Gravity-driven membrane filtration of primary wastewater effluent for edible plant cultivations: Membrane performance and health risk assessment

In this study, gravity-driven membrane (GDM) systems packed with Icelandic lava stones (0%, 26% and 53%) were employed to treat primary wastewater for reuse. The three GDM systems could achieve > 75% removal of organics. While, the lava stone biocarriers facilitated higher nitrogen removals (>51% vs. similar to 37% without biocarriers), which were related to the presence of manganese oxide (MnOx) in the lava stones. A combined mechanism of pore constriction and cake fouling was predominant during flux stabilization, regardless of lava stone packing ratios. Compared with non-cleaning or geothermal water-/persulfate-based cleaning, sodium hypochlorite-based cleaning (0.5% in geothermal water at 50 degrees C, 10 min per 3-4 days) was more effective in improving flux recovery (similar to 46-79%) and water permeability (similar to 23-79%). Furthermore, GDM permeate was used to irrigate two vegetables (tomato and basil), with tap water and commercial fertilizer as comparison baselines. Statistically insignificant (p > 0.05) results were observed for vegetable growth profiles and non-essential heavy metal contents in soils. Even though the GDM permeate-irrigated vegetables had a slightly higher uptake of non-essential metal cadmium (Cd) compared to those with tap water, the hazard quotient index was still less than 1, indicating negligible human health risk.