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 CH 4 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 CH 4 , 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.