Beyond The Energy Bill: Direct Emissions, Sludge and The Climate Footprint of Wastewater Reuse Trains in Lebanon

Water reuse is central to water security in Lebanon, but the climate burden of producing reclaimed water is rarely assessed under the country's conditions of diesel-dependent electricity and uncontrolled sludge disposal. This study compares seven complete treatment trains, from conventional activated sludge (CAS) with UV disinfection to reverse osmosis, each matched to a Lebanese reuse category. The functional unit is 1 m³ of reclaimed water. A streamlined LCA combines a technology inventory with IPCC 2019 methods for direct N₂O and CH₄, dissolved methane and sludge disposal, five improvement scenarios and a 10,000-run Monte Carlo analysis. Gross climate impacts range from 1.24 kg CO₂ eq/m³ for an anaerobic membrane bioreactor (AnMBR) to 2.34 for a train with reverse osmosis. Direct emissions and sludge disposal account for 50–91% of the impact of every train except reverse osmosis. AnMBR, CAS + UV and a constructed wetland overlap within uncertainty, while reverse osmosis is the highest in 94% of runs. Upgrading from restricted to unrestricted irrigation quality adds only 0.15–0.20 kg CO₂ eq/m³ without reverse osmosis. Recovering 70% of dissolved methane lowers AnMBR to 0.68, a vertical-flow wetland reaches 0.98, and solar power cuts the MBR and reverse osmosis trains by 40–46%. Displacing groundwater pumping roughly halves every train's net footprint. The results indicate that methane and N₂O control, controlled sludge management and renewable electricity should be core design criteria for wastewater reuse in Lebanon.