Efficiency And Mechanism Of Anaerobic Release And Crystallization Recovery Of Phosphorus From Excess Activated Sludge

A large amount of phosphorus?P?can be recovered from the sludge in wastewater treatment plants,which is considered as as an rich potential res ource for phosphorus recovery.The anaerobic phosphorus release technology presents a quite high performance for phosphorus release and hence has been attracted most of attentions recently.However,there are still some important research gaps existed on this technology,including:how to increase the fraction of available phosphorus;phosphorus release mechanism between intracellular polyphosphate bacteria and extracellular polymeric substace;identification of main impact factors on phosphorus absorption/release in extracellular polymeric substances?EPS?phase.Moreover,how to reduce the cost of the phosphorus recovery technology and how to reduce the usage of traditional chemical reagent?such as ammonium chloride as nitrogen source?are essential for an efficient and ecomomic technology of anaerobic phosphorus release technology.Therefore,the phosphorus release mechanism of different phosphorus fractions in excess activated sludge,the effect of EPS on phosphorus release performance,the mechanism of acid/alkali pretreatment and phosphorus recovery by struvite crystallization during anaerobic process were reported in this study.In this study,phosphorus fractions in the sewage sludge was investigated using Standards in Measurements and Testing?SMT?protocol.The results showed that inorganic phosphorus?IP_s?was the main component of phosphorus in the sludge,which accounteds for 87.82%of total phosphorus.Non-apatite inorganic phosphorus?NAIP_s?accounted for 77.50%of inorganic phosphorus,while apatite phosphorus?AP_s?was the most stable form.Organic phosphorus?OP_s?was obtained by sludge disintegration or microbial metabolism.Phosphorus in the EPS phase mainly existed as the form of ortho-P,pyro-P and polyphosphate?poly-P?by the ³¹P-nuclear magnetic resonance(³¹P-NMR)experiments.Most of short-chain polyphosphates were intercepted by EPS and remained in the EPS phase.While most of the long-chain poly-P were decomposed into short-chain poly-P and phosphate during the anaerobic phosphorus release process.The efficiency of anaerobic phosphorus releasing from sewage sludge under different temperature conditions was studied by batch experiments so as to identify the optimal temperature condition.The relationship between the metabolism of polyphosphate accumulating organisms?PAOs?and phosphorus release/accumulation in EPS was revealed.The results showed that anaerobic phosphorus release depends on both phosphorus-accumulating organisms and extracellular polymeric substances.Temperature not only affected the metabolism of PAOs,but also significantly influenced the EPS components and the hydrol ysis of EPS-associated poly-P.The highest concentration of total P in the supernatant?TP_L?occurred at 15^oC,and the TP_L release from the solid to liquid phase was well fitted into a pseudo-second-order kinetic model.More OP_s in the sludge released from the sludge phase at 35^oC and converted into IP_s.The non-apatite inorganic phosphorus was the most labile P fraction for P release.The hydrolysis of EPS-associated poly-P was enhanced by higher temperatures due to the degradation of the long-chain poly-P by PAOs.Meanwhile,a lower temperature could obviously improve the performance of P release because the dominance of PAOs would be likely to be shifted to GAOs along with the temperature increase.But the very-low temperature?5^oC?suppressed the microbial activities and therefore was not beneficial for the P release.Phosphorus in EPS and transported by EPS were closely related to the PAOs activity and the intracellular P dynamic metabolism played a decisive role for phosphorus release.In addition,to obtain phosphate-rich supernatant,anaerobic P-release treatment of sewage sludge enhanced by acid/alkali pretreatment was investigated.The influence of acid/alkali pretreatment on the performance of anaerobic phosphorus release was explored,and the changes of sludge floc structure and microbial community were investigated by molecular biological technology.The results showed that the acid/alkali pretreatments enhanced the structures of both EPS and microbial cells,and anaerobic P release was dependent on the phosphorus fractions and pH values.Acid pretreatment lead to the dissolution release of inorganic phosphorus?NAIP_s and AP_s?,while the release of NAIP_s and the conversion of organic phosphorus to inorganic phosphorus lead to the enhancement of phosphorus release by alkali pretreatment.The form of phosphorus in the EPS phase was organic phosphorus.Both acidic and alkaline treatments strengthened the crack of sludge floc structure with the richness and diversity of the microbial community.The alkali pretreatment was better for the enhancement of the community richness than the acid pretreatment.The highest community richness was achieved at pH 10.0.The relative abundances of Proteobacteria and Firmicutes were reduced by the acid pretreatment,while the predominant phylum changed from Proteobacteria to Chloroflexi and Firmicutes was due to the alkali pretreatment.The dewaterability of sewage sludge was improved by acid pretreatment?pH=5.0?and the capillary suction time was linearly related to the ratio of protein/polysaccaride in the LB-EPS phase.The acid pretreatment+ultrasonic treatment with low specific energy showed a stronger dewaterability.The shortest capillary suction time was 13.3 s when the treatment condition was pH=5.0 and SE=800 kJ/kg TS.Furthermore,phosphorus recovery by anaerobic phosphorus supernatant and mixed human urine was studied and the optimal phosphorus recovery condition was determined based on the orthogonal tests.The feasibility and further prospects are assessed based on acombined A/O treatment plant with the struvite recovery process.This study for the first time significantly improved the phosphorus recovery efficiency in the form of struvite using the mixture of two kinds of phosphorus-rich wastewater?i.e.,anaerobic sludge supernatant and human urine?.An optimal volumetric ratio(VR_s/f)of 1:9 of fresh and 10-d stale human urine could yield phosphorus recovery of 90.76%at[Mg²⁺]:[NH₄⁺-N]:[PO₄^3--P]=1.2:1.05:1,pH 10,and reacting time 15 min.The results of SEM and XRD analyses showed that the white struvite crystal was precipitated,and the characteristic peaks in the precipitates were consistent with the standard peaks of struvite.Struvite precipitation was determined by relative supersaturation in both nucleation and crystal growth phases,with homogenous nucleation predominated in the latter phase.Under the optimal conditions,the concentrations of each ion in the anaerobic pshosphorus release solution and the mixed human urine were high and close,thus the struvite crystallization showed a obvious potentiality.One liter of mixed urine could recover the PO₄^3--P content in 41.9 liters of anaerobic sludge supernatant to form 182.3 g of struvite and the purity of struvie was 83.12%.The results in this study could provide technical support of phosphorus recovery from sewage sludge and meanwhile the simulation of A/O treatment process in the wastewater treatment plant confirmed the struvite recovery by anaerobic phosphorus supernatant and mixed human urine had the promising prospect of engineering application.