Factors influencing the rate and stability of the anaerobic digestive process : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology at Massey University
Three factors affecting the rate and stability of the methane fermentation of a readily-hydrolysable feedstock were investigated. The aim of this work was to develop improved processes and control strategies to facilitate economic treatment of industrial wastes by anaerobic digestion. A comparison was made between the performance of a continuously-fed digester and semi-continuous digesters slug fed every second day. A semi-synthetic medium with glucose as the major carbon and energy source was used and seed material was transferred between the digesters, which were operated under similar loading conditions. The continuous digester repeatedly failed even when operated at dilution and loading rates much lower than the maximum values commonly reported. In contrast, the semi-continuous units provided satisfactory performance and could be easily and rapidly recovered from retarded operation. Failure of the continuous digesters was characterised by a steady fall in volatile suspended solids concentration followed by a rapid accumulation of acetate, and was attributed to a deficiency in the medium of one or more essential nutrients. These were thought to be provided in the semi-continuous digester by lysis of acidogenic bacteria or luxury uptake from the medium. Degradation of acetic and propionic acids was investigated in batch culture. Increasing the concentration of either acid from low levels decreased the rate of utilisation of the acid, but the proposed inhibitory role of un-ionised acids was not conclusively supported. Increasing the initial acetate concentration above 1000 to 1500 mg.l-1 significantly reduced the rate of degradation of propionate added at 500 mg.l-1. When acetate was added at 2000 mg.l-1 the rate of propionate utilisation was approximately half of that when acetate was present at 500 mg.l-1 or lower. In batch culture experiments, addition of up to 3.2 mM cysteine-hydrochloride or sodium sulphide, or 4.4. mM sodium thioglycollate did not inhibit total gas production from samples drawn from the continuous digester. However thy rate of methane production in effluent samples from a semi-continuous digester was inhibited by 25 % to 30 % by addition of 3.2 mM cysteine or sulphide. Inhibition was attributed to the sulphide ion. Sodium thioglycollate did not inhibit methane production from acetate but propionate degradation was markedly reduced, with increasing inhibition noted with increasing incubation time. The work adds to a considerable body of investigation into the factors influencing anaerobic digestion and the unresolved problem of process stability in long-term operation of conventional stirred tank digesters has again been highlighted. Indicators and possible causes of process failure have been suggested and further development of these should assist in the continuing increase in the rate of treatment while ensuring acceptable working margins of safety for the process.