Phosphorus Release During Alkaline Treatment And Subsequent Anaerobic Digestion Of Waste Activated Sludge From Wastewater Treatment Plants:Speciation And Mechanism Clarification

Phosphorus is an essential nutrient element for all living creatures.However,phosphate rocks used for phosphorus fertilizer is non-renewable.Thus,phosphorus release and recovery is fundamentals for a viable agriculture and society worldwide.Previous studies have demonstrated that the sludge as a novel absorbent have potential for phosphorus removal from wastewater.Chemical phosphorus removal(CPR)is one of the gradually implemented technique to increase phosphorus removal for perceiving with rigorous discharge limits in wastewater treatment plants,whereas comprehensive clarification into the phosphorus release and recovery is deficient.Thus,the main purpose of this work is to subsidize a mechanistic understanding of phosphorus release from waste activated sludge of wastewater treatment plants using A1 salt stimulated CPR through alkaline treatment.Waste activated sludge were collected from three WWTPs to study the effect of alkaline treatment on the dynamics of phosphorus distribution and fraction associated with the mobilization/immobilization of phosphorus,the metals dissolution and the sludge cell integrity as a function of pH being modified.The results revealed that the aqueous TP(TPaq)concentration in the sludge increased significantly with an increase of pH to 12(>200 mg/L),particularly with the ascendency of PO4-P at high pHs.Overall results showed a significant correlation between aqueous PO4-P and A1 concentration in the entire range of alkaline pH.The adjustment of pH 12-8 had an important explanation for the phosphorus mobilization reversibility:(i)the accountability of the dissolution of Al-bound phosphorus for the release of sludge-P,and(ii)the insignificant correlation between the sludge cell integrity and the release level of TPaq.Overall outcomes demonstrate that the concentration of released TP in this study had been amongst the highest as compared to previous studies.However,further well-organized and cost-effective efficient approaches need to be developed when the TP contents(TPmx>800 mg/L in this study)in waste activated sludge is considered,to stimulate the release of phosphorus from sludge for further beneficial phosphorus recovery.In this study,we also thoroughly analyzed phosphorus species of the sludge from three wastewater treatment plants by anaerobic digestion coupled with adjustment and readjustment of pH(8,10-8 and 12-8).Results showed that TPaq and PO4-Paq released concentration with all three pH kept increased during anaerobic digestion.The maximum PO4-Paq release was observed at day-15 of digestion.In this study,the proportions of apatite phosphorus(AP)increased and non-apatite inorganic phosphorus(NAIP)declined?7%at the end of anaerobic digestion.The standard measurement and testing protocol(SMT)demonstrated that the inorganic phosphorus(IP)and organic phosphorus(OP)transformed with another during the process of anaerobic digestion.The sole alkaline treatment dissolved most of NAIP and readjustment of pH(10-8,12-8)causes apatite phosphorus(AP)formation,which was negative to more and non-apatite inorganic phosphorus(NAIP)dissolution,demonstrating that an alkaline-acidic environment in the sludge mixture stimulated the conversion from NAIP to AP.The inorganic orthophosphate(ortho-P)content in the sludge is significantly increased from?69%to 87%.It indicates that the pretreatment has a promoting effect on the conversion of other forms of phosphorus to inorganic orthophosphate.Although the content of inorganic orthophosphate in the sludge with three-pH adjustment(pH8,10-8,12-8)increased in a quite similar trend.Consequently,pH adjustment is more conducive to the transformation of other phosphorus species to inorganic phosphorus.