WASTE ACTIVATED SLUDGE DIGESTION
WITH THERMOPHILIC ATTACHED FILMS
Ung Jun Han, Visiting Fellow
R. M. Kabrick, Research Assistant
W. J. Jewell, Professor
Department of Agricultural Engineering
Cornell University
Ithaca, New York 14853
INTRODUCTION
High rate anaerobic methane fermentation of waste activated sludge and other organic slurries
could lead to substantial improvements in waste treatment as well as energy producing processes.
Conventional sludge digesters designed for hydraulic retention times of 30 days have sludge stabilization rates of about 1.5 gm COD/l/d. New developments in high temperature attached film processes indicate that conversion rates 50 times faster than this may be achieved. This study was conducted
to determine the potential for high rate treatment of an organic slurry.
Waste activated sludge represents a substantial problem in many treatment facilities because of
its bulky nature and its resistance to rapid digestion. It was chosen because it is a "real world"
problem, yet it has uniform characteristics and can be generated in a well defined form in the laboratory. In addition recent studies emphasizing biological phosphorus removal show that an anaerobic high rate sludge processing alternative would be useful [1,2].
GOALS AND OBJECTIVES
The goal of this study was to determine the feasibility of using a high temperature attached microbial film reactor to stabilize organic slurries. The specific objectives of this study were to: 1) test
the thermophilic attached film expanded bed reactor at 55 C using waste activated sludge as a typical
organic slurry; 2) define the need for particulate separation from the expanded bed by comparing
direct feeding of the activated sludge to the attached film reactor to parallel testing with similar
substrate that had received hydrolysis followed by liquid/solid separation; 3) compare the interactions
of well-defined laboratory generated waste activated sludge (LWAS) to that obtained from a domestic
sewage treatment facility (RWAS); 4) define the site and limits of the reaction; and 5) estimate kinetics
of the process.
BACKGROUND
The stabilization of particulates in the anaerobic methane fermentation process involves a complex
array of microorganisms that participate in a four-step conversion [3]: 1) hydrolysis of solid organics
to complex soluble organics; 2) acidification of these soluble organics; 3) formation of acetic acid,
hydrogen, and carbon dioxide; and 4) the use of the products of step 3 to form methane. Data for
the microbial characteristics of the organisms that carry out steps 1, 2, and 3 are limited, especially
for thermophilic applications to waste activated sludge. The organisms in the first three steps have
reproduction times substantially less than one day when growing on readily available substrates.
Conversely, the final stage of methane production is carried out by microorganisms that have reproduction times of one to three days at 55 C. When complex communities of these organisms are
combined, the resulting hydraulic retention time is usually longer than 10 days for sludge treatment.
Obvious alternatives to the conventional one-stage digestion with microorganisms with varying
reproduction rates are to separate the reactors according to the specialized needs of the microorganisms. The separation of phases was suggested a number of years ago and has been examined by a
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