Removal Of Refractory Organics And Perfluoroalkyl Substances In Nanofiltration Concentrates In Municipal Solid Waste Leachate Treatment Plants

Leachate is commonly generated in the incineration of municipal solid wastes.The nanofiltration（NF）concentrates are usually derived from the NF memebrane separation units in the typical solid waste leachate treatment.Generally,the NF membrane concentrates containe high concentations of a variety of refractory organic pollutants and low concentrations of organic micro-pollutants.Therefore,the effective control and treatment of NF concentrates is an urgent issue in the secure operation of solid waste incineration facilities.Based on the combined technology of"oxidative flocculation-Fenton advanced oxidation",this paper aims to realize the NF membrane concentrate can be recycled to the leachate treatment system for circulation treatment,and carried out experimental research on oxidative coagulation and Fenton advanced oxidation,respectively.This paper compared the removal performance of different kinds of oxidative coagulation processes and Fenton advanced oxidation process on refractory organic matter,investigated the effects of p H,Fe SO₄·7H₂O and H₂O₂ dosage on the removal efficiency of organic matter.At the same time,the removal characteristics of organic matter and the concentration change of perfluoroalkyl substances（PFASs）during the combined process were studied.The following research results were obtained:（1）For a NF membrane concentrate with COD of 5000±100 mg/L from a waste incineration power plant,the combined process achieved more than 90%organic matter removal.COD in the treated effluent was reduced to 225±20 mg/L.The combined process provided an option for the cyclic treatment of NF membrane concentrate.（2）The optimal operating conditions for“oxidative coagulation-Fenton advanced oxidation”process were:the initial p H was 4;the dosage of Fe SO₄·7H₂O and H₂O₂ were30 m M and 40 m M,respectively,in the stage of H₂O₂/Fe²⁺oxidative coagulation.The dosage of H₂O₂ was 100 m M and no Fe SO₄·7H₂O was added in the stage of UV-Fenton.Oxidative coagulation achieved more than 75%removal efficiencies in COD,UV₂₅₄ and TOC,while UV-Fenton stage could reduce the above indicators by 66.7%～74.6%further.（3）In the stage of H₂O₂/Fe²⁺oxidative coagulation,fulvic acid substances（FA）can basically be completely removed by coagulation adsorption,and the content of small molecular weight organics and carboxyl groups increased;In the stage of UV-Fenton,some refractory aromatic hydrocarbon and carboxylic acid structures were degraded into aliphatic compounds.（4）In the stage of H₂O₂/Fe²⁺oxidative coagulation,the removal efficiency of different PFASs varied from-14.80%to 100%.The coagulation removal of PFASs was mainly affected by dissolved organic matter（DOM）and iron hydroxide precipitates,DOM can promote the removal of PFASs;In the stage of UV-Fenton,PFASs are defluorinated and degraded under the action of·OH radicals and PFASs-Fe³⁺electron transfer complexes.The combined process can remove 2/3 of the total amount of PFASs in NF membrane concentrate,and the long-chain PFASs with a carbon chain length greater than 7 can be completely removed.