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Teil 3 Seite 4-47
ciently to be more than a pretreatment. It is reported to be capable of removing 73 - 76 %
of COD in a rendering plant if 2 reactors are used, one operating in down-flow mode and
the other in up-flow mode.
Achieved environmental benefits
Partial removal of some of the organic load from waste water, prior to further treatment
on the site or at a municipal WWTP. Removal of BOD and COD compounds from high
loaded waste water steams. The load can be reduced by this process without air intake.
The produced biogas contains more energy than anaerobic plants need for the treatment.
Cross-media effects
The ammonia content released from organic nitrogen combinations during anaerobic
treatment exceed the nitrogen fixation through new biomass formation., so the level of
ammoniacal nitrogen increases during the treatment.
Operational data
As a rule, almost 75 % of the COD during the down-flow stage is soluble, with the re-
mainder being solids. The dissolved COD is approximately 85 % volatile organic acids,
especially ethanoic and propionic acid. Anaerobic micro-organisms convert approximately
95 % of the in-flowing organic impurities into biogas and only 3 - 5 % into new biomass.
Low volumes of biomass (excess sludge) are created during the acidification or methana-
tion stage and these can be channelled into an aerobic biological purification stage. The
biogas created is 85 % CH4, which can be used for heating and power generation.
In an example rendering plant the specific CH4 production is reported to reach on aver-
age 0.32 m 3 /kg COD input. The biogas comprised 86 - 87 % CH4 and 0.3 – 0.7 % H2S.
The high H2S content in the crude gas had to be removed using a desulphurisation plant
to prevent emissions and corrosion.
The achievable volume load depends on the specific surface, open space and biomass
concentration within the reactor. The reactor configuration, the environmental conditions
existing within the reactor, such as the temperature and pH, as well as the substrate spe-
cific decomposition ability of the micro-organisms are also important.
To treat waste water originating in rendering plants, a mesophilic method of operation at
35 - 37 ºC is reported to be effective. A temperature of 32 - 42 ºC can largely be main-
tained through the heating energy contained in the raw effluent. The residence time in the
fixed bed reactor can be 11 – 30 h, depending on biomass concentrations.
A prerequisite for the trouble-free operation of fixed bed reactors is the removal of solid
and lipophilic substances from the effluent, in order to avoid the accumulation of materials
and blockages. The anaerobic process is relatively susceptible to failure from load fluc-
tuations, so volume and concentration equalisation of the effluent is required. A mixing
and equalisation tank with a stirring device can also facilitate gradual pre-acidification.
The pH needs to be around neutral, to prevent inhibition of the anaerobic mixed biocoe-
nosis. Stable CH4 production takes place within a pH range of 6.8 – 7.8. The optimum pH
for a separate pre-acidification process, depending on the substrate, is between 3.5 – 6.5.