The Use of Ultrafiltration
Membranes for Activated
Sludge Separation
CLIFFORD V. SMITH, JR., Manager of Sanitary Technology
DAVID DI GREGORIO, Sanitary Engineer
ROBERT M. TALCOTT, Assistant Laboratory Manager
Dorr Oliver Incorporated
Stamford, Connecticut
INTRODUCTION
For the past four years Dorr-Oliver has been deeply involved in a program to
develop waste-water management systems employing ultrafiltration membranes as a
phase separation device. The primary objectives of this program are twofold; first, to
develop compact systems that specifically avoid dependence upon gravity separation;
and, second, in the field of waste treatment, to produce an effluent of higher quality
than that normally obtained from secondary treatment. Through the use of ultrafiltration membranes, it is feasible to remove all of the residual suspended solids, a
substantial fraction of the residual BOD, and practically all of the coliform bacteria
normally contained in secondary effluent. Inherent in these systems is the utilization
of rather open, skinned membranes having high flux-retention characteristics at
modest pressures (20 to 50 psi).
During the past year a prototype system was built and operated in the State of
Connecticut for treating raw sewage. The discussion that follows is a description of
this installation and an appraisal of its performance. A detailed description of ultrafiltration membranes and the mechanics of membrane transport is beyond the scope
of this report.
The objectives of the Dorr-Oliver research program were to develop a compact
waste treatment system which did not depend upon gravity for separation, and which
would produce an effluent of better quality than that normally obtainable from
secondary treatment. With these objectives in mind, the decision to investigate the
use of ultrafiltration membranes as a phase separation device was logical. The ultrafiltration membranes used in these systems are capable of extremely efficient separation. In order to evolve as compact a system as possible, it was decided to carry the
activated sludge at concentrations much higher than those normally found in conventional or extended aeration systems. Not only does this approach substantially reduce the aeration tank volume, but it also permits one to maintain a low food-to-
mass ratio (in the range of extended aeration or lower). (See Figure 1.)
In other words, the utilization of ultrafiltration membranes for activated sludge
separation allows one to design a plant requiring a minimum of space which: 1) can
minimize sludge wastage by maintaining low food-to-mass ratios; and 2) can
minimize reactor size by maintaining high solids concentrations.
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