Development and application of a mechanistic model to study substrate degradation, microbial synthesis and gas production

A mechanistic model was developed to study the relationships betweensubstrate degradation, microbial synthesis, and gas production in thein vitrogas production technique. The model was based on standardsubstrate and microbial growth kinetics and consisted of six state vari-ables that represented undegradable substrate (QU), insoluble degrad-able substrate (QD), soluble substrate (QS), microbial biomass (QM), volatile fatty acids (QVFA), and gas (QG). Hydrolysis of QDwas as-sumed to obey mass-action kinetics. Microbial uptake of QSwas repre-sented by saturation kinetics. The substrate taken up was utilized formicrobial growth and non-growth requirements. A fixed microbial non-growthrequirementwasassumed. TheamountofVFAandgasproducedwere calculated based on stoichiometric principles. Simulations wereterminated when non-growth requirements exceeded substrate uptake,representative of the situation of maximal microbial biomass. Gas pro-duction profiles were simulated for five feedstuffs with different degra-dation characteristics. The feedstuffs evaluated were a good (G1) anda poor quality grass silage (G2), sugarbeet pulp (SB), pressed potatopulp (PP) and soy hulls (SH). All simulated cumulative gas productionprofileshadrealisticsigmoidalshapes. Fastdegradingfeedsreachedthepoint of maximal microbial biomass more quickly than slow degradingfeeds. The incubation time at maximal microbial biomass varied be-tween 15.3 (SB) and 27.8 (SH) h. Besides, fast degrading feeds tendedto have a higher efficiency of microbial synthesis compared with slowdegrading feeds. Microbial efficiency at the point of maximal biomassvaried between 257 (SH) and 317 (SB) g microbial biomass / kg sub-strate truly degraded. The simulations provided a quantitative under-standing of the relationships between substrate degradation, microbialbiomass, and gas production.