Nitrogen Removal System Impacts on Secondary Treatment Greenhouse Gas Production and Whole Plant Carbon Footprint

Traditional top-down methods of determining the carbon footprint of a wastewater treatment plant does not provide the detailed feedback needed by operations and design to minimize carbon footprint at the facility. A detailed bottom-up approach based upon whole plant modeling has been developed that gives the detail needed by designers and operators. This bottom up approach is based upon the ASMN model where two step nitrification and four step denitrification is used for secondary treatment. This model releases N2O as a part of the denitrification system. Autotrophic N2O production is not considered.This modeling study of nitrogen removal and its impacts upon whole plant carbon footprint clearly show the importance of biogenic N2O production in the secondary treatment system. This production accounted for between 35% and almost 65% of the total plant footprint.In this model, N2O release is primarily by stripping in the aerobic zone by the fine bubble aeration system. As a result the lowest N2O production levels were seen in systems that minimized the amount of N2O entering that first aerobic zone. The last anoxic zone nitrate/nitrite level was used as a surrogate for N2O levels since it is more easily measured. Essentially if there are significant nitrate/nitrite levels in the last anoxic zone, the denitrification intermediate, N2O, is also likely to be present at higher levels.If the designer's, or operator's, goal is to minimize their plant's carbon footprint, the goal should be to minimize the concentration of nitrate leaving the plant's anoxic zones. While the plant influent COD/N ratio is not under the control of either party the design and operation of the nitrogen removal system can be adjusted for the desired goals.On a whole plant basis, the production of N2O in the bioreactor system is the single largest variable on the plant carbon footprint, with it accounting for between 25% and 65% of the total. This paper has shown that this carbon impact can be minimized during design and operation ofthe facility.