Reactor configuration-Part II Comparative process stability and efficiency of thermophilic anaerobic digestion

Abstract

Comparative process stability and efficiency of thermophilic anaerobic digestion (55 °C) has been evaluated for four different reactor configurations, which are: daily batch-fed single-stage continuously stirred tank reactor (CSTR) (TB), continuously-fed single-stage CSTR (TC), daily batch-fed two-phase CSTR (TTP), and daily batch-fed non-mixed single-stage reactor (TNMR). The results are discussed for 2 periods, 1) start-up until steady state (Days 0 - 200) and 2) OLR increase from 4 % solids at steady state to reactor failure by increasing solids concentration in feed and decreasing the HRT (Days 201 -). During the start-up period, the TB showed the worst stability with a pH drop whenever the solids concentration in the feed was increased. Conversely the TNMR reached steady state with 4 % feed solids in the shortest time with relatively stable pH and very low VFA. The superior performance of the TNMR confirms the importance of microbial consortia proximity especially for the removal of propionate. The cocktail of inorganic nutrients (Ca, Ni, Fe, and Co) was added daily into all reactors showing high VFA. In the case of the TNMR, complete removal of propionate occurred after supplementation of nutrients. The results indicated that adding nutrients stimulated gas production and facilitated removal of almost all VFA confirming the importance of inorganic nutrients bioavailability. During the long-term operation with OLR increases until reactor failure (pH below 5.5), the results show that TB failed at the lowest OLR while the TC and the TNMR reached the highest OLR. Compared with the daily batch fed reactors, the constant pH of the MC seems to be the reason why the MC reached the highest OLR. The superior performance of the TNMR during both the start-up and the long-term period confirms the importance of microbial consortia proximity. Two-phase digestion showed little benefit over single stage during the start-up period and no benefit was observed during the long-term period. Additional experiments in which the reactor configuration was changed from CSTR to non-mixed reactor showed significant benefit with respect to gas production and VFA threshold concentration. This once again manifests the importance of microbial consortia proximity.