Life cycle assessment of vertical flow constructed wetlands vs. conventional treatment in hot-arid climates: a Saudi Arabian case study

Water scarcity and the carbon intensity of conventional treatment are coupled constraints in arid regions, where reclaimed effluent is a strategic resource under Saudi Vision 2030. Vertical flow constructed wetlands (VFCWs) perform well in hot climates, but no life cycle assessment (LCA) has compared them with conventional treatment using primary kinetic data measured in a hot-arid setting, and none has adjusted the conventional benchmark for the same climate. This study reports the first comparative cradle-to-grave LCA of a pilot VFCW and a functionally equivalent conventional activated sludge (CAS) system treating institutional wastewater in Riyadh, Saudi Arabia, over a 20-year design life. The functional unit is 1.0 m³ of wastewater entering the treatment system and treated to the Saudi standard for restricted irrigation; every inventory flow, including the reuse credit, is expressed on this influent basis. The systems were compared on the operational elements that distinguish them: aeration and pumping electricity, chemical dosing, sludge production and handling, direct methane and nitrous oxide emissions, construction media and haulage, and end-of-life management. Primary kinetics and measured material quantities were combined with a regionalized inventory: grid carbon intensity of 0.69 kg CO₂-eq·kWh⁻¹, aeration energy raised by 15% for reduced oxygen transfer at high temperature, and avoided seawater desalination credited in proportion to the reusable effluent each system delivers (0.935 m³ for the VFCW after evaporative loss; 1.00 m³ for CAS). Excluding the reuse credit, the VFCW returns a global warming potential of 0.39 kg CO₂-eq·m⁻³ against 0.67 for CAS, a 42% reduction rising to 52% at community scale (10 m³·day⁻¹); paired Monte Carlo simulation over 1,000 iterations gives a discernibility proportion above 99.9%. Because the VFCW delivers less reusable water, crediting avoided desalination by effluent volume halves the absolute margin, from 0.28 to 0.14 kg CO₂-eq·m⁻³, without reversing it. The wetland removed 92.7% of ammonia-nitrogen, but total nitrogen was not measured; under a conservative nitrogen mass balance in which nitrified nitrogen leaves as nitrate, the marine eutrophication potentials of the two systems are similar (0.022 and 0.020 kg N-eq·m⁻³), and no nitrogen advantage is claimed. Net water savings through displaced desalination are 0.93 m³ (VFCW) and 0.98 m³ (CAS) per m³ treated. The advantage is conditional rather than general: phosphorus retention of 59% produces a freshwater eutrophication potential about 6.5 times that of chemically dosed CAS, the carbon advantage is lost where aggregate haulage exceeds approximately 180 km, and it disappears where the conventional plant is supplied by solar photovoltaic electricity. The findings rest on a single pilot in one climate and on a design-based conventional benchmark, and are bounded accordingly.