64 CHARACTERISTICS OF RESIDUES
FROM THERMALLY TREATED ANAEROBIC SLUDGES
A.A. Friedman, Professor
Department of Civil Engineering
James E. Smith, Associate Professor
Department of Biology
Jacquelin DeSantis, Graduate Student
Department of Civil Engineering
Syracuse University
Syracuse, New York 13244
Timothy Ptak, Environmental Engineer
Ch2M-Hill
Gainsville, Florida 32602
Robert C. Ganley, Managing Engineer
O'Brien & Gere Engineers
Syracuse, New York 13221
INTRODUCTION
Sludge management and disposal are probably the most difficult and expensive operations involved
in wastewater treatment today. To minimize final disposal costs many waste treatment facilities
practice some form of anaerobic digestion and dewatering to reduce the volume and offensiveness of
their by-product sludges. One potential alternative for reducing sludge volumes consists of high
temperature, partial oxidation of these previously digested sludges (PDS) and subsequent anaerobic
biological conversion of resulting soluble organics to methane. These thermal conditioning techniques
involve heating wastewater sludges to 170° to 240°C at pressures of 250 to 400 psig. Greater solubilization of organic material is achieved when oxygen or air is injected into the reactor, a process termed
wet air oxidation (WAO).
This paper describes solids destruction, residue characteristics and biodegradability factors that
should be considered in the design of liquid thermal treatment processes for the management of
anaerobic sludges. Ideally, the processes should reduce sludge volumes by at least 80% and result in
significant quantities of usable methane. Since WAO treatment produces high strength side-streams,
their impact must be considered if they are recycled directly to the head of the plant. Recently the first
detailed information concerning WAO treatment of anaerobic sludges has been published '■*. These
data indicated that WAO can destroy 80% of the volatile suspended solids (VSS) and 73% of the
chemical oxygen demand (COD). To date only very limited information is available concerning the
suitability of thermally treated PDS to serve as a substrate for the generation of methane 3'4.
The primary objective of this research was to determine the feasibility of producing methane
efficiently from the residual VSS in anaerobically digested sludges. Secondary goals were to establish
the "best" conditions for thermal treatment for solubilizing PDS, to observe the effect of the soluble
products on methanogenesis and to evaluate process sidestreams for dewaterability and anaerobic
biodegradability.
THERMAL CONDITIONING OF WASTEWATER TREATMENT SOLIDS
Since the VSS contains almost all of the organic matter present in PDS, it is necessary to solubilize
and/or destroy most of the VSS as the first step toward generating methane. Unlike other wastewater
629