Pre-acidification of a Synthetic Dairy Wastewater in Continuously Stirred Tank Reactors

Abstract: Two-stage anaerobic digestion is considered as a sustainable and efficient way to deal with dairy effluent, and is commonly applied. The pre-acidification happening during the first stage of this process could be used to buffer the complex conditions in the initial dairy effluent, as well as produce intermediary product such as volatile fatty acids (VFA), and it is essential to be controlled. Various types of reactors have been used studying this process, among which, continuously stirred tank reactor (CSTR) is common with its special benefits. During the pre-acidification process, the adjustment of hydraulic retention time (HRT) and pH is crucial to affect the performance of reactors. In order to compare the performance under different operation conditions, CSTRs were used to treat a synthetic dairy effluent in this work. This experiment was divided into two periods. One is under the pH of 4.9, another is under the pH of 4.0. The first condition was operated for 55 days. Then the pH in both reactors was adjusted at 4.0 for another 22 days. The volatile fatty acids (VFA) as the major fermentation product was analyzed. The results shows that at the pH of 4.9, the acidification degree in the CSTR with the HRT of 18h reached 80%, while the one with lower HRT could only reach 18%. Besides, results also indicate that HRT also has impact on the distribution of acids. As for the influence of pH, the results show that after the pH was adjusted down to 4.0, the total amount of VFA production decreased significantly especially for the one with the HRT of 18h (the acidification degree dropped down from 80% to 15%). Moreover, the decreased pH had a distinct impact on the VFA distribution. Before reducing the pH butyric acid was the most generated acid, followed by propionic acid and acetic acid. After the pH dropped, the mostly produced VFA was acetic acid, and at the same time, the proportion of butyric acid dropped significantly. The fermentation product consists not only of VFA which was mainly analyzed in this work, but also of other compounds such as formic acid and lactic acid. However, other fermentation compounds were not specifically analyzed in this work.