Digester effluent’s agronomic and odor emission potential: A swine case study

Authors: STOWELL, RICHARD - University Of Nebraska; Miller, Daniel; POWERS, CRYSTAL - University Of Nebraska

Submitted to: Waste to Worth Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/15/2017
Publication Date: 5/16/2017
Citation: Stowell, R., Miller, D.N., Powers, C. 2017. Digester effluent’s agronomic and odor emission potential: A swine case study. Waste to Worth Conference. Digester Effluent's Agronomic and Odor Emission Potential: A Swine Case Study.

Interpretive Summary: Anaerobic digestion may help manage manure as a fertilizer by reducing odor and other air emissions and increasing ammonia availability. Manure slurry and digester effluent samples were collected from three sites (pit, digester, and holding pond) over a 15-month period at a swine operation in eastern Nebraska that utilizes a complete-mix anaerobic digester that produces biogas for generating electricity. Manures were analyzed for common agronomic measures (N-P-K, pH, micronutrients, etc.) and measures of biological decomposition (chemical oxygen demand, volatile solids content). Gases released by manure samples were analyzed for odor compounds. Manure was applied at N-based rates comparing chemical fertilizer, raw pit manure, digested manure, and stored digested manure. Digester effluent had greater ammonia N content compared to raw manure, but NH3-N dropped substantially during subsequent storage. Volatile solids and chemical oxygen demand decreased from undigested manure in the below-barn pit. Loss of volatile solids and COD as the manure moved through the digester and during storage in the basin is consistent with consumption of organic matter and production of methane and other biogases. Similarly odorous compounds decreased as the manure slurry moved through the digester and during basin storage. When the digester was operating as designed, chemical oxygen demand was reduced by an average of 45%, odorous volatile fatty acids were reduced by an average of 66%, and ammonia increased by an average of 58%. Similar corn yields were obtained for each treatment. Digesters increase the availability of nitrogen in manure for plant growth, which unfortunately may also increase losses of this valuable plant nutrient via ammonia volatilization.

Technical Abstract: This on-farm study looked at the full-scale treatment effects of anaerobic digestion on the composition of manure effluent from an agronomic and air quality perspective. The goal was to improve our understanding of the role that anaerobic digestion may play in managing manure as a fertilizer and in reducing odor and other air emissions. Manure slurry and digester effluent samples were collected from three sites (below-barn pit, digester outlet, and holding pond) over a 15-month period at a swine production operation in eastern Nebraska that utilizes a complete-mix anaerobic digester to treat the manure and produce biogas for generating electricity. Manure analyses included common agronomic measures (N-P-K, pH, micronutrients, etc.) and measures of biological decomposition potential (i.e. chemical oxygen demand, volatile solids content). Gases released by manure samples (‘head-space air’) were analyzed for odor compounds (i.e. volatile fatty acids, aromatic compounds, and ammonia). Manure was applied at a nitrogen-based rate in duplicate plots comparing chemical fertilizer, undigested manure from the pit, digester effluent, and mixed manure from the earthen storage. A trend was observed for ammonia nitrogen content of the digester effluent to be greater than in the raw manure [influent], but then NH3-N dropped substantially during subsequent storage in the earthen basin. Volatile solids and chemical oxygen demand decreased from undigested manure in the below-barn pit. Loss of volatile solids and COD as the manure moved through the digester and during storage in the basin is consistent with consumption of organic matter and production of methane and other biogases. Similarly odorous compounds decreased as the manure slurry moved through the digester and during basin storage. When the digester was operating as designed, chemical oxygen demand was reduced by an average of 45%, odorous volatile fatty acids were reduced by an average of 66%, and ammonia increased by an average of 58%. Similar corn yields (156 to 163 Bu/Ac) were obtained for each treatment. Digesters increase the availability of nitrogen in manure for plant growth, which unfortunately may also increase losses of this valuable plant nutrient via ammonia volatilization.