Recycling Nitrates to Headworks for Multiple Benefits in a Fixed Film Plant

This paper describes the results obtained at the Littleton/Englewood wastewater treatment plant using an intra-plant nitrified effluent recycle (NER). A NER capacity equal to 50-percent the design maximum month flow was implemented. By pacing the NER flow the plant can be essentially base-loaded thereby attenuating influent diurnal peaks and providing overall plant stability. The removal of nitrates in the headworks, recycled back via the NER was approximately 35-percent the mass returned. During the period studied the mass of nitrate removed through the headworks averaged about 580 lb/d as nitrogen. The corresponding mass of total inorganic nitrogen removed through the trickling filters averaged 1,230 lb/d. On average a total of 1,900 lb/d of nitrogen was removed upstream of the “official” nitrogen removal processes. For an example removal of 1,500 lb/d nitrogen, the annual cost of the NER system would be one-third the cost of the denitrification filter system using methanol. Implementation of the NER system significantly reduced odors and H2S levels measured at the primary clarifiers, thereby saving on odor control chemicals. By incorporating the intra-plant recycle, redundant and costly denitrification filter units were avoided.