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April 8, 2015

Nutrient Removal: Starting With An “End-In-Mind” Focus

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Donohue & Associates has assisted countless communities in developing nutrient removal plans to meet current and future requirements. When evaluating phosphorus removal techniques, strategies are developed with an “end-in-mind” focus. The specific end goal may be loosely defined but often ties to effluent permit requirements and operational preferences. The demands placed on the overall treatment facility can challenge specific nutrient removal goals, especially phosphorus.

When a facility is seeking to reduce phosphorus, two options exist: biological and chemical. Oftentimes, this becomes a balancing act of both options; however, properly designed facilities have demonstrated the ability to meet end goals with little to no chemical use. For extremely low phosphorus end goals where the effluent is less than 0.10 milligrams/liter (mg/L) total phosphorus (TP), a chemical system is imperative.

Chemical phosphorus removal end goals are often achievable regardless of the effluent nitrogen requirement, whereas biological phosphorus removal has a significant relationship to nitrogen removal goals. Donohue has been working with communities to develop biological phosphorus removal concepts for communities with varying levels of nitrogen removal, for example:

Conventional – For communities that do not have an effluent ammonia limit, the activated sludge process can be operated to avoid ammonia to nitrate conversion. The lack of nitrates in the recycle stream allows full utilization of influent volatile fatty acids to be processed biologically using a special phosphorus-accumulating mechanism. Controlling a facility to avoid nitrification takes special attention and system knowledge. Modern advances such as in-situ nutrient analysis have proven to be invaluable tools for understanding and troubleshooting biological nutrient removal systems. Donohue has performed extensive process modeling, troubleshooting, and training to optimize existing facilities.

Nitrification – Facilities with an end goal to minimize effluent nitrogen are further challenged. Unlike phosphorus, nitrogen removal is only accomplished through biological means. Thus, when implemented with biological phosphorus removal, the biological and chemical systems must be compatible or end goals will not be met. Nitrification converts ammonia to nitrate, which, when improperly introduced to the biological phosphorus removal zone, can negatively compete for volatile fatty acids. To prevent concerns with nitrates, Donohue has successfully applied denitrification reactors to the return sludge, most recently at Eau Claire, Wisconsin. For alkalinity sensitive facilities, the addition of denitrification restores a portion of the alkalinity consumed by nitrification.

Denitrification – More facilities are facing total nitrogen (TN) and TP removal. Removal is traditionally achieved through a series of unaerated and aerated activated sludge zones to controllably enable nitrification and denitrification. This approach limits the amount of nitrates returned to the anaerobic zone for optimal biological phosphorus removal. Donohue assisted with startup activities at a 5-stage Bardenpho biological phosphorus and nitrogen removal facility in Cashmere, Washington that has exhibited effluent TN less than 2 mg/L and TP less than 0.2 mg/L with no chemical addition.

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