Extracellular polymers of hydrogen-utilizing methanogenic and sulfate-reducing sludges

Extracellular polymers (ECP) play an important role in biological wastewater treatment. They are believed to be responsible for the formations of flocculent activated sludge as well as anaerobic granules. In this study, the ECP characteristics of two types of enrichment cultures were examined. One enriched culture was composed of hydrogenotrophic methanogens (HM) alone while the other was composed of HM and hydrogenotrophic sulfate-reducing bacteria (HSR). Both cultures were enriched through a series of 107 repeated batches using plus, in the HM/HSR series, sulfate and nutrients. At various stages of the enrichment process, the ECP were extracted from the sludge samples and analyzed for their protein (ECPp) and carbohydrate (ECPc) contents. Results showed that in the mixed culture HM consumed 67% of hydrogen and HSR 33%. The net yields of biomass for HM and HSR cultures were 0.046 g-VSS/g-COD and 0.059 g-VSS/g-COD, respectively. Productions of ECP were dependent upon substrate. Glucose-degrading sludge produced more ECP than the HM culture, which in turn produced more than the HSR culture. The net ECP yields for HM culture were 1.06 mg-ECPp/g-COD and 0.64 mg-ECPc/g-COD, respectively; the corresponding yields for HSR culture were 0.74 mg-ECPp/g-COD and 0.52 mg-ECPc/g-COD. For the enriched HM culture, more ECPp, as well as ECPc but to a lesser degree, were produced at the beginning of each batch when high concentration of hydrogen was available. This was, however, not noticeable for the HSR culture