Research On Biological Filler Derived From Sewage Sludge And Its Application In Urban Runoff Pollution Control

Municipal sewage sludge generated from domestic sewage treatment must be disposed properly,or else it will cause secondary pollution.It is a sustainable development way of sludge disposal and resource utilization to employ sewage sludge to make environmental friendly materials such as lightweight aggregate ceramsite,packing medium for wastewater treatment and so on.However,there are some drawbacks in the current ceramsites,such as relatively less sludge content,high sintering temperature,and poor water absorption capacity,which will affect runoff reduction and pollutants removal as filler or filtration medium.The present paper attempts to make a kind of sewage sludge based biological filler?SSBF?with the characteristics of light weight,high permeability and biological activity,rich pore structure and proper intensity by using sewage sludge and other materials,and to assess the performance of SSBF applied in low impact development facilities such as bioretention pools to control the pollution of urban runoff.It will not only to consume a large amount of sludge,but also to improve the filter structures and properties of low impact development facilities,therefore to achieve rapid and effective control of non-point source pollution.The main research contents and conclusions are as follows:?1?Preparation of SSBF.SSBF was prepared by using sewage sludge,fly ash,river silt,Solidago canadensis L.and calcium carbonate,and the factors?such as material ratio,sintering temperature,sintering time,preheating temperature and heating rate?influencing main performance indexes?water absorption capacity,apparent density and cylinder compressive strength?of SSBF were studied.The optimal formula and sintering technology of SSBF were determined by orthogonal test and the characterizations of SSBF samples were analyzed.The results showed that the optimum formula of SSBF for sludge,fly ash,river silt was 3?5?4.5,with 9 % Solidago canadensis L.and 5 % calcium carbonate added.The best preparation conditions for SSBF were 950 ?of sintering temperature,10 min of sintering time,500 of preheating temperature,and 7? ?/min of heating rate.The main performances of SSBFs with the optimum conditions were water adsorption of 65 %,the apparent density of 630 kg/m³,the cylinder compressive strength of 0.42 MPa,the specific surface area of 0.2 m²/g.A number of irregular pores with different shapes and sizes were observed from the SEM images of SSBF.XRD analysis indicated that the filler was mainly composed of ionic crystals,such as oxide and silicate structures.?2?Adsorption of heavy metals from aqueous solution by SSBF.The sorption effect of SSBF on heavy metals?lead,nickel,cadmium and chromium?from aqueous solution was evaluated,and the sorption characteristics and the mechanisms of SSBF for heavy metal removal were studied by using trivalent chromium as model.The results showed that the addition of calcium carbonate was beneficial to improve the adsorption capacity of SSBF on Cr³⁺ and Pb²⁺.SSBF showed selective adsorption characteristics,and the removal ability of SSBF for the four heavy metals followed the order of Cr³⁺>Pb²⁺>Ni²⁺>Cd²⁺ on the competition of adsorption.The maximum adsorption capacity of SSBF for Cr³⁺ was 4.32 mg/g and the fit models for SSBF isotherm were the Langmuir model and the Freundlich model.The adsorption mechanism of SSBF for trivalent chromium was mainly ascribed to 1)surface precipitation by forming insoluble chromium compounds in alkaline condition induced by biochar;2)electrostatic adsorption by combination of surface negative potential with positive chromium ions;and 3)displacement and substitution reaction between chromium ions and cations.?3?The effect of SSBF applied to bioretention system.Bioretention facility?medium column?filled respectively with SSBF,commercial ceramsite and gravel were comparatively studied,the effects of SSBF on the reduction of runoff,the lag of runoff peak and the removal of pollutants were evaluated,and the influences of different pollution loads and retention time on the removal of pollutants from waste water were explored.The results showed that SSBF column was more effective in reducing the runoff and lagging the appearance of flood peak than commercial ceramsite and gravel packed column.The values of the peak reduction,runoff reduction and peak lag were respectively 3.25 %¹2.9 %,20 %⁸0 %,8⁴0 min in the simulation of different rainstorm intensities.The peak of SSBF column lagged 25 min,and the total runoff and the maximum peak value of SSBF column was respectively reduced by 54 %,6.25 % in the rainstorm intensity of 1a recurrence period.With relatively long dry period,SSBF column was conducive to rainwater storage in bioretention facilities.When the hydraulic load were 0.5 m³/?m²·d?,the removal capacity on CODCr by SSBF column were increased 28 %⁴4 % and 48 %⁶5 % respectively,compared to that of commercial ceramsite and gravel packed column.The removal efficiency of NH₄⁺-N by SSBF column was similar to that of ceramsite packing column,and higher than that of the gravel packed column in low concentration inflow.The best hydraulic retention time?HRT?of SSBF column was 4 h,and the removal rates of CODCr,NH₄⁺-N,TN and SS were respectively 60 %,68.4 %,52 %,and 88 %.HRT could be decreased to 1² h under the lighter pollution levels of rainwater runoff.?4?On-site disposal device based on SSBF for urban runoff pollution control.An on-site wastewater treatment device based on Moving Bed Biofilm Reactor?MBBR?was constructed using SSBF and commercial ceramsite as fillers.Factors influencing the pollutants removal were researched to find the optimum operation conditions of the device.The results showed that the average removal rates of CODCr,NH₄⁺-N,TN and turbidity were 78.4 %,69.1 %,49 %,and 89.8 % respectively at HRT=2 h.The removal rate of pollutants increased slowly at HRT>2 h.The optimal volume loads of CODCr,NH₄⁺-N and TN were 166.7 g/?m³·h?,8.2 g/?m³·h?,and 8.1 g/?m³·h?.The device,with a strong resistance to shock loading,is suitable for rapid treatment of surface runoff,and can be considered as the initial treatment of rainwater runoff.