Anaerobic Co-digestion of Swine Manure and Agricultural Residues for Biogas Production.

摘要

Anaerobic digestion process to treat organics has become attractive because of environmental and economical reasons, since it not only reduces the amount of material to be disposed, but also is a source of energy by producing biogas. An interesting option to improve yields of anaerobic digestion is the treatment of different substrates in a process called anaerobic co-digestion. This is a promising technology whose purpose is to enhance biogas production and stabilize the process. In most of the cases addition of co-substrates improves the biogas yield due to a better balance of nutrients and positive synergisms in the digestion medium. The overall goal of this research is to gain new insight into the impact of anaerobic co-digestion of swine manure and agricultural residues on biogas production.;In this study, the performance of anaerobic co-digestion of swine manure and corn stover in two 14 liters continuous stirred tank reactors (CSTR) under mesophilic conditions (35°C) with an hydraulic retention time (HRT) of 25 days was investigated. One reactor (Reactor 1) was fed with swine manure alone (C:N ratio of 2.3) and the other reactor (Reactor 2) was fed with swine manure and corn stover (C:N ratio 10). When corn stover was co-digested with swine wastewater, methane production increased 11 times compared to methane production from swine wastewater alone (1980 versus 186 ml of CH4 per day). However, there was no significant difference on methane yield (0.19 versus 0.18 m3CH4/kg CODremoved). Because of the better balance of nutrients during co-digestion the ammonium accumulation in the Reactor 2 decreased, reducing the risk of inhibition.;The effect of alkali pretreatment (solution of 5% NaOH and 5% CaO) and additional enzymes (cellulase complex) on biogas production from anaerobic co-digestion of swine wastewater and corn stover was investigated in the same reactors under mesophilic conditions (35°C) with a HRT of 25 days. While both reactors were fed with 560 ml of swine manure and 14 g of corn stover, three different treatments where applied only to reactor 2 (1: addition of 140 mg per day of cellulase; 2: pretreated corn stover; and 3: pretreated corn stover and addition of 140 mg per day of cellulase). Even though all the treatments improved methane yield compared, the best result was obtained with treatment 3 (31.9% of increase), followed by treatment 2 (21.8% increase) showing that alkali pretreatment was effective improving methane generation. Cellulase complex also showed to be effective, increasing methane production by 10.4%. However, cost analysis showed that, for all the treatments studied, the benefit obtained because of the extra methane produced is lower than the cost implied by the addition of either chemicals or enzymes because of the low price of methane.;The effect of total solids (TS) content on anaerobic co-digestion of swine manure with corn stover and Bermuda grass for methane production was investigated. Three sets of experiments at different total solids concentrations were carried out in batch reactors under mesophilic conditions, and cumulative methane production was examined to evaluate their performance. For both co-substrates, methane production was successfully produced at 2% and 3% of TS concentration, with the maximum methane production of 1800 ml of methane after 29 days. However, methane production was inhibited at 4% TS because of acid accumulation in the reactors. When no inhibition occurs (at 2% and 3% TS) methane yield from corn stover was higher than that from Bermuda grass.;Four mathematical models (First order, modified Gompertz, Logistic, and Transference model) were applied to determine kinetic parameters and evaluate methane production from anaerobic co-digestion of swine wastewater and six agricultural residues (corn stover, Bermuda grass, switchgrass, wheat straw, rice straw, and cocoa husk) in batch reactors at 2%, 3%, and 4% of TS concentration. The parameters estimated where B0 (methane production potential), k (first-order kinetic constant), Rm (maximum specific methane production rate), and lambda (lag-phase time). All models showed a good fit with actual data (R2 > 0.9) of cumulative methane production for all substrates at the 3 different TS concentrations. Considering all the conditions studied with the six substrates, the highest lag-phase time was less than one day (18 hours), being negligible in most of the cases, which is a sign of high activity of the methanogens in the reactors. Rm was consistently higher at 2% TS followed by 3% TS and 4% TS for all the substrates, which means that a shorter period of time is necessary to digest the substrates at 2% TS concentration.