Navigating Process Challenges of Sidestream Nitrogen Removal: Insights from the Fond du Lac WRRF

Background
As regulatory pressures and environmental stewardship drive stringent effluent quality targets across North America, wastewater utilities are forced to manage more complex nutrient removal processes. Fond du Lac Wastewater Treatment & Resource Recovery Facility (FDL) operates a low DO, anoxic-aerobic (AO) process, rated for 44,000 m3/day [~12 mgd] (Figure 1). Additionally, FDL commissioned Paques' ANAMMOX® process for sidestream nitrogen elimination in 2018 to manage dewatering centrate nitrogen loads.

ANAMMOX® is a well-established, upflow granular sludge bed process used for single-stage partial nitritation and anammox (PNA). Tilted plate settlers are used to reduce influent solids, retain biomass granules, and waste lighter floc (Figure 2). Some common process challenges associated with PNA systems in general include control of nitrite-oxidizing bacteria (NOB), temperature sensitivity of process microbiology, sufficient alkalinity for aerobic ammonia oxidizing bacteria (AOB), and retention of slow-growing anammox organisms [1].

Recently, FDL observed a drop in both nitrogen removal efficiency and the granule fraction of biomass. This presentation and paper will discuss observed process challenges, key optimization steps, and impacts on nitrogen elimination.

Operations and Timeline
In 2021, FDL upgraded their solids processing equipment, which impacted dewatering operations. With consistently higher loading and airflow to their ANAMMOX® process, FDL observed granule washout. Adding granules and a new screening process for biomass retention overcame washout concerns, but removal efficiencies unexpectedly decreased. Strategies included addition of iron, micronutrients, and caustic for pH adjustment to reduce effluent nitrate and ammonia. DO control was also optimized to control NOBs, and activity tests returned normal results. With monitoring of alkalinity and dilution water (for temperature control) and ending caustic and iron addition, FDL has mostly restored anammox activity in the system. Temperature setpoint was historically 36°C, until 2024 when the following changes were initiated (Table 1).

Observations and Discussion
Granule washout occurred in July 2021 (Figure 3), with granule counts stabilizing quickly with minimal floc accumulation. In May 2022, granule count began to decline, and Imhoff cone results reached a minimum in early 2023. Coinciding with this, flocculant biomass flourished, reaching a maximum quantity (88% of Imhoff cone) in May 2023 (Figure 3, shaded). This suggests poor granule retention and that process conditions promoted growth of light, flocculant biomass. Flocculant biomass does not support the growth structure (anoxic layer) required by anammox.

Effluent nitrogen as ammonia, nitrite, and nitrate are shown below (Figure 4). Minimum granule quantity coincided with high concentrations of effluent nitrite, suggesting there was little to no anammox activity (shaded area of Figure 4). Nitrite consumption was recovered shortly after. As granule count increased through late 2023, FDL began to see elevated effluent ammonia and nitrate from ANAMMOX®. While granule counts have rebounded, anammox activity may not be optimized despite process improvement efforts to dose iron and control the pH in the reactor. High effluent nitrate suggests poor NOB control, and high effluent ammonia suggests poor AOB performance. As effluent ammonia decreased in summer 2024, FDL decreased their DO setpoint to continue to drive out NOB.

There is a positive correlation of influent alkalinity concentration with TIN elimination (Figure 5). FDL uses heated plant effluent to control temperature in the ANAMMOX® reactor, which dilutes the influent alkalinity concentration. There was no clear correlation between dilution water flow rates and TIN elimination. There is a slight decrease in the ratio of influent alkalinity to ammonia over time.

Theoretical stoichiometry of single-stage PNA suggests a 0.1 molar ratio of nitrate produced to ammonia consumed. This can increase or decrease with the presence of other organisms, like NOB and denitrifiers, and a higher nitrate:ammonia ratio suggests the presence of NOB and can be used as a process indicator of NOB control in the ANAMMOX® process. The trend of this ratio over time was reasonably well-controlled until late 2023, following a sharp increase in floc and effluent nitrate (Figure 6). NOB may have outcompeted anammox bacteria for available nitrite, a theory also supported by high effluent nitrate concentrations (Figure 4).

FDL's ANAMMOX® biomass and nitrogen elimination performance displayed a compounded response to DO, temperature, and alkalinity changes. This presentation will discuss FDL's optimization strategies, perplexing system responses, and operational conclusions. Next steps include process modeling focused on granular and flocculant fractions observed in the field to further understand performance and guide optimization.

Relevance
- Sidestream nitrogen removal technologies are becoming more commonplace and process upsets are often still poorly understood and characterized.
- Determining which process variables are available, reliable, and important to track in specific systems is critical to informing process control decisions and evaluating system response.
- Presentation and discussion of full-scale case studies are valuable mediums for sparking innovation and conversation.