Alkaline Fermentation Of Excess Sludge With Calcium Hydroxide To Achieve Efficient Extraction Of Short-chain Fatty Acids By LDHs

The traditional biological denitrification method is considered to be the most cost-effective,resultful and frequently-used nitrogen removal method.In this process,a sufficient and effective organic carbon source is vital importance.In the actual project,in order to ensure the complete denitrification smoothly,COD/TN is generally required to be 4-15.However,with the improvement of residents’living standards,getting lower and lower of the COD/TN in urban sewage,the organic carbon content in the sewage is difficult to meet the demand of denitrifying bacteria for carbon source,and the biological denitrification process is inhibited.To improve biological nitrogen removal efficiency with the low carbon ratio of wastewater,the development of carbon source with a low cost and high efficiency is decisive,which can not only improve the efficiency of biological denitrification,but also enhance using efficiency of carbon source.The fermentation liquid from excess sludge contains short chain fatty acid（SCFAs）of high concentration.These organic acids can be directly used as organic carbon sources in the biological denitrification process of wastewater.However,fermentation liquid is rich in high concentration of ammonia nitrogen,heavy metals,etc.Direct dosing fermentation liquid as a denitrification carbon source may lead to an increase of nitrogen load in reactor,secondary pollution of effluent,and low utilization efficiency of carbon source.In order to effective use SCFAs of fermentation liquid,in the early stage of the research group,SCFAs were used as target anions,SCFAs-LDH was successfully synthesized in a simulation system and successfully used as a solid controlled release carbon source of denitrification.The actual liquid system contains a large amount of carbonate and phosphate,which will affect the extraction efficiency of LDHs on SCFAs.Combined with the previous experimental results of the research group,for increasing the extraction rate of SCFAs,using Ca（OH）₂ as a base-altering agent for the alkaline fermentation of excess sludge expect to improve SCFAs concentrations,and simultaneously to remove the carbonate and phosphate from the fermentation liquid.The effects of Ca（OH）₂ and NaOH hydrolysis and acid production,CO₃^2-and PO₄^3-removal effects,and extraction efficiency of SCFAs in fermentation liquid were compared by different alkali fermentations.In addition,in order to improve the extraction efficiency of SCFAs in Ca（OH）₂ fermentation liquid,the effects of different methods and different conditions on the extraction of SCFAs were compared.The main findings and discussions are as follows:（1）Using Ca（OH）₂ and NaOH as alkali-altering agents of alkali anaerobic fermentation for excess sludge,the dissolution of SCOD and SCFAs under alkaline anaerobic conditions at pH 10 were studied.The dissolution of SCOD was 3.4 and4.5 times that of control,respectively.The concentrations of total SCFAs from Ca（OH）₂ and NaOH conditions were significantly higher than the control group.The main constituents both were acetic acid,which accounted for 66.1%and 65.8%of the total acid,respectively.（2）The release of CO₃^2-and PO₄^3-under different alkalis anaerobic conditions at pH 10 was studied by alkaline anaerobic fermentation of excess sludge.The results showed that the Ca（OH）₂ fermentation group had a low PO₄^3-release amount,the maximum was 7.56 mg P/L,the NaOH fermentation group maximum PO₄^3-release amount was 250.8 mg P/L.By calculation,the amount of CO₃^2-release in the Ca（OH）₂ fermentation group could was negligible,and the sludge residue from Ca（OH）₂ fermentation mainly was CaCO₃ precipitation,and NaOH was dominated by SiO₂ crystal.This shows that,using Ca（OH）₂ as a alkali-altering agents,large amounts of CO₃^2-and PO₄^3-can be removed.（3）The SCFAs from Ca（OH）₂ and NaOH fermentation liquid were extracted by using LDH,and the extraction effects of SCFAs were compared.The results showed that only Ca（OH）₂ fermentation liquid successfully synthesized CaAl-LDH,and the extraction amount and extraction rate of SCFAs were the highest,respectively up to17.38 mg COD/g and 4.1%.（4）The extraction efficiency of SCFAs from the actual Ca（OH）₂ alkaline anaerobic fermentation liquid were studied by twice co-precipitation methods.The COD concentration of SCFAs-LDH1 was 157.8 mg/g,the extraction rate of SCFAs and COD respectively were 0.5%and 3.5%.In the second extraction,SCFAs-LDH2obtained had lower COD concentration than SCFAs-LDH1,which was only 44.5mg/g,and the extraction rate was 4.1%,and the extraction rate of SCFAs was 2.5%.The extraction rate of total SCFAs by twice co-precipitation methods was 3.0%,and the total extraction rate of COD was 7.6%.（5）By ion exchange method to extract SCFAs from Ca（OH）₂ fermentation liquid,the extraction effects at different temperatures and pH were studied.The results showed that the carbon loading of LDHs increased with the increase of temperature,and at the temperature of 80°C,the COD and the carbon loading both reached the maximum value.The LDHs from ion exchange at different pH,have lower loading of carbon and COD concentration,which is much lower than that of LDHs without pH adjustment.And the intercalation rate of SCFAs decreased with the increase of pH,and compared to not changing the pH,the intercalation rate is reduced by half.