Capacity at wastewater treatment facilities is more complex than often assumed. While many unit processes are governed by peak day or peak hour flow rates, the largest unit processes—secondary treatment and the solids train—are governed by the influent load received at the facility. Secondary Treatment Systems: Secondary treatment systems face three primary capacity constraints: 1.Maintaining sufficient volume to uphold the facility's design Solids Retention Time (SRT) at a settleable mixed liquor suspended solids (MLSS) concentration. 2.Effectively settling the MLSS during peak flows without blanket loss. 3.Providing adequate aeration capacity for peak biochemical oxygen demand (BOD5) loading. The solids train, including digestion and dewatering, is designed based on loading rather than influent flow rates. Importance of Evaluating Loading: Population growth in service areas increases BOD5 loading, while the corresponding flow increase may be less pronounced due to water efficient fixtures and stable infiltration and inflow (I&I) patterns. Consequently, facilities in high growth areas often operate below design average flow rates while exceeding design loadings. Two case studies demonstrate how increasing influent BOD5 concentrations, driven by water efficiency and I&I reduction, can exceed treatment capacity before reaching design average flow rates. Case Study: City of Maryville WWTP From 2014 to 2023, the City of Maryville's WWTP experienced a 21% increase in average influent flow rate but the facility's BOD5 loading increased by 70%. By 2023, the facility was at its stated load capacity, despite being a “17 MGD plant” with average influent flow rates of 11 MGD. Evidence of a facility at capacity included near maxed-out surface aerators, digestion capacity limitations forcing the facility to limit secondary sludge wasting, and solids-overloaded clarifiers during wet weather events. Case Study: City of Sevierville WWTP Similarly, the City of Sevierville's WWTP saw a 38% increase in flow from 2013 to 2024, while the average influent BOD5 load grew by an astonishing 130%. Sevierville’s WWTP has been over capacity for some time, with a new wastewater plant nearing completion. However, load growth has been so dramatic that expansion planning for the next phase is already required. The max month BOD5 load today is already 50% higher than it was during the design of the facility under construction, and the full design load will be exceeded within six years at current growth rates. This rapid load growth is exemplified by the plant now treating wastewater from the world’s largest gas station, which opened with 71 toilets in June 2023. Conclusion: This presentation will discuss how a focus on influent flow can be misleading in a utility’s planning efforts and will use the two cities as illustrations. Given the typical 3-5 year time frame from the start of an expansion design to an online facility, utilities should check if their facilities are close to their load capacity, regardless of current average flows.