For this study 2 laboratory-scale reactors were run under three
different conditions for about 7 months. To simulate the operation
of the full-scale biowaste reactor in Karlsruhe-Durlach, the reactors
were only fed during working days from Monday to Friday, twice a
day (i.e. 9.00 a.m. and 16.00 p.m.), in a semi-continuous mode. No
feeding was provided on Saturday and Sunday. COD, pH and VFA in
the effluent, as well as biogas production and composition were
analysed daily before addition of fresh substrate. The basic feeding
of the reactors was fixed to 2 0.5 L of biowaste suspension per
day throughout the whole experiment. Before new feed addition,
the same amount of digested suspension was withdrawn. In the
first period the reactors were fed with a biowaste suspension at
an OLR of 12.3 kg COD m3 d1 (HRT 8 d, OLR and HRT calculated
for 5 d per week feeding if not stated otherwise) to determine
methane productivity and the efficiency of degradation of biowaste.
After process data for biowaste feeding were analysed for
variations (weeks 1–3), the OLR was increased step-wise by means
of biowaste in one of the reactors and with press water (weeks 4–
17) or food waste addition (weeks 18–30) to a final OLR of more
than 20 kg COD m3 d1 in the other reactor.