Effects of dairy manure addition from methane production under semi-solid state condition

Abstract: Anaerobic digestion (AD) technology has been given great attention recently for its important role in converting crops residues into methane-rich biogas in China. The methane content in biogas is a carbon-neutral source of renewable energy and has contribute to the reduction of pollution . However, the mono-digestion of crops straw as feedstocks has the low methane yield because it has a complexity and compactness of lignocellulosic biomass structure and the high carbon to nitrogen(C/N) ratio. Several previous studies showed that anaerobic co-digestion had the high methane yield due to the synergistic effect by adding dairy manure (DM) or cattle manure (CM) to crops straw. The synergistic effect was mainly attribute to shorten digestion period, more balanced nutrients and increased buffering capacity. Nevertheless, these studies have focused on improving methane production of anaerobic co-digestion of crops straw with livestock manures based on operating the low total solids (TS) content namely liquid AD (L-AD) with TS concentration of feedstocks less than 10%. Compared to L-AD, the semi-solid AD(S-AD) is ideal for agri-residues and manures with high solids organics, which has TS content between 10% and 20%, and has some advantages such as higher volumetric methane production, higher organic loading rate, lower biogas slurry production, minimized reactor volume, lower energy requirements for warming. There is no literature so far on the evaluation of AD stability and methane production of rape stalk (RS) and DM with different mix ratios in S-AD system. Therefore, in order to improve methane production and provide some reliable technical parameters to efficiently produce methane from rape stalk through AD, the bath anaerobic digestion tests were conducted at semi-solid state(15% total solids content ) using RS and DM at six addition amount (0, 20%, 40%,60%, 80% and 100%) of DM(based on mass ratio of volatile solid),effects of addition amount of DM on the methane production performance of RS were studied, and then the dynamic characteristics of producing methane process were analyzed by the modified Gompertz equation. The results showed that increasing addition amount of DM to RS would balance C/N of digestion feedstock and improve buffering capacity, methane production rate of mixing feedstock was increased with increasing addition amount of DM and pure DM was against to methane production. The highest special methane production rate(SMPR) and methane volumetric production rate(MVPR) of 239.87mL/g and 1.01L/(L·d) were obtained at 80% addition amount of DM and C/N ratio of 33.60, which was 1.2-2.0 times for SMPR and 1.3-3.6 times for MVPR higher than other treatments, respectively. The analysis results of modified Gopermpertz equation indicated that the model had feasibility to describe the dynamic process of AD. Some dynamic parameters were achieved at 80% addition amount of DM and the maximum methane production rate(Rm), the lag phase(λ) and the shortest technical digestion time(T80) was separately 31.19 mL/(d·g), 1.21d and 8.59d. These simulated parameters were very close to the experimental values and had guiding significance in the scale of biogas production from co-digestion with RS and DM.