Effect of the recirculation of a reverse osmosis concentrate on leachate generation: A case study in an Italian landfill

•In “Fossetto” landfill concentrated leachate from RO is recirculated to landfill.•Leachate characteristics before and after recirculation beginning are analysed.•Leachate amount increased moderately.•Ammonia Nitrogen and Chloride concentrations increase were the most significant.•Changes in leachate composition did not affect the quality of RO plant effluent. “Fossetto” landfill has been operating in the municipality of Monsummano Terme (Pistoia Province, Italy) since 1988; the authorized volume for landfilling is about 1,000,000 m3; at the moment the plant is being mainly used to dispose of mechanically and biologically treated residual municipal solid waste. Since September 2006, an in-situ reverse osmosis leachate treatment plant has been operating to treat leachate. The treated water is being discharged into a small nearby stream while the concentrated leachate is being recirculated back into the landfill body following Italian Regulations and an authorization from the local authority (Pistoia Province). This paper presents monitoring results on leachate generation rates and composition for the past fifteen years. A moderate increase of the concentration of some of the monitored parameters occurred (e.g. ammonium, chlorides) and a decrease for most heavy metals. The increase of concentrations for Cl− and NH4+ was more evident in the leachate coming from the wells closer to reinjection area. However, the change in leachate composition did not affect the quality of the effluent from the leachate treatment plant. The annual volume of the generated leachate increased significantly right after the recirculation started.