Effect of Humic Acids with Different Characteristics on Fermentative Short-Chain Fatty Acids Production from Waste Activated Sludge

Abstract

Recently, the use of waste activated sludge to bioproduce short-chain fatty acids (SCFA) has attracted much attention as the sludge-derived SCFA can be used as a preferred carbon source to drive biological nutrient removal or biopolymer (polyhydroxyalkanoates) synthesis. Although large number of humic acid (HA) has been reported in sludge, the influence of HA on SCFA production has never been documented. This study investigated the effects on sludge-derived SCFA production of two commercially available humic acids (referred to as SHHA and SAHA purchased respectively from Shanghai Reagent Company and Sigma-Aldrich) that differ in chemical structure, hydrophobicity, surfactant properties, and degree of aromaticity. It was found that SHHA remarkably enhanced SCFA production (1.7-3.5 folds), while SAHA had no obvious effect. Mechanisms study revealed that all four steps (solubilization, hydrolysis, acidification, and methanogenesis) involved in sludge fermentation were unaffected by SAHA. However, SHHA remarkably improved the solubilization of sludge protein and carbohydrate and the activity of hydrolysis enzymes (protease and α-glucosidase) owing to its greater hydrophobicity and protection of enzyme activity. SHHA also enhanced the acidification step by accelerating the bioreactions of glyceradehyde-3P → d-glycerate 1,3-diphosphate, and pyruvate → acetyl-CoA due to its abundant quinone groups which served as electron acceptor. Further investigation showed that SHHA negatively influenced the activity of acetoclastic methanogens for its competition for electrons and inhibition on the reaction of acetyl-CoA → 5-methyl-THMPT, which caused less SCFA being consumed. All these observations were in correspondence with SHHA significantly enhancing the production of sludge derived SCFA.

A New Approach to Assess the Effect of Various Humic Compounds on the Metabolic Activity of Cells Participating in Methanogenesis

“…The observed inhibition was explained by the adaptation capacity of the microbial community to the elevated HC concentrations. Additionally, it was found that granular sludge using H 2 , formate, or acetate as substrates provided the maximum observed reduction in the total amount of the produced CH 4 (24%) at 5 g/L HC [28]. In our work, the decrease in CH 4 content as a part of biogas was more pronounced for some variants of the anaerobic consortia used (Figure 1).…”

“…A decrease in the portion of methane in biogas was observed due to an increase in the amount of accumulated CO 2 . A decrease in the accumulation of CH 4 was also noted in another study on the effect of HCs on methanogenesis with anaerobic sludge when using cellulose or xylan as a substrate [28]. It was shown that an increase in the concentration of HCs to 8 g/L inhibited the efficiency of hydrolysis of the substrates by 40% with a simultaneous decrease in the CH 4 yield.…”

“…It was shown that an increase in the concentration of HCs to 8 g/L inhibited the efficiency of hydrolysis of the substrates by 40% with a simultaneous decrease in the CH 4 yield. Average daily biogas production and the methane content obtained with the addition of HCs was 2680 ± 10 mL, which was significantly lower than that in the case without HCs (3995 ± 362 mL) [28]. The observed inhibition was explained by the adaptation capacity of the microbial community to the elevated HC concentrations.…”

“…The difference in the effect of the natural HCs on the microbial consortia studied is probably due to the differences in the composition of these HCs. Recently, researchers have noticed that aromatic and aliphatic groups similar to those found in humic acids are found in the molecules of FA, but the aromatic part of FA molecules is less pronounced [28]. Since aliphatic groups are carriers of hydrophilic properties (as opposed to hydrophobic aromatic groups), FA, with a wider ratio of aliphatic groups to aromatic groups, are different from humic acids and more hydrophilic.…”

Effect of DEHP on SCFA Production by Anaerobic Fermentation of Waste Activated Sludge

“…Anaerobic fermentation of WAS can produce energy biogas methane and achieve sludge reduction while minimizing the number of pathogenic microorganisms in the sludge [35][36][37][38]. A large number of microorganisms, such as acetogens, hydrogenogens, and methanogens, are linked to the anaerobic fermentation process of WAS [1,39,40]. Although anaerobic fermentation is an effective means of dealing with WAS, the existence of a large amount of DEHP in the WAS may affect the microbial community in the WAS anaerobic fermentation system.…”