ACTIVATED SLUDGE PLANT EFFLUENT POLISHING
William A. Parsons, Director
Corporate Environmental Services
Arthur G. McKee & Company
Cleveland, Ohio 44131
INTRODUCTION
Activated sludge processes employ a heterogenous community of microorganisms
that operate at dynamic equilibrium rather than at steady state.   Many activated sludge
plants experience occasional shifts in the equilibrium that cause increased discharge of
suspended solids (SS) in the effluent.   Fluctuation of activated sludge culture settleability
is particularly characteristic of industrial wastes containing an abundance of readily
assimilable carbohydrate.   Increased effluent SS can also be caused by fermentive action
in stagnant areas of clarifiers or other parts of activated-sludge systems.
The NPDES permit program mandates consistent low SS performance from wastewater treatment facilities.   The daily maximum values for effluent SS discharge in permits are particularly difficult to comply with if activated sludge operations are subject
to fluctuation of culture settleability.   Such fluctuations can cause "dribbles" and
"bumps" of SS that exit with the clarifier overflow.   The implementation of effluent-
polishing facilities may be necessary at some plants in order to meet the stipulations of
discharge permits.   The purpose of this chapter is the presentation of the findings of
a study conducted to evaluate alternatives for control of excessive effluent SS discharge.
The alternatives evaluated were lamella-stabilized dissolved air flotation and dual-media
pressure filtration.
METHODS
The activated sludge unit servicing a prepared foods plant receives a flow of about
0.3 mgd of wastewater containing food residues and cleaning agents, and operates on
the step aeration mode.   During the period of the study, cationic polymer (Primafloc
C-3) was added to the feed to the final clarifiers to assist with effluent clarification.
The polymer dosage was approximately 0.005 lb/lb of mixed liquor suspended solids
(MLSS) fed to the clarifier.   The MLSS concentration was about 4000 mg/1, the Sludge
Volume Index was stable at 170 ml/g, and the SS in the clarifier overflow averaged
about 35 mg/1.   Wastewater temperature ranged from 18 to 22 C.
Dissolved Air Flotation
Dissolved air flotation is an attractive concept for separation of poor settling activated
sludge culture inasmuch as the strong driving force of the buoyancy of air bubbles replaces a negligible difference in specific gravity.   The process has been employed for
separation of activated sludge in lieu of sedimentation [1,2], but the performance has
sometimes been erratic in respect to effluent clarity.   The pilot plant unit employed in
the studies was a FAVAIR Mark II, Model OB-10.   The FAVAIR Mark II dissolved
air flotation system employs lamellas to control turbulence during the separation process.
The unit has been employed for the direct separation of activated sludge culture from
mixed liquor.
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