Passive Permeate-Side-Heated Solar Thermal Membrane Distillation: Extracting Potable Water from Seawater, Surface Water, and Municipal Wastewater at High Single-Stage Solar Efficiencies

Globally, hundreds of millions of people still drink untreated surface water due to the lack of even a basic drinking water service, and urgently need economical off-grid water treatment devices. A passive, single-stage, permeate-side-heated solar thermal membrane distillation system is developed for extracting potable water from seawater, surface water, and municipal wastewater. The carbon black-coated permeate side of 0.45 µm PVDF membrane absorbs solar radiation and evaporates the feed water within the pores of the membrane. Under natural sunlight, the distillate flux was 8.56 kg/(m²∙day) at an average daytime irradiance of 652 W/m², equivalent to the system energy efficiency of 67.5%, the highest so far for single-stage solar distillation under natural sunlight. This system removed all the heterotrophic bacteria, 99.9% turbidity, and 99.6% chemical oxygen demand (COD) from wastewater, and reduced electrical conductivity by 99.9% from seawater, during the first 8 hours of operation under simulated sunlight (1800 W/m²). The operation continued for 32, 18, and 10 days on average for seawater, canal water, and wastewater, respectively, until the feed water penetrated the membrane. Throughout long-term experiments, distillate had decreasing flux within 0.98–1.55 kg/(m²∙h), steady pH of 7.2–7.9, steady turbidity of 0.09–0.21 NTU, steady electrical conductivity within 0.003–0.451 mS/cm, and increasing COD within 1.9–9.2 mg/L regardless of the type of feed water. Comprehensive water quality tests show that the distillate extracted from all types of feed water meets U.S. drinking water standards for total coliform, 22 heavy metals and minerals, 7 anions, 5 physical factors, and 50 volatile organic compounds, and will be safe to drink in real-world applications.