A Rational Approach to Size Denitrification Filters to Meet TMDL Requirements

Both the Littleton/Englewood WWTP in Colorado, and the OWASA Mason Farm WWTP in North Carolina are being called upon to meet more stringent effluent nutrient limits in the future. The configurations and treatment processes at these two plants are quite different, but they are similar in that both require post-denitrification to meet future effluent limits. This paper will describe the process approach used to determine the sizes of the post denitrification facilities in both cases and the development of kinetic denitrification equations and their incorporation into the process models developed for the two plants.

This paper will present a new rational approach to size post-denitrification facilities based on accurate predictions of secondary effluent nutrient loads. This approach allows the correct total number of filters to be used for partial or full flow treatment and blending and predicts the effluent quality after blending under all conditions of flow through the plant (including peak flows). This approach is particularly important when maximum day effluent limits need to be met.

The Littleton-Englewood (L/E) WWTP currently comprises preliminary and primary treatment, trickling filters and solids contact (TF-SC) treatment, secondary clarifiers, nitrifying trickling filters (NTF), and chlorine disinfection. The plant is currently under construction for expansion to 50 mgd. As part of the expansion, tertiary deep-bed Severn Trent/Tetra DeniteĀ® denitrifying filters are being added. Figure 1 presents a process schematic of the plant after expansion.

Currently the plant meets in-stream ammonia water quality standards. A Total Maximum Daily Load (TMDL) study is nearing finalization and stricter effluent nutrient requirements are indicated, partially because of recent drought low flow conditions in the South Platte River. A first phase requires stricter ammonia and, in addition, total inorganic nitrogen limits. It is possible that in the future, phosphorus removal may be required.

Both 30-day Average and Maximum Day limits will be imposed. Currently, ammonia limits are seasonal and range from 4.9 mg/l in April to 11.1 mg/l in December on a 30-day average basis, and 15.2 mg/l to 22.5 mg/l on a maximum day basis. These limits could be reduced in the future based on the results of the TMDL study.

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