47    HYDRAULIC CHARACTERISTICS OF AN UPFLOW
ANAEROBIC SLUDGE BLANKET (UASB) REACTOR
WITH GRANULAR SLUDGE
M. M. Wu, Graduate Student
T. C. Voice, Associate Professor
Dept. of Civil and Environmental Engineering,
Michigan State University,
East Lansing, Michigan 48824
R. F. Hickey, Senior Engineer,
Michigan Biotechnology Institute,
Lansing, Michigan 48909
INTRODUCTION
A number of high-rate anaerobic treatment systems have been developed over the past two decades.
Despite the demonstrated benefits of these processes, hydraulic mixing efficiency problems are frequently observed. These must be taken into account when scaling up from laboratory reactors to
pilot- and full-scale systems. Clogging and short-circuiting in anaerobic reactors are a major concern
since these problems could lead to reduction in the volume of active biomass and poor removal
efficiency. The upflow anaerobic sludge blanket (UASB) system is the most widely used high-rate
anaerobic process, and it is generally reported to have good hydraulic mixing.1,2 The design of UASB
reactors has been primarily based on empirical relationships or using the assumption that the process
behaves as a continuously stirred tank reactor (CSTR). How well these systems approximate an ideal
CSTR has not been adequately evaluated under different organic and hydraulic loading rates.
Several studies have been conducted in attempts to characterize reactor hydraulics. Macmullen and
Weber presented an analytical study for short-circuiting in CMF (completely mixed flow) reactors in
series in early 30's.3 Cholette and Cloutier4 described three factors influencing reactor fluid flow
mixing patterns: effective volume, short circuiting, and plug flow. Van der Meer,5 studying the
hydraulic characteristics of UASB reactors at bench-, pilot-, and full-scales, observed that biogas
production, due to increases in organic loading rate, did not result in significantly altered fluid flow
patterns. He further suggested that dead volume can be reduced by control of sludge amount and
internal recirculation. In treating cane sugar wastewater, it was observed that at low organic loading
rates, adequate mixing of an UASB reactor contents did not take place.6 Bolle et al.7 developed a
model in which both sludge bed and sludge blanket can be described as completely-mixed flow
reactors (CSTRs) with short-circuiting flow paths, while the settler volume acts like a plug flow
reactor. Short-circuiting flow over the sludge bed was found to be a function of the bed height. Hall1
suggested that gas production has a major impact on mixing under low hydraulic retention time
(HRT< 1 day) conditions. Other researchers considered dead volume and mixing-zone volume without
by-passing flow,8,9
Three different models which have been reported are summarized in Figure 1. Cholette and
Cloutier4 modified the CSTR model to include the effects of dead volume and short circuiting
resulting in the following expression:
Q        fi f,Q
cri^1^ (1)
where:        C, = effluent concentration at tj
C0 = effluent concentration at t = 0 in an ideal CSTR
f| = VI /V, VI = mixing volume (Vm)
V = total volume
f2 = Ql/Q, Ql = working flow rate
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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