Investigating the hydrolysis step of staged anaerobic digestion and the possibility of using rumen microorganisms to improve hydrolysis

The treatment of wastewater is essential for protecting the environment and public health. However, wastewater treatment produces a large amount of sludge that also poses a threat if not properly handled. Therefore, sludge treatment is carried out to reduce sludge volume and ensure a well-stabilized final product. This tends to be a long process usually achieved by anaerobic digestion. Thus sludge conditioning and pre-treatment methods are being applied to help speed-up and improve the anaerobic digestion process. These applications involve either physical, chemical, or biological means aimed at enhancing the hydrolysis of organic matter for increased biodegradability. While physical and chemical pre-treatments are effective, they require energy and resources that increase treatment costs. On the other hand, biological pre-treatments rely on the natural biological processes of microorganisms to improve hydrolysis, making it an attractive option. Two such pre-treatments are phased anaerobic digestion and bioaugmentation. Phased anaerobic digestion involves separating the hydrolysis and acidogenesis phases of anaerobic digestion into their own pre-treatment stage ahead of the main digester. Bioaugmentation involves pre-treating the sludge with hydrolytic microorganisms or their extracted enzymes. In this study, the retention time of a pre-treatment stage was lowered from 5 days to 1 day to evaluate the effects it had on the phased anaerobic digestion of mixed sewage sludge. This was carried out by operating a laboratory-scale two-stage anaerobic digester with a pre-treatment stage (acidogenic stage) initially set at 5 days retention time and a methanogenic stage maintained at 15 days retention time. Once stable conditions were observed, the acidogenic stage retention time was reduced to 3 days. This resulted in improved hydrolysis, leading to a greater acidogenic stage methane production (239 mLCH4/gVSdestroyed versus 109 mLCH4/gVSdestroyed at 5 days), a higher extent of COD solubilisation (14% versus 11% at 5 days), and increased total volatile solids reduction (57% versus 52% at 5 days). A change in the volatile fatty acids distribution was also witnessed, shifting from mostly propionic acid to a much closer acetic acid to propionic acid ratio. But when the system was operated at a 1 day acidogenic stage retention time, hydrolysis decreased. Solubilisation reduced to 7% and the acidogenic stage methane production dropped to 79 mLCH4/gVSdestroyed. However, acidogenesis improved showing increased VFA concentrations, particularly acetic acid, and a better VFA/SCOD ratio. Therefore, lowering the pre-treatment stage (acidogenic stage) retention time to 3 days was the best option for improved hydrolysis and performance whereas operating the system at a 1 day retention time would only be advisable if volatile fatty acids recovery was the main purpose. Also in this study was the investigation into whether rumen fluid as a biological pre-treatment on mixed sewage sludge would be feasible. This was done by conducting batch pre-treatment tests involving different volumes of rumen fluid inoculum combined with the same volume of mixed sewage sludge substrate and incubated for 24 hours at 35°C on a rotary shaker. Unfortunately, the results were not conclusive. The extent of solubilisation was approximately the same as the 1 day pre-treatment stage of the phased anaerobic digester while the VFA/SCOD ratio was worse. Therefore, the tests were not evident in proving feasibility and more tests need to be conducted