Evaluation of Solids and Liquids from Anaerobic Digesters for Use as Bedding and Fertilizer
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Summary: Solids and liquids from digestion of manure at 17 farms were analyzed for bacterial concentration, physical properties and/or metal concentrations. Four of these farms digested manure only and the rest accepted some type of food or food processing waste as co-digestion material. As expected, bacterial concentrations were variable and no clear trends were discovered for higher populations in solids that were digested with food and food processing waste versus those that came from manure only. In addition, no single type of waste yielded higher bacterial concentration than any other. Season played a much larger part in bacterial concentration than did co-digestion. Physical properties (% moisture and % of fine particles) of the solids were not significantly different whether digested with a co-digestion material or not. Organic matter, which may lead to increased bacterial growth in the bedding, but could be helpful for conditioning of the soil, was significantly lower in solids that were digested with manure only versus co-digestion. There were differences in the chemical properties of the solids that were co-digested versus those that were not. However, co-digestion should not affect the fertilizer value of the solids in normal farm operations as the difference favors them as fertilizer. The only one that may be of concern is increased phosphorus in runoff. Bacterial concentration in liquid digestate was similar to that of solids. Rather than co-digestion, differences in bacterial concentration were based on season, with summer having the highest bacterial load. Concentration of metals in all liquid digestate was well-below regulatory limits set for sewage sludge. Liquid digestate from co-digestion with food and food processing waste had higher concentrations of copper and zinc than liquid from digestion of manure only, but were still 5 and 10 times lower, respectively, than the New York State Department of Environmental Conservation sludge and sludge product maximum (NYSDEC 360 Rules). There was no difference in fertilizer value (N, P, K) of liquid digestate from digestion with or without co-digestion materials. The concentration of boron, a micronutrient, was significantly lower in digestate from co-digestion of all wastes studied versus manure alone and the concentration of Zn was significantly higher from co-digestion with grocery store waste than any other liquid digestate. However, neither of these would result in deficiency or toxicity. Many of the other micronutrients found in the liquid digestate, regardless of co-digestion, were above limits set by EPA for use as irrigation water. The objective of this study was to see if the addition of food and food processing wastes to agricultural digesters has an adverse or positive effect on the end products. Taking into account all of the above information, it appears that co-digestion does not change either the solid or liquid digestate and thus can be used as animal bedding and for fertilization/irrigation of farm fields.