The Fate of Nutrients: How do Nitrogen and Phosphorus Affect Biosolids Production, Treatment, and Handling?

If you are a major municipal utility with average wet weather wastewater flows greater than one (1) million gallons per day, or a minor municipal facility planning to increase design loads, the letters N and P could greatly influence your treatment process, biosolids handing, and city budget. How?

These letters represent nutrients present in wastewater effluent that will come under regulation as part of Iowa’s Nutrient Reduction Strategy for major municipal facilities. For major municipal facilities governed under the Nutrient Reduction Strategy, funding will need to be devoted to monitoring nutrient levels, studying options for nutrient removal, and possibly upgrading treatment plants or making operational changes to meet the new nutrient removal goals.  Scheduling upgrades would be negotiated with the Iowa Department of Natural Resources (IDNR) and based on what is “reasonable”. The main goal of the Nutrient Reduction Strategy is for all major municipal utilities to commit to a schedule for implementation of nutrient removal that works for their community. This strategy is not a one size fits all.

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North Liberty gate to membrane tank

Nutrient Reduction Strategy Impact on Biosolids

What is the potential impact of the Nutrient Reduction Strategy (NRS) on biosolids?  Often discussions of nutrient removal focus on the liquid treatment train at facilities and the technologies used to remove nutrients to meet the NRS annual average effluent goals of 10 mg/L total nitrogen (N) and 1 mg/L phosphorus (P). However, a holistic approach to planning for nutrient removal that includes evaluating both liquid and biosolids treatment is important in developing the best solution for meeting the NRS’s effluent goals and building a more effective nutrient removal facility. Two important considerations when evaluating nutrient removal at a facility are: the impact of nutrients on biosolids production and handling and the management of high nutrient loading in return streams from biosolids treatment processes.

The Fate of Nitrogen

Nutrients removed from wastewater end up in one of two places: in the biosolids or released to the atmosphere. Biological nitrogen removal processes first convert ammonia to nitrate in an aerobic environment, which is a process called nitrification. A majority of nitrogen is then biologically removed in an anoxic (not aerated) environment where nitrogen is converted to nitrogen gas and released to the atmosphere, which is called denitrification.


North Liberty Water Pollution Control Facility digesters, aeration basins, and membrane bioreactor treatment building.

Many different biological treatment technologies exist for reducing nitrogen to less than 10 mg/L. The following are examples of BNR processes that can be retrofitted into existing treatment facilities or built in new facilities.

  • Denitrifying filters
  • Sequencing Batch Reactors (SBRs)
  • Preanoxic or postanoxic treatment in conventional activated sludge treatment
  • Cyclic aeration
  • Anoxic/Aerobic zones + Membrane Bioreactor Treatment

The Fate of Phosphorus

All phosphorus that enters a treatment facility leaves through the effluent or in the biosolids. Phosphorus can be removed either chemically or biologically from wastewater.  Biological removal of phosphorus is achieved by adding anaerobic treatment to an activated sludge treatment process. Many of the BNR processes previously listed can also be designed to remove Phosphorus. Phosphorus can also be removed through chemical precipitation with the addition of chemicals like iron salts and polymers to treatment systems.

Chemical phosphorus removal can be implemented in addition to or in lieu of biological treatment. Chemical phosphorus is typically easier to control and can be easier to implement in an existing treatment process because it only requires the addition of chemical storage and feed equipment.

Implementing an effluent treatment goal of 1 mg/L phosphorus means that whatever phosphorus was previously leaving the plant through the effluent will be removed in the biosolids. Biosolids production is expected to increase due to the mass of phosphorus taken up by organisms in biological systems or by the formation and removal of chemical solids in chemical phosphorus removal systems.

Land Application

Nutrient content in biosolids at nutrient removal facilities is higher than at facilities that have not implemented nitrogen and phosphorus removal. An important aspect of nutrient removal is the impact it has on the total land area required for biosolids application. Biosolids produced from nutrient removal facilities have higher concentrations of phosphorus in their biosolids and see an increase in total biosolids production compared to an activated sludge plant that has nitrification and BOD treatment. 

In facilities that have implemented nitrification but not nitrogen and phosphorus removal, the nitrogen content in the biosolids is often the limiting parameter that determines the minimum land area required for application of biosolids.

Consider a conventional activated sludge municipal treatment facility with nitrification and BOD removal processes and average influent nutrient loading of 40 mg/L TKN and 10 mg/L TP. This facility would require almost two times the land area for biosolids application when nutrient removal is implemented. The increase in land area required can be attributed to two factors: an increase in biosolids production from the nutrient removal process and an increase in the biosolids phosphorus content which could be at least two times higher than a conventional activated sludge plant.

Biosolids Return Stream Handling

The other important aspect of biosolids handling at a nutrient removal facility is management of supernatant return nitrogen and phosphorus concentration from biosolids treatment processes. These recycle streams come from thickening and dewatering processes an can account for less than 1% of the total plant influent but contain 20 to 30% and 5 to 35% of the total plant influent ammonia and phosphorus load, respectively. Working with your engineer to manage these return stream nutrient loads is important to minimize high nutrient loading slugs on the main treatment process. Options for managing the loading can include the following:

  • Aerobic biosolids treatment processes – Aerobic treatment is more efficient at keeping phosphorus bound in the biosolids compared to anaerobic processes.
  • Add chemical to bind phosphorus in biosolids – Biosolids dewatering equipment would be installed downstream.
  • Sidestream nutrient removal treatment – Sidestream treatment can be used to remove nitrogen and phosphorus prior to returning the sidestream to the head of the plant
  • Design for return stream nutrient loading – The main treatment process can be designed to handle the increased nutrient content in the recycle stream.
Biosolids Return Stream

Thickened Biosolids Waste


As your facility starts planning for nutrient removal, consider how nutrients will be treated or managed in the liquid treatment and biosolids handling processes. Nutrient removal can increase biosolids production, required land area for biosolids application, and sidestream loading from biosolids treatment. Nutrient removal and biosolids handling processes should be selected to meet the operational and maintenance goals of your facility and produce effluent that meets the Nutrient Reduction Strategy goals of 10 mg/L nitrogen and 1 mg/L phosphorus.

Rotary press pilot test at North Liberty Water Pollution Control Facility

Rotary press pilot test at North Liberty Water Pollution Control Facility

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.