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. Introduction and Problem Statement Nitrogen is one of the most important pollutants in municipal wastewater effluent. Depending on its form, nitrogen can threaten human health or damage the integrity of aquatic ecosystems. Nitrate, which is the predominant form of nitrogen in nitrified wastewater effluent, is one of the most prevalent drinking water pollutants in California. Nitrite, a form of nitrogen that typically accounts for less than 5% of the total inorganic nitrogen in wastewater effluents, also can be a problem in drinking water because it is more toxic to humans than nitrate. Ammonium, the predominant form of nitrogen in secondary wastewater effluent, is toxic to fish and aquatic organisms at concentrations typically present in wastewater effluent. All three forms of nitrogen can lead to cultural eutrophication in estuaries and in California's nitrogen-limited lakes. Organic nitrogen is the fourth form of nitrogen present in wastewater effluent. Organic nitrogen often is ignored by scientists because it typically accounts for less than 10% of the total nitrogen in secondary wastewater effluent or surface water and is not as well understood as inorganic forms of nitrogen. However, recent developments in watershed management, indirect potable water reuse and improvements in analytical chemistry necessitate further study of the nature and behavior of wastewater-derived organic nitrogen. Several recent developments necessitating a reexamination of organic nitrogen are summarized below. One reason why it is important to reexamine organic nitrogen in wastewater effluent is that it plays an increasingly important role in Total Maximum Daily Load (TMDL) regulations. To avoid complications associated with biogeochemical cycling of nitrogen, many TMDL plans are based on total nitrogen (i.e., the sum of all inorganic and organic forms of nitrogen). This simplification implies that all forms of nitrogen contribute equally to cultural eutrophication or that all forms of nitrogen are readily interconverted. While this assumption probably is reasonable for inorganic forms of nitrogen, it is questionable for organic nitrogen. Despite the potential shortcomings of this simplifying assumption, scientists and regulators use it because organic nitrogen is believed to be relatively unimportant. However, after installation of nutrient removal systems at wastewater treatment plants, organic nitrogen can be very important. An extreme example of such a situation is provided by the TMDL for the Truckee River, which originates at Lake Tahoe and terminates at Pyramid Lake, NV. Since the Truckee Meadows Wastewater Treatment Plant (WWTP) installed a nitrification/denitrification system in 1988, organic nitrogen has accounted for approximately 85% of the total nitrogen discharged by point sources in the watershed (TMWRF 2001). Engineers and regulators now are faced with questions about the merits of installing advanced treatment systems (e.g., reverse osmosis) to remove the organic nitrogen from the effluent or implementing control measures for non-point sources of inorganic nitrogen. Decisions about which approach will yield the most cost-effective control of cultural eutrophication depend upon the ability of organic nitrogen to stimulate the growth of algae and periphyton in the river. Another reason to examine wastewater-derived organic nitrogen is that it serves as a precursor toxic disinfection byproducts. In California, municipal wastewater effluent is disinfected prior to discharge or reuse. Despite the recent proliferation of ultraviolet disinfection systems, most wastewater treatment plants still use chlorine for disinfection. Furthermore, indirect potable water reuse projects (i.e., the practice of employing wastewater effluent for groundwater recharge or augmentation of surface water flow) frequently employ chlorine disinfection. Chlorination of nitrogen-containing components of municipal wastewater effluent results in the formation of three major types of toxic byproducts: (1) organic chloramines, which are toxic to aquatic organisms and resistant to transformation by dechlorinating agents such as sodium bisulfite (i.e., NaHSO3) (Jensen and Helz, 1998; Jameel and Helz, 1999); (2) trihalomethanes (THMs), which are carcinogens of concern in drinking water (Scully et al.,1988a); (3) N-Nitrosodimethylamine (NDMA), which is an extremely potent carcinogen formed in conventional and advanced wastewater treatment plants (Mitch et al. 2003). The formation of chloramines disinfection byproducts that present human health risks, such as THMs and NDMA may not present a problem if they are removed during transport in surface and groundwater systems. However, if the disinfection byproduct precursors are not removed prior to drinking water treatment, indirect potable reuse migh tstill pose a human health risk. Until recently, measurements of organic nitrogen in wastewater effluents were restricted to quantification of total organic nitrogen, which is measured after converting the organic nitrogen into an inorganic form that is readily measured. For example, organic nitrogen is measured in the Kjeldahl nitrogen test after conversion of organic nitrogen into NH3 followed by distillation and subtraction of NH4 + present prior to the conversion step (APHA, 1998). Other, more sensitive approaches for quantifying organic nitrogen use persulfate digestion to convert organic nitrogen to NO3 (APHA, 1998). New analytical methods, developed by biologists and marine chemists, have been adopted to identify specific organic nitrogen compounds in wastewater effluents. Using these techniques, scientists have characterized organic nitrogen in a limited number of wastewater effluent samples (Grohman et al. 1998; Confer et al., 1995; Scullyet al. 1988b; Burleson et al., 1980). Combining measurements made on different wastewater effluent samples, it is possible to identify approximately 10% of the organic nitrogen species (Table 1). Other synthetic organic compounds account for less than 5% of the organic nitrogen (Grohman et al. 1998). It is likely that much of the remaining organic nitrogen species consists of polymeric species (e.g., humic substances). However, additional research is needed to characterize the nature of the remaining organic nitrogen species. Table 1: Typical concentrations of organic nitrogen species in secondary wastewater effluent. Nitrogen Species Concentration Range (mM) % of Total Organic Nitrogen Free Amino Acids 0.2-2.1 0.5% Combined Amino Acids 3.9-19 5% EDTA 0.1-15 3% Dimethylamine 5.1 2% 1 references: Free and combined amino acids: Burleson et al., 1980; Confer et al., 1995; Scullyet al. 1988b; EDTA: Kari and Giger, 1996; Bedsworth and Sedlak, 1999; Dimethylamine: Hwang