Assessment of Multiple Anaerobic Co-Digestions and Related Microbial Community of Molasses with Rice-Alcohol Wastewater

Abstract

Molasses is a highly dense and refined byproduct produced in the sugarcane industry, and it contains high amounts of degradable compounds. Through bioconversion, these compounds can be transformed into renewable products. However, the involved biological process is negatively influenced by the high chemical oxygen demand (COD) of molasses and ion concentration. The co-digestion of molasses with rice-alcohol wastewater (RAW) was compared with its mono-digestion at an increasing organic loading rate (OLR). Both processes were assessed by detecting the COD removal rate, the methane contents of biogas, and the structure and composition of microbial communities at different stages. Results showed that the co-digestion is stable up to a maximum OLR of 16 g COD L−1 d−1, whereas after the acclimatization phase, the mono-digestion process was disturbed two times, which occurred at a maximum OLR of 9 and 10 g COD L−1 d−1. The volatile fatty acids (VFAs) observed were 2059.66 mg/L and 1896.9 mg/L, which in mono-digestion causes the inhibition at maximum OLRs. In the co-digestion process, the concomitant COD removal rates and methane content recorded was 90.72 ± 0.63% 64.47% ± 0.59% correspondingly. While in the mono-digestion process, high COD removal rate and methane contents observed were 89.29 ± 0.094% and 61.37 ± 1.06% respectively. From the analysis of microbial communities, it has been observed that both the bacterial and archaeal communities respond differently at unlike stages. However, in both processes, Propionibacteriaceae was the most abundant family in the bacterial communities, whereas Methanosaetaceae was abundant in the archaeal communities. From the current study, it has been concluded that that rice-alcohol wastewater could be a good co-substrate for the anaerobic digestion of molasses in terms of COD removal rate and methane contents production, that could integrate molasses into progressive biogas production with high OLR.

Multiple Effects of Different Nickel Concentrations on the Stability of Anaerobic Digestion of Molasses

“…The main purpose of adding rice alcohol wastewater with molasses was to stabilize the process. In addition, Khan et al (2020) confirmed that the RAW could be a good co-substrate for stable anaerobic digestion of molasses [22]. As a result, the COD removal rate at day 45 increased in control was (86.74 ± 0.41%), R1 (87.65%), R2 (87.65%), R3 (87.77%), and R4 (87.71%) (Figure 1).…”

“…The inoculum sludge was taken from the last experiment conducted on anaerobic co-digestion of molasses and rice alcohol wastewater [22]. Molasses was taken from the Guangxi Guitang (Group) Co., Ltd. (Guigang, China).…”

“…This decreasing trend in alkalinity could be due to the suppression of methanogens in the sludge that cannot consume the reactors' accumulated volatile fatty acids (VFAs). This could also be due to molasses' mono-digestion, which can suppress methanogenic activity [22]. Li et al (2014) suggested that alkalinity could be an adequate early warning for a stable digestion process [38].…”

“…Molasses-based environmental problems could be significantly reduced by anaerobic technology, considered a source of high-value by-products such as biomethane, hydrogen, and organic acids obtained through anaerobic digestion [21,22]. Moreover, anaerobic digestion of molasses is difficult due to high COD, melanoidin contents, and high ion concentrations.…”

“…Moreover, anaerobic digestion of molasses is difficult due to high COD, melanoidin contents, and high ion concentrations. For example, high potassium ions in molasses has shown adverse effects on microbial communities and leads to carboxylate accumulation during anaerobic digestion [22]. Oleszkiewicz and Sharma (1990) reported that methanogen activity in anaerobic digestion could be increased or decreased by the concentration of nickel and other trace elements [14].…”