Screening for electrical conductivity in anaerobic granular sludge from full-scale wastewater treatment reactors

Granulation in bioreactors for the treatment of industrial wastewater is a common phenomenon. Anaerobic granular sludge grown on brewery wastewater is known to manifest electrical conductivity, a characteristic that previously has been related to direct interspecies electron transfer. To assess if this characteristic is more widespread, we determined the electrical conductivity of 28 anaerobic granular sludges grown on a wide variety of industrial wastewaters in full-scale reactors. This revealed that it is a highly variable characteristic: one sludge had a conductivity of 171 mu S cm(-1) which is 4 times higher than the maximum reported for industrial anaerobic granules, 5 sludges had moderate conductivity ranging from 9.6 to 39 mu S cm(-1), while the conductivity of the remaining 22 sludges was below 4.5 mu S cm(-1). Additionally, we studied the possible relation between electrical conductivity and biological, chemical and physical properties of the granules. No clear relationship was found between electrical conductivity and microbial composition, while a positive correlation was found with both the iron and sulfur content. Other correlations were not found. Results suggest that electrical conductivity is a fairly rare characteristic of anaerobic granular sludge. The factors triggering its occurrence, as well as its significance, are not well understood yet.