ACCLIMATION OF DIGESTED SEWAGE SLUDGE
DURING START-UP OF AN UPFLOW
ANAEROBIC SLUDGE BLANKET
(UASB) REACTOR
Willem de Zeeuw, Engineer
Gatze Lettinga, Doctor of Engineering
Department of Water Pollution Control
Agricultural University Wageningen
6703 BC Wageningen
The Netherlands
INTRODUCTION
The use of anaerobic digestion for the direct treatment of low- and medium-strength
wastewater has been greatly stimulated in the Netherlands by the development of the upflow
anaerobic sludge blanket (UASB) process [ 1 ] and its recent successful full-scale application
[2]. Detads of this process are presented elsewhere by Lettinga et al. [3] and Pette et al. [4].
The potentials of an UASB reactor, for a given wastewater and temperature, are mainly
dictated by the amount of sludge that can be retained in the reactor on the one hand, and
the specific activity of the sludge on the other hand. In turn, the sludge retention is strongly
dependent on the sludge settleabdity. Provided the process is operated properly, the settle-
ability of the sludge that develops in an UASB reactor with a waste like sugar beet and
potato processing wastewater is excellent, i.e., SVI values will be in the range of 20 to 40
ml/g for flocculated sludge and less than 20 ml/g for granular sludge, which was found to
gradually develop in the reactor. Loading rates of 30 to 40 kg COD/m3/day—at sludge
loading rates of 1.5 kg COD/kg VSS/day—have already been achieved in 6-m UASB reactors
[5], and even then the process is not operated at its maximum capacity. Hydraulic loading
rates of 8 m3/m3/day have been successfully applied, but even higher loading rates may be
feasible.
One of the main problems stdl remaining in the application of anaerobic treatment is to
enhance the start-up of the process in using digested sewage sludge as seed sludge.
It is well known in the literature [6,7] and also our experience, that considerable time is
involved in the first start-up of the process. This especially is the case when certain restrictions set for the initial loading rate and its increase are not obeyed. Evidently the main task
in the start-up is to develop in a period as short as possible a highly active and settleable
sludge from the poor quality seed sludge that as a rule is avadable (viz., digested sewage
sludge). Moreover, we preferentially want the sludge in a granular form, such as has been cultivated in most of the pdot- and full-scale UASB reactors operated thus far. In earlier
research we obtained indications that a well-adapted sludge (i.e., a sludge capable of degrading all volatile fatty acids (VFA) from a VFA feed simultaneously and at an overall rate
exceeding about 0.75 kg COD/kg VSS/day) can be developed within a period of a few
months.
The objective of this chapter is to present the main results of a comprehensive study concerning the start-up of a digestion process using a very poor quality digested sewage sludge as
seed material.
As anaerobic treatment presumably wdl become increasingly popular in the very near
future, the results obtained may be of great practical interest; the more because we found
that a highly active sludge can be cultivated within a period of six weeks using a mixture of
acetate and propionate as feed. An acetate and propionate feed was chosen in these experiments, because acetate is the main precursor of methane for most complex substrates (viz.,
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