31    RECIRCULATION IMPACT BY INFLUENT
METHANOGENIC SLUDGES DURING TWO-PHASE
ANAEROBIC WASTE DIGESTION
J. Hermana, Research Associate
G.K. Anderson, Professor of Environmental Engineering
Department of Civil Engineering
University of Newcastle-upon-Tyne
NE1 7RU, United Kingdom
ABSTRACT
An attempt to improve process efficiency was carried out by applying recirculation of
methanogenic sludge to the influent of an acidogenic reactor during two-phase anaerobic digestion of a soluble organic wastewater. It was intended to investigate its impact as well as to determine its performance at different hydraulic retention times (HRTs). The two-phase system used
consisted of an anaerobic packed bed reactor (APBR) as the acidogenic reactor and an upflow
anaerobic reactor (UAR) as the methanogenic reactor.
INTRODUCTION
Two-phase anaerobic digestion has been proposed as an alternative for treating industrial
wastewaters since the 1970s.1,2 It is considered to be more advantageous than a single-phase
process because it is more stable (particularly during variable process conditions); has a shorter
recovery time after failures caused by organic shock loadings; is more resistant to inhibitory conditions; has a higher treatment efficiency, and less total reactor volume is required.3-6
The disadvantage of phase separation, however, is that the acidogenic organisms may cause
the settleability of granular sludge to deteriorate and also adversely affect the formation and
growth of granular sludges when they are not removed from the effluent of the acidogenic reactor.7 Currently, a staging process in anaerobic digestion is proposed instead of phase separation
for treating slightly or nonacidified wastewaters, because it provides a higher treatment efficiency and a greater process stability.79 This study was based on the assumption that a high
treatment efficiency can be achieved by maintaining the immobilization of a balanced microbial
community inside the reactor, so that the concentration of intermediate products will be sufficiently low.
Nevertheless, taking into account the potential of an acidogenic reactor to act as a buffer to either toxic effects or discontinuous wastewater flow in a separated reactor, the significance of the
two-phase anaerobic process still has to be considered.5
Although there have been many publications dealing with two-phase anaerobic digestion, information on the possibility of enhancing its application is scarce. Understanding the reaction kinetics as well as metabolism has not been confirmed. An appreciation of the limiting factors, hydrogen flow mechanisms, types and concentration of intermediate products, and interspecies
transfer between the microorganisms becomes of importance for the overall improvement of a
two-phase process.10
The separation of the acidogenic and methanogenic processes results in the accumulation of
hydrogen due to the suppression of growth of methanogens in the acidogenic reactor." This may
lead to an alteration of the fermentation pattern tending toward increased utilization of those
metabolic routes that do not employ H+ reduction as a means of disposing of electrons but in-
52nd Purdue Industrial Waste Conference Proceedings. 1997. Ann Arbor Press, Chelsea. Michigan 48118. Printed in
U.S.A.
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