Search

Several completely stirred tank reactors (CSTRs) connected in series for corn stover anaerobic digestion was devised to obtain more methane yield and increase conversion rates. Liquid fraction of the digestate (LFD) was recirculated from the second-stage reactor to first-stage reactors to reuse LFD and improve system performance. LFD recirculation didn’t inhibit methane production of serial digestion, and methane and biogas production were increased by 2.3% and 10.8%, respectively. Moreover, LFD recirculation increased pH and alkalinity concentration (AC) and decreased volatile fatty acids (VFAs) concentrations and the ratio of VFAs to AC, which means a significant increase in system stability of anaerobic digestion (AD). Ammonia concentrations gradually increased with LFD recirculation, but was far lower than the inhibition concentration. Microbial analysis indicated that the recirculation increased the richness, but decreased the diversity of both bacterial and archaeal community in the first and second stage of serial system. More specifically, LFD recirculation enriched the dominant bacterial phyla (Bacteroidetes and Firmicutes), but had little influence on the dominated archaeal genus (Methanobacterium). The results showed that the recirculation of LFD during serial AD was technically suitable by minimizing both discharge of LFD and possible pollution related with discharging LFD.

The effects of different potassium and nitrogen pretreatment strategies on the anaerobic digestion (AD) performance of rice straw (RS) were investigated. KOH, NH3·H2O and KOH + NH3·H2O combined pretreatments were applied. The results showed that KOH + NH3·H2O combined pretreatment achieved the highest biomethane production and TS (TS: total solid) removal rate of 274 mL g VS−1 and 43.9%, which were 6.2–75.8% and 4.3–29.5% higher than that of single alkali pretreatments and untreated RS, respectively. The NH3·H2O groups improved the process stability, which maintained the NH3–N concentration in the range of 265–580 mg L−1. It was also found that Bacteroidetes and Firmicutes were the dominant bacterial at phyla level, and the populations of acetate methanogen (Methanosarcina and Methanosaeta) were enriched in the AD system by KOH + NH3·H2O pretreatment. Furthermore, the cost of pretreatment agents can be recovered by the increased digestate nutritional value due to the K and N remaining in the digestate after AD. The results indicated that the KOH + NH3·H2O combined pretreatment might be a promising method for efficient AD of straw in future industrial applications.

Temperature has different effects on anaerobic digestion (AD) of various biomasses, which could bring out changes in microbial communities. The relationship between microbial community and methane production at 35 °C (R35), 38 °C (R38), 41 °C (R41), and 44 °C (R44) was analyzed during AD of corn stover (CS). The results showed that the daily biogas and methane production from R44 were 16.6%–42.4% and 16.2%–40.6% higher than yields from R35, R38 and R41, respectively. The abundance of Bacteroidetes in R35, R38 and R41 was relatively close (30.70%–39.36%), which was low in R44 (16.00%). The abundance of Firmicutes in R35 was 32.30%, however, Firmicutes was the most dominant phylum at R44 (66.58%). The abundance of Miscellaneous_Crenarchaeotic_Group and Euryarchaeota were 54.63 ± 6.47% and 44.43 ± 6.73% across all digesters. This research demonstrated that among all temperatures studied, 44 °C could enhance the conversion efficiency of the substrates to methane and be recommended for better conversion of CS in AD process.

The effect of ammonia pretreatment on the anaerobic digestibility of corn stover was investigated. Corn stover with different moisture contents (30%, 50%, 70%, and 90%) was pretreated with three concentrations of ammonia (2%, 4%, and 6%) at 35 ± 2 °C for the following batch digestion. Results showed that the reagent of 4% ammonia and 70% moisture content could achieve the highest anaerobic digestibility. In comparison with the untreated, the time needed to produce 90% of the maximum digester gas production (T90) shortened from 52 d to 37 d. The total biogas production and the unit volatile solids (VS) biogas yield were 20,740 ml and 427.1 ml respectively, both 26.70% higher than the untreated. It was found that the digesters with high moisture contents of 70% and 90% were more stable and had shorter acidification periods relative to the low moisture contents of 30% and 50%. The decreases in cellulose, hemicelluloses and lignin indicated that ammonia pretreatment could destroy the lignocellulose (LCH) structure and furthermore enhance the biogas production. Following anaerobic digestion, 80.6% of cellulose and 68.52% of hemicelluloses were consumed where there was 4% ammonia and 70% moisture content, indicating why these conditions produced the highest level of biogas.

Several completely stirred tank reactors (CSTR) connected in series for anaerobic digestion of corn stover were investigated in laboratory scale. Serial anaerobic digestion systems operated at a total HRT of 40 days, and distribution of HRT are 10 + 30 days (HRT10 + 30 d), 20 + 20 days (HRT20 + 20 d), and 30 + 10 days (HRT30 + 10 d) were compared to a conventional one-step CSTR at the same HRT of 40 d. The results showed that in HRT10 + 30 d serial system, the process became very unstable at organic load of 50 gTS·L−1. The HRT20 + 20 d and HRT30 + 10 d serial systems improved methane production by 8.3–14.6% compared to the one-step system in all loads of 50, 70, 90 gTS·L−1. The conversion rates of total solid, cellulose, and hemicellulose were increased in serial anaerobic digestion systems compared to single system. The serial systems showed more stable process performance in high organic load. HRT30 + 10 d system showed the best biogas production and conversions among all systems.

This research applied sodium hydroxide (NaOH) pretreatment and trace elements to improve biomethane production when using corn stover for anaerobic digestion. Full-factor experimental tests identified the best combination of trace elements with the NaOH pretreatment, indicating that the best combination was with 1.0, 0.4, and 0.4 mg·L−1·d−1of elements Fe, Co, and Ni, respectively. The cumulative biomethane production adding NaOH pretreatment and trace elements was 11,367 mL; total solid bioconversion rate was 55.7%, which was 41.8%–62.2% higher than with NaOH-pretreatment alone and 22.2%–56.3% higher than with untreated corn stover. The best combination was obtained 5–9 days shorter than T90and maintained good system operation stability. Only a fraction of the trace elements in the best combination was present in the resulting solution; more than 85% of the total amounts added were transferred into the solid fraction. Adding 0.897 g of Fe, 0.389 g of Co, and 0.349 g of Ni satisfied anaerobic digestion needs and enhanced biological activity at the beginning of the operation. The results showed that NaOH pretreatment and adding trace elements improve corn stover biodegradability and enhance biomethane production.

The effect of ammonia pretreatment on the anaerobic digestion performance of rice straw was investigated. The rice straw was pretreated with four different moisture contents (30%, 50%, 70%, and 90%) and three concentrations of ammonia (2%, 4%, and 6%). The results showed that the anaerobic digestion achieved best performance for the rice straw pretreated by ammonia concentration of 4% and moisture content of 70%, and biogas yield achieved the highest value (396.92 mL•g-1 total solid), which was 21.9% higher than that of the untreated rice straw. Higher moisture was recommended as it could increase system stability, shorten anaerobic digestion time, and increase biogas production. The composition analyses indicated that ammonia pretreatment could effectively destroy the chemical structure of lignocellulose, and partially decompose cellulose, hemicellulose, and lignin. Cellulose and hemicellulose were converted by 79.2% and 63.2%, respectively, at optimal ammonia amount and moisture content. The ammonia pretreatment proved to be a simple and effective methods to improve the anaerobic digestion performance of rice straw.