fact, strict adherence to this
add-on philosophy eliminates consideration of suspended solids reductions through modification of secondary system operation [1,2].
Because either proper design of a new system or in-plant modifications to an existing
system depend upon detailed analysis of the specific waste problem, this paper cannot
cover all alternatives and will be limited to the five cases listed below:
Case I      Modification of Existing System at Secondary Treatment Level Only
Case II     Modification of Existing System by Add-On Filtration Only
Case III    Modification of Existing System at Secondary Level Plus Add-On Filtration
Case IV   Modification of Existing System by Add-On Coagulation, Sedimentation and
Filtration
Case V     New System Design Optimizing Biological Removal of Soluble Substrate and
Chemical Removal of Suspended Solids
Case I involves two fundamental notions:    (a) the secondary clarifier must be operated properly if low suspended solids are to be expected; and (b) the biological reactor
must utilize organisms which have good settling characteristics.   Cases II, III, and IV
recognize that a reasonably well operated biological system may not be able to produce
suspended solids concentrations that are sufficiently low and that an   add-on system
must also be designed.   Case V departs from the long held requirement that organisms
must have good settling characteristics, eliminates the unaided sedimentation which
serves as the basis for the requirement and provides for solids separation by coagulation
and filtration operations only.
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