22    DEVELOPMENT OF LOW-COST FLOTATION TECHNOLOGY
AND SYSTEMS FOR WASTEWATER TREATMENT
Milos Krofta, President
Daniel Guss, Vice President
Krofta Engineering Corporation
Lenox, Massachusetts 01240
Lawrence K. Wang, Director & Professor
Lenox Institute for Research, Inc.
Lenox, Massachusetts 01240
INTRODUCTION
Activated sludge consists of biological floes which are matrices of microorganisms, non-living
organics and inorganic substances. The microorganisms include bacteria, fungi, protozoa (Sarcodina,
Mastigophora, Sporozoa, Ciliata, and Suctoria), rotifers, viruses and higher forms of animals such as
insect larvae, worms and crustaceans. The activated sludge process is one of the most common
biological wastewater treatment processes, and can be defined as a suspended-growth system in which
biological floes are continuously circulated to come into contact and to oxidize the organic waste
substances in the presence of oxygen and nutrients. The waste organic matter is aerobically converted
to gaseous carbon dioxide, cell tissue of microorganisms (C5H7NO2), and other simple soluble end
products. Part of microorganisms (i.e. activated sludge) are returned to the aeration basin in order to
maintain a constant microbial population (i.e. constant mixed liquor suspended solids). The wastewater is considered to be adequately treated when the excess microorganisms (i.e. excess waste sludge)
and residual suspended solids are separated from the aqueous phase by clarification, and the clarified
effluent meets the Federal and State Effluent Standards. The most common clarification used today is
sedimentation.
The recent and accelerating emphasis on water pollution control has necessitated the rapid development of improved biological waste treatment systems to aid in cost and energy savings. The use of
secondary flotation clarification in place of, or in assisting secondary sedimentation clarification in
the activated sludge process system is one recent advancement in this basic process. The potential of
this development, in terms of higher suspended solids and BOD removals from existing plants and
expansion of hydraulic capacity at significantly reduced cost, is expected to result in extremely rapid
acceptance by municipalities and industry.
The primary distinguishing feature of the improved activated sludge treatment system is that high
rate dissolved air flotation, known as Supracell, is the secondary clarifier for separation of suspended
solids from the aeration basin effluent, as opposed to secondary sedimentation alone in conventional
activated sludge systems. The concept of using flotation for water-solid separation is not new at all;
many engineers have applied the flotation technology in sludge separation since the early 1920's. The
major deterrent to flotation use in the municipal and industrial processes envisaged by these early
practicing engineers was economics, with objections centering mainly around the cost of gas bubble
generation and retention. There has been significant evolution of dissolved air flotation clarifiers
during the last 50 years. The following progress has been made: 1) Specific clarification load
increased from 1.5 gpm/sq ft (60 Lpm/sq m) to 3.5 gpm/sq ft (140 Lpm/sq m) and for triple stacked
unit to 10 gpm/sq ft (420 Lpm/sq m); 2) The retention time of water in the flotation clarifier
decreased from 30 min to 3 min; 3) The largest unit size increased from 260 gpm (1000 Lpm) to 7900
gpm (30,000 Lpm) and for triple stacked units to 23,700 gpm (90,000 Lpm); 4) The size of modern
DAF units is much smaller. (It allows construction predominantly in stainless steel prefabricated for
easy erection); 5) The smaller size and weight, 120 lb/sq ft (60 kg/m^), allows installation on posts
leaving free passage under the unit, therefore, it is easier to find available space for indoor installation
and to construct inexpensive housing; 6) Air dissolving is improved and now requires only 10 seconds
retention time in the air dissolving tube instead of the previous 60 seconds, accordingly this reduction
in retention time results in smaller air dissolving tubes which are predominantly built from stainless
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