Development And Modification Of Alum Sludge Sponge And Its Application In Combined Floating Treatment Wetland

Substrate plays an important role in the construction of the floating treatment wetland(FTWs)and can effectively improve their purification effect on polluted water bodies.In addition,the substrate within the FTWs also plays an important role in supporting the growth of plants and providing a place for microorganisms’attachment and growth in the water.As a by-product of the water purification process,alum sludge from water treatment plants has a special material structure and chemical composition that makes it a good adsorption and removal agent for phosphorus.In recent years,there have been numerous studies on the use of alum sludge as a wastewater treatment material,but the efficiency of its use is greatly limited by its inherent problems of high density,high water content and the tendency to cause blockage of water treatment facilities.Based on the above problems,this study combines alum sludge with the process of synthesizing polyurethane flexible foam by the prepolymer method for the first time to prepare a lightweight water treatment material with low density,high water absorption,good phosphorus absorption and loose and porous:alum sludge sponge(ALSP),the material was also modified with lanthanum by the impregnation method,which greatly improved the phosphorus adsorption capacity of the material.The modified materials were also used in the construction of combined floating treatment wetland and their effect on the purification capacity improvement of the FTWs was verified.The effects of alum sludge particle size,water-prepolymer ratio(mass ratio of water to prepolymer)and solid-liquid ratio(mass ratio of alum sludge to water)on the density,water absorption and phosphorus absorption capacity of the ALSP were investigated by single-factor experiments.The optimum parameter of alum sludge particle size(in terms of sieve mesh)250mesh,water-prepolymer ratio 2.156 and solid-liquid ratio 1 was then obtained using the response surface method to obtain alum sludge sponge with a density of 0.132 g/cm~3,water absorption of 4.589 g/g and equilibrium phosphorus adsorption of 0.812 mg/g.The results of the characterization of the apparent morphology,surface composition and chemical composition of the alum sludge sponge show that the ALSP has a loose and porous structure,and its elemental composition and chemical groups have not changed significantly compared to the original alum sludge.The results of the lanthanum modification experiments and the characterization of the modified materials show that the lanthanum can be successfully loaded onto the alum sludge particle and sponge by impregnation,thus confirming the feasibility of the modification method.The analysis of the adsorption kinetics of alum sludge particle,alum sludge sponge,lanthanum modified alum sludge particle and lanthanum modified alum sludge sponges and the results show that the adsorption of phosphorus is dominated by chemisorption for all four materials.The results of the Langmuir equation fits of the materials yielded the theoretical saturation capacity of phosphorus adsorption for each material:3.189 mg/g,1.776mg/g,5.869 mg/g and 6.481 mg/g respectively.In this study,two sets of combined floating treatment wetlands were constructed using alum sludge sponges and lanthanum-modified alum sludge sponges as substrates,respectively.A comparison of the purification effects with plant FTW on simulated micro-polluted water showed that under two stages of continuous operation with influent specifications of 60-80mg/L COD,9-11 mg/L TN,2 mg/L TP and a hydraulic retention time of 72 h for 120 days.The average removal rates of COD,TN and TP for the alum sludge sponge FTW were 74.66±2.1%,55.66±1.6%and 56.26±3.9%(Phase I)and 77.64±4.6%,59.17±1.8%and 46.04±3.4%(Phase II).The average removal rates of lanthanum modified alum sludge sponge FTW for COD,TN and TP were 73.98±3.7%,56.06±1.5%and 84.91±3.8%(Phase I);78.43±3.7%,58.96±1.7%and 76.47±2.1%(Phase II).This was much higher than the 28.91±2.8%,16.62±1.5%and6.45±1.7%(Phase I);32.71±3.2%,19.77±1.8%and 5.83±2.2%(Phase II)for the plant FTW.In terms of plant growth,the aquatic plants grew well on the two combined FTWs,the plant roots could easily pierce the substrate and continue to grow,with substrates providing some support and protection for plant growth.In addition,a large number of water purification related microorganisms could be detected on the surface of the substrate at different locations within the combined FTWs.This is due to the loose and porous structure of the ALSP,which provides a large number of sites for microorganisms to attach and grow,improving the purification capacity of the FTWs for micro-polluted water and demonstrating the biosafety of the lanthanum modified material.In conclusion,alum sludge sponge has the characteristics of low density,outstanding water absorption and phosphorus absorption abilities,its loose and porous structure is also conducive to microorganisms’growth and reproduction,is a kind of polluted water treatment material with excellent performance and huge application potential.