Revealing the Underlying Mechanisms of How Sodium Chloride Affects Short-Chain Fatty Acid Production from the Cofermentation of Waste Activated Sludge and Food Waste

Recently, anaerobic cofermentation of waste activated sludge (WAS) and food waste for short chain fatty acid (SCFA) production has drawn growing attention. However, the details of how sodium chloride (NaCl) in food waste affects SCFA generation from the cofermentation remain largely unknown, which provides limited understanding of the cofermentation process. This work therefore aims to provide such support. Experimental results showed that the effect of NaCl on SCFA production was dosage dependent. With the increase of NaCl level from 0 to 8 g/L SCFA production increased from 367.6 to 638.5 mg chemical oxygen demand (COD)/g of volatile suspended solids (VSS). However, further increase of NaCl caused severe inhibition of SCFA production. The presence of NaCl not only accelerated the release of soluble substances from food waste and disruption of both extracellular polymers and cell envelopes in sludge but also promoted the conversion of protein released from the disintegration process, thereby causing more substrates for SCFA production. It was also found that low NaCl levels improved hydrolysis and acidification processes but inhibited methanogenesis while both acidification and methanogenesis processes were seriously inhibited by high NaCl levels. Further investigation with enzyme analysis showed that the activities of protease and α-glucosidase were in the order of high NaCl > low NaCl > blank while the activities of oxaloacetate transcarboxylase and CoA transferase were in the sequence of low NaCl > blank > high NaCl. However, the activity of coenzyme F420 decreased with increasing NaCl level. This work reveals the underlying mechanism of how NaCl affects SCFA production from the cofermentation process and might be of significance for the operation of the cofermentation system.