An integrated strategy for sequential nitrite removal and methane recovery: Sludge fermentation driven by nitrite reduction

Abstract

Although the treatment of sludge with free nitrous acid can effectively recover short chain fatty acids, the feasibility of sequential nitrite reduction and methane recovery without acidic pH adjustment is still scarcely studied. Therefore, this study aimed to provide insights into the effect of nitrite at different levels on nitrite reduction and methane production. The results showed that the nitrite concentrations of 100, 200, 400 and 800 mg/L were completely reduced in 1, 2, 2 and 4 days, respectively. The nitrite reduction process stimulated the fermentation of sludge to produce more organic matters, which served as electron donors for denitrification. With the nitrite concentrations increasing from 200 to 800 mg/L methane production decreased from 128.7 to 0 mg/L at the digestion time of 15 d. The toxicity of nitrite to methanogenic microorganisms and the nitrite reduction process competing with methanogens for carbon sources may lead to the inhibition of methane production by excessive nitrite. Moreover, the methane production reached 184.4 mL with 100 mg/L nitrite reduction, which was increased by 83.2 % compared with that without nitrite addition (101.1 mL). Nitrite reduction stimulated hydrolysis without negatively impacting acetogenesis, thereby providing more substrates for subsequent methanogenesis. Model-based analysis indicated that nitrite reduction enhanced the maximum methane yield and methane production rate, aligning with the aforementioned analysis. 16S rRNA analysis unraveled that the bacterial abundance associated with hydrolysis increased. This anaerobic digestion technique driven by nitrite reduction is both environmentally and economically attractive for increasing methane production.