Study On Purification Effect Of Zeolite Substrate Coated With LDHs Being Synthesized By Alum Sludge In Constructed Wetlands

Constructed wetlands were one of the best technologies for rural domestic sewage because of its low construction and operation cost,easy maintenance and high treatment efficiency.As a commonly used wetland filler in constructed wetlands,zeolite shows a better performance on ammonia nitrogen removal while its performance on phosphorus removal is poor.To improve the phosphorus removal capacity and reduce the cost of modification,the alum sludge in the water supply plant was obtained to prepare the modified zeolite coated with layered double hydroxide?LDHs?which was synthesized by acidified extraction liquid.The surface characteristics and chemical compositions of raw zeolites,Al-Zn modified zeolites and alum sludge modified zeolites were determined,while the isothermal adsorption and adsorption kinetics were analyzed.The performance and mechanism of phosphorus removal of modified alum sludge modified zeolites were discussed.The submerged constructed wetlands test system was set up with domestic sewage from a village in Xi'an to study the purification effect of alum sludge modified zeolite on rural domestic sewage.The removal effect of acidification extraction-LDHs modified zeolite substrate on COD?NH₄⁺-N?TP and PO₄^3--P was analyzed.The feasibility of acidification alum sludge-LDHs coated zeolite for constructed wetlands was discussed to provide a certain theoretical basis in enhanced phosphorus removal of constructed wetlands and in waste being controled by waste.The main conclusions were as follows.?1?As the object of alum sludge acidification extraction-LDHs coated modified zeolite,the effect of different acidification extraction conditions on alum salt recovery rate in alum sludge was determined.The results of acidification extraction-LDHs coated modified wetland zeolite substrate test showed that the optimum acidification conditions were 60 min,150 r·min^-1and pH 1.0.Under this condition,1 g of alum sludge?dry weight?could extract 77 mg of alum.The surface characteristics of the original zeolite and the modified zeolite?Al-Zn modified zeolite and alum sludge modified zeolite?were observed by scanning electron microscope and X-ray fluorescence spectrometer?XRF?was used to determine the changes of the main chemical components of the three.The results showed that the surface characteristics of the modified zeolite have changed significantly and the change of surface characteristics was related to the type of alum source.The LDHs were successfully coated on the surface of the original zeolite.The filming effect of the alum sludge used as alum source was more significant than the chemical agent as alum source.?2?The study on the phosphorus removal characteristics and phosphorus removal mechanism of zeolite before and after modification showed that the LDHs coated modified zeolite was synthesized by hydrothermal-co-precipitation method.The isothermal adsorption and desorption characteristics of the original zeolite and modified zeolite were analyzed to obtain that the LDHs coating can improve the adsorption performance and maximum adsorption capacity on zeolite,while desorption performance of zeolite on phosphate was also improved.Different types of LDHs coated modified zeolite had different adsorption properties.The alum sludge modified zeolite had better adsorption performance and higher recyclability,and its theoretical maximum adsorption capacity was increased from 30.24 mg·kg^-1 to 170.40 mg·kg^-1.Comparing the adsorption kinetics of phosphate of modified zeolite that use the alum sludge as alum source with chemical agent as alum source,the phosphorus removal mechanism of acidification extraction-LDHs coated modified zeolite was analyzed to obtain that the main adsorption type of acidification extraction-LDHs coating modified zeolite was changed from physical adsorption to chemical adsorption.The kinetics of phosphate adsorption in water before and after modification can be divided into three stages,namely rapid adsorption stage,slow adsorption stage and adsorption equilibrium stage.The adsorption rate of the slow adsorption phase was limited by membrane diffusion and intraparticle diffusion.The LDHs prepared by using alum sludge as the alum source was a multi-component LDHs material,and the membrane-modified zeolite has increased the adsorption active site of the zeolite and has enriched the pore structure of the zeolite,thus improving the adsorption performance of the alum sludge modified zeolite on phosphate.?3?The test results of sewage purification by subsurface flow constructed wetlands system show that the alum sludge modified zeolite substrate had the effect of improving by being compared with the original zeolite.The removal efficiency of COD?NH₄⁺-N?TP and PO₄^3--P was obviously improved.After adding alum sludge modified zeolite,the average removal rates of COD,NH₄⁺-N,TP and PO₄^3--P in the subsurface flow constructed wetlands system were 91.41%,98.84%,70.09%and 69.25%,respectively.The average concentration of effluent COD and NH₄⁺-N reached the Class A emission standard of Pollutant Discharge Standard for Urban Sewage Treatment Plant?GB18918-2002?.