A MEMBRANE TECHNIQUE FOR BIOMASS RETENTION
IN ANAEROBIC WASTE TREATMENT PROCESSES
C. B. Saw, Senior Research Associate
G. K. Anderson, Senior Lecturer
A. James, Reader
Department of Civil Engineering
University of Newcastle Upon Tyne
Newcastle Upon Tyne
United Kingdom NE1 7RU
M. S. Le, Head
Biotechnology Group
Domnink Hunter Filters Limited
Birtley, Durham Co.
United Kingdom DH3 2SF
INTRODUCTION
In the current energy and pollution situation, the cost of waste treatment is becoming more acute
every year. Anaerobic digestion is economically and ecologically a significant step forward in industrial effluent treatment. The anaerobic method of treating high strength industrial wastes offers
a number of significant advantages with few serious or insuperable drawbacks compared to other
treatment methods. A wide variety of anaerobic treatment processes has been evolved to meet the
various needs of industrial effluent treatment and to attempt to overcome any inherent disadvantages
of the anaerobic process [I].
One of the major difficulties in anaerobic digestion has always been to retain a sufficient quantity
of active biomass in the reactor. In anaerobic systems, the microorganisms reproduce less rapidly
than in aerobic systems and a longer minimum SRT (solids retention time) is required to accomodate
the slower net growth rate. Numerically, the reciprocal of the solids retention time is the net growth
rate of the system. For the system to function satisfactorily, the reciprocal of the design SRT of the
process must be in excess of the minimum time it takes for the microorganisms to reproduce in the
process. Below this minimum SRT, the microbial cells will be washed out of the system at a faster
rate than they can multiply and the system fails.
The loading rates attainable in an anaerobic waste treatment process are primarily dictated by the
biomass retention in the reactor. Loss of biomass will affect the digester performance in terms of
gas yield and will also result in a poor quality effluent due to the high solids content. For a completely
mixed digester, the slow growth rate of the methane producing bacteria necessitates maintenance of
reactor solids retention times generally of 12-15 days. This would lead to reactors of very large
volume and hence unsuitable for industrial wastes. The capital cost of an anaerobic digestion process
is most sensitive to the size of the digester.
These limitations can be overcome by retaining the biomass within the digester for periods longer
than the waste undergoing digestion thereby increasing the density of the bacterial populations in the
reactor. In the contact process, a settlement tank is used to concentrate and recycle the biomass to
the digester. However, settling is not an efficient process at high MLSS (above 10000 mg/1), and
hence it limits the range of loading rates which can be used. This paper presents the development of
a new concept of operating completely mixed digesters using crossflow filtration units for biomass
retention.
805