Purification of landfill leachate with the reverse osmosis and nanofiltration membrane process - practical experiences and results with regard to applications in Scandinavia

Abstract

The most important membrane process for purification of landfill leachate is reverse osmosis. With this technology the negative impact of leachate on the environment can be nearly fully eliminated. The first large-scale plants went into operation in the mid eighties. Today, approximately one third of the installed capacity worldwide bases on this technology. Based on experiences with numerous systems which have been in operation for many years, results concerning the main parameters such as COD, ammonia etc. are presented. Depending on how the permeate is discharged, either directly into a surface water or indirectly into a municipal sewage treatment plant (i.e. RO works as pre-treatment), excellent quality parameters for the permeate may be achieved. With such systems working within a pressure range up to 65 bar, recovery rates between 65% and 80% permeate may be achieved with leachates with normal salt concentrations. In order to achieve higher recovery rates, the treatment of landfill leachate by RO has been significantly improved by the introduction of high-pressure osmosis. This results in a minimised concentrate amount which considerably eases disposal. The utmost reduction of concentrate is achieved in the additional combination with nanofiltration which may yield a recovery rate of more than 90%. The application of nanofiltration makes sense for the concentration of such leachates where not the osmotic pressure due to high salt concentration but the presence of membrane sealing factors such as earth alkaline ions limit the recovery rate. Nanofiltration selectively retains two or more valent ions and thus enables high concentration factors to be achieved. For Scandinavian applications with dilute leachate simpler concepts with single stage units have been realized. By demonstration and commercial size applications it could be proven that such membrane systems are suitable for purifying high amounts of leachate with low concentration of salt and organic components.