Presence of Legacy Nitrogen and Its Impact on Nitrogen Release in Onsite Wastewater Treatment Systems

Conventional onsite wastewater treatment systems (OWTSs) provide limited nitrogen removal and the legacy nitrogen (N) accumulated may act as a nitrogen releasing source that impact groundwater quality. However, there has been no rigorous assessment of the presence and impact of legacy N on groundwater quality. In this study, two cesspools (A: under operation; B: abandoned pool) were selected to evaluate the legacy N quantity, composition and its impact on water quality at two environmental-relevant scenarios: (I) the cesspool serves as the final treatment unit for advanced treatment unit (ATU) effluent; (II) extreme weather events, such as flash flood/ heavy rains. The total amount of legacy N beneath the pools measured at two sites were 4.1 ± 1.6 mg N/g sediment, and 3.0 ± 1.0 mg N/g sediment, respectively. Organic N accounts for the majority of legacy N at both sites (97.3% at site A; 99.7% at site B), while ammonium (NH₄⁺) was the major form of inorganic N (site A: 0.030 ± 0.005 mg NH₄⁺-N /g sediment, pH =6.9; site B: 0.003 ± 0.001 mg NH₄⁺-N /g sediment, pH =3.7). In the defined study area, the majority of legacy N was present beneath the pool (87.2%) compared to the surrounding environment. The amount of legacy N accumulated beneath the pool equals to 20 40 days of total N loading from a single house at wastewater loading rate 220 gallon/day, and nitrogen concentrations 40 60 mg N/L.

In the desorption tests, 50.2% (site A) and 82.7% (site B) of the inorganic N (the sum of inorganic N in the influent and the amount adsorbed in the column) was eluted in scenario I. The accumulative total N released from site A was 3.8 times higher than that in site B. In scenario II, 96.8% (site A) of the inorganic N was eluted with synthetic rain (pH 4.6). With the same volume of desorption effluent collected (60 mL), there was no significant difference in terms of the accumulative total nitrogen (TN) released at scenario I (10.7 mg-N) and scenario II (6.9 mg-N), demonstrating the amount of TN desorbed from the column was not affected by the flow rate or flow pattern. Less than 10% of total N in the matrix was released during the experimental period and the inorganic nitrogen accounted for 23.7% 59.5% (scenario I) and 46.7% (scenario II) of the effluent from the desorption test.