Effects And Mechanisms Of Residual Al-coagulant In Reclaimed Wastewater On Bioclogging And Biofilm Formation

Coagulation process is used widely in wastewater reclaimation.Approximately 80%of wastewater purification coagulants are based on aluminum.Most studies demonstrated that coagulation is a feasible process to remove common contaminants effectively,such as SS and COD.However,there is limitation of knowledge on the principle of removing soluble microbial products.Morever,the harm of the residual Al（Ⅲ）in the reclaimed water has not been recognised.This work discussed on the soluble microbial products removing efficiency and concentration of residual Al when reclaimed the secondary effluent using coagulation.Additionally,this work reveals the mechanism of residual Al（Ⅲ）on biofilm formation and aquifer clogging acceleration.We identified the characteristics of the EfOM from a typical WWTP and investigated the performance of coagulation on suspended solids（SS）and dissolved organic matter（DOM）removal.The effluent could even satisfy the highest national standard of China（Class 1 A）for WWTP effluent,as evaluated by the traditional parameters such as SS and chemical oxygen demand（COD）.However,the DOM in the EfOM we studied contained considerable biomass-associated products（BAPs）,which were dominated by proteins with a molecular weight of approximately 150 kDa.In addition,protein also dominated the DOM after coagulation.Fulvic acid and humic-like acid organics were poorly removed by either AlCl₃ or polyaluminum chloride（PAC）coagulation,even with a dosage as high as 24 mg Al L^-1.Biodegradability was very poor,as the ratio of biological oxygen demand（BOD₅）to COD was less than 0.17.After coagulation the typical BAPs,protein and polysaccharide,remained as high as1.6 mg L^-1 and 1.2 mg L^-1 respectively.In this study we found coagulation was ineffective for removal of SS and recalcitrant BAPs simultaneously.In addition,the residual Al after coagulaltion could attain to 0.54-2.12 mg/L,which depended on different coagulant dosages.Recharging reclaimed water is an efficient strategy to simultaneously address the issues of water quality deterioration and groundwater levels declining.Residual Al-coagulants in the recovered water may induce serious clogging problems due to the hydrolysis characterization of Al（Ⅲ）,however,this has been ignored in the past.In this work,we elaborated the mechanisms of Al（Ⅲ）induced aquifer biological clogging,and the roles of Al（Ⅲ）in Si dissolving and precipitation in the simulated aquifer columns.We found Al（Ⅲ）,in concentration of as low as national drinking water standard,could result in serious clogging in¹40 hours without disinfection.The hydraulic conductivity decreased by 40%,96%and 97%in the bacteria attached aquifer when Al（Ⅲ）concentration was 0.05,0.2 and 0.6 mg/L respectively,which were 30%,27%and 2%greater than the corresponding bacteria-free protocols,respectively.Such Al（Ⅲ）related clogging was caused by modifying quartz sand to form Si-O-Al（OH）n,so that biofilm attachment was improved.Al（Ⅲ）also enhanced biomass production and retaining by binding with microbial extracellular polymeric substances.Biomass in the aquifer with bacteria attached was 21%,23%and 80%greater for Al（Ⅲ）concentrations of 0.05,0.2 and 0.6 mg/L than those of Al-free protocols,respectively.In turn,the corresponding retained Al（Ⅲ）increased by 29.1%,45.2%,91.8%due to microbial products-Al binding.The enhanced biomass production and retaining,increased Si dissolving and precipitation which potentially damaged the aquifer structure.Above results showed that the reclaimed wastewater,treated by chemically advanced treatment of Al-coagulation,should be employed with caution for recharging.Residual Al（Ⅲ）（at low concentration）is common in wastewater treatment plants（WWTPs）and is associated with bacteria.We hypothesize that Al（Ⅲ）accelerate biofouling due to its hydrolysis and hormesis characteristics,as compared with other cations.To verify this,we elaborated the roles of Al（Ⅲ）at low concentrations on the biofilm formation.Al（Ⅲ）flocculation resulted in the suspended cells precipitation instantly,for Al（Ⅲ）dosages of 0.6 and 2.0 mg/L and the concentration of Al（Ⅲ）decreased by 0.07 and 0.14 mg/L,respectively.Al（Ⅲ）poisoned the bridged bacterial cells and decreased their ATP by 22.36%and 55.91%,respectively.Al（Ⅲ）formed polymer presented strong affinity with bacterial outer membrane,and this damaged the bacterial outer membrane.This caused proteins to leak at the combined point.Al-polymer bound to-NH₂ and/or-NH-on the leaked protein,contributed to biofilm formation.Biofilm maturity was aided by polysaccharides,which shielded Al（Ⅲ）toxicity for the formed biofilm.Thus,the biofilm exhibited a distinguished double-layer microstructure,principally with proteins and inactivated cells at the bottom,polysaccharides and activated cells at the top.Thus,hormesis and flocculation caused by low concentration Al（Ⅲ）mutually promoted each other,and together accelerated biofilm formation.This work found the limitation of Al salts coagulation on the removing of soluble microbial products and the low concentration of residual Al exist in reclaimed water.This work reveals the mechanism of residual Al on biofilm fromation and aquifer clogging,that provides theoretical foundation.