Section Four
BIOLOGICAL SYSTEMS-C. ANAEROBIC
40    IMPROVED ANAEROBIC DIGESTION WITH LOW LEVEL
CHEMICAL PRETREATMENT
R. V. Rajan, Doctoral Student
University of Massachusetts at Amherst
Amherst, Massachusetts 01003
Jih-Gaw Lin, Doctoral Candidate
Bill T. Ray, Assistant Professor
Southern Illinois University at Carbondale
Carbondale, Illinois 62901
INTRODUCTION
Sludge handling costs represent 30 to 40% of the capital cost and about 50% of the operating costs
of a typical wastewater treatment facility.1 About two thirds of the volume of sludge in secondary
treatment plants is waste activated sludge (WAS). WAS has to be stabilized sufficiently to reduce its
organic content, odor problems and pathogen contamination, before its ultimate disposal. Thus, any
new process or process modification that significantly improves WAS stabilization will reduce overall
wastewater treatment plant costs.
Anaerobic digestion is a commonly used form of secondary sludge stabilization. Digestion of WAS,
in which most of the substrate is enclosed within cell membranes, requires conversion of cell bound
substrate into more readily accessible forms before it can be utilized by viable anaerobes. In the
context of this chapter, mechanical and chemical unit operations on sludge that improve substrate
availability are referred to as "pretreatment" methods. Past research in the area of substrate modification has led to the development of pretreatment schemes that are energy intensive and impractical for
WAS processing. These include thermal pretreatment of WAS at moderate (60 to 100°C)2 and high
(175 to 225°C)3 temperatures and thermochemical pretreatment using high temperatures and chemical
levels (300 mEq/L).*
In this research project, a relatively simple, less expensive WAS pretreatment process was evaluated
for its influence on particulate hydrolysis and the performance of anaerobic digesters. It was the
intent of the first phase of this project to study the kinetics of alkaline hydrolysis and to determine the
parameters that control the process. The rationale behind the use of an alkali, instead of an acid, is
that residual alkalinity is compatible with subsequent anaerobic digestion, while acidity is not. This
ambient temperature, alkaline pretreatment procedure is discussed and preliminary results of its effect
on anaerobic digestion are presented in this chapter.
METHODOLOGY
WAS used in this project was collected from the Carbondale Northwest wastewater treatment plant
in Carbondale, Illinois. Sludge was collected a day before subsequent hydrolysis experiments were
performed. Samples in 20 liter carboys were immediately transferred to the laboratory to be stored at
1°C, while the sludge settled. The sludge readily settled to over 1.0% total solids (TS). When feed
sludge over 2.0% TS was required, a portion of the sludge was centrifuged at 5000 rpm and adjusted
to the appropriate strength with the settled sludge. Analyses performed in this study were based on the
procedures recommended in the 16th edition of Standard Methods.*
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
327