It’s a holiday weekend, a storm is squalling above the public utility process facility and suddenly, as expected, power fails. Effluent water keeps streaming downstream yet it’s doing so untreated. What do you do to re-start water process operation, and how critical is it? The answer lies on the power source feeding the electrical loads of the facility. One question you may be asking yourself is, “How reliable is the power supply to our water and wastewater treatment facilities?”
EPA Reliability Design Criteria
In 1974 the U.S. Environmental Protection Agency published a reliability design criteria for mechanical and electrical support systems in wastewater facilities called Design Criteria for Mechanical, Electrical, and Fluid System and Component Reliability. The reliability criteria standards delineate three classes of wastewater treatment facilities:
In general, the power supply in a Class I facility will be able to provide power for the operation of all vital process components during peak wastewater flow conditions, including lighting and ventilation. A Class II facility is the same as Class I except that the vital components used to support secondary processes (i.e., mechanical aerators or aeration basin air compressors) need not to be included as long as equivalent to sedimentation and disinfection is provided. A power supply serving a Class III facility needs to be able to operate the screening, main wastewater pumps, primary sedimentation basins and disinfection facility during peak wastewater flow condition including critical lighting and ventilation. Essentially, two separate power sources should be provided to the facility: a power feeder as the primary source and a second one as a backup. This can be accomplished by bringing in two separate primary feeders from distinct power substations. Another option is to provide a single primary feeder from the local power substation as the primary source and a standby generator as the backup source.
Power Distribution Configurations
The power distribution within the facility can be provided in several different configurations such as Primary Selective, Secondary Selective or Radial Feed System with Standby Generator Backup.
Primary Selective: The Primary Selective configuration contains two primary feeders coming into the facility and connected into a selective primary switchgear. The power feeders are provided by the utility company. The switchgear then connects to a single electrical bus that distributes power to all the loads of the facility.
Secondary Selective: In this distribution configuration the electrical equipment contains two buses, each supplying 50% of the electrical load. Typically an open bus tie is provided. If a feeder fails, the faulty feeder is disconnected and the bus tie-breaker is closed to the second feeder so it can carry the entire electrical load.
Radial Feed with Standby Generator Backup: In such a system a single utility source feeder is provided to an automatic transfer switch that switches power between utility power source or an onsite standby power generator.
Combining all three methods increases reliability considerably. Per an electrical power service reliability study from the Design of Municipal Wastewater Treatment Plants Fourth Edition Manual (by ASCE), the following is a power source reliability study table. The study shows that two independent power sources provide a high level of reliability.
An additional source of power is justifiable only during severe reliability constraints. Power utility companies can provide you with a reliability report that describes the amount of failures and duration the power source feeding your new or existing facility has had in the past. That information may help in determining the type of power system that will supply power to your facility. Consideration of the facility class and reliability level of the power source and power distribution system need to be reviewed in order to select the right power system for your facility.
Halff is fully invested in sharing valuable information that can help our clients solve immediate issues and plan for the future through innovation and smarter solutions. If Halff can assist your team with a water utility issue, please call Executive Vice President Jessica Baker, PE, CFM, PMP, at (214) 217-6692.
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