Characteristic of leachate distribution at profile in straw anaerobic digestion with high solid content

Abstract: High solid content anaerobic digestion is an environment friendly and cost efficient way to dispose the organic wastes because of little water added and renewable energy produced. In high solid digestion system, substrate has little free water so that the mass transfer is inhibited frequently, leading to volatile fatty acids (VFAs) accumulation and fermentation system failed easily. Many researchers report that leachate recirculation in reactors is an efficient way to enhance mass transfer leading the increase of anaerobic digestion efficiency. Most of the investigators are keen on making mathematics model to indicate the movement of leachate in substrates, these mathematical models require skillful person to play a key role in practices. Therefore, it is important to have better and easier understanding of leachate distribution characteristics in substrates. The trial was designed to determine such distribution. Lithium-Ion tracing method and contour map were used to achieve these goals. A batch wheat straw anaerobic digestion experiment was conducted at 14% total solid content and mesophilic conditions in special designed batch fermentation device, three treatments (T1, T2 & T3) were set to investigate the leachate distribution characteristics, and anaerobic digestion substrates for T1, T2 & T3 were all same. For T1, we divided the substrates into three equal parts by two separator plates. T2 was divided into two equal parts by one separator plate. T3 was not divided but as a whole. A certain amount of lithium chloride solution instead of leachate was circulated in T1, T2 & T3 at the end of fermentation. When the circulation process was over (no water dropped from the fermentation device bottom), we took the inner part of fermentation device out and froze them. The substrate profile was obtained by sawing off the substrate. Samples for measuring residue-lithium-Ion and Extracellular Polymeric Substances (EPS) were taken from profile substrate regularly. The distribution characters of leachate in substrates were expressed by the contour map obtained by SUFER 8.0 based on residue-Lithium-Ion and EPS data. At the same time, the daily biogas production, methane content, layer wet bulk density before and after digestion were analyzed. The results showed that there were no significant differences among total biogas production, accumulation of TS biogas production, and methane content in T1, T2 & T3 treatments. At the end of fermentation, the layer wet bulk density of T1, T2 and T3 was high at the top layer and decreased generally. The wet bulk density of T3 decreased in substrate depth before reaching a constant of 0.6 g/cm3, but it was larger than the initial substrate wet bulk density of 0.58 g/cm3. From the Li+-residue contour map, the area of residue Li+ content below 2 mg/kg in T1, T2 and T3 profile took up 2.32%, 12.69% and 20.66% of total profile area respectively, indicating that the “leachate dead zone” existed in three treatments. The conclusions drawn from profile EPS contour map were the same as these conducted from Li+-residual contour map, they both showed leachate recirculation was not homogeneous substrates high in straw solid content anaerobic digestion. These results provided scientific evidences on improvement of leachate recirculation technologies in high straw solid anaerobic digestion.