Mechanism And Efficiency Of Nitrogen And Phosphorus Recover From Wastewater Based On Bipolar Membrane Electrodialysis Coupled Magnesium-air Cell System

Nitrogen and phosphorus are the main pollutions of surface water in China,and the discharge of them often causes“bloom”and“red tide”and other pollution events,causing great harm to the environment,economy and human health.In addition,with the global economic development and population increase,the demand for nitrogen and phosphorus is increasing.However,rock phosphate is a kind of non-renewable resource with limited reserves and uneven distribution in the world.It is expected that the rock phosphate resource will be exhausted in 100 years.Nitrogen resource is abundant,however,the overuse and loss of nitrogen fertilizer in agriculture has caused a serious imbalance in the global nitrogen cycle.Struvite(Mg NH₄PO₄·6H₂O,MAP)crystallization is considered to be an effective method for the recovery of nitrogen and phosphorus,which can simultaneously recover ammonia nitrogen(NH₄⁺)and phosphate(PO₄^3-)from wastewater,and also MAP is a good agricultural slow-release fertilizer.However,the recovery of NH₄⁺and PO₄^3-from different wastewater as MAP still has some challenges,including the addition of alkali(OH^-)and magnesium(Mg²⁺)sources,the serious interference of Ca²⁺,and also which is not suitable for wastewater with low concentration of NH₄⁺and PO₄^3-.In this study,a magnesium-air cell(MAC)was used to in-situ produce OH^-and Mg²⁺.This provides theoretical and technical basis for the recovery of NH₄⁺and PO₄^3-from different wastewater.A bipolar membrane electrodialysis(BMED)was used to separate and enrich NH₄⁺and PO₄^3-,and remove Ca²⁺and other metal ions that are easily co-precipitated with MAP.The main research results are as follows:(1)Research on the performance of nitrogen and phosphorus recovery from wastewater based on MAC system and analysis of its influencing factors.In MAC system,NH₄⁺and PO₄^3-were recovered as struvite.The effects of initial solution p H,temperature and NH₄⁺/PO₄^3-molar ratio,initial concentration,stirring intensity and Ca²⁺concentration on NH₄⁺and PO₄^3-removal were investigated.The main influencing factors on NH₄⁺and PO₄^3-removal were analyzed using a linear regression method.The physical and chemical properties of the crystallization products were characterized,and the electrical performance of MAC system was analyzed.The results showed that initial solution p H,NH₄⁺/PO₄^3-molar ratio,initial concentration and Ca²⁺concentration were the four main influencing factors.Without addition of alkali and Mg²⁺source,the removal rates of NH₄⁺and PO₄^3-were 54.9%and 99.9%,respectively,after 6h,and the purity of MAP was 93.8%.The presence of Ca²⁺ion will greatly reduce the purity of MAP.The output power and energy density of MAC were 3.5?5.5 m W and1.46?2.29 W/m²,respectively.The results also showed that the system has a low removal capacity for wastewater with low concentration of NH₄⁺and PO₄^3-.(2)Separation and enrichment performance of BMED system for nitrogen and phosphorus.In terms of separation and enrichment of NH₄⁺and PO₄^3-in wastewater,the effects of current density,chemical oxygen demand(COD)and other parameters on the separation and enrichment of NH₄⁺and PO₄^3-were studied.The separation and enrichment efficiency and migration rate of NH₄⁺and PO₄^3-were measured.The energy consumption and current efficiency of separation and enrichment of NH₄⁺and PO₄^3-were analyzed.Finally,the kinetic process of separation of NH₄⁺and PO₄^3-was investigated.The results showed that the separation efficiencies of NH₄⁺and PO₄^3-were 100%and88.3%,respectively,under a low current density of 1 m A/cm² for 140 min.Compared with one module equipped BMED system,the three-module equipped BMED system can reduce the energy consumption from 3.56 to 2.74 k W·h/m³.It can be seen that the energy consumption of separation and enrichment of NH₄⁺and PO₄^3-effectively reduced and current efficiency can be improved in multi-module equipped BMED system.(3)Study on the mechanism of nitrogen and phosphorus recover from wastewater based on BMED-MAC coupling system.The coupling system of BMED and MAC was established to study the influence of current density,initial concentration,and hydraulic residence time(HRT)on the separation and enrichment of NH₄⁺and PO₄^3-in BMED,and the separation rate was calculated.The removal performance for Ca²⁺from wastewater and the adaptability to low concentration of NH₄⁺and PO₄^3-wastewater were analyzed.The results showed that MAP purity was related to current density of BMED,initial NH₄⁺and PO₄^3-concentrations and HRT.MAP with a purity of 86.2%was obtained from low concentration of NH₄⁺(2 mmol/L)and PO₄^3-(1 mmol/L)wastewater by intermittent start-up of MAC.The MAP purity was 89.1%when the concentration of NH₄⁺,PO₄^3-and Ca²⁺in wastewater were 10,5 and 2 mmol/L,respectively,indicating that the coupling system has a good anti-Ca²⁺interference performance.(4)Nitrogen and phosphorus recovery from anaerobic fermentation supernatant of domestic sludge based on MAC system and BMED-MAC coupling system.The recovery of NH₄⁺and PO₄^3-from anaerobic fermentation supernatant of domestic sludge was studied using MAC system and coupled systems of MAC and BMED,respectively.The removal rates of NH₄⁺and PO₄^3-were investigated.The physicochemical properties of the precipitations,the purity of struvite and the electricity generation of the two systems were analyzed.The results showed that the MAC system could remove 42.5%NH₄⁺and 97.0%PO₄^3-from the sludge supernatant after 8 h without additional alkaline and Mg²⁺sources,the MAP with purity of 95.7%,output power of2.53±0.25 m W and energy density of 1.05±0.11 W/m² were obtained,respectively.The total removal rates of NH₄⁺and PO₄^3-in BMED-MAC coupling system were 93.5%and67.1%,respectively.The MAP purity of the precipitation was 90.7%,and the output power and energy density were 5.08±2.18 m W and 2.12±0.91 W/m²,respectively.