A complex medium promotes fermentation to VFAs and ethanol with little hydrogen while a mineral medium promotes acetate-butyrate-hydrogen formation
Two anaerobic SBRs were operated with either a mineral or complex medium. The enrichments displayed distinct fermentation patterns after 20 SRTs. Initially, the mineral-medium enrichment showed a 1:2:1 acetate:propionate:butyrate product spectrum with little ethanol and no lactate (Figure S1). This shifted after 10 SRTs to primarily acetate and butyrate, with a small amount of lactate and ethanol (Figure 1). Hydrogen was the major gaseous product in the mineral medium enrichment (13%±1% of incoming COD). Up to 96% ± 3% of the incoming COD could be recovered for this enrichment, which indicates that a minor by-product might have been missed. Succinate, valerate or formate concentrations were below the detection limit of 50 µM.
The complex medium enrichment showed a more dynamic product spectrum development. Initially lactate and acetate were the dominant products (Figure S1). After 3 SRTs, the product spectrum shifted to acetate, propionate, butyrate, valerate, and lactate. After 31 SRTs, 0.19 Cmol ethanol Cmol-1 sugar was produced, and only a minor amount of hydrogen were detected in the off-gas (1%±0% of the incoming COD) (Figure 1). This product spectrum was more diverse than for the mineral medium enrichment. The carbon balance (based on glucose as the only substrate) displayed a recovery of 113% ± 2%, which most likely is caused by the uptake of tryptone for biomass production. Taking into account that tryptone uptake is equivalent to the nitrogen requirements for biomass production, the carbon recovery would be 100% ± 2%.
The mineral medium enrichment showed a 25% higher biomass yield on glucose then the complex medium enrichment culture. The µmax values for the cultures were derived with a cut-off at 20 SRTs (Figure S2). The µmax in the complex medium enrichment was 58% higher than the mineral medium enrichment, while the maximal biomass specific substrate uptake rate (qsmax) was even 94% higher (Table 1).