Sanitary landfills are the primary method currently used for municipal solid waste disposal in many countries. The landfill regulation state that the waste disposed must be previously pretreated, with a biologic or incineration treatment) and the organic content must not be more than 3% (determined as TOC) to assure a reduction of methane production. Moreover the landfill has to fulfill several requirements according the sealing (liner systems and surface system) and construction in order to guarantee a decomposition of the waste contained in it, as well as to minimize the generation of leachate. This leachate is formed as a consequence of the rainwater that infiltrates and percolates through the waste (primary leachate), as well as from the degradation of the biologic components inside the landfill (secondary leachate). There are lots of factors that interfere in the formation and composition of the leachate. The chemicals within the leachate vary over the time depending on the physical, chemical, and biological activities occurring inside the landfill. The quality and quantity of leachate vary depending on the age of the landfill, the climate, the landfill morphology, the surface conditions, the operation of it, the size of the landfill, the composition of the waste such as its compaction, as well as the moisture inside and the degree of rainwater infiltration. The result is a high concentrated leachate that must be treated before discharging it to the nature or to a common clarification plant. There are different methods that can be applied, this are mainly two, degradation methods (biological such as anaerobic or aerobic decomposition) and separating methods (like coagulation-flocculation, adsorption, flotation, chemical oxidation and advance oxidation process as well as several filtration technologies). This case study is about a municipal solid waste landfill, which contains two different parts of landfill with two kinds of waste. Neither of both parts of the landfill have surface sealing. The leachate generated is treated in a treatment plant that consists basically of three consecutive filters (a fabric filter, sand filter and fabric filter) and a further treatment consisting of a twostep reverse osmosis (RO) membranes. The concentrate expected out from the second RO is 10% concentrated respect the 100% of initial leachate that enters to the plant. But this percentage has raised to aproximately a 20%, and blocking problems have been occuring. The disposing of this output is very expensive, as well as the often cleaning and changing of the membranes due to the blocking of these ones. It has been seen they are two kinds of leachate, the black one and the brown one. When the leachate is brown, the second step of reverse osmosis is not able to operate because the membranes get blocked. The analysis of pH, conductivity, ions, organic content and suspended solids, of the black leachate were carried out and the results showed a reasonable reduction through the treatment process. Due to the observations it is believed that the brown leachate is generated because of an increase of the infiltration rainwater flow into the landfill, caused by an abrasion and elution of the ash wood from the incinerated waste. This water flow drags with it mineral matter such as inorganic compounds and salts, that probably be causing the blocking of the membranes. It is believed that the quality of the brown leachate is the one causing blocking problems in the RO membranes. It is proposed to proceed to follow the sample analysis procedure for the brown leachate than for the black leachate, adding potential redox and turbidity and color parameters. As alternatives it is recommended a new biological treatment or an additional flotation plant with chloridric acid. On the other hand, the capping of the landfill to prevent the infiltration of rainwater to avoid the generation of brown leachate.