Regulation Of The Concentrations Of Divalent Ions For Recovering Humic Substances From Leachate MBR+NF Concentrate

Membrane bioreactor(MBR)and nanofiltration(NF)hybrid process has dominated the treatment of leachate.However,NF concentrate generated from the process is still a challenge for treatment.Given NF concentrate with a high content of humic substance(HS),a NF and two-stage ultrafiltration(UF)hybrid process for recovering HS from NF concentrate as a water-soluble fertilizer is proposed as an advisable and economic method.However,during this process,an increasein concentration of divalent ions with the increase of concentration factor(CF)leads to a heavy membrane fouling and adversely affects the stability of the system as a whole.In order to attain a high stability for this process with a closed loop,a pre-treatment process should be taken in a certain part of the process to regulate divalent ion concentrations in the feed,especially calcium,magnesium and sulphate ions.Firstly,the removal of calcium and magnesium ions by using Na3PO4·12H2O as a precipitator was performed for the feed of the second stage UF in the presence of a high content of HS.The optimal dosage of precipitatorwas n(Ca2+,Mg2+)/n(PO43-)=2.1,and pH of feed was 6.45 after precipitation.The removal efficiencies of calcium and magnesium ions were 79.4%and 37.6%,respectively,TP concentration was up to 357.6 mg/L,and the loss rate of TOC was 11.5%.Concerning the effect of the high content of HS on precipitation,not only was the removal efficiency of calcium and magnesium ions impaired but an apparent increase in TP was also observed.Inhibitor showed no effect on precipitation.Secondly,the removal of calcium and magnesium ions by using Na3PO4·12H2O as a precipitator was also investigated for the permeate from the second stage UF in the presence of fulvic acid(FA)and magnesium ion.Although the removal efficiency of magnesium ion was worse than calcium ion,it increased with the rising concentration of magnesium ion in the permeate.The amount of theoretical residue of calcium and magnesium ion at 300mg/L was the optimal dosage.The removal efficiency of calcium and magnesium ions in the permeates with different magnesium concentrations was all above 80%,and TP concentration all below 60 mg/L.A positive correlation between pH value and the dosage of precipitator was found.The loss of TOC decreased as pH increases.Inhibitor could not effectively prevent the reaction of precipitation although it showed strongly inhibiting effect on the removal of magnesium ion compared to calcium ion.Finally,good performance of the reduction of calcium and sulphate ions by a precipitation process of calcium sulphate dihydrate in the permeate of the second stage UF could be expected.The operating parameters(temperature and pH),mechanism of precipitating behaviour,and kinetics of the precipitation in the presence of FA and magnesium ion were conducted.The removal of calcium ion increased with decreasingpH and increasingtemperature's.Especially,facilitation of calcium sulphate dihydrate crystallisation was observed through adjusting pH in the range of 2-3.The best removal of calcium ion was at pH around 2.5,which is the point of zero charge of calcium sulphate dihydrate.At low pH adsorption of FA onto gypsum surface playing a significant role in calcium sulphate dihydrate precipitation relied mainly on weak interactions of hydrogen bonding and hydrophobic effect.At neutral pH electrostatic interaction became dominant.The optimal operating parameters were found at 70? and pH 2.5,andthe crystallisation process of calcium sulphate dihydrate in the presence of FA and magnesium ion corresponded to 1st-order kinetics.