Effects Of Sludge Biochar On Volatile Fatty Acid Production From Sludge Anaerobic Fermentation

In this paper,waste activited sludge(WAS),taken from the secondary sedimentation tank of the wastewater treatment plant,was used as the base material to prepare the biochar.The biochar was prepared by two methods: hydrothermal carbonization method and pyrolysis carbonization method,and the products were named as hydrochar and pyrochar,respectively.The effects of preparation methods,preparation temperatures,and biochar dosage on the production of short-chain fatty acids(SCFAs)from WAS were evaulated,and further research was carried out to the biochar which tremendously promoted the production of SCFAs.The main findings were as follows:(1)When the ratio of the hydrochar to the volatile solid(VS)in sludge was0.1:1,there was almost no effect on the production of SCFAs.When the ratio reached 1:1,the hydrochar could promote the production of SCFAs,and could effectively change the proportion of the acid.The proportion of acetic acid and propionic acid in the whole fermentation process was above 60%.And the hydrochar prepared at 220 °C can obviously promote the production of SCFAs.The maximum yields of SCFAs was 4595.02 mg COD/L,which is 5.8 times that of the control group(784.89 mg COD/L).Besides,hydrochar could significantly enhance the dissolution of protein and SCOD,and the amount was about 4.75 and 2.8 times that of the control group respectively.The hydrochar prepared at 260°C was slightly inferior to the hydrochar prepared at 220°C.In the experiment added hydrochar,the consumption trend of SCFAs was slower,because hydrochar inhibited the activity of methanogens and reduced the total acid consumption.(2)Directly pyrolyzed biochar had no significant effect on the production of SCFAs.When the ratio of pyrochar to VS was 0.1:1,only pyrochar prepared at300°C could influence the production of SCFAs(789.57 mg COD/L),which was only 1.2 times that of the control group.But in the experimental group added with pyrochar prepared under 450°C and 600°C,the maximum yield of SCFAs was not much different from that of the control group.Meanwhile the pyrochar could not change the proportion of acid.In the whole fermentation process,the proportion of acetic acid and propionic acid was about 30%,and isovaleric acid always was dominant,which was not conducive to the subsequent methanogenesis process.Moreover,pyrochar could not significantly promote the dissolution of organic matterin the sludge,and the dissolution of protein,polysaccharide,and SCOD were similar to those of the control group.(3)By analyzing the elemental composition of the hydrochar prepared at the two temperatures,it was found that the elemental composition of the two hydrochars was not much different,which was not the main factor to promote the production of SCFAs.Based on the three-dimensional fluorescence spectrum analysis,it was confirmed that the content of humic acid in two kinds of hydrochar had significant difference.The effect of humic acid and fulvic acid presented in hydrochar on the SCFAs production was further investigated,and the results showed that the effect of humic acid was more obvious.The maximum yield of SCFAs was 130.95 mg COD/L,which was 4.6 times that of the control group(28.71 mg COD/L).The maximum production of SCFAs induced by fulvic acid was only 31.88 mg COD/L,which was1.11 times that of the control group.Experimental results have shown that humic acid can promote sludge acid production,mainly through the transmission of electrons,and methanogens compete for electrons to inhibit methanogenesis,thereby reducing the comsuption of short-chain fatty acids.