TREATMENT OF DILUTE SOLUBLE AND COLLOIDAL
WASTEWATER USING AN ANAEROBIC BAFFLED
REACTOR: INFLUENCE OF HYDRAULIC RETENTION
TIME
ALETTE A.M. LANGENHOFF*
M
, NARISARA INTRACHANDRA and DAVID
C. STUCKEY{{
M
Department of Chemical Engineering, Imperial College, London SW7 2BY, UK
(First received 1 November 1998; accepted in revised form 1 May 1999)
AbstractÐThe start-up and performance of four anaerobic baed reactors (ABR, each of 10 l and
eight compartments) treating a complex dilute soluble or colloidal wastewater (milk and colloidal rice
and dog food Ð 500 mg COD/l) was studied. All four were started with a hydraulic retention time
(HRT) of 80 h at 358C and this was steadily reduced to 6 h; all the HRTs tested resulted in more than
80% COD removal. In one ABR an HRT as low as 1.3 h was tested over a period of 2 d and even at
this high ¯ow rate a COD removal of 40% was obtained. The production of soluble microbial products
(SMP) was found to be greater in the reactors being fed colloidal material, possibly due to `dierences'
in solubility and composition, and these were potentially refractory resulting in slightly lower COD
removals than in the milk reactor. The biomass content of the reactors did not appear to in¯uence
SMP production, however, as HRTs were reduced, more SMPs were produced. Anaerobic bioassays
were used to evaluate the activity of the biomass on the dierent wastewaters and their
biodegradability. It was found that the initial activity on both the soluble and colloidal wastewater
were of the same order of magnitude and hence solid hydrolysis was not limiting the rate or extent of
degradation. Residence time distribution studies were carried out to monitor the eect of system
characteristics such as HRT, solids content and feed composition on the hydrodynamic ¯ow
characteristics and dead space in each ABR. Little dierence in mixing or dead space was observed
under the dierent conditions tested. The average dead space measured was 20±37% and the ¯ow
pattern within the reactors showed an intermediate behaviour between plug±¯ow and ideally mixed.
# 2000 Elsevier Science Ltd. All rights reserved
Key wordsÐanaerobic wastewater treatment, low strength feed, soluble, colloidal, low hydraulic reten-
tion time
INTRODUCTION
The treatment of both domestic and industrial
wastewaters is usually carried out using biological
methods due to their lower costs compared to
chemical methods. However, due to a number of
misconceptions, aerobic (activated sludge) rather
than anaerobic treatment is often the method of
choice. These misconceptions include the under-
standing that anaerobic treatment is; poor for treat-
ing low strength wastes (<1000 mg COD/l), cannot
tolerate inhibitory compounds in the wastes, cannot
operate at low temperatures (<358C) and has poor
removal eciencies. Nevertheless, aerobic treatment
produces large amounts of activated sludge which
then has to be anaerobically degraded before dispo-
sal to an increasingly more stringently controlled
environment, and consumes substantial amounts of
energy.
Great advances have been made in anaerobic
reactor design in the last 20 yr and in understanding
the complex microbial processes that occur in an-
aerobic digestion. These advances have enabled
reactors to maintain a high solids retention time
(SRT 20±100 d), while keeping the hydraulic reten-
tion time (HRT) to a minimum (1.3±20 h). These
designs have enabled a variety of dilute soluble and
colloidal wastes, such as sewage, to be treated econ-
omically (Speece, 1996). Short HRTs result in a
smaller reactor size and thus a more economical
treatment scheme.
Anaerobic digestion of low strength wastewater
(<1000 mg COD/l) results in the production of sig-
ni®cantly smaller amounts of biological sludge com-
pared to aerobic systems, thus resulting in less
Wat. Res. Vol. 34, No. 4, pp. 1307±1317, 2000
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*Author to whom all correspondence should be addressed.
Tel.: +44-171-594-5591; fax: +44-171-594-5629.
{Present address: TNO-MEP, P.O. Box 342, 7300 AH
Apeldoorn, The Netherlands.
{ e-mail: d.stuckey@ic.ac.uk