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Enrichment of 15N/14N in wastewater-derived effluent varies with operational performance of treatment systems: implications for isotope monitoring in receiving environments

Munksgaard, Niels C., Warnakulasooriya, Kanchana N., Kennedy, Karen, Powell, Lynne, Gibb, Karen S.
Environmental monitoring and assessment 2017 v.189 no.1 pp. 36
ammonia, emissions, monitoring, nitrogen, ponds, stable isotopes, waste lagoons, wastewater, wastewater treatment, Australia
Stable nitrogen isotope ratios are routinely used to trace the dispersion and assimilation of wastewater-derived N in receiving environments, but few isotope studies have investigated wastewater treatment plants and ponds themselves. An improved understanding of N isotope compositions in effluent will help assess treatment plant processes and performance and will help trace sources of excess nutrients in receiving environments. Here, we assess N budgets and treatment processes in seven wastewater treatment plants and wastewater stabilisation ponds in northern Australia based on concentrations and isotope ratios of N in effluent. We show that δ¹⁵N values in effluent are linked to treatment type, effectiveness of conversion of ammonia and levels of gaseous N emissions. These relationships suggest that N isotope monitoring of wastewater treatment plants and ponds can provide an integrated assessment of treatment performance and gaseous N emissions on a pond- or plant-wide scale that is not readily available through other methods. Our findings further imply that monitoring N isotope ratios in receiving environments cannot be assumed to be universally effective as their sensitivity to uptake of wastewater-derived N will vary with the characteristics of individual treatment systems. Paradoxically, N isotope monitoring is less effective where treatment systems are functioning poorly and where monitoring needs are the greatest.