Fertilizer Recovery from Anaerobic Digesters

Abstract

ABSTRACT AGRICULTURAL wastes can be anaerobically digested to product methane. Fertilizer nutrients in the wastes are recovered after the digestion process. The objectives of this study were to evaluate nutrient recovery, extractability and distribution between supernatant and sludge fractions. Three influent types prepared to 60 g/L total solids (TS), poultry manure-thin stillage mixture, dairy cattle and swine manure (PMT, CM and SM), were utilized in laboratory (35 C) digestions with two retention times (16 and 24 days). While total Kjeldahl N of these wastes was approximately 100% recovered after digestion, 2N KCl extractable NH4+N was recovered in excess of 100% (229 to 240% for PMT and 136 to 152% for CM and SM digesters) due to the mineralization of influent organic N. The extractable:total ratios of influent P, Ca and Mg were reduced (P by 19 to 30%, Ca by 19 to 24% and Mg by 6 to 20%) after digestion of PMT and SM. These ratios of K and Ca were reduced (by 9 to 11%) after digestion of CM. Reduced extractability may have resulted from increased nutrient sorption by manure solids after digestion. Centrifugation distributed NH4+N and K between supernatant and sludge fractions in proportions that paralleled the effluent total mass distribution between those fractions and distributed P, Ca and Mg in proportions that paralleled effluent TS distribution. Since supernatants accounted for most of the mass, a majority of the effluent NH4+N (58.9 to 83.4) and total K (60.0 to 80.0%) was present in the supernatants. Since the sludges accounted for most of the TS, a majority of the effluent total P (59.6 to 85.8%), total Ca (56.6 to 81.0%) and total Mg (58.8 to 75.1%) was present in the sludges.