Volatile fatty acids and methane production from protein

Abstract

Many industrial wastes and wastewaters, e.g. dairy, beverage, slaughterhouse wastewater, and food processing waste streams contain appreciable quantities of protein. Anaerobic treatment is appropriate for wastewaters and wastes rich in organics to achieve pollution control and resource recovery, e.g. energy rich methane or important chemical platforms such as volatile fatty acids (VFA) or added-valuable amino acids. However, the understanding of the hydrolysis, especially of proteins, is still limited, resulting in non-optimized anaerobic systems dealing with these protein rich wastewaters. Therefore, this thesis aimed to investigate anaerobic conversion of proteins, focussing on the hydrolysis process to accomplish sufficient protein degradation for VFA and methane production. Batch and completely-stirred tank reactor (CSTR) experiments were carried out to study the effect of different environmental and microbial factors on hydrolysis and further degradation of gelatine, a model protein, at mesophilic conditions. Results show that in contrast to earlier suggestions in literature, carbohydrates did not directly affect the protein hydrolysis rates either under methanogenic or non-methanogenic conditions at neutral pH. However, the high VFA concentrations strongly inhibited the protein hydrolysis rate in the batch experiments. Methanogenesis did not stimulate the rate of hydrolysis and acidification of protein at pH 7. Yet, protein hydrolysis was inhibited at pH 5. The hydrolysis rate constant for protein at pH 5 (0.05 L g -1 VSS day -1 ) was much lower than at pH 7 (0.62 L g -1 VSS day -1 ). Even long-term exposure (480 days) of the microbial population to pH 5 did not result in an enhanced hydrolysis of dissolved protein. Hydrolysis always is the rate-limiting step of protein degradation at pH 5 between a solid retention time (SRT) of 12 and 30 days. At pH 7, protein degradation was limited by hydrolysis at SRTs >8 days or acidogenesis at SRTs ≤ 8 days. The pH also determined the VFA product spectra.Different mathematical models for kinetics of hydrolysis were tested for the steady states of the CSTR pH 7 during 600 days. The modelling results indicated that the high concentrations of amino acids may inhibit hydrolysis of proteins. The findings in this study give direction how to solve problems associated with insufficient protein degradation and how to design and operate anaerobic treatment processes for protein rich wastewaters. For methane production, the reactor design should be based on methanogenesis being the slowest process if the wastewater is mainly comprised of dissolved proteins. Regarding the VFA production, the optimum volumetric VFA productivity was 2.3 g CODVFA L -1 day -1 at SRT 10 days and pH 7. Recommendations for improving the VFA productivity include design of anaerobic granular-based reactors to attain high biomass concentrations, operation at an optimum pH between pH 5 and pH 7 to avoid methanogenesis and active separation of VFA to avoid the inhibitory effect of the VFA on the protein hydr...