Mechanisms Of Separation And Recovery Of Humic Substances From Leachate MBR-NF Concentrates

Landfill leachate is difficult to be treated due to high content of humic substance (HS) with low biodegradability. Therefore, treatment of leachate is a problem of the secondary pollution in landfill or incineration plant. The concentrate of leachate generated from a typical membrane bio-reactor (MBR) and nanofiltration (NF) combined process for leachate treatment contains higher content of HS, indicating the potential of its recovery. In this study, a pilot-scale NF and ultrafiltration (UF) combined process was used to separate and thicken HS from MBR-NF concentrate of landfill and incineration plant mixed leachate. Characteristics of NF and UF process for the separation of the components of MBR-NF concentrate were investigated, and the separation mechanism was also discussed to optimize the processes.The NF experiments showed that the NF membrane can effectively retain organics in the concentrate. The retention of COD was as high as 98%. The pressure and pH had little effect on COD retention rate, and the effluent of NF process can meet the professional discharge standard. In the NF process, concentrate COD and UV254 had a good linear relationship. The inorganic ion permeability were decreased with the increase of the concentration factor (CF), instead the Cl- ion. The NF membrane had high retention rates for heavy metals.Separation and recovery of HS from the retentate obtained from the NF process by UF indicated that part of the small molecular fulvic acid (FA) could permeate the UF membrane, while the humic acid (HA) was almost rejected. Both HA and FA concentration enhanced in the UF process, and FA accounted for the majority of HS. But HA enrichment multiple was higher than FA. In the UF process, the penetration rate of inorganic salt first increased and then decreased. However, the desalination rate is on the contrary. The UF membrane could effectively separate the organics and inorganic salt in the concentrate, and the separation factor (SF) was strongly correlated with CF. The heavy metal ions enrichment factor is relatively low in the final concentrate, except the Cu, Co and Ni. The HS recovery ratio decreased with the increase of the CF, with a percentage of 74.37% in the final concentrate. The content of HS in the final concentrate could meet the requirement of water-soluble fertilizers containing HA, and the concentrations of the heavy metals were lower than the content-limit of related standards.XRD pattern of the rententate and permeate precipitates In the UF process showed that their main components were CaSO4. Na2CO3 precipitation could effectively remove Ca ion, but the organic matter is also substantially absorbed by the precipitation in the process. Adding BaCl2 could effectively precipitate SO42- ion in the concentrate. As a result, the NF membrane flux can improve significantly.