Introduction to Wastewater Flow and Capacity

June 13, 2024

Infrastructure: The Foundation Business Is Built Upon - Understanding Flow and Capacity in Union County

In our ongoing series (part 1, part 2) on infrastructure in Union County, this post dives into the specific metrics of wastewater management—flow and capacity. Both terms are crucial for understanding how the water reclamation facilities (WRFs) operate and their ability to handle current and future demands.

Flow vs. Capacity: The Basics

    • Flow: This is the actual volume of wastewater that is being treated, moved, or reused by the facilities daily. In Union County, we measure flow in millions of gallons per day (MGD).
    • Capacity: This refers to the total volume of wastewater that the facilities are capable of handling. Like flow, capacity is typically expressed in MGD.

Normally, the flow should be less than the capacity to ensure that the system can handle sudden increases in wastewater volume, which might occur during heavy rainfall or due to seasonal variations in water use.

Understanding the Chart: WRF Data

The county provides a chart (an example from June 2024 is posted here) that details various aspects of the WRFs. Let’s break down the columns to clarify what each one represents:

    • 1. WRF (Water Reclamation Facility): Lists the names of the facilities within Union County.
    • 2. Permitted Capacity (MGD): The maximum amount of wastewater each facility is authorized to treat on a daily basis.
    • 3. Actual Average Daily Flow (MGD): The average volume of wastewater actually treated by the facility each day, based on a 12-month rolling average.
    • 4. Percent of Actual Flow Used: Indicates what percentage of the permitted capacity is being used by the actual flow, providing insight into how close a facility is to reaching its capacity.
    • 5. Actual + Permitted Obligated Flows (MGD): This figure combines the actual flow with any committed flows not yet contributing to the system (e.g., from new developments that have been approved but not yet built).
    • 6. Percent of Permitted Flow Used: This shows the percentage of the facility’s total capacity that is either currently being used or is obligated to future developments.
    • 7. Actual Rainfall (in): Rainfall data is crucial as heavy rain can significantly increase flow rates temporarily, especially in systems where stormwater and sanitary sewers are combined.

Example Data Analysis

Let’s apply this understanding to one of the facilities:

Twelve Mile Creek WRF

      • Permitted Capacity: 7.5 MGD
      • Actual Average Daily Flow: 5.202 MGD
      • Percent of Actual Flow Used: 69.4%
      • Actual + Permitted Obligated Flows: 6.724 MGD
      • Percent of Permitted Flow Used: 89.7%
      • Actual Rainfall: 6.7 inches

From this data, we can see that Twelve Mile Creek is operating below its total permitted capacity but has little room for additional flow. Nearly 90% of its capacity is either used or obligated, indicating limited space for new developments without upgrades or expansions.

Development Flows and Backlog

The chart also details the backlog of projects for each WRF, categorized by their approval status and potential added flow:

Approved – Permit Submittal Pending: Projects approved but awaiting final permits.

Engineering Plan Review: Projects currently under technical review.

Sketch Plan Review: Initial proposals under consideration.

For instance, at Twelve Mile Creek, there are 30 projects in the backlog that could add up to 1.239 MGD if all are realized, underscoring the need to manage both current capacity and future demand strategically.


Understanding these figures helps us anticipate future challenges and opportunities in wastewater management. As Union County grows, so does the demand on our infrastructure. By keeping informed about these details, the business community can better engage in discussions about development and infrastructure planning.

What’s Next

Our next installment will discuss current challenges for Union County’s system and planned investments by the County along with recent funding from the General Assembly.


Editor’s note: This blog is part of an occasional series published by the Chamber to provide information to our members and promote discussion on public policy issues affecting businesses in Union County.  You can find past installments here. Future blogs will focus on infrastructure issues such as broadband, roads, energy and water, workforce issues such as childcare and worker skills and other policy issues such as tax and regulatory policy.

Last modified: June 19, 2024

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