Modeling of soluble microbial products in anaerobic digestion: the effect of feed strength and composition

Continuously stirred tank reactors (CSTRs) and fill-and-draw reactors were used to investigate soluble microbial product (SMP) production during anaerobic digestion. Continuously stirred tank reactors with a glucose feed at three different strengths (5, 10, and 20 g chemical oxygen demand [COD]/L) and fill-and-draw reactors with four different feed compositions (glucose, glycerol, lactic acid, and starch at 10 g COD/L) were used with a solids retention time (SRT) of 15 days. The SMP production ranged from 102 to 588 mg COD/L for the glucose-fed CSTRs and between 157 and 1192 mg COD/L for the fill-and-draw reactors and was found to increase with increasing influent COD (SO). Normalized production of SMPs (SMP/So) ranged from 1.4 to 3% for the CSTRs and 12.0, 1.7, 14.7, and 3.6% for the glucose-, glycerol-, lactic acid-, and starch-fed reactors, respectively. A model incorporating SMP production and degradation was fitted to results from carbon-14 tracer studies in all of the reactors. The best-fit parameters from this model revealed that the type of SMP that dominates in any particular system depends not only on the strength of the feed but also on the composition of the feed and the type of reactor.