Biodegradability of Dissolved Organic Nitrogen: Adaptation of the BOD Test

To prevent eutrophication in receiving waters, many astewater treatment plants are being firected by regulatory agencies to achieve very low levels of total nitrogen (3–4 mg/L TN or lower) in the effluent. Dissolved organic nitrogen (DON) component is included within these total nitrogen limits. The regulations assume that DON is completely biodegradable, and therefore needs to be included in any total nitrogen control. In many cases the effluent DON concentrations can approach 1 mg/L or higher and can constitute one-third to one-half of the effluent total nitrogen that a wastewater treatment plant will need to attain to maintain permits. The objectives of the research were to develop a biodegradable dissolved organic nitrogen (BDON) test to determine the degradability of organic nitrogen containing constituents in wastewater effluents using US Envirionmental Protection Agency approved analytical procedures and to assess the BDON for four advanced wastewater treatment plants in the Chesapeake Bay region. The developed test is an adaptation of a commonly used biochemical oxygen demand test. Several 20-day batch BDON (BDON20) experiments were conducted to determine the biodegradability of effluent samples from the four plants. Effluent samples were obtained prior to filtration and from the final effluent. The BDON20 for effluent samples prior to filtration varied between the four plants examined. A decrease in DON was observed for prefiltration samples from two out of four plants, and between one-quarter to one-third decrease in organic nitrogen was observed for these two plants over the 20-day period. In the remaining two plants, there was insignificant decrease in DON over the 20-day period. The BDON20 for final effluent samples was not significant and below the detection limits of the BDON test for all four plants examined. There was a substantial decrease in dissolved oxygen of between 4-6 mg/L over 20-day period, for all samples examined, but this decrease did not correspond to a similar decrease in DON, suggesting other less refractory carbonaceous sources for the decrease or non-growth linked carbon degradation. Much additional work remains, including assaying the BDON in wastewater effluents using alternate inoculum and using longer term assays. This adapted BDON test is a convenient method to conduct these assays.