Section Four
WASTE TREATMENT PROCESSES
B. AEROBIC BIOLOGICAL
38    IMPROVE THE PERFORMANCE AND INCREASE THE
CAPACITY OF AN ACTIVATED-SLUDGE
PLANT WITH A DISSOLVED-AIR FLOTATION CLARIFIER
K.A. Graham, Environmental Process Engineer
John Zink Company
Tulsa, Oklahoma 74102
M. Venkatesh, Manager
Waste Management Services
Badger Engineers, Inc.
Cambridge, Massachusetts 02142
When greater removal efficiencies or an increase in treatment capacity are required from a conventional activated-sludge treatment system, use of a dissolved-air flotation clarifier ought be considered.
The return activated-sludge from a dissolved-air flotation clarifier is several times more concentrated
than that from a gravity clarifier; consequently, higher biomass concentrations can be maintained in
the aeration tank. An increase in the biomass concentration in the aeration tank will increase plant
capacity. Cost comparisons show that replacement of the gravity clarifier in a conventional activated-
sludge system with a dissolved-air flotation clarifier is a less expensive route to achieve an increase in
performance or capacity than installation of additional conventional equipment. In addition, typical
activated-sludge problems, such as floating and bulking sludge, are easily overcome with a dissolved-
air flotation clarifier making the system more reliable and easier to control. Thus, dissolved-air
flotation is an attractive alternative to sedimentation for biomass separation.
INTRODUCTION
In recent years wastewater effluent quality requirements have become increasingly stringent. Not
only have the acceptable levels of the biological oxygen demand (BOD), chemical oxygen demand
(COD), total suspended solids (TSS), oil and grease (O&G), ammonia, sulfide, phenol and cyanide
been reduced, but toxicity limits may require the reduction of other components in the effluent. If a
toxicant is organic, treatment for toxicity may translate into even further reductions in the acceptable
effluent BOD or COD. Finally, many wastewater treatment facilities built several years ago are now
overloaded and are finding it difficult to meet even their previous discharge requirements. Thus, for
numerous reasons, many industrial facilities are finding it necessary to upgrade their existing wastewater treatment systems. This paper discusses the use of Dissolved Air Flotation clarifiers (DAF) for
secondary clarification. Two examples are given where systems with both gravity and DAF clarifiers
are considered and costs compared.
BACKGROUND
Activated-sludge treatment for removal of dissolved biologically degradable organic compounds is
one of the most common types of wastewater treatment. As shown in Figure 1, the wastewater
entering a conventional activated-sludge treatment system is mixed with a concentrated stream of
recycled biomass, known as return activated-sludge (RAS). After aeration, the wastewater-biomass
mixture, also called the mixed liquor, undergoes gravity clarification to remove biomass from the
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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