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A Case Study: How the Marshalltown Water Pollution Control Plant Can Meet the Iowa Nutrient Reduction Strategy

To reduce the size and severity of the hypoxia zone in the Gulf of Mexico, a 2008 EPA action plan called for each of 12 states along the Mississippi River to develop a Nutrient Reduction Strategy (NRS). In 2012, Iowa adopted an NRS for wastewater plants with average wet weather (AWW) flows greater than 1.0 million gallons per day (MGD) and required that a nutrient removal feasibility report be prepared as part of their operation permit.

The NRS requires an evaluation of current nutrient removal process efficiencies, identification of operational changes and new technologies to enhance removal, analysis of the economics justifying implementation, and a proposed schedule to accomplish strategy limits. *

*For “typical domestic” wastewater, NRS goals for Total Nitrogen (TN) and Total Phosphorus (TP) are 10 mg/L and 1 mg/L, respectively. Typical domestic wastewater for TN is 25-35 mg/L while TP is 4-8 mg/L.  For non-domestic wastewater, NRS goals are 66% reduction of influent TN and 75% reduction of influent TP.

 

Marshalltown Case Study:

FOX Engineering has a long history of work at the Marshalltown wastewater plant including numerous design and construction projects. The facility has complex liquid and solids treatment processes so FOX engineers worked with the City to develop an NRS unique to their treatment process.

The current wastewater process consists of preliminary treatment via mechanical bar screens and grit removal followed by primary clarification. Wastewater proceeds through pre-aeration and conventional activated sludge treatment in three parallel trains, designated as plants 1 through 3. In addition, a significant industrial user anaerobically and aerobically pre-treats their pork packing plant wastewater before sending the waste to the city’s separate industrial treatment plant, which consists of two sequencing batch reactors (SBRs). The influent between the domestic and industrial plants is kept separate but their effluent combines before UV disinfection.

Solids handling consists of dissolved air floatation (DAF) thickening for waste activated sludge (WAS) from the domestic and industrial plants.  Gravity thickeners are utilized for the domestic primary sludge. After thickening, the WAS and primary sludge streams combine in a temperature-phased anaerobic digestion (TPAD) process with methane capture for biogas recovery and utilization in co-generation engines (heat and electricity).

The combined permitted capacity of all plants is an average wet weather flow of 13 MGD and maximum wet weather flow of 17.4 MGD. The permitted design BOD5 loading is 24,077 lbs/day and TKN is 4,311 lbs/day.

The objectives of the NRS study for Marshalltown generally included the following:

  • Analysis of nutrient data to determine nutrient reduction targets
  • Assessment of operational changes to enhance removal
  • Assessment of various nutrient removal technologies
  • Value engineering and economic analysis

Methodology and Implementation:

To assess plant performance, the study included analysis of current flows and loads for the study year 2017. Additional total nitrogen (TN) and total phosphorus (TP) monitoring were integrated into their testing routine to facilitate this preliminary analysis. The composite influent (flow weighted between the two plants) TN and TP were 64.8 mg/L and 10 mg/L, respectively. Based on the composite influent, Marshalltown’s wastewater is not considered domestic wastewater. Marshalltown’s composite system may be categorized as non-domestic, but the TP concentrations on the municipal side may be considered typical domestic. However, it was reasonable to classify the combined composite influent as non-domestic type wastewater resulting in an NRS goal of 66% removal of Total N and 75% removal of Total P.

As a result, the following scenarios for removal efficiencies were evaluated: changing the industrial SBR plant only, changing the domestic plant only, changing the SBR and domestic plant, and changing the offsite industrial pretreatment system and domestic plant.

After identifying the required removal efficiencies under each reduction scenario, operational changes to each system were addressed to verify little to no-cost solutions. SBRs have flexibility because cycle times can be altered to include anoxic periods for TN removal and static fill times for TP removal.

Marshalltown’s plant experienced progressively lower strength wastewater during the study period, so the SBR’s nutrient removal performance declined over time. FOX analyzed the operational changes to the SBR and concluded the only feasible method to increase TN removal was to construct a third SBR; however, the economics and site constraint made the alternative unreasonable.

The domestic plant is operated under cyclic aeration with dissolved oxygen (DO) probes to limit blower energy usage and achieve denitrification. Maximum TN removal efficiencies of cyclic aeration are typically around 58%, and the plant already achieves approximately 51% TN removal. TP removal was around 46%. As a result, there were three treatment alternatives for converting the activated sludge process to the Modified Ludzack-Ettinger (MLE) process:

  1. Convert primary clarifiers into anoxic basins and add chemical to the SBR units for TP removal.
  2. Convert primary clarifiers into anoxic basins and add chemical to both the industrial pretreatment system and the city’s SBR units.
  3. Convert primary clarifiers into anoxic basins. Convert the industrial activated sludge pretreatment process into an A/O process for enhanced biological phosphorus reduction (EBPR) while adding chemical at the city’s SBR units to polish.

Challenges:

The following challenges became evident during the study and were addressed with the city along each stage of the process:

  • The industrial user changed waste characteristics by overloading their pretreatment aeration basin during the study to avoid paying BOD surcharges sent to the city. As a result, the wastewater had significantly lower BOD and higher nitrates than anticipated from normal operations. This led to poor nutrient removal performance due to insufficient BOD to drive denitrification and excess nitrates which poison the polyphosphate accumulating organisms (PAOS) responsible for EBPR.
  • The NRS study required additional N and P sampling and testing.
  • If enhancing the industrial pretreatment system for nutrient removal, a process not physically located at the city’s plant, the wastewater to the city’s plant could have lower TN and TP concentrations. Thus, the classification of the effluent strategy limits could become more stringent, so additional clarification on boundaries for nutrient removal was required.
  • Converting the primary clarifiers to anoxic would lower the quantity of primary sludge thus resulting in lower biogas production for the city’s biogas generator engines.

FOX assisted the city in working with the industry to modify their pretreatment agreement to improve the waste composition to the SBRs. As a result, the SBRs are performing closer to their design. This was one low-cost operational adjustment the city could utilize during the study period and demonstrate improved nutrient removal.

 

Recommendation and Conclusion:

FOX recommended Alternative 3 due to lower operational and maintenance costs.  However, this alternative requires the city to work with the industry to lower their effluent TP by construction of an anaerobic selector at the industry’s pretreatment plant. This cost-effective alternative utilizes existing infrastructure and minimizes annual chemical costs incurred by the City by implementing biological removal methods of TP to achieve strategy limits. In addition, the phosphorus rich sludge produced by the proposed EBPR industrial pretreatment plant can be directly land applied, thus avoiding problems with phosphorus recycle from within the City’s anerobic digestors.

FOX analyzed the economics and worked through the City’s sewer expense and debt obligations to justify delaying the implementation of the strategy to the Year 2031. At that time, substantial amounts of debt service will have been retired; this will provide room in the City’s budget for implementing the Iowa Nutrient Reduction Strategy.

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.