Stage temperature boosting in solar membrane distillation systems for agricultural wastewater treatment using waste heat recovery

Solar-powered membrane distillation for agricultural wastewater is limited by temperature polarisation and solar intermittency. This study evaluates stage temperature boosting – injecting low-grade agricultural waste heat at intermediate stages – in an eight-stage air-gap membrane distillation (AGMD) pilot operated at an agricultural research facility in the Fergana Valley (Uzbekistan). Under high irradiance (>800 W∙m−2), distillate flux increased by 88% from 6.5 ±0.4 to 12.2 ±0.6 kg∙m−2∙h−1 (n = 3, p < 0.001), and under low irradiance (<500 W∙m−2) from 2.8 ±0.3 to 9.1 ±0.5 kg∙m−2∙h−1 (n = 3, p < 0.001). Uptime (flux ≥80% of nominal) improved from 67 to 92%. Energy efficiency rose markedly: gained output ratio (GOR) increased by 161% from 3.1 ±0.3 to 8.1 ±0.5 (p < 0.001). Total specific energy consumption decreased by 62% to 145.0 ±11.4 kWh∙m−3, driven by lower thermal demand (specific thermal energy consumption, SECth: from 375 ±28 to 138 ±11 kWh∙m−3) with a modest electrical increase (specific electrical energy consumption, SECel: from 2.8 ±0.2 to 7.0 ±0.4 kWh∙m−3). Permeate quality remained high (total dissolved solids (TDS) rejection >99.5%; Escherichia coli not detected). By sustaining the thermal driving force and mitigating polarisation, stage temperature boosting couples solar input with site-available waste heat to deliver robust, energy- efficient water reclamation for agriculture.

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