Recovery Of Humic Substances From Leachate Nanofiltration Concentrate By A Two-stage Process Of Ultrafiltration Membrane

The recovery of humic substance?HS?from leachate MBR+NF concentrate as a water-soluble fertilizer could give a potential alternative to solve the problem of its handling,in view of its high concentration of HS.A combined processes of two-stage ultrafiltration?UF?process,consisting of first stage ultrafiltration membrane and second stage ultrafiltration membrane with molecular weight cut off of 1000 Da and 2500 Da,respectively,were performed to recover HS from MBR+NF concentrate of landfill site and incineration plant mixed leachate,and to investigate the separation performance of the process on pilot scale.Due to divalent ions in the concentrate are partly rejected by the UF membrane,the effect of these ions(Ca²⁺,Mg²⁺ and SO₄^2-)on membrane performance were also investigated and the transport mechanism of ions through UF membranes were further studied.Data on the effect of divalent ions(Ca2,Mg²⁺ and SO₄^2-)on UF membrane performance indicated that the flux of the two membranes was decreased linearly with the addition of divalent ions.When SO₄^2-was added,the flux decreased was the most serious,and the membrane flux recovered to some extent after adding Mg²⁺.The decline in flux for three ions was SO₄^2->Ca²⁺>Mg2.When the first ion was added,the rejection decreases with the increase of concentration.When second ions was added,except for in Ca²⁺+Mg²⁺,the Mg²⁺ retention first increased and then decreased with adding Mg²⁺;and the rest were decreased.When adding cations before adding SO₄^2-,their rejection increased continuously,and the rejection of SO₄^2-decreased continuously.When SO₄^2-was added first,the rejection of cation decreased firstly and then increased with the addition of Ca²⁺,and the rej ection of SO₄^2-was decreased.For recovery of HS from leachate MBR+NF concentrate by a two-stage process of ultrafiltration membrane,the membrane flux decreased linearly with the increase of CF,and the concentration of organic matter in the feed continued to rise.The retention of organic matter in the first stage UF increased with the increase of CF,while that in the second stage UF was maintained at 88%-92%.Moreover,the recovery of organic matter in the second stage UF were better than that in the first stage UF.As for the inorganic ions in the feed solutions,the concentration of Na+,K+,Ca²⁺,Mg²⁺ and SO₄^2-in the feed increased with an increase in CF,while Cl-was negative rejection.Additionally,the retention of SO₄^2-in the first stage UF was increased with increasing CF,which is opposed to that in the second stage UF.The content of humic acid and heavy metal concentration of two-stage UF final concentrate could meet the requirements of relevant standards.The concentration of organic matter in the permeate originated from first stage UF decreased logarithmicly with the amount of recirculating,suggesting that the recovery of organic matter in the permeate by flowing back to the UF process is feasible.The transport mechanism of main ions in the extraction process of HS from the leachate concentrate by a two-stage UF process was examined by a ion transport model.The results showed that the main inorganic ions in the concentrate were good agreement with the model,and the mass transfer coefficient Keffi and ion transmittance?i of the divalent ions were less than those of monovalent ions.For the mass transfer behavior of HS molecules,this model was applicable to the first stage UF process and does not apply to the sceond stage UF process.