Mixed ensiling quality of maize straw with waste cabbage and biogas production potential analysis

Abstract: Growing energy consumption and diminishing fossil fuel supplies have encouraged increased research on renewable energy sources and development of new energy production processes. Biogas production from agricultural biomass is of growing importance as it offers considerable environmental benefits and alleviates the environmental pollution. Maize straw (MS), a substantial source of lignocellulosic biomass is the prevailing material producing biogas for large and medium sized biogas plants because of abundant cellulose and hemicellulose. In the majority of cases, the MS collected after grain harvest are available once per year in China, a strongly collection seasonal character entails preservation and storage to be fed for biogas plant continuously. Hence low-loss preservation of MS is essential for economical and sustainable use of biogas crops for anaerobic digestion throughout the year. Generally, the MS were wilted and dried to yellow stalks and collected after corn ears harvested in autumn. As a result, the free sugars are consumed, the water evaporates during wilting and field drying, giving rise to fail to achieve the demand of silage. On the other hand, a large sum of vegetable wastes, which contain a mass of water and sugars, often give rise to environment pollution, such as leaf vegetable. This study put forth assume of trans-seasonal mixed storage of MS and cabbage waste (CW) by silage theory on the basis of strong complementary in terms of physical structure, nutrient content and moisture content. The objective of the current work was to explore its feasibility for mixed silage of dried MS and CW and evaluate the effects of different mixed ratio (29:19, 27:21, 25:23, 23:25, 21:27 and 19:29) of MS and CW on silage quality, including four aspects of sensory quality, chemical composition, fermentation quality and microbial numbers. Moreover, the appropriate storage condition was selected by the dynamic analysis of mixed silage quality. The results indicate that the pH value and the ammonia nitrogen (AN) content of six mixed silages was significantly lower than sole silage of MS (P<0.05). The lactic acid content was significantly higher than sole silage of MS (P <0.05), which indicated the mixed silage quality was better than that of single silage. The pH value and the lactic acid content of six mixed silages reached its minimum level when the storage period was 30 days, which indicated good fermentation quality at the time of 30 d. With the storage period time prolonged, the pH value increased and the lactic acid content decreased, which resulted in lower silage quality. As the proportion of cabbage wastes increased in mixed silage, the moisture content increased gradually, the pH value decreased gradually, the lactic acid content increased gradually. When the mixed mass proportions was 21:27, that is the moisture content of 73%, the dried MS and CW can preserved for 60 d continuously to maintain good silage quality. In other words, the higher proportion of CW was beneficial to preserve the digestible components such as cellulose and hemicelluloses. The results of scanning electron microscope (SEM) indicated that the original dense complex lignocellulose structure was disrupted during ensiling, which lead to a lot of cracks and holes generated on the straw silage surface. These changes of microstructure increased the reaction area of microbes or enzymes. The results of anaerobic digestion showed that the accumulative biogas production of maize straw silages was slightly higher than that of dried maize straw, moreover, the accumulative methane production of maize straw silages was increased 61.67% compared with dried maize straw. Modified Gompertz model could fit well the change of the accumulative biogas or methane production with time, its kinetic parameters such as maximum biogas or methane yield, maximum biogas or methane production rate and constant of reaction rate can be regarded as the important evaluation index for metabolic process of methane production. In conclusion, this research found a suitable method to preserve dried maize straw and reduce pollution of waste vegetable in rural areas. More importantly, the methane production potential after storage was increased, which was important for storage and energy conversion of maize straw in large-scale biogas projects.