On many landfill sites the most environmentally friendly and economical way to treat landfill leachate is to reduce its volume by 75 to 80% using reverse osmosis and then return the concentrate to the landfill by controlled reinjection. If this procedure is not yet authorized by local authorities then the treatment process must achieve very high rates of recovery by using a combination of reverse osmosis and nanofiltration technology with controlled crystallization to reduce the volume of concentrate for further processing.