Nutrient Reduction Strategies – Two Case Studies

The Iowa Nutrient Reduction Strategy was created in November 2012. It was revised in May 2013 and October 2014 to reduce nitrogen and phosphorous in Iowa waterways which lead to negative environmental impacts to watersheds and the Gulf of Mexico due to algal blooms. These blooms cause hypoxic zones, which lead to fish kills and other ecological damage.

The Nutrient Reduction Strategy applies to all wastewater plants with an average wet weather (AWW) flow of 1.0 million gallons per day (MGD) or higher. All plants that meet this flow threshold will be required to file a Nutrient Reduction Strategy Feasibility report with IDNR as part of their NPDES permit renewal.

New permits typically include language that requires communities to submit a report that includes “A description and evaluation of new or additional treatment technologies that would achieve significant reductions in the amounts of total nitrogen and total phosphorus discharged in the final effluent ….”

Important Features

The system must be designed to meet annual average effluent total nitrogen (TN) concentration of 10 mg/L and phosphorus (P) concentration of 1.0 mg/L. The permit limits will be based on mass loads calculated at the AWW flow. While the system must be designed to meet the effluent concentration, compliance with permit limits will be based on the total mass discharge compared to the mass limit calculated at the AWW flow on an annual average basis.

The Nutrient Reduction Strategy feasibility report must include an evaluation of the existing facility, operational changes that could be implemented, and an evaluation of new or additional treatment technologies to significantly reduce TN and P.  The evaluation must include capital costs, annual operating costs, and impact-on-user charges to determine feasibility, reasonableness, or practicability.  A schedule must be proposed for implementing the required improvements to meet the nutrient reduction strategy, or a detailed financial justification must be provided if meeting the nutrient reduction strategy goals is determined to be unreasonable.

There are several possible outcomes for this feasibility report. One is that the existing system meets the goals of the nutrient reduction strategy. In that case, the permit will be modified to include the nutrient limits.

Another possible outcome is the existing system does not meet the requirements, but can meet them with operational modifications. In that case, the operational changes must be implemented, and the facility will be monitored for a year to determine its capabilities prior to nutrient limits being added to the permit.

Another possible outcome is the existing system does not meet the requirements, but can be modified, upgraded, or replaced to meet the Nutrient Reduction Strategy goals, and it has been determined to be reasonable, feasible, and practical to do so. In that case, the city must develop a schedule implementing the improvements necessary to meet the Nutrient Reduction Strategy goals.

Finally, if the system does not meet the requirements, and it is determined to be unreasonable to do so and would impose a high financial burden on the community, no further action is required.  However, in the next permit renewal, another feasibility report will be required. Following are two case studies of Nutrient Reduction Strategy Feasibility Reports completed by FOX.

New Hampton, IA

New Hampton operates a trickling filter plant with a permitted AWW flow of 2.2 MGD and MWW flow of 3.0 MGD. The system includes screening, grit removal, primary clarification, roughing filter, trickling filter, secondary clarification, and new UV disinfection. In 2015, the system had an average influent TN concentration of 39.4 mg/L and an average effluent concentration of 18.5 mg/L. The influent P concentration averaged 3.69 mg/L and the effluent concentration averaged 3.28 mg/L.

New Hampton

New Hampton

This system does not meet either the TN or P Nutrient Reduction Strategy goals. Due to the inherent limitations of a trickling filter system, there are no simple modifications or operational changes to improve nutrient reduction.

FOX evaluated several options for meeting the nutrient reduction goals, including denitrification filters, chemical phosphorus removal, and total plant replacement. Capital costs for the necessary improvements ranged from $5.5 to $14.4 million. When considering additional operating costs and other needed improvements to the facility, implementing nutrient reduction would increase the cost of wastewater treatment per household to 1.5% to more than 2% of the median household income.

Due to the high cost, implementing nutrient reduction was determined to be not feasible at this time. The city is planning to replace the facility and incorporate nutrient reduction over the next 20 years, when the existing facility reaches the end of its useful life.

Charles City, IA

Charles City has a trickling filter system that has a permitted AWW flow of 4.66 MGD and a MWW flow of 9.18 MGD. The system has screening, grit removal, primary clarification, trickling filter, biotower, secondary clarification, and UV disinfection. In 2015, the average influent TN concentration was 22.1 mg/L and the average TN effluent concentration was 15.3 mg/L. The average influent P concentration was 4.21 mg/L and average effluent concentration was 2.70 mg/L.

This system does not meet the Nutrient Reduction Strategy goals for TN or P.

Charles City

Charles City

Again, the trickling filter system does not have many operational parameters that can be changed to enhance treatment. It would be possible to install chemical phosphorus removal and denitrification filters; however, the plant is at the end of its useful life and a new major industrial contributor will increase the load to the plant above its design capacity.

It is therefore more reasonable to replace the system with a new plant that can meet the nutrient requirements as well as these new challenges. The City has tasked FOX Engineering with preparing a new facility plan under the assumption that the plant will be replaced. The new plant will be designed to provide sufficient treatment to meet the goals of the Nutrient Reduction Strategy by 2021 to comply with IDNR guidance.


Every wastewater plant with an AWW flow of 1.0 MGD or higher will be required to provide a study showing how the system will meet the nutrient reduction strategy goals in the next five years or show that such changes will not be feasible. For some treatment plants, it may be as simple as making some operational changes. Other facilities may be able to meet the goals with relatively minor modifications.  Some facilities will require a whole new treatment system.

A thorough evaluation of the costs associated with the necessary improvements will help determine if nutrient reduction is feasible. If it is shown not to be feasible within the five-year timeframe, another study will be required when the permit is renewed.

IDNR’s goal is that eventually all plants with AWW flow of 1 MGD or more will meet the nutrient reduction strategy goals.

FOX Engineering is an environmental engineering firm based in Ames, Iowa. We specialize in water and wastewater solutions for our diverse municipal and industrial clients. Our work varies in size and scope and can be found throughout the Midwest and beyond.