What are the requirements related to the rated capacity of a wastewater treatment plant?
The rated capacity of a wastewater treatment plant is based upon the ‘firm capacity’ of the facility. Firm capacity refers to the available capacity of a system or process with the largest unit out of service. Thus, the reliability and redundancy of a water treatment plant's equipment and process units are integral to the plant's firm capacity.

As noted in previous question, reliability and redundancy requirements are based on installed redundant equipment and process units. Using analytical or statistical methods such as ‘predicted downtime,' ‘mean time between failure’ or other types of failure mode analysis to justify the reliability of systems or processes within a wastewater treatment plant is not an acceptable means of determining the rated capacity of the plant. A wastewater treatment plant's rated capacity is solely based on its firm capacity.

The rated capacity of a wastewater treatment plant is critical in determining when an expansion of a plant is required. There are 2 stipulations normally included in a wastewater treatment plant's operating permit are based on the rated capacity of the plant. Those stipulations are:
- "By the time the average day maximum month flow to the Facility reaches 80% of the Facility's rated capacity, the Owner shall have initiated planning and design of the next expansion of the Facility."
- "By the time the average day maximum month flow to the Facility reaches 90% of the Facility's rated capacity, the Owner shall have initiated construction of the next expansion of the Facility."

Besides being used in determining when an expansion of a facility is required the rated capacity is also used as a basis of design for determining peaking factors. Since short term (instantaneous) influent flows may exceed the rated capacity of the facility, peaking factors are used during the design process to determine if flow equalization or additional equipment is required to handle the peak flows to the plant. For more information, please call 602-372-2861.

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1. Is emergency power required for a wastewater treatment plant and collection system lift stations?
2. Is emergency power required for collection system odor control stations?
3. What are the design requirements for equipment and process redundancy for a wastewater treatment plant?
4. What are the requirements related to the rated capacity of a wastewater treatment plant?
5. What are the minimum setback requirements for a wastewater treatment plant?
6. What are the design requirements for venting wet wells, dry wells, basins, tanks and reservoirs?
7. What are the design requirements for wet well, basin, tank or reservoir overflows?