MEMBRANE-ENHANCED ANAEROBIC DIGESTERS
William T. Choate, Program Manager
Abcor, Inc.
Wilmington, Massachusetts 01887
David Houldsworth, Project Engineer
Abcor Environmental Systems, Ltd.
Oakhill Grove, Surbiton
Surrey, KT6 6DU
United Kingdom
G. A. Butler, Product Manager
Tenstar Products Ltd.
Ashford
United Kingdom
Anaerobic processing is becoming recognized as a simple and energy-efficient means of
treating and stabilizing many high strength organic wastes. This process uses relatively
simple equipment to directly produce a usable fuel, and finds application in a wide variety
of industries. The anaerobic system is very well suited for treating wastes with biochemical
oxygen demand (BOD) in excess of 10,000 mg/1. One of the drawbacks of treating wastes
with lower BOD is that insufficient methane is produced to heat and maintain the digester
at its optimal operating temperature.
The variable most significant in controlling anaerobic process performance and stability
is the solids retention time (SRT). Most of the anaerobic process design improvements
in recent years have concerned themselves with retaining suspended biomass and organic
solids to increase the SRT relative to hydraulic retention time (HRT). Organic wastes from
many sources, particularly the food industry (such as potato or wheat processing wastes),
are generally water-soluble, which means that it is not possible to increase the soluble BOD
by conventional clarification, thickening, sludge blanket or other common techniques.
Membrane filtration not only offers a very stable and simple means of increasing the
biomass and suspended SRT, but further enhances the process by retaining many soluble
organic components. The retention of soluble organics can make the anaerobic process
more energy efficient even for low-strength organic wastes.
This chapter concerns the operation of a commercial anaerobic reactor that was retrofitted with ultrafiltration for solids and dissolved solids retention, and expands on the potential use of membranes in the anaerobic field.
COMMERCIAL DIGESTER MEMBRANE RETROFIT
The Tenstar Products Ltd. factory is located near the town center of Ashford, England
and processes 400-450 ton/week of wheat flour. The flour is ground to produce starch and
two distinct types of starch granules result. The A starch granules are 25-30 pm in size and
shaped like rugby balls while B starch granules are 1-2 pm in diameter and spherical. These
starch granules are spun off and the 6-to 7-m3/hr water stream is discharged as waste.
This wastewater stream has a pH of 3.2-3.4 and 1.4% total solids, of which only 0.2%
is suspended. The waste stream's COD averages 17,000 mg/1. These waste solids consist of
"" 80% starch, mostly soluble, but containing some broken bits of A starch and B starch
granules. The remaining percentage is in the form of soluble and suspended proteins.
Tenstar was charged heavily for this effluent, and in the early 1970s their production
represented   more  BOD  than the rest of Ashford combined. The municipal authorities
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