Effects of enzyme addition on biogas and methane yields in the batch anaerobic digestion of agricultural waste (silage, straw, and animal manure)

The anaerobic digestion (AD) of the organic fractions of solid wastes has been recognized as an economical and highly renewable resource for biogas production. The challenges lie in maximizing the biodegradation of lignocellulose-rich biomasses, which requires effective strategies to prevent the underutilization of feedstocks. Enzymatic treatments for lignocellulosic degradation have huge potential because of their highly versatile and selective compounds. Moreover, such treatments mimic natural processes and require low energy inputs. This study focuses on the batch AD of a variety of lignocellulosic substrates typically used in agriculture, such as silage, straw, and the manure of various animals. The effects of five different enzyme mixtures on the increase in the gas yields of batch AD were investigated. Biogas and methane yields were systematically obtained, and then the effects on total maximum gas yields and biodegradability were determined on the basis of organic dry matter. In almost all the assays between test days 5 and 15, the biodegradability was accelerated, leading to increases of 0.3%-21.1% in methane yields as appropriate to the substrate. However, after 60 days of testing, increases (-2.7%-9.4%) in the total maximum gas yields were not detectable in most cases. It was assumed that the main effects of the enzyme treatments would be faster degradability. Furthermore, the test series with hay and different enzyme dosages showed that a higher dosage does not necessarily result in a higher effect.