9    DESIGN CONSIDERATIONS FOR CYCLICALLY OPERATED
ACTIVATED SLUDGE SYSTEMS TREATING DOMESTIC
WASTEWATERS
Mervyn C. Goronszy, Executive Technical Manager
Transfield, Inc.
Irvine, California  92715
INTRODUCTION
Variable volume or cyclically operated activated sludge technology has emerged as a viable alternative to the use of constant volume, constantly aerated conventional activated sludge methodology.
The development and application of the variable volume approach has been well documented in the
literature [1,2,3,4,5,6]. A wide array of wastewaters including municipal, food processing and many
industrially derived wastes are amenable to treatment in a variable volume facility [7,8] for which
there are a number of proprietary and well-developed systems. While previously considered as an
appropriate technology for small treatment applications, the need for cost effective and high quality
treatment performance has seen the adoption of this simple and reliable methodology for much larger
installations. Capital cost effectiveness over conventional systems of around 30% has been demonstrated for variable volume municipal wastewater systems to a size of around 15 mgd. Similar savings
in both capital and operational costs can be realized in many industrial wastewater treatment applications.
The emergence of this treatment methodology, apart from the cost savings, stems primarily from
process advantages that can be realized. A major advantage derives from the configuration and
aeration sequencing whereby initial floc-loading conditions are generated which maximize biosorptive
transport of soluble substrate. These conditions can be used to favour the generation of floc-forming
microorganisms over most filamentous forms [9]. This principle is used to good advantage in the
treatment of wastewaters which have traditionally been associated with filamentous bulking. Long
sludge age systems operating at low temperatures treating domestic wastes and systems treating
essentially carbohydrate wastes having a high soluble organic content are typical situations where
bulking sludge is frequently encountered.
System design requires the determination of a basin volume in which a desired degree of treatment
can be achieved, a mass and rate of application of oxygen, the generation of biomass, the rate at
which surface liquors can be withdrawn and a method for optimizing the use of the basin volume for
hydraulic load equalization. A design procedure based on material balances, active biomass, floc-
loading parameters, biosorptive capacity with related biosorptive regeneration criteria, and biomass
zone settling velocity is presented.
CYCLIC OPERATIONAL PRINCIPLES
All systems use a common basin in which to carry out the biodegradation reactions together with
the separation of biomass and liquid in order to produce a treated effluent. Biodegradation is
achieved during an aeration sequence which is followed by a non-aeration sequence which is necessary
to enable the same basin to function as a solids-liquid separation unit. Time sequences are therefore
used to accomplish in a single basin what is spatially achieved in separate basins in conventional
constant volume activated sludge systems. Time sequencing treatment provides an operational flexibility which is not available in conventional systems. The basic cycle consists of three discrete periods
which accomplish AERATION, SETTLEMENT, DECANTATION (effluent removal through surface skimming).
During the period of a cycle, the liquid volume within the basin increases from a set minimum
operating bottom water level in response to a varying influent flow rate. Aeration and mixing ceases
at a predetermined period of the cycle to allow the biomass to flocculate and settle under quiescent
conditions. After a specific settling period, the treated effluent is removed as supernatant returning
the liquid level in the basin to the minimum operating bottom water level after which the cycle is
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