Comparative Assessment of Solar Distillation of Greywater With and Without Boiling for the Production of Distilled Water

Abstract

Water scarcity in arid urban environments has driven the utilization of graywater for potential reuse. Among lowcost water treatment technologies, solar distillation is an attractive option; however, its field performance with real effluents and the direct comparison between boiling and non-boiling configurations remain scarcely documented. In this study, the solar distillation of domestic graywater was compared using (i) a boiling system, consisting of a CK-002 solar cooker coupled to a black-painted copper still, copper coil, and condenser, and (ii) a non-boiling system, consisting of a single-slope solar still with a glass cover. Graywater was collected from five households in Tacna (Peru), and 5 L batches were treated per trial for 11 h on sunny days, recording the distillate volume, physicochemical parameters (pH, conductivity, turbidity, nitrates, nitrites, sulfates, BOD₅, and COD), total metal concentrations, and microbiological indicators of fecal coliforms, Escherichia coli, and heterotrophic bacteria. Results showed that the boiling solar distillation system produced 2,790 mL of distillate, compared to 1,725 mL from the non-boiling system, with a statistically significant difference (α = 0.05). Regarding  physicochemical results, significant differences were found only in turbidity, conductivity, and nitrates, with the non-boiling system achieving the best performance (removal > 90%). Total metal removal from graywater (initial concentration 445.03 mg/L) was 98.63% (6.06 mg/L) for the boiling system and 98.56% (6.37 mg/L) for the non-boiling system. Microbiological results for both distillate samples were similar: fecal coliform counts (MPN/100 mL) were reduced by 99.76%; heterotrophic bacteria counts (CFU/mL at 35 °C) by 99.99%; and E. coli counts (MPN/100 mL at 44.5 °C) by 98.04%. Solar distillation, with and without boiling, enabled substantial reduction of microbiological contaminants and total metals; however, due to the persistence of non-volatile organic compounds, such as biodegradable organic matter, which remain above the limits established in the Peruvian Environmental Quality Standards, its optimal use is as a barrier within a treatment train intended for restricted reuse, or in combination with post-treatments to meet regulatory requirements.