Increasing the loading rate of continuous stirred tank reactor for coffee husk and pulp: Effect of trace elements supplement

In this study, the anaerobic performance and stability of coffee husk and pulp with and without trace element (TE) supplement was investigated, using 20 L mesophilic continuous stirred tank reactors for 140 days of experiment (DOE). The TE was cocktail of trace metals composed of Fe, Ni, Zn, Co, Mn, Mo, Se W and B. The organic loading rate (OLR) was increased stepwise from 2.5 (HRT = 40 d) to 6.0 kg VS m^-3 d^{- 1}(HRT = 16.7 d). The highest methane productivity from pulp with and without TE was 1.272 and 0.965 m³ m^-3 d^-1 at an OLR of 6.0 and 5.0 kg VS m^-3 d^-1; while the husks performed 0.895 and 0.795 m³ m^-3 d^-1 respectively, both at an OLR of 6.0 kg VS m^-3 d^-1. The specific methane yield (SMY) of pulp (at OLR = 5 kg VS m^-3 d^-1) with and without TEs was 217.9 ± 4.7 and 193.1 ± 8.2 L kg^-1 VS; while husk yielded 149.2 ± 6.0 and 132.5 ± 4.9 L kg^-1 VS, respectively. The effect of TEs on SMY was statistically significant (p < 001) at higher OLRs (5.0 - 6.0 kg VS m^-3 d^-1). The TEs improved the anaerobic stability through an optimum alkalinity ratio (VFA/TIC < 0.3) and suppressed the accumulation of volatile fatty acids. Mono digestion of husks and pulp are prone to lack Mo, Zn, Ni and Fe in long-term anaerobic fermentation. Further studies on co-digestion of husk/pulp with animal manure and dry fermentation helps to efficiently use this biomass resource.