The Speciation and reactivity of wastewater-derived organic nitrogen
Nitrogen often is the limiting nutrient for the growth of algae and phytoplankton in estuaries and surface waters in California. To control cultural eutrophication (i.e., excessive growth of algae and plankton related to anthropogenic sources) regulatory agencies often focus on the control of point source discharges, including municipal wastewater effluent. Recent attempts to control cultural eutrophication in nitrogen-limited systems have focused on the simultaneous control of all forms of inorganic nitrogen with the underlying assumption that inorganic and organic nitrogen are equally bioavailable. To assess the validity of this assumption, algal growth bioassays were conducted using denitrified wastewater effluent samples that contained mainly organic nitrogen. The growth assays were performed with a species of algae (Selenastrum Capricornutum) that is commonly used for regulatory compliance monitoring. Results of the study indicate that the wastewater-derived dissolved organic nitrogen (DON) is not bioavailable to the algae in the absence of bacteria. However, approximately half of the wastewater-derived organic nitrogen was available to the algae in the presence of bacteria during a two-week incubation.
In conjunction with the experiments on bioavailability, the nature and properties of wastewater-derived organic nitrogen was characterized by measuring concentrations of free and combined amino acids and by subjecting wastewater effluent samples to ultrafiltration. Results of these experiments indicate that most of the wastewater-derived organic nitrogen was associated with unidentified compounds that are capable of passing through a 1 kDa ultrafilter.
Wastewater-derived organic nitrogen also plays an important role in the formation of the carcinogenic disinfection byproduct, N-nitrosodimethylamine (NDMA). NDMA is formed when wastewater effluent is disinfected with chlorine. It also can be formed when surface water or groundwater that has been impacted by wastewater effluent discharges undergoes disinfection in drinking water treatment plants. Recently, concerns associated with NDMA have caused great concern among utilities that practice indirect potable water reuse. Most of the attention to date has focused on the removal of NDMA from the treated wastewater effluent. However, the presence of NDMA precursors also could be a problem if the NDMA precursors in drinking sources are stable after they are discharged because they could result in NDMA formation during drinking water treatment. To assess the stability of NDMA precursors associated with wastewater-derived organic nitrogen, NDMA precursor concentrations were measured in effluent samples before and after incubation with bacteria under aerobic conditions. Results of the experiments indicate that the NDMA precursors are stable for at least 30 days.
These results suggest that wastewater-derived organic nitrogen consists of complex compounds that are not very reactive. However, under certain conditions, wastewater-derived organic nitrogen species can serve as a source of nutrients in nitrogen-limited systems and as disinfection byproduct precursors. The results of this research will be useful in the development of indirect potable water reuse systems and the design of watershed protection plans designed to protect aquatic ecosystems from the effects of cultural eutrophication.