Methane biogas production is rather slow [89], and large digesters are often required to produce enough biogas to be recovered cost-effectively as energy. AD processes are also susceptible to a host of factors, which can render AD suboptimal or some­times lead to unpredictable upset or total failure of AD [17]. The re-startup and recovery process after failure are often slow [17, 64, 84]. These limitations and other barriers can severely undermine the economic viability of AD processes and make many industries and farmers reluctant to implement this biotechnology. These issues and the research required to improve AD for bioenergy production are briefly discussed below.