Co-digestion of agricultural and municipal waste to produce energy and soil amendment

Authors: MACIAS-CORRAL, MARITZA - New Mexico State University; SAMANI, ZOHRAB - New Mexico State University; HANSON, ADRIAN - New Mexico State University; Funk, Paul

Submitted to: Waste Management and Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2017
Publication Date: 7/12/2017
Citation: Macias-Corral, M.A., Samani, Z.T., Hanson, A.T., Funk, P.A. 2017. Co-digestion of agricultural and municipal waste to produce energy and soil amendment. Waste Management and Research. 35(9):991-996. https://doi.org/10.1177/0734242X17715097.
DOI: https://doi.org/10.1177/0734242X17715097

Interpretive Summary: In the past, the economic viability of anaerobic digestion has been limited by process instability and low methane yield. Yet disposing of agricultural wastes is increasingly important due to economy-of-scale concentration of processing and production. Urban agriculture also produces waste that is no longer are welcome by landfill operators. Combining cotton gin trash and dairy manure with grass clippings from a golf course created a favorable ratio of carbon to nitrogen; anaerobically fermenting the mixture in a two-phase system improved process stability and increased gas yield and energy value. Multi-substrate co-digestion produced gas containing 70.5% CH4, yielded 68 m3 CH4/t dry waste and reduced volume 75% and mass 57%. Nutrients were concentrated during the process, resulting in a soil amendment that was high in fertilizer value and relatively free from pathogens.

Technical Abstract: In agriculture, manure and cotton gin waste are major environmental liabilities. Likewise, grass is an important organic component of municipal waste. These wastes were combined and used as substrates in a two-phase, pilot-scale anaerobic digester to evaluate the potential for biogas (methane) production, waste minimisation, and the digestate value as soil amendment. The anaerobic digestion process did not show signs of inhibition. Biogas production increased during the first 2'weeks of operation, when chemical oxygen demand and volatile fatty acid concentrations and the organic loading rate to the system were high. Chemical oxygen demand from the anaerobic columns remained relatively steady after the first week of operation, even at high organic loading rates. The experiment lasted about 1'month and produced 96.5'm3 of biogas (68'm3 of CH4) per tonne of waste. In terms of chemical oxygen demand to methane conversion efficiency, the system generated 62% of the theoretical methane production; the chemical oxygen demand/volatile solids degradation rate was 62%, compared with the theoretical 66%. The results showed that co-digestion and subsequent digestate composting resulted in about 60% and 75% mass and volume reductions, respectively. Digestate analysis showed that it can be used as a high nutrient content soil amendment. The digestate met Class A faecal coliform standards (highest quality) established in the United States for biosolids. Digestion and subsequent composting concentrated the digestate nitrogen, phosphorus, and potassium content by 37%, 24%, and 317%, respectively. Multi-substrate co-digestion is a practical alternative for agricultural waste management, minimisation of landfill disposal, and it also results in the production of valuable products.