In order to ensure the security for a sanitary landfill site and obtain social reassurance, a stable and efficient landfill leachate treatment system, which responds to the quality of water of landfill leachate as changed by the life cycle of the landfill, needs to be built. What usually wind up as landfill are municipal solid wastes such as incinerated ash and incombustible residues. The ratio of inorganic salts and persistent organic pollutants (POPs) in the leachate is currently increasing (Table 1).
Harmful substances like boron and boron compounds can be contained in inorganic salts. Inorganic salts per se exhibit a deleterious effect on the freshwater ecology. They cause certain problems such as corrosion of processing units and calcium scaling . Furthermore, the remaining ammonium ions until the final stage of their discharge elevate the cost of the treatment process of biological nitrification denitrification process. Various organochemicals derived from petrochemicals are also included in the leachate 1).
In the recent typical landfill leachate treatment, the biodegradable organic pollutants and nitrogen are removed during the biological treatment process. Subsequently, the residual suspended solids, heavy metal, and colorable COD component are removed with coagulation setting, sand filtration, and activated carbon absorption. Thus, the development of a technology which responds to water quality matrix for new leachate components like inorganic salts and POPs is required. In this paper, the employment of the reverse osmosis (RO) Membrane, its application and efficiency to landfill leachate treatment using membrane separation, sample application cases, and future expectations are outlined.
2. Characteristic of RO membrane separation
The RO membrane, which is made from cross-linked aromatic polyamide, is largely sold in the market. The operating pressure is 1-10Mpa. The ion component or molecular weight (more than 50-100) can be separated. Since RO membrane separation is produced by converting pressure into driving force without phase change, it has the characteristics of creating better energy efficiency, nonspace-consuming, and easy automatization and scale-up 1). The membrane modules have 3 types of configurations, which are Tubular, Spiral，and Plate-and-Frame modules.
Boron usually exists as boric acid. It does not dissociate within neutral pH conditions. Since its molecule is 0.4 mm in diameter, the removal rate with RO membrane is approximately 60%. To obtain high boron removal rate, it needs alkaline treatment conditions 2). On the other hand, ammonia has approximately 90% removal rate with RO membrane treatment (Table 2). The occurrence of organic fouling is treated with coagulation/sedimentation and utilization of MF membrane. On the other hand, inorganic scaling is counteracted with the alkali sedimentation method, ph control, and scale dispersant supplementation. However, regular agent washing is needed for a long operation. Therefore, the choice of a chemical-resistant membrane in the design of process is important, a membrane which responds to transmission coefficient and operating pressure，separation performance of solute, and membrane cleaning.
3. Application of reverse osmosis to landfill leachate treatment
The landfill leachate treatment facility was first built in Switzerland in 1984, where the Tubular RO membrane module for the first stage was used 4）. In 1995, Germany Rochem developed the DT-Module, which was a tubular flat membrane. The DT-Module can solve fouling and enhance the recovery rate of treated water by using turbulence effect caused by surface projection of the disk. In northern Europe, DT-Modules are being employed in more than 100 sanitary landfill sites 5). More than 95% recovery of treated water was achieved at lhlenberg, Germany (Fig. 1). The landfill leachate treatment facility (disposal capacity at 70 m3/day) using DT-Module was built at a landfill for municipal solid waste, “Clean Park, Kinu”, in Ibaragi, Japan (Fig. 2). The facility records a recovery rate of treated water at 80-95% 7). The “Shiso Kankyo Bika Centre” in Hyogo, Japan installed the DT-Module (10 m3/day) as the final part of the conventional process of desalination and removal of dioxins. The treated water is subsequently reused as water supply for the facility. In addition to the DT-Module, Haase built a facility for the treatment of landfill leachate where a Tubular module for the first stage and a Spiral module for the second stage were installed at Rastorf, Germany in 1991 8). VSEP developed by New Logic could remove fouling from the RO membrane surface by shearing actions of 50 times / per second. VSEP was installed at a sanitary landfill site (30 m3/day) in Korea in 1999.
4. Treatment of concentrate
The whole membrane treatment system for the Membrane Separation is completed by treating the concentrate properly. Since the cost for concentrate treatment accounts for most of the entire treatment cost, various tests are performed to improve the concentration rate. In the process shown in Fig. 2, the RO membrane, which can respond to high voltage (120 bar) for concentrate treatment, increases the water recovery in the whole process to 75-90％ 7). In the Treatment Flow shown in Fig. 1, after the concentrate is sent back to the crystallization tank and becomes supersaturated as CaSO4, solidification is conducted and the solid residues are drained out 6). Other ways to treat the concentrate are reported as follows: sent back to landfill in liquid form; stored after evaporation and drying; treated in commission; co-incinerated with other waste; and used as kneading water when fly ash was solidified 7)． In Japan, means of recycling the salts for industrial use are also being examined. Concurrently, it is reported that the refined salts obtained by Haagenmeire Process have similar component to the native salts．
5. Future challenges
In the RO membrane treatment process for landfill leachate, the following challenges are foresighted. The advancement of the whole process to enhance the recovery and concentration rates of water, the choice of rational treatment for concentrate, the development of contamination-resistance against fouling and scaling as well as chemical resistance against washing, and the development of membrane material responding to low solute with low removal rate of boron are answers to the current dilemmas which need to materialize. In order not to follow in the wake of the current biotreatment process, it is important to simplify pre-and post treatments as the total processing. The development of a membrane designed for the landfill leachate treatment, which can improve the function of the main process, is a breakthrough for large share.
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