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.

Potential release of legacy nitrogen from soil surrounding onsite wastewater leaching pools.

This study examined whether the accumulation of nitrogen (legacy nitrogen) within and surrounding leaching pools for onsite wastewater treatment may act as a source of nitrogen contamination to groundwater upon changes to the quantity and/or composition of the influent to the pool. In this study, one concrete leaching pool with neutral pH (A, pH 6.9) and one leaching pool after acid washing (B, pH 3.7) were selected to examine the quantity and composition of legacy nitrogen in the surrounding soil, as well as evaluate the potential release of this nitrogen under two environmentally relevant leaching scenarios: (i) the concrete leaching pool serves as the final discharge unit for aerobic treatment unit (ATU) effluent; (ii) extreme weather events (flash flood/heavy rains) act to increase the quantity and dilute the composition of flow to the pool. Core sample analysis showed that organic nitrogen accounts for the majority (97.3-99.7%) of the total nitrogen (TN) at site A (4.1 ±â€¯0.6 mg N/g soil) and site B (3.0 ±â€¯0.4 mg N/g soil); while ammonium was the major form of inorganic nitrogen present at the sites. The TN accumulated under the two leaching pools was equivalent to approximately 17-39 days of nitrogen loading to the system. pH had a significant impact on the mass of TN leached from the soil, while no significant difference in leached TN was observed for the two leaching scenarios. The amount of TN leached from the soil matrix was not affected by the flow rate (18.6 mL/d in scenario i vs. 547.2 mL/d in scenario ii) or flow pattern (intermittent dosing vs. continuous flow). The quantity of TN leached from soils in both scenario (i) and (ii) was low and accounted for 2.6-8.9% of the total nitrogen in the soil.