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.  The mass of the chloride portion reduced was estimated to be the chloride reduction.  There may be modest additional phosphorus savings in cases where biological removal removes more phosphorus than the existing chemical process.

Results

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: Mechanical Wastewater Plant Recommended Project Savings

Total Mechanical Plant Recommended Project Savings

Total N (lb)Total P (lb)Chamical Reduction (lb)Energy (kWh)Additional Required Mix Energy(kWh)Annual Savings ($)
M17,400200245,700580,0000$89,200
M26,3002,40027,100150,0000$16,500
M3109,6001,100271,000393,000-65,000$74,000
M456,5003,7000412,700-49,200$29,500
M587,80003,855,40035,000-23,600$259,000
M651,1002,300487,0001,300,000-72,000$248,00
M79,3001,20050,40000$16,800
M87,8001,40040,00000$13,000
M9300,00079,00001,300,000-340,000$74,000
M1044,30000178,0000$10,000
Total680,10091,3004,976,6004,348,700-549,800$830,000
Table 2: Wastewater Pond Recommended Project Savings

Wastewater Pond Steady State Primary Method & Best Practices Recommendations

SitePhosphorus Reduction (lb/yr)Nitrogen Reduction (lb/yr)Chemical Needed (lb/yr)Chemical Cost per YearChemical Savings vs Chemical treatment Only per Year (lb/yr)Csot Savings vs Chemical Treament Only per Year
Pond 19202,9500000
Pond 2*3,6004,35027,000$8,100178,000$53,400
Pond 31,1001,8700$042,700$12,800
Pond 44,5258300$080,900$24,300
Pond 54601,8900$021,000$6,300
Pond 67001,2500$033,600$10,100
Pond 77501,0500 $0 33,000$9,900
Pond 87752,3400 $0 22,800$6,800
Pond 92,8004,8000 $0 0 $0
Pond 106401,3100 $0 15,500$4,700
Pond 115,90010,7000 $0 109,900$33,000
Pond 123,70011,1000 $0 114,000$34,600
Pond 131,2204,3600 $0 26,700$8,000
Pond 141,9606700 $0 94,000$22,000
Total29,05049,47027,000$8,100772,100$225,900
*Additional chemical added for Pond 2 that still requires supplemental chemical to reach a 1 mg/L effluent phosphorus limit.
Table 3: General Project Outcomes

General Project Outcomes

# of StudentsEducational MaterialsEvents & Presentations# of Marketing PromotionsOther Resources Generated
11717317
Table 4: Student Assessment Outcomes

Student Assessment Outcomes

# of Sites Engaged# of Sites Visited# of Recommendations
282361

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 low-cost solution is to purchase and install rubber diffusor caps that prevent airflow into the tank.  A three-hole punch can be used to punch ¼’’ holes in some of these diffusor caps in order to create course bubble mixing in the tank while minimizing oxygen transfer.  This strategy allows operators to trial a low-oxygen, mixed tank for low-cost.  This strategy is also reversible – if it doesn’t work as expected, operators can remove the caps and return to baseline operation.  This strategy was used by the Saint 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 allows the team to estimate the statewide reduction opportunity.  In developing this estimate, the project site with the largest savings was removed from the average, as that site is treating a very heavy industrial load that is unlikely to be representative of other sites in the state.

Table 5: Statewide Savings Estimate for Statewide Biological Nutrient Removal Implementation in Minnesota Mechanical Wastewater Treatment Facilities

Statewide N Savings Estimate (lb)Satewide P Savings Estimate (lb)Statewide Chemical Solution Savings Estimate (lb)Statewide Chloride Savings Estimate if ALL Mechanical Plant Chemical REduction is Ferric Chloride (lb)
15,100,000500,000193,400,00048,200,000

Barriers

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:

  • ComplacencyMost Minnesota wastewater plants do not have limits on total nitrogen and are adequately meeting existing ammonia and phosphorus limits.  While sites are interested in learning strategies to achieve better treatment through BNR, there is currently limited driving force to motivate cities to implement this treatment strategy.
  • Lack of understanding – The current standard WWTP design in Minnesota does not utilize biological nutrient removal.  It is a less common and generally less well understood operational strategy.  Biological nutrient removal wastewater modeling software will be extremely useful in helping wastewater professionals to learn the mechanics of BNR and to help answer complex questions that arise while learning this treatment process.
  • Split incentives – As 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 graceTrialing a new operational strategy is a risk for wastewater plant managers.  Plant managers are expected to meet permit limits and ensure water is being properly treated in the facility.  Trialing a large scale treatment modification carries a risk of the trial not working as planned which could cause permit exceedance.  The Minnesota Pollution Control Agency guidance for wastewater pilot testing can be found in the “Pilot testing for wastewater treatment facilities” fact sheet.

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 ‘steady state primary’ 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 ‘steady state primary’ 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 ‘steady state primary’ 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 addition 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 ‘steady state primary’ 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, as the barriers to biological nutrient removal are reduced, some sites 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 nitrate from the wastewater.  As 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 modification to improve nutrient treatment, there are some options.  First, if plants are assigned total nitrogen limits that require nitrate removal, this will need to be accomplished using biological 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.

There is funding available for mechanical wastewater treatment facilities that receive a new or more stringent limit.  This funding is knowns as Point Source Implementation Grants (PSIG).  These grants are eligible to cover 80% of project costs up to $7 million.

A Note of Caution

The project team completed simulation modeling suggesting that it is possible to complete these types of nutrient optimization changes without reducing design capacity, but the team cannot guarantee this is always true. 

The MPCA comments that cities need to be aware that making these changes at a fraction of design flow may mean that they cannot meet permit limits at design flow, shortening the life of the plant if expansion and growth is expected.

Resources

PDFs

Videos

Acknowledgement

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
612-624-4683
jvanyo@umn.edu