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.

Automated summary

The study found that using GDM systems packed with Icelandic lava stones can efficiently treat wastewater and facilitate nitrogen removal efficiency through the presence of manganese oxide. The cleaning method significantly affects flux recovery and water permeability. Irrigating vegetables with GDM permeate has minimal impact on plant growth and heavy metal contents in soils compared to using tap water.