The outlet water quality of many municipal wastewater treatment plants (MWWTPs) is characterized by high nitrogen strength, which can cause culture eutrophication. Due to the more and more stringent effluent total nitrogen discharge standards in China, MWWTPs need to upgrade and reconstruct. Postdenitrification biofilters (DNFs) have been used for tertiary nitrogen removal in MWWTPs because of their high efficiency and low cost. In this process, oxidized nitrogen is denitrified to nitrogen gas under anoxic conditions, with organic carbon as the electron donor.
Effluent nitrogen is composed of inorganic and organic nitrogen. Dissolved organic nitrogen (DON) can account for 52% and even up to 80% of the effluent nitrogen in low effluent total nitrogen MWWTPs. Effluent DON is the key factor to limit the total nitrogen concentration in the effluent of WWTPs. Moreover, it has been shown that a considerable fraction of effluent DON can be more bioavailable to bacteria and algae than inorganic nitrogen. Bioavailable DON (ABDON) is the component of DON which supports the growth of bacteria and algae. Unfortunately, most researchers and engineers working on DNFs have primarily focused on decreasing the inorganic nitrogen, whereas concern is rarely paid to the DON and ABDON.
Recently, a team of researchers led by professor Hongqiang Ren from the State Key Laboratory of Pollution Control and Resource Reuse at Nanjing University evaluated the effect of the carbon-to-nitrogen (C/N) ratio (3, 4, 5, and 6) on the removal characteristics of DON and ABDON in the pilot-scale DNFs treating a real secondary effluent. The availability of an electron donor, conveniently expressed in terms of the C/N ratio, is the main control parameter for the DNF process. Optimizing external carbon addition not only improves the NDF process efficiency but also saves chemical costs and reduces secondary pollution (e.g., chemical oxygen demand). The chemical composition of DON in DNF effluent was also investigated with the purpose of facilitating a better understanding of the impact of the C/N ratio on DON bioavailability (ABDON/DON).
Results showed that DNFs effluent DON accounted for 31.2-39.8% of the effluent total nitrogen. The maximum effluent DON and ABDON concentrations both occurred in DNF operated at a C/N ratio of 3. Effluent DON from DNFs did not significantly differ when the C/N increased from 4 to 6; however, effluent ABDON and DON bioavailability significantly decreased with C/N ratios (p ˂ 0.05, t-test). Effluent ABDON is an important issue and desirable to be removed by wastewater treatment plants because ABDON can be bioavailable to natural algae and support eutrophication in effluent receiving waters.
Overall, this study supports the benefit of a high C/N ratio during the DNF process in term of controlling the DON forms that readily stimulate algal growth. The results of this study provided a solution to control the eutrophication and would be helpful for the researchers and engineers working on the upgrading of municipal wastewater treatment plants. The work is currently published in the research journal Environmental Science & Technology.