Characterization of soluble microbial products from anaerobic treatment by molecular weight distribution and nickel-chelating properties

Nine anaerobic chemostats fed glucose as the sole carbon and energy source were used to investigate the characteristics of soluble microbial products (SMP). These reactors were maintained at solids retention times (SRT) of 15, 25, and 40 days (3 reactors for each SRT) with organic loading rates of 0.25 or 0.50 g COD/1-d. Results showed that the concentration of SMP increased with increasing SRT (steady-state SMP for 15-, 25-, and 40-d systems were 54, 126, and 291 mg COD/1, respectively). Distribution of SMP among three molecular weight categories (MW < 1000 daltons, 1000 < MW < 10,000, and MW > 10,000) was bimodal. The fraction of SMP with MW > 10,000 was found to increase with increasing SRT (34 mg/l and 48% of total SMP for a 15-d SRT, 83 mg/l and 62% for a 25-d SRT, and 242 mg/l and 76% for a 40-d system). The stability constant (cK) between Ni and SMP was approximately 103.62, a value very similar to some naturally occurring organic acids. The total SMP-ligand concentration (CT,L) ranged from 0.07 to 1.33 mM as Ni and increased with increasing SRT. The complexation capacity ranged from 0.65 to 5.97 μmol of Ni/mg SMP, and was independent of SRT with an average of 2.49 μmol of Ni/mg SMP.