Phosphorus attenuation and mobilization in sand filters treating onsite wastewater.

The effectiveness of phosphorus (P) removal by sand filters is limited during septic tank effluent (STE) treatment. The elevated effluent P concentrations pose threats to drinking water quality and contribute to eutrophication. The concern of P leaching from sand filters is further exacerbated by the increased frequency of flooding and natural precipitation due to climate change. This study aimed to understand P attenuation and leaching dynamics, as well as the removal mechanisms in sand filters treating STE, offering insights into the design and implementation of P removal/recovery modules to onsite wastewater treatment systems. P attenuation and leaching during STE treatment and rainfall were studied in bench-scale columns (new vs. aged sand). At standard STE loading (1.2 gallon d−1 ft−2), 24–32% removal of total phosphorus (TP) was achieved, while increased P removal efficiency (35–53%) was observed at low loading (0.6 gallon d−1 ft−2) with influent containing 10.3–20.0 mg P L−1. Complete breakthroughs were observed in both aged (12–70 days) and new columns (27–73 days) at test hydraulic loadings. The maximum TP attenuation level was 20.6–45.3 mg P kg−1 and 25.3–33.0 mg P kg−1, in aged and new sand columns, respectively. When simulated rain was applied (15–60 mm h−1), 80–97% of the attenuated P leached out and the leaching dynamics were impacted by rainfall duration rather than the intensity. The highest concentrations of TP (15.6–15.9 mg L−1) were leached out from both columns within the first 2–6 h. Orthophosphate was the dominant P species in treated effluent (83–84%) and leachate (69–88%), demonstrating its significance as the major P form in the discharge. In addition, aged sand (>5 years) accumulated higher levels of Mg, Al, Ca, and Fe, thus enhancing the P attenuation level during STE treatment. Collectively, this study underscored the importance of frequent field monitoring for reliable long-term P removal estimates. [Display omitted] • P attenuation in sand filters is reversible and is impacted by STE loadings. • The extent of P leaching is more impacted by rainfall duration than intensity. • Adsorption is the main P removal mechanism in sand filters treating STE. • P attenuation is enhanced by metal accumulation in sand after long-term STE treatment. • Frequent monitoring is essential to understand the fate and dynamic of P in OWTSs. [ABSTRACT FROM AUTHOR]