Variations in nitrate isotope composition of wastewater effluents by treatment type in Hong Kong

Stable isotopes (δ¹⁵N, δ¹⁸O) can serve as tracers for sources of nitrogen in the receiving environment. Hong Kong discharges ~ 3 × 10⁶ m³ d⁻¹ of treated wastewater into the ocean from 68 facilities implementing preliminary to tertiary treatment. We sampled treated sewage from 18 plants across 5 treatment types and examined receiving seawater from northeast Hong Kong. We analyzed nitrate and nitrite (NO₃⁻ + NO₂⁻, hereafter NO_x) ammonium (NH₄⁺), phosphate (PO₄⁺) concentrations and δ¹⁵N_NOx, δ¹⁸O_NOx. Sewage effluents contained high mean nutrient concentrations (NO₃⁻ = 260 μmol L⁻¹, NH₄⁺ = 1400 μmol L⁻¹, PO₄⁺ = 50 μmol L⁻¹) with some indication of nitrogen removal in advanced treatment types. Mean δ¹⁵N_NOx of sewage effluents from all plants and treatment types (12‰) was higher than natural sources and varied spatially and seasonally. There was no overall effect of sewage treatment type on δ¹⁵N_NOx. A mass balance model indicated that sewage (> 68%) remains a dominant source of nitrate pollution in seawater in Tolo Harbor. - Highlights: • Hong Kong is an ideal site to study sewage treatment impacts. • This study is among the first to conduct wastewater δ¹⁵N_NOx analysis by treatment type. • Other nitrogen sources alter the isotopic composition of treated sewage and seawater. • Sewage contributed to > 68% of nitrate in a harbor with no known point sources of sewage.