Long-term performance of three mesophilic anaerobic digesters to convert animal and agro-industrial wastes into organic fertilizer

Authors: ZHANG, XIAOQIAN - Purdue University; LOPES, IGOR - Purdue University; NI, JI-QIN - Purdue University; YUAN, YONGPING - Environmental Protection Agency (EPA); Huang, Chi Hua; Smith, Douglas; CHAUBEY, INDRAJEET - Purdue University; WU, SHUBIAO - Aarhus University

Submitted to: Journal of Cleaner Production
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/22/2021
Publication Date: 4/26/2021
Citation: Zhang, X., Lopes, I., Ni, J., Yuan, Y., Huang, C., Smith, D.R., Chaubey, I., Wu, S. 2021. Long-term performance of three mesophilic anaerobic digesters to convert animal and agro-industrial wastes into organic fertilizer. Journal of Cleaner Production. 307. Article 127271. https://doi.org/10.1016/j.jclepro.2021.127271.
DOI: https://doi.org/10.1016/j.jclepro.2021.127271

Interpretive Summary: Anaerobic digestion (AD) is a process used to convert animal and agro-industrial wastes to organic fertilizers. Since digester effluents are used as for cropland applications, we are interested to find out whether the quality of these organic fertilizers are impacted by the AD process. In addition, few studies have investigated the performance of AD in field conditions. In this study, we monitored the performance of three operational AD systems for two years. These three systems have different retention times (9, 12 and 34 days), that is how long the waste materials stay in the digester. We collected influent and effluent samples every two months and analyzed for pH, nutrients and metals. We found that the two shorter retention systems slightly decreased effluent pH while the long retention system significantly increased the effluent pH, i.e., by 1.8 unit. The 34-day retention system also removed more solids and increased ammonium-nitrogen in the effluent. We found no significant changes in the metals that we analyzed. These findings can assist in developing strategies for cleaner production using AD in an environmentally sustainable manner.

Technical Abstract: Few studies have investigated the performance of anaerobic digestion (AD) to convert animal and agro-industrial wastes to organic fertilizers over a long-term under field conditions. This paper studied three large-scale mesophilic digesters (D1–D3) over two years for their effects on feedstocks, which were dairy manure for D1 and D2 and codigestion mixed manure and agro-industrial wastes for D3. Hydraulic retention times (HRT) were 9 d for D1, 12 d for D2, and 34 d for D3. All digester effluent was ultimately used as organic fertilizer for cropland application. Digester influent and effluent samples were taken every two months from the digesters and analyzed for pH, and concentrations of total solids (TS), ammonium nitrogen (NH 4 -N), total Kjeldahl nitrogen (TKN), total phosphorus (TP), and eight metals (Cu, Fe, Mg, Mn, K, Na, Zn,and Ca). The study revealed high variability in converting feedstock in the three digesters. Compared with their respective influent, the digester effluent pH decreased by 0.6 in D1 (p < 0.01) and 0.3 in D2 (p < 0.01), but increased by 1.8 in D3 (p < 0.01). Digester effluent TS increased by 0.1% (p > 0.05) in D1, but decreased by 1.3% in D2 (p < 0.05) and 4.8% in D3 (p < 0.01). All three digesters significantly increased NH 3 -N concentrations by 21.4–81.8% (p < 0.05), but insignificantly changed TKN and TP concentrations (p > 0.05). Effects of digestion on all metal concentrations were mixed and were insignificant (p > 0.05) due to large concentration variations. However, study of a ratio quotient (q Mg) using magnesium (Mg) as the reference discovered accumulation of NH 4 -N, Cu, K and Na, but loss of TKN, TP, Fe, Mn, and Ca during AD for D2 and D3. The impact of AD conversion was closely related with types of feedstock (on pH) and HRT (on TS and NH 4 -N). The results of this study can assist in developing strategies for cleaner production using AD in an environmentally sustainable manner.