Wastewater Nutrient Optimization

LCCMR Wastewater Nutrient Optimization Project

Overview | Results | Key Findings | Barriers | Next Steps | Resources | Acknowledgement

Project Overview

The purpose of this project was to work with 6-10 mechanical wastewater treatment plants and 6-10 wastewater pond sites in order to identify low and no-cost strategies to achieve better treatment for nutrient pollution.  Additionally, this project completed a series of comprehensive testing in six Minnesota wastewater pond sites in each season in order to gather information to compare characteristics between ponds that naturally achieve good nutrient treatment and those that do not.  Nutrient pollution in environmental water bodies can cause algal blooms through a process called eutrophication.  If left unchecked, these algal blooms will consume the oxygen in the water, creating a dead zone which is not suitable for life of typical aerobic organisms such as fish.  Better nutrient treatment will result in cleaner lakes and rivers in Minnesota, and help to reduce eutrophication issues for waterbodies downstream of Minnesota.

Savings Numbers Explanation

Many of the mechanical plant sites that were worked with are already treating a portion of their phosphorus chemically.  By implementing these recommendations to promote biological nutrient removal (BNR), these plants will largely have nitrogen savings and chemical savings, as phosphorus is being removed biologically instead of chemically.  The most common chemical used for phosphorus treatment in mechanical plants is ferric chloride, so this chemical reduction will also result in chloride reduction.  There may be modest additional phosphorus savings in cases where biological removal removes more phosphorus than the existing chemical process.


The project team successfully completed one-on-one technical assistance assessments with 10 mechanical wastewater treatment facilities and 14 wastewater pond sites.  The suggested saving, implemented savings, and other project outcomes are shown in the following tables. Project results can be found in the Resources section.

Table 1: Total Recommended Project Savings

Table 2: Total Implemented Project Savings
Total N (lb)Total P (lb)Chamical Reduction (lb)Energy (kWh)Annual Savings ($)
Table 3: General Project Outcomes
# of Sites Engaged# of Sites Visited# of Recommendations
Table 4: Student Assessment Outcomes
# of Students# of Partners# of Event Activities# of Presentations# of Marketing Promotions# of Resources Generated

Key Findings

Mechanical Wastewater Treatment Plants

Through modeling using the Activated Sludge SIMulation Model (ASIM) software, the team was able to identify low-cost operational changes for each pilot site to achieve better nutrient treatment through biological nutrient removal (BNR).  Typically, the modifications include converting some treatment tank volume currently used for aeration to low-oxygen tank volume instead.  A simple solution for operators is to simply purchase and install diffusor caps to prevent airflow into the tank.  A three-hole punch can be used to punch ¼’’ holes in some of the diffusors in order to create course bubble mixing in the tank while minimizing oxygen transfer.  This strategy allows operators to create a low-oxygen, mixed tank for low-cost.  This strategy was used by the St. Cloud Wastewater Treatment Facility team in their initial BNR pilot.  Some sites will also benefit from reducing aeration to the secondary aeration tanks in order to prevent excess oxygen from recirculating back to the low-oxygen zones.  Finally, some plants will greatly benefit from accepting a readily bioavailable source of industrial COD – it is believed that brewery and dairy waste tend to be particularly good for this purpose.  Adding a COD source will help drive the BNR microbial processes to completion, but will also increase aeration energy requirements.  Computer simulation modeling will help plants to develop an operational modification that will allow their plants to achieve BNR.

On average, mechanical plants in this pilot were modeled to have average nitrogen reduction of 14.14 mg/L, average phosphorus reduction of 1.84 mg/L (most sites already treat phosphorus chemically to 1 mg/L) and chemical reductions of 886 lb chemical/MGal flow.  If all of that chemical is ferric chloride (the most common chemical used in Minnesota mechanical plants in this study) this also results in a reduction of chloride of 221 lb / MGal flow.

Scaling these findings up to statewide implementation reduction results in the following statewide potential savings:

Individual site summaries are included as an attachment
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Mechanical Plants

There is considerable opportunity for mechanical wastewater treatment facilities to achieve much better nutrient treatment through low and no-cost operational changes resulting in biological nutrient removal.  Through observations made over the course of this project, several key barriers to the implementation of biological nutrient removal have been identified:

  • ComplacencyEffluent nitrogen limits are extremely rare for Minnesota wastewater treatment facilities. For this reason, there is little reason for plant operators and their consulting engineers to seriously consider modifying the existing operations to achieve BNR.  The current most common design in Minnesota meets the current permit limits.  The current design is one that both operators and wastewater engineers in Minnesota are very experienced and comfortable with using.  This lack of driving force is the most critical barrier in the way of the broad use of biological nutrient removal to achieve both nitrogen and phosphorus treatment which would result in much better treatment of nutrient pollution here in Minnesota.
  • Lack of understanding – The current industrial standard design in Minnesota does not utilize biological nutrient removal. Therefore, this team perceives lack of deep understanding of the pros and cons of the biological nutrient removal design, and the relative ease with which the current typical design can be retrofitted to achieve BNR.  This team has heard concerns related to the re-release of phosphorus from PAOs, to this type of retrofit reducing treatment capacity, to other possible issues which tend to have fairly trivial solutions but may not be immediately obvious without experience or training in BNR.
  • Split incentivesAs the current typical wastewater plant design in Minnesota does not utilize BNR, it would take considerable effort for the design engineering teams to learn to design for BNR. Because of this, there is a perception that some design engineers would prefer to maintain the status quo.  This relates to complacency but also has financial motivations, as it will take considerable effort and training to learn this more efficient treatment process.  Furthermore, modifying operations to achieve biological nutrient removal can often be achieved through low-cost operational changes, but a design engineering firm has financial incentive to promote the design of a new plant.  Additionally, fully implementing BNR for both nitrogen and phosphorus treatment will also likely hurt the sale of chemical phosphorus removal chemicals, and therefore may also be discouraged by those who sell them.
  • Lack of regulatory grace – Champion wastewater plant managers who do want to pilot operational changes to achieve biological nutrient removal would greatly benefit from a prescriptive process to achieve a period of regulatory grace or variance for peace of mind while piloting an operational change from a traditional treatment system to one that achieves biological nutrient removal. Whenever something is changed, there is risk that the change will not work as planned – prescriptive regulatory grace would help encourage plant managers to know that they will not receive punishment if their work towards a better treatment strategy does not immediately work as planned.  Knowing that they would not be punished if something failed would make optimization piloting much safer and more accessible for wastewater plant managers.

Wastewater Ponds

Wastewater pond systems have fewer barriers to implementation, but there are two critical barriers to be addressed in future work.

  • Failing Infrastructure Implementing the ‘flow through method’ requires working transfer structures and slide gates. For this project, the team specifically chose plants with mostly working transfer structures as those sites would be able to implement recommendations regarding modifications to the flow of water through the ponds.  The MRWA portion of the project team strongly believes that many sites have transfer structures which are not working well enough to implement this method.
  • Lack of knowledge – The ‘flow through’ method was developed over the course of this project. The project team has been able to share the concept with most of the project sites, and has created two case studies to showcase the benefits, however, most pond operators in the state are still unaware of it as an operational strategy to achieve better nutrient treatment.  Additional assessments, presentations, and case studies would help to spread this operating strategy to operators throughout the state.

Next Steps

Wastewater Ponds

The first objective moving forward as a result of this project is to drive implementation of the ‘flow through’ method with more of the pilot project sites.  Several sites are in the process of implementing this method over the summer of 2021.  Continued guidance, technical assistance, and follow up will help these sites to complete their pilot test of the method, quantify the results and develop additional case studies to continue highlighting the savings opportunity for Minnesota ponds.

A second objective is to determine whether this method or a slightly modified version of it is suitable for winter operation in additoin to warm weather operation, and whether it has a positive impact when used over the winter.

A third objective will be to continue reaching out to additional wastewater pond systems to share the findings of this project and to provide technical assistance to sites interested in modifying operations to utilize the ‘flow through’ method.   This project provided assessments to 14 of the 391 wastewater pond systems in the state.  A subsequent project could sort the pond systems from highest to lowest in terms of effluent nutrient concentrations, and schedule one-on-one consultations with site operators in order to discuss this strategy as an option to improve nutrient treatment.

The team is also aware that there are many wastewater pond sites with failed infrastructure used to control the movement of water between the pond systems.  The team would like to acquire a source of funding to help cities with wastewater pond sites to have these control structures repaired, and then to connect that repair process with improved operational strategies that are made possible through these repairs.  Having control structures repaired and then teaching operators how to utilize the repaired structures to achieve better nutrient treatment would help these wastewater pond systems to achieve great nutrient treatment moving forward.

Mechanical Wastewater Plants

There is benefit in having funding available to complete computer simulation models of mechanical wastewater treatment facilities that are specifically interested in the opportunity to achieve biological nutrient removal through relatively low-cost operational change.  While the project has shown that implementation rates have been low through simply reaching out to facilities and offering a no-cost biological nutrient removal assessment, as the barriers to biological nutrient removal are reduced, sites that do not want to completely redesign their mechanical wastewater plants would benefit from having the option to explore low-cost operational change options.  Should wastewater treatment facilities begin to receive total nitrogen limits, biological nutrient removal is the only commonly used strategy to remove nitrogen from the wastewater.  Additionally, should additional regulatory grace be put in place for wastewater operators interested in exploring biological nutrient removal to empower operators to start achieving nitrogen treatment before it is mandated, that would also likely spur interest in low-cost options for operators to achieve biological nutrient removal.  Regardless, as these barriers are reduced, there is benefit in having a statewide resource that can complete wastewater simulation modeling and provide guidance on nutrient optimization strategies for wastewater treatment plants.

In terms of increasing the interest of wastewater plant operators in this type of operational modificaiton to improve nutrient treatment, there are some options.  First, if plants are assigned total nitrogen limits that require nitrogen removal, this will need to be accomplished using biologicla nutrient removal.  If the project sites from this project are representative of the state, most sites can accomplish this for relatively low cost through operational changes, primarily by reallocating some secondary aeration tank volume for use as low-oxygen volume.

Alternatively, or perhaps as an interim incentive, perhaps plant operators can be rewarded for achieving lower total nitrogen concentrations in the effluent.  An award or special recognition for operators who successfully implement biological nutrient removal may help to promote a culture of improvement surrounding wastewater treatment.

Regulatory grace from the MPCA to operators that choose to pursue biological nutrient removal pilot projects would also help empower operators to make changes that are expected to improve long term treatment quality.

It would also be beneficial if there was full or partial infrastructure upgrade funding that could be made available to mechanical wastewater treatment facilities that would benefit from some funding for equipment similar to tank mixers or baffles in order to facilitate the creation of conditions required for BNR.





The authors would like to acknowledge support for this project by the Minnesota Pollution Control Agency (MPCA) with funding provided by the Minnesota Environmental and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR). The Trust Fund is a permanent fund constitutionally established by the citizens of Minnesota to assist in the protection, conservation, preservation, and enhancement of the state’s air, water, land, fish, wildlife, and other natural resources. Currently 40% of net Minnesota State Lottery proceeds are dedicated to growing the Trust Fund and ensuring future benefits for Minnesota’s environment and natural resources.

Wastewater Nutrient Optimization Contact

Jon Vanyo – Engineer