Characterization And Recycling Of The Bottom Ash Of Municipal Solid Waste Incinerator For Wastewater Treatment

Municipal solid waste incineration can greatly reduce waste volume, and in most cases also recover energy. After incineration, about 20-30% of the waste is transferred to the bottom ash and fly ash. While fly ash belongs to hazardous waste, bottom ash is considered to be a non-hazardous waste, though some developed countries may take the bottom ash as a hazardous waste and cannot be recycled before proper detoxification.In this work, the bottom and fly ashes, taking from two Shanghai MSW Incinerators, were well characterized. It was found that the bottom ash comprised mainly of glass, synthetic ceramics, metals, minerals such as silicates, phosphates, sulfates or carbonates, and some unburned organic matters. Over 60% of the bottom ash was >4 mm in size.The chemical composition of bottom ash was analyzed by X-ray fluorescence (XRF) method. The results indicated that the bottom ashes were mainly composed of SiO₂ (35.3%⁴2.3%), CaO (19%²7.2%), Al₂O₃ (7.4%⁷.8%) and Fe₂O₃ (3.9%⁵.1%). The contents of CaO, P₂O₅, MgO, TiO₂ increased and SiO₂, Al₂O₃, Fe₂O₃, Na₂O, K₂O decreased, as the particle size of bottom ash decreased. Moreover, the concentrations of heavy metals also increased as the particle size of bottom ashes decreased. It may be considered that the coarser particles may originate from the wastes incinerated and the smaller ones may form during the incineration process. The contents of heavy metals in both bottom ash and fly ash were found to be inter-correlated and depended on the volatilization of heavy metal involved. The heavy metals with low volatilization mainly remained in bottom ash.The heavy metals in bottom ash were mainly present as carbonates and residual forms with high stabilization.The leachability of heavy metals in the bottom ashes were lower than the leachability toxicity limit of wastes according to the China leaching standard and USA TCLP and could be treated as a non-hazardous waste for uses as buildingmaterials, etc.Bottom ash had a 1.8-2.0 mol/kg acid neutralization capacity (ANC), the ability of resistance to acid attack affecting matrix durability was very largeThe characteristics of phosphate adsorption by bottom ash were investigated. Bottom ash had an excellent phosphate removal capacity, could be used as media to remove phosphate in water. The P can be quantitatively removed from aqueous solutions by the bottom ash when its initial concentrations were lower than 200 mg/L, and the absorption capacity increased as pH decreased. After absorption, the P was changed into the chemical forms of Ca2-P> Cag^ Al-P> Fe-P and Ca)0-P through chemical reactions.The bottom ash was used as the media for the biological treatment of feedlots wastewaters. It was found that 85.2%, 94.2% and 98.6% of COD, NH3-N and TP in the wastewaters could be removed when the bottom-ash-based biofilter was inoculated for somewhile, and the effluent can meet the local environmental constraints at a hydraulic load of 0.189cm/min. It was proposed that the pollutants were removed biologically as the biofilter had been used for the test for long time without stop.