Application of a Lower Energy Pressurized Gas
Transfer System to Dissolved Air Flotation
And Oxygen Transfer
R.E. SPEECE, Betz Chair Professor
R.H. SIDDIQI, Visiting Research Associate
R.P. AUBERT, Research Engineer
Drexel University
Philadelphia, Pa. 19104
OVERVIEW
This paper describes a proposed low energy pressurized gas transfer (LEPGT) system
and its applications in wastewater treatment practice. The system has potential for use in
two situations particularly: upgrading of existing sewage treatment plants utilizing
Dissolved Air Flotation (DAF) and oxygen transfer using either commercial oxygen or air.
Upgrading Sewage Treatment Plants
Numerous "sewer bans" are in effect across the country because treatment plants are
overloaded. Normal plant expansions are costly, often involve years to materialize and
frequently require more land than is available. Maximum utilization of existing capacity
with minor flow modifications appears quite promising without sacrificing effluent quality.
An overloaded activated sludge plant normally experiences one or more of the
following limitations: 1) Inadequate oxygen transfer capacity; 2) Insufficient Solids
Retention Time (SRT); 3) Excessive hydraulic loading on the secondary clarifier; and 4)
Excessive solids loading on the secondary thickener.
Insufficient SRT is sometimes referred to as an excessive Food/ Microorganism (F/ M)
ratio or as excessive loading of BOD per 1,000 cubic feet of aeration tank volume.
Overloading of BOD on the existing biomass most directly leads to a deterioration of the
settling characteristics of the sludge and an associated deterioration in effluent quality. The
remedy for an organically overloaded plant is to increase the biomass or mixed liquor
volatile suspended solids (MLVSS). This can be accomplished by carrying a
proportionately higher concentration of biomass in the existing aeration tank volume or
holding the biomass concentration constant and increasing the tank volume. Increasing the
aeration tank volume is costly in time and money. Additional oxygen transfer capacity is
also required as mentioned above. The higher MLVSS imposes an additional solids load on
the secondary sedimentation system. Subsequently, a process flow scheme will be discussed
to reduce solids loading to a subcrit ica I level to the secondary sedimentation system without
requiring an increase in the size of the secondary clarifiers.
The excessive hydraulic loading and to a certain extent, the solids loading on the
secondary clarifier is normally associated with the temporary diurnal flow peaks and does
465