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

Stable isotopes (delta N-15, delta O-18) can serve as tracers for sources of nitrogen in the receiving environment. Hong Kong discharges similar to 3 x 10(6) m(3) 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 NOx) ammonium (NH4+), phosphate (PO4+) concentrations and delta N-15(NOx), delta O-18(NOx). Sewage effluents contained high mean nutrient concentrations (NO3- = 260 mu mol L-1 = 1400 mu mol L-1, PO4+ = 50 mu mol L-1) with some indication of nitrogen removal in advanced treatment types. Mean delta N-15(NOx) of sewage effluents from all plants and treatment types (12 parts per thousand) was higher than natural sources and varied spatially and seasonally. There was no overall effect of sewage treatment type on delta N-15(NOx). A mass balance model indicated that sewage (>68%) remains a dominant source of nitrate pollution in seawater in Tolo Harbor. (C) 2016 Elsevier Ltd. All rights reserved.