Development Of A Novel Lightweight Polyurethane-Alum Sludge Substrate And Its Application In Constructed Wetlands And Ecological Floating Beds For Wastewater Treatment

Substrate is the main component of constructed wetlands(CWs).It is responsible for purifying pollutants in wastewater,supporting plant growth,providing a place for microbial metabolism,and creating proper hydraulic conductivity.As a by-product of water purification process in drinking water treatment plant,alum sludge contains a large amount of element Al,which has strong adsorption affinity for phosphorus(P).However,the density of alum sludge is relatively great,which caused the difficulty of transportation and use as main substrate in CWs.In order to solve these problems,this study aims to combine polyurethane and alum sludge to produce a novel lightweight substrate called polyurethane-alum sludge(PU-Al),which is characterized and used in constructed wetlands and ecological floating beds(EFBs)for wastewater treatment.The response surface method was used to optimize the synthesis ratio of PU-Al lightweight substrate.The results showed that when the particle size of alum sludge is 60-100mesh,the mass ratio of white material to black material is 1.4(white material 35 g;black material 25 g).In making the PU-Al,250 g alum sludge,3.5 g water,0.1 g silicone oil(L580),0.5g triethylenediamine(A33),and 0.35 g stannous octoate(T9)are quickly stirred,which cause to form the foamed mixture.Then,the mixture is placed in an oven(at a constant temperature of 45℃)for 30 min,a new PU-Al lightweight substrate with good appearance and dense pores can be obtained.In examining the PU-Al,the FT-IR,BET and SEM tests show that the main chemical groups are not much different from the raw alum sludge.Compared with the raw alum sludge,PU-Al surface area is reduced to 3.4960±0.0239 m~2/g(from BET test),while the density is reduced to 0.463 g/cm~3.Through adsorption kinetic experiments and model fitting,the P adsorption by PU-Al belongs to chemical adsorption.The Langmuir equation fits the saturated P adsorption capacity of 1.599 mg/g,which is lower than 2.011 mg/g of the raw alum sludge.A vertical flow single-stage CW experiment with PU-Al,gravel,alum sludge and polyurethane sponge as the substrate was constructed,respectively(water inlet index:COD=300 mg/L,TP=10 mg/L,TN=25 mg/L;hydraulic retention time:48h),the operating results of 185 days continuous flow trial show that increased the height of the siphon port and connecting two different wetland systems in series can significantly increase the removal effect of the wetland system on COD and TN,while increased the height of the siphon port can increase the COD removal rate by about 5%and the TN removal rate by about 9%.The two wetland systems in series can also increase the removal rates of COD and TN by 5%and 12%,respectively.EFBs experiment with PU-Al and polyurethane sponge as the substrate and the traditional plant floating bed as the reference(water inlet index:COD=60 mg/L,TP=2 mg/L,TN=10 mg/L;hydraulic Retention time:48 h;150 days of operation)shows that the addition of PU-Al lightweight substrate can significantly improve the removal effect of the floating bed on water pollutants,and the highest removal rates of COD,TN and TP by the PU-Al floating bed were 71.48%,57.63%and 51.24%,respectively,which are much higher than 36.6%,34.51%and 17.2%,respectively,of plant floating beds.In terms of plant growth,scindapsus aureus grew well in both CWs and EFBs.Its roots can penetrate into the interior of the PU-Al lightweight substrate.The PU-Al lightweight substrate supports and protects plants.More significantly,the PU-Al lightweight substrate did not show disintegration,fragmentation etc.after soaking in wastewater,while a large amount of microorganisms adhered to its surface,which significantly improved the diversity of the microorganisms in the systems,and increased the ability to remove pollutants in water bodies.Overall,PU-Al lightweight substrate has the characteristics of high porosity,light weight,and good P adsorption performance,which is conducive to the adhesion and growth of microorganisms,and can effectively remove COD,N and P.It is an excellent wetland substrate which has good practical application value.