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AN INNOVATIVE DESIGN FOR ANAEROBIC CO-DIGESTION OF ANIMAL WASTES FOR SUSTAINABLE DEVELOPMENT IN RURAL COMMUNITIES
Impact/Purpose:
Animal waste poses one of America's most serious pollution problems, because the natural decomposition of livestock manures releases large quantities of pathogens, excess nutrients, and greenhouse gas methane, among other pollutants. To overcome this challenge, the objective of this project is to design and assess the technical and economic feasibility of an innovative anaerobic co-digestion process for the simultaneous treatment of dairy waste and poultry litter
Description:
With the aim of the Phase I project to develop an innovative anaerobic co-digestion design for the treatment of dairy manure and poultry waste, our Phase I team has evaluated the technical and economic feasibility of the anaerobic co-digestion design concept with a thorough investigation into the waste characteristics, batch treatability tests, continuous digestion optimization, and economic feasibility analysis.
Analyses of waste composition show poultry waste contained much higher total solids (~56%) than dairy manure (2.3%), an indication of higher organic content and biogas potential. In contrast, the higher nitrogen content of poultry waste suggests potential inhibition of the digestion process by excess ammonia, which supports the expectation of the co-digestion design concept that the dilution of nitrogen-rich poultry waste by nitrogen-poor dairy manure would reduce the risk of inhibition of ammonia from poultry waste. These results suggest that the use of dairy manure and poultry waste as co-substrates in anaerobic digestion would have the benefit of enhanced process efficiency and biogas yield. Batch treatability tests confirmed that poultry waste has higher methane potential than that of dairy manure. Thus, better process anaerobic digestion performance could be achieved by maximizing poultry waste loading. Moreover, complete digestion of animal wastes was shown to be accomplished with a retention time of 20 days at operational temperatures above 20°C.
Building upon the operational parameters from batch tests, multiple lab-scale continuous anaerobic co-digesters were developed and operated for more than 5 months to determine critical process parameters such as hydraulic retention time and organic loading rates. Optimal hydraulic retention time was selected as 20 days as biogas yield and organic removal did not improve when hydraulic retention time was greater than 20 days. As a result, a 20-day hydraulic retention time was selected in all subsequent continuous digesters. Optimal organic loading rates of dairy manure and poultry waste were determined by step-wise increase in the organic loading of manure-fed continuous digesters by the addition of organic-rich poultry waste. Our results show that the addition of organic-rich poultry waste as a co-substrate resulted in increases in biogas production, with the highest biogas production rate observed at the organic loading of 1.5 gVS/L·d. However, organic loading rates higher than 1.5 gVS/L·d led to the collapse of digester performance, likely a result of process inhibition by excess levels of ammonia produced during the breakdown of poultry waste in anaerobic digestion, which is consistent with the high nitrogen content of poultry waste. Furthermore, the volatile fatty acids (VFA) concentrations also rose dramatically from less than 5 mg/L to greater than 1,000 mg/L when organic loading reached 1.8 gVS/L·d, another evidence that the digestion process was inhibited at higher loadings of poultry waste. These results indicate that the optimal organic loading rate is 1.5 gVS/L·d, which corresponding to the optimal ratio of approximately 2:1 (gVS/gVS) between dairy manure and poultry waste in the feed. Overall, the addition of organic-rich poultry waste improved biogas yield by more than 75% at optimal conditions.
While the anaerobic co-digestion design is shown to be technically feasible, the economic feasibility is paramount in the decision-making process for the implementation of this innovative design in rural communities. Our Phase I team has collaborated with USDA Natural Resources Conservation Services (NRCS) EQIP grant program and Farm Credit Services of Mid-America to determine the economic feasibility of the anaerobic digestion design developed in the Phase I project. The EPA AgStar Farmware analysis indicates that the anaerobic digestion design was, in general, more feasible in larger farms with a payback period of less than 10 years. The recovery of bedding material from anaerobic digestion was a key factor that would strengthen the economic feasibility of anaerobic digestion with the current green energy rates and financial terms. Given the importance of the scale of farms to the economic feasibility, it is critical that the design developed in this project is implemented as a centralized facility that serves multiple farms to improve economic sustainability.