Nitrogen mineralization and energy from anaerobic digestion of jatropha press cake

Abstract

Expelling oil from jatropha seeds leaves a dry press cake which is rich in energy and plant nutrients. Anaerobic digestion of the press cake decomposes the organic matter into biogas and digestate. The digestion process mineralizes organic nitrogen to plant available nitrogen found in the digestate. In this experiment press cake was digested in mesophilic batch reactors with 1% VS and 71days incubation. Anaerobic digestion of jatropha press cake produced 289L·kg−1 VS methane. Concentration of methane in the biogas after gas generated had stabilized was 60% to 65%. Energy in biogas from jatropha press cake was 4.7MJ·kg−1 VS; 1/5 of the energy in the undigested jatropha press cake. From the total nitrogen found in jatropha press cake (40g·kg−1 TS), 76.6% had been converted to ammonium-N and 1.6% to nitrate-N through the anaerobic digestion process. 21.8% remained as organic-N. Ammonium and nitrate are valuable as plant fertilizers since they are absorbed directly by plants. Fertilization with 32tons digestate (10% TS) per hectare corresponds to 100kg N/ha mineral fertilizer. Water requirements and the digestate's liquid form makes anaerobic digestion appropriate technology in agriculture where water and infrastructure is available. These pre-conditions may infrequently be met in semi-arid environments where jatropha is promoted for rural development.