Pollutants Removal Performance And Mechanisms Of Coagulation Sludge-based Granular Ecological Substrate

In recent years,many wastewater treatment plants have added coagulation-sedimentation as the advanced water treatment process,resulting in the generation of a large amount of wastewater（WPCS）,which becomes another source of coagulation sludge that has not been paid attention to.WPC S is rich in aluminum（Al）/ iron（Fe）hydrolysate,and more organic matter and pollutants are contained in WPCS than drinking water treatment sludge（DWTS）.Improper disposal not only pollutes the ecological environment,but also leads to the waste of chemical resources,like Al/Fe salts.In view of the porous structure and enormous Al in WPCS,the WPCS was pyrolyzed and granulated into an anti-erosion and efficient multi-pollutant adsorption substrate in this study,aiming at solving the poor removal of di ssolved phosphate（PO?3?-P）and heavy metals in runoff and alleviating the risk of clogging of powdered sludge in traditional ecological facilities.Meanwhile,in order to make full use of the limited organic matter in the advanced treatment sludge,CO₂ was adopted as the pyrolysis atmosphere instead of traditional N?atmosphere.The effects of CO₂ on the pyrolysis characteristics,physicochemical and adsorption properties of WPCS were investigated.Moreover,the adsorption property of optimized granular su bstrate,as well as the removal efficiency and accumulation of pollutants by granular coagulation sludge amended rainwater filter were also investigated.The effects of using CO₂ instead of N? on the pyrolysis characteristics,physicochemical and adsorption properties of WPCS were investigated with DWTS as reference.The WPCS had about 2.5 times the total organic carbon content and 8 times the total pore volume of DWTS.WPCS was an Al-rich and porous precursor for pyrolysis,with a large amount of carbon containing functional groups and high specific surface area.The thermal decomposition rates of both sludges under CO₂ were higher than those under N?,which indicated that CO₂ promoted the cracking of 4.4 wt.% of volatile organic compounds.Hence,CO₂ could make full use of the limited organic matter in the coagulation sludge compared with N?.The catalytic effect arising from high concentration Al/Fe in the coagulation sludge lowered the temperature of CO₂-mediated reactions to 260℃～275℃.The specific surface area of WPCS biochar in CO₂（121.41 m2 g?1）was 37% higher than that in N? atmosphere and the amorphous Al content increased to 197.89 mg g ?1.Therefore,this product exhibited the highest PO?3?-P adsorption capacity of 15.58 mg g?1.Furthermore,pyrolysis led to the loss of carboxylic,amino,and other functional groups in the coagulation sludge,while enhanced the stability of heavy metals and aromaticity of organic compounds in sludge.WPCS was co-pyrolyzed and granulated with bentonite（binder）and corncob（pore forming agent）under CO₂ to prepare ecological substrate.Using the mixed design method and response surface analysis method,the optimal preparation condition was determined,that is,pyrolyzing 70 wt.% WPCS,25 wt.% bentonite and 5 wt.% corncob for 2 hours at 400°C under CO₂.The response surface plots showed that PO?3?-P adsorption capacity was positively correlated with the content of WPCS,and bentonite and corncob were critical to enhance erosion wear resistance.Although pyrolysis process was conducive to improving the specific surface area and the removal of PO?3?-P,the granulation process inevitably decreased adsorption capacity of PO?3?-P by 10% than powdered WPCS.Comprehensively affected by these process es,the optimized WBG was a porous and anti-erosion substrate with low heavy metal leaching toxicity.Pre-cleaning before application can eliminate the risk of PO?3?-P leaching risk.The adsorption properties of WPCS-based granule（WBG）were comprehensively evaluated.The adsorption data of PO?3?-P by WBG conformed to the pseudo second-order kinetic model and Langmuir adsorption isotherm,indicating that removal processes were controlled by chemical adsorption.The strong selectivity for PO?3?-P adsorption and a theoretical maximum PO?3?-P adsorption capacity of 34.44 mg g?1 made this substrate outperform traditional ones.An acidic environment is more encouraged to enhance the adsorption of PO?3?-P,while the co-existing HCO?? and humic acid will compete adsorption sites with PO?3?-P.The adsorption mechanism of PO?3?-P to WBG was dominated by the ligand exchange between PO?3?-P and-OH and-Cl on the surface of WBG.The mechanism also included electrostatic adsorption,surface precipitation and internal diffusion.Finally,a simulated rainwater filter modified by WBG was established to investigate the permeability and stormwater purification results.The granule amended column manifested good permeability and stable hydraulic conductivity(2.74～2.52 m d^-1)over a 54-day operation period,which was higher than the powdered coagulation sludge column and beneficial to address the clogging issue.Under various stormwater runoff conditions,the involvement of WBG in stormwater filtration systems exerted consistent ly fancy performance of Cu（II）,Pb（II）,Cd（II）and PO?3?-P removal,with average removal rates of over 85.0%,and the average effluent concentrations were all lower than the limitation of the Chinese standard for surface water.The desorption rates of all adsorbed pollutants were less than 3.0%.The accumulation of PO?3?-P and heavy metals in the short-term laboratory study and the long-term accumulation simulated by HYDRUS-1D model both proved that pollutants tended to accumulate in the upper layer of the filter system.The lifespan of the top-layer substrate was almost 9 years.The substrate replacement cycle could be delayed by only removing the top-layer media with massive accumulation of pollutants.Hence,the development of WPCS-based granular substrate provides a potential recycling method with both environmental and economic benefits for waste treatment.