| Polyscrylamide(PAM),polymerized from acrylamide monomer,is a water-soluble linear polymer,which can be dissolved in water in various percentages.It is one of the most widely used polymer compounds,and enjoys the name of "all kinds of auxiliary agents".PAM is widely used in oil extraction,textile,sugar production,medicine,food,cosmetics and other fields,which will inevitably result in the release of PAM,and flows into sewage.As the treatment process "concentrates" various pollutants in wastewater,PAM will inevitably accumulate in waste activated sludge(WAS).Especially,as the flocculant widely applied in wastewater pretreatment,sludge thickening and dehydration,PAM's content in WAS is high(especially the sludge from mechanical dehydration).It is reported that the content of PAM in WAS is frequently up to 5-10 g/kg TS.With the increase requirements of sludge dehydration by transportation,treatment and disposal,PAM's content in WAS may continue to increase.Anaerobic digestion,one biological technology that can realize the stabilization,harmless,reduction and resource recovery of organic matter,has been widely used in the WAS treatment.Anaerobic digestion of WAS is driven by a series of biochemical processes or functional microorganisms,and the presence of PAM is likely to have a certain impact on the microbial activities.However,to date,the effect of PAM on sludge anaerobic digestion has not been systematically revealed.Firstly,this paper investigated the effect of different levels of PAM(i.e.,0,3,6,12,and 24 g/kg TSS)on the methane yield and the typical intermediate metabolites(e.g.,protein,carbohydrate,short-chain fatty acids,ammonia)concentrations from WAS during anaerobic digestion through short-term batch tests,with the kinetic model being employed to reveal the effect of PAM on the kinetic parameters of anaerobic digestion.Based on the results of batch tests,the long-term operation of semi-continuous reactors were performed to investigate the effect of PAM on the treatment efficiency,concentrations of typical intermediate metabolites,as well as the stability of reactors.Results showed that PAM inhibited the anaerobic digestion of WAS,and the degree of inhibition became more significant with the increase of PAM dose,but had no significant effect on the proportion of methane in the digestive gas.The presence of PAM not only decreased the maximal biochemical methane potential of WAS,but also slowed the maximum methane-producing rate,and increased the delay period of digestion.it was found that the 50% inhibition dose of WAS anaerobic digestion was 164 mg/L,i.e.,8 g/kg TSS.The presence of PAM decreased the concentration of organic substrates or intermediate metabolites such as protein and short-chain fatty acids(SCFAs)in the anaerobic methane-producing system,and reduced the stabilization of WAS(i.e.,VSS reduction).The variations of physical and chemical properties of WAS(e.g.,particle size distribution,Zeta potential,extracellular polymer substance,surface morphology,etc.)after contacting with PAM were investigated simultaneously,as well as the effect of PAM on the sludge solubilization,hydrolysis,acidogenesis,and methanogenesis processes.Results showed that PAM flocculated the WAS through charge neutralization and adsorption,interparticle bridging,which made the sludge flocs be bigger,thereby increased the mass transfer resistance of anaerobic digestion and prevented the relase of organics involved in LB-EPS(loosely-bound extracellular polymer substance)and/or TB-EPS(tightly-bound extracellular polymer substance)of WAS to the liquid phase,resulting in the decrease of soluble organics concentrations and VSS reduction,and additionally,the degree of inhibition became more significant with the increase of PAM dose.The presence of PAM significantly restrained the hydrolysis,acidogenesis,and methanogenesis processes,and its inhibition to protein degradation was significantly higher than that of p carbohydrate,to methanogenesis process was significantly higher than that of hydrolysis and acidogenesis processes,the reason were that the PAM had stronger ability to adhere to protein molecules surface,and the methanogens was sensitive to the external environment and was also the most vulnerable to the effect of poisonous and harmful substances in the wastewater or sludge.Based on the above research,the degradation of PAM in the anaerobic digestion of WAS was investigated and the influence of PAM's key degradation products on the typical processes of anaerobic digestion was analyzed.Results showed that PAM was degraded in the anaerobic digestion of WAS,but compared with the major components of WAS,protein and polysaccharide,it was more difficult to be degraded and converted into methane by anaerobic microorganisms.After the semi-continuous reactor with WAS containing 12 g PAM/kg TSS as the substrate being operated steadily,the PAM concentration in the reactor was 158.8 ± 5.6mg /L,and the acrylamide,acrylic acid and polyacrylic acid concentrations were 14.9 ± 2.1,22.1 ± 1.6 and 32.3 ± 3.2mg /L,respectively.The three degradation products accounted for ~50% of the total degradation amount of PAM.Under the concentrations,PAM and polyacrylic acid significantly inhibited the organic matter solubilization,hydrolysis,acidogenesis and methanogenesis processes in the anaerobic digestion of WAS,while the corresponding concentrations of acrylic acid and acrylamide only inhibited the methanogenesis process to some extent.The inhibition constants of PAM and its degradation products on the methane-producing process of sodium acetate degradation were lower than that of organic matter solubilization,hydrolysis and acidogenesis.Under the same concentration,the toxicity of PAM to sodium acetate degradation and methane-producing process was lower than that of its degradation products.The effect of PAM on microbial characteristics of the ananerobic digester was clarified through the analysis of microbial key enzyme activity and community composition,it was found that the relative activities of protease,acetate kinase,and F420,which are the key enzymes responsible for hydrolysis,acidogenesis,and methanogenesis processes,were inhibited to some degree,and this degree improved with the increase of PAM dose.The presence of PAM reduced the species and diversity of bacteria-archaea community in reactors.The presence of PAM changed the community structure of bacteria and archaea,reduced the relative abundance of bacteria involved in the hydrolysis and acidogenesis of WAS,and induced the generation of bacterial species that might have the ability to degrade PAM.The presence of PAM induces the archaea community to transfer from the methane-producing pathway of reduced carbon dioxide to the methane-producing pathway of methyl nutrition and reduced carbon dioxide or the methane-producing pathway of acetic acid nutrition.Finally,the feasibility of microwave,heat-alkali and alkaline hydrolysis pretreatment to regulate the PAM's inhibitory effect on anaerobic digestion and methane yield of WAS was investigated.Results showed that all of the microwave,heat-alkali and alkaline hydrolysis pretreatment improved the anaerobic digestion of PAM-flocculated sludge,and the optimal conditions were respectively 800 W for 12 minutes for microwave treatment,17.5 h at 75.5 °C + pH 11.0 for heat-alkali treatment,and 12 d at pH 10.0 for alkaline hydrolysis treatment.These regulations not only effectively broke PAM-sludge floccules,weakened the effect of PAM on anaerobic digestion,but also destroyed the WAS itself,promoted the solubilization,hydrolysis and acidogenesis of macromolecular organics,improved the biochemical methane potential and the hydrolysis rate of PAM-flocculated sludge.Microwave pretreatment required the highest energy consumption,alkaline hydrolysis pretreatment showed the best performances on digestion enhancement,and heat-alkali pretreatment had the most significant effect on shortening anaerobic digestion time.This study comprehensively investigated the effect behavior of PAM in the process of WAS anaerobic digestion,mechanism and microbial characteristics,and explored the feasibility of pretreatment technologies to regulate the PAM's inhibitory effect on anaerobic digestion,filling a fundamental gap between PAM and sludge anaerobic digestion,providing some necessary theoretical basis references for the control of WAS anaerobic digestion in the future. |
PDF Full Text Request