Formation of soluble microbial products during anaerobic treatment

Stock (fill-and-draw) reactors and chemostats (CSTR) fed acetate and glucose were used to investigate the production of soluble microbial products (SMP) during anaerobic treatment. These reactors were maintained at solids retention times (SRT) from 15 to 56 days with organic loading rates (OLR) from 0.18 to 0.50 g chemical oxygen demand (COD)/L . d. Results showed that longer SRTs resulted in higher levels of SMP, with SMP ranging from 17 to 59 mg COD/L when acetate was the sole carbon and energy source and 50 to 291 mg COD/L when glucose was the substrate. Normalized production of SMP (SMP/influent COD = SMP/S-o) ranged from 0.2% to 1.0% for acetate reactors and 0.6% to 2.5% for glucose reactors. Steady-state results from chemostats fed glucose showed that as SRT increased, SMP/S-o decreased to a minimum and then increased, indicating the existence of an optimal operating SRT that would result in a minimum normalized production of SMP. Effluent soluble COD was mostly SMP; the fraction of effluent soluble COD made up of SMP increased with increasing SRT. Production of SMP from glucose reactors was modeled. Normalized production of SMP during anaerobic treatment appears to be lower when compared with aerobic production of SMP reported in the literature.