Humic Substance Recovery from Reverse Osmosis Concentrate of a Landfill Leachate Treatment via Nanofiltration

Landfill leachate reverse osmosis (RO) treatment generates a highly concentrated stream rich in recalcitrant organic matter, particularly humic substances (HS), which present potential for recovery and reuse as a biofertilizer. This study evaluated HS recovery from the RO concentrate of the Seropédica Landfill (Rio de Janeiro, Brazil) using a nanofiltration (NF) process with a polyethersulfone membrane (MWCO = 1000 Da) operated at 9 bar. The NF system achieved a volume reduction factor of 2.5, rejecting 70–75% of the organic matter. At the same time, salts were predominantly transmitted to the permeate. The phytotoxicity of recovered HS solution was evaluated through maize (Zea mays L.) germination assays at concentrations ranging from 20 to 100 mg HS/L. All treatments showed germination indices above 100%, indicating the absence of phytotoxicity, and seedling growth significantly improved relative to the control, especially at 20 mg HS/L. Trace metal concentrations in the recovered HS complied with Brazilian standards for irrigation water. Overall, the results show that nanofiltration is highly effective in concentrating humic substances from leachate RO concentrate, minimizing the presence of salts, and contributing to strategies for landfill leachate management.