Leachate treatment in landfills is challenging and costly due to changes in quality and quantity over time. Recycling reverse osmosis concentrate back into the landfill can have benefits like improving leachate quality and gas production. Lab tests on landfill simulation reactors showed that recirculating the concentrate did not consistently change COD emissions or methane production. However, it did increase ammonium ion levels in leachate by over 25% in anaerobic conditions and doubled free ammonia gaseous emissions. Chloride also accumulated in the reactor.