Comparative evaluation of sludge surface charge as an indicator of process fluctuations in a biogas reactor

The current political situation imposes high demands on the economic feasibility of biogas plants. High prizes for biogas substrates and a trend to reduced feed-in tariffs generated an increasing need to optimize substrate exploitation and operation conditions. This includes a comprehensive and reliable biogas process monitoring. For that purpose a number of different process monitoring methods like CH₄ production rate, FOS/TAC (ratio of organic acid/total inorganic carbon alkalinity), pH or (auto)fluorescence are successfully applied. This paper will evaluate whether the surface charge - a parameter, which has not been in use so far - might also be suitable for biogas process monitoring. Since it is known that the surface charge is correlated with the adherence and floc formation capability of microbial cells, a change in surface charge might also reflect a change in the biogas process efficiency, or vice versa. To test this hypothesis, samples for the investigations were taken from a continuously stirred laboratory-scale tank biogas reactor with continuously increased substrate load. The impact of the load change was measured with both, surface charge and a number of more established monitoring parameters as given above. It was found that the "surface charge" reflected well short-term process changes (within hours) caused by an increasing substrate load in the reactor, though the highest short-term monitoring sensitivity was obtained with the "FOS/TAC" monitoring. Different from other monitoring parameters like CH₄, pH, or FOS/TAC the value of the parameter "surface charge" decreased with every feeding, eventually indicating a continuous deterioration of the biogas process conditions. Surface charge might therefore be of particular use as a complementary tool especially for the long-term monitoring of biogas process conditions.