Enhanced Anaerobic Fermentation Of Waste Activated Sludge Through Cation Adjustment

Recently,increasing amount of waste activated sludge(WAS)was produced in wastewater treatment plants(WWTPs),while the requirement for WAS resource treatment and disposal have been raised.Anaerobic fermentation is one of the promising technologies for WAS resource treatment,by which the orgamic matters in WAS could be transferred to short-chain fatty acids(SCFAs)and could be recovered as high-quanlity carbon source.This study focused on the anaerobic fermentation of WAS,which built the the strategy for facilitating WAS solubilization and SCFAs production in anaerobic fermentation process based on cation regulation.In this study,the promotion of cation-exchange resin(CER)and Na⁺regulation to WAS hydrolysis and SCFAs production in anaerobic fermentation process were explored.Besides,the responding mechanism of microbial community was revealed,and the feasibility of CER reuse and high salinity waste as alternative Na⁺source were analyzed.The facilitated sludge solubilization by CER-mediated removal of multivalent cations was proposed to improve WAS hydrolysis efficiency.The process parameters(CER dosage,treatment period and stirring strength)were optimized using response surface methodology(RSM),while the patterns of sludge solubilization and SCFAs production in anaerobic fermentation process at different CER dosages were explored through batch anaerobic fermentation experiments.The research results indicated that CER significantly removed multivalent cations(e.g.Ca²⁺,Mg²⁺,Fe³⁺,etc.)from sludge liquid and extracellular polymeric substance(EPS),which induced microbial cell lysis and EPS disruption.The WAS flocs were deflocculted to loose and small particles with dedreased particle size,numerous organic matters(e.g.proteins,carbohydrates,etc.)and nutrients(i.e.nitrogen and phosphorus)were released,indicating prominent sludge solubilization performance.At the optimal CER dosage(1.75 g/g SS),the optimum sludge solubilization performance was obtained after 2-day treatment,i.e.the SCOD was 5944 mg/L,the soluble NH₄⁺-N and total phosphorus(TP)were 158 and 248 mg/L,respectively.Sludge solubilization improved the SCFAs accumulation(mainly acetic and propionic acids),the maximum SCFAs production of 334.5 mg COD/g VSS was achieved after 4-day anaerobic fermentation,which was 3.5 times higher than that in the control.Furthermore,the operation mode was optimized since the CER-induced sludge solubilization was highly efficient in the first day,i.e.the CER treatment period was shortened to 1 day,then the CER was recovered and the sludge itself was proceeded in anaerobic fermented.In the optimized operation mode of CER-enhanced anaerobic fermentation,significant sludge solubilization and SCFAs production could be achieved.The SCOD release and SCFAs production were 81.9-82.2%of those of the conventional operation mode(sludge was treated by CER during the whole fermentation period),while the sludge reduction of 5.08 g VSS/L(40.9%of total VSS)could be acieved.After anaerobic fermentation process,the used CER could be easily separated from fermented sludge for CER regeneration.Significant performances of CER cleaning and regeneration after anaerobic fermentation were achieved by HCl and Na Cl solutions,which could elute organic pollutant and cations.However,Na Cl solution has better elution performance for low molecular weight organic matters.The regenerated CER by Na Cl solution could be reused in anaerobic fermentation process,which has similar performances of SCOD release and SCFAs production compared with new CER.The effects of Na⁺content on sludge solubilization,hydrolysis,acidification and methanogenesis were further explored,then the enhanced anaerobic fermentation by Na⁺regulation was proposed.It was found that Na⁺regulation facilitated EPS disruption and microbial cell lysis through the osmotic pressure and multivalent cation release,leading to destroyed structure of sludge flocs,decreased particle size and organic matter release(e.g.proteins,carbohydrates,etc.).Moreover,proper Na Cl concentration(10-20 g/L)could promote hydrolysis and acidification of dissolved organic matters(DOMs),while the methanogenesis was simultaneously inhibited.As such,the SCFAs accumulation efficiency could be significantly improved.At the optimal Na Cl concentration(20 g/L),efficient sludge solubilization could be achieved in the first 2 days,i.e.the SCOD could reach2724.9 mg/L.The SCFAs production reached 288.2 mg COD/g VSS within the optimal anaerobic fermentation period(4 days),while 56.68%of the produced SCFAs were composed by acetic and propionic acids.Numerous NH₄⁺-N(553.3 mg?L)and TP(308.4 mg/L)were observed in the fermentative liquid.In order to further improve the performances of sludge hydrolysis and SCFAs production,the coupled approach of CER hydrolysis and Na⁺regulation for enhancing anaerobic fermentation was explored.Apparently,Na⁺regulation has significant secondary hydrolysis performance for CER-solubilized sludge.At the optimal coupled process parameters(CER dosage 1.75 g/g SS,treatment period 1 day,then the CER was separated and the sludge was treated by Na⁺regulation treatment at the Na Cl concentration of 20 g/L),SCOD increased to 6588 mg/L after 2-day treatment(40.6%of TCOD),SCFAs production of 432.8 mg COD/g VSS was achieved after 4-day anaerobic fermentation.The organic matter composition distribution in fermentative liquid was:SCFAs(75.7%)>proteins(14.5%)>other organic matters(4.0%)?humic acid(3.6%)>carbohydrates(2.2%).The sludge reduction of 6.41 g VSS/L could be achiveved.Numerous Na⁺was presented in RO brine and CER regeneration wate liquid,which could be used as alternative Na⁺source for facilitating WAS solubilization and SCFAs production in anaerobic fermentation.After 4-day anaerobic fermentation,the SCOD and SCFAs production were 4615-5203 mg/L and 27936-284.1 mg COD/g VSS,respectively.More than 96%of the organic matters in fermentative liquid was composed by proteins,carbohydrates and SCFAs.On the basis of the above research,the microbial community dynamic during the cation regulation-enhacned anaerobic fermentation process of WAS was explored.The microbial community structure and dominant bacteria distribution related to CER-mediated multivalent cation removal and Na⁺regulation were revealed using grey relational analysis(GRA),principal component analysis(PCA)and redundancy analysis(RDA).The research results indicated that Na⁺regulation treatment increased protease relative activity(126-160%),which was beneficial for DOMs hydrolysis.CER-mediated multivalent cation removal and Na⁺regulation(Na Cl concentration of 20 g/L)both inhibited survival and growth of some microbes,which decreased species abundance and bio-diversity of microbial community.This changed microbial community composition distribution and promoted dominant bacteria evolution.The lack of multivalent cations(e.g.Ca²⁺,Mg²⁺,etc.)and Na⁺salinity were the major inducements for microbial community evolution,which have positive correlations with fermentative bacterial responsible for DOMs hydrolysis and SCFAs production(e.g.Macellibacteroides,Bacteroides,Proteiniclasticum,etc.)and have negative correlations with SCFAs-consuming bacteria(e.g.Simplicispira,Ottowia,Ideonella,etc.).In this process,the SCFAs-producing bacteria were enriched and the SCFAs-consuming bacteria were inhibited with decreased abundances,which promoted the microbial community evolution in favor of sludge hydrolysis and SCFAs accumulation in anaerobic fermentation.