The most economical and environmentally friendly way to treat landfill leachate is to reduce ifs 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 (RO) and nanofiltration DT-module technology with controlled crystallization. Dr-Ing. Thomas A. Peters discusses.