60    LOW TEMPERATURE TREATMENT OF DILUTE
WASTEWATERS USING THE ANAEROBIC
SEQUENCING BATCH REACTOR
Udeme J. Ndon, Graduate Research Assistant
Richard R. Dague, Professor of Environmental Engineering
Department of Civil Engineering
Iowa State University
Ames, Iowa 50011
INTRODUCTION
In all high-rate biological wastewater treatment processes it is possible to achieve long solids
retention times (SRT) while maintaining short hydraulic retention times (HRT). Compared with
aerobic processes, high-rate anaerobic methanogenic processes are capable of operating at much
higher organic loadings. However, the common experiences of the past indicate that anaerobic processes are not capable of achieving efficient organic removals when treating dilute wastewaters, e.g.,
wastewaters with chemical oxygen demands (COD) less than about 1,000 mg/L. Also, it is commonly
believed that anaerobic methanogenic processes are only applicable to the pretreatment of wastewaters. It is common practice to provide some further treatment for anaerobic process effluents prior
to ultimate discharge to streams.
Another perceived problem with anaerobic methanogenic processes is temperature. It is commonly
believed that anaerobic processes must be operated in the mesophilic range of temperatures (30° to
40°C) in order to achieve efficient conversion of organics to methane and carbon dioxide. This is a
serious problem when attempting to apply high-rate anaerobic processes to dilute wastes at low
temperatures. The cost of heating such wastes to the mesophilic temperature range is prohibitive. The
recent literature contains reports on full scale anaerobic plants for the treatment of low-strength
wastewaters, but such plants are presently operated in tropical countries (Brazil,1,2,3,4 Colombia,5 and
India6).
Dague and coworkers at Iowa State University have been conducting research on a new methanogenic process called the Anaerobic Sequencing Batch Reactor7"11 (U.S. Patent No. 5,185,079). The
ASBR has some unique characteristics that enable the process to achieve high levels of organic
removals, even when treating dilute wastes at low temperatures. This paper presents the results of
recent research on laboratory applications of the ASBR to a dilute synthetic wastewater at temperatures ranging from 35°C to 15°C.
THE ASBR PROCESS
Operating Principles
The operating principles of the ASBR have been described in detail by Sung and Dague10 and
Herum and Dague." Each sequence of the reactor includes four steps: feed, react, settle and decant,
as illustrated in Figure 1. This results in a variable substrate concentration in the reactor, alternating
from high concentrations of substrate during and immediately after feeding and low substrate concentrations at the end of the react cycle, just before biomass settling and effluent decanting. This results
in a variable food-to-microorganism (F/M) ratio in the reactor: High F/M just after feeding and low
F/M just before settling, as illustrated in Figure 2.
Effect of Variable F/M
In the 1960s, Dague and coauthors McKinney and Pfeffer1213 reported on solids retention in
anaerobic methanogenic processes and emphasized the importance of low F/M ratios in achieving
biomass flocculation and internal settling in batch-fed reactors. Although not recognized at the time,
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
563