COMPARISON OF DIFFERENT METHODS FOR
ANAEROBIC TREATMENT OF DILUTE WASTEWATERS
P. M. Heertjes, Professor and R. R. van der Meer, Engineer
Laboratory of Chemical Engineering
Delft University of Technology
Delft, The Netherlands
INTRODUCTION
The fact that anaerobic bacteria are able to convert a large variety of organic substrates
has been used in wastewater treatment for the stabilization of primary and secondary
sludges and for the purification of the wastewater itself [1,2]. The second process is the
subject of the study presented in this chapter. This process has been studied intensively
with respect to its microbiological and kinetic aspects [3-7], but there is stdl a lack of
general technological knowledge, although many technological studies have been performed [8-20]. These studies were mostly undertaken to find out whether the anaerobic
process was appropriate for treatment of a specific wastewater. However, no systematic
studies were performed to obtain information about the best type of reactor and method
of process performance for anaerobic treatment. Because such information would be very
useful, a comparative systematic study was set-up with two methods of process performance in three different types of reactor. The types and methods to be studied were chosen
according to the conclusions of a literature study performed as an introduction to the
experiments.
LITERATURE SURVEY
Methods of process-performance
For anaerobic treatment of wastewater two methods have to be considered:
1. The process-performance in which the conversion of the waste into fatty acids and
these into me thane and carbon dioxide take place in one reactor. In this case the
acid-forming bacteria and the methane-forming bacteria live and grow under the
same circumstances. This process will further be called the "one-reactor process."
2. The process-performance in which the reactions of the waste with the acid-forming
bacteria and thereafter of the products (mainly fatty acids) with methane forming
bacteria take place in separate reactors. In that case the effluent of the reactor in
which acid formation takes place is used as the influent of the reactor for the second
step of the process. This process will further be called the "two-reactor process."
In general, the bacterial population for biological wastewater treatment adapts itself
easdy to changes of the composition, flow and concentration of the wastewater [21-23].
However, the two main groups of bacteria involved in anaerobic treatment do not always
adapt themselves with the same rates. This appears from the various examples of anaerobic
treatment by the one-reactor process in which the adaptation of the acid-forming bacteria to a change of the influent is too rapid with respect to that of the other bacteria
[24-26]. The fatty acids are then produced faster than they can be converted by the
methane-forming bacteria, which results in an accumulation of fatty acids and a decrease
in pH. Consequently, the methane-forming bacteria wdl be inhibited in their functioning
and their adaptation wdl be interrupted.
The different adaptation rates of the two groups of bacteria are a consequence of the
difference in growth rates. This difference may introduce another problem for the
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