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1. Is emergency power required for a wastewater treatment plant and collection system lift stations?

Yes in most cases. Maricopa County Environmental Health Code (MCEHC) Chapter I, Section 2, Regulation 5 states: "For systems that treat, or which are designed to treat greater than 10,000 gallons/day, a standby power source shall be provided at all sewage treatment systems and/or pump stations where a temporary power failure may allow a discharge of raw or partially treated sewage. Standby power may be via a standby generator, separate feeders from separate substations, a loop feeder on separate transformers from a common substation, or a high-level alarm with portable generators. Standby power also shall be provided to any sewage treatment systems and/or pump stations, regardless of size, if a temporary power failure may allow a discharge into surface waters classified as 'Unique Waters', by the Arizona Department of Environmental Quality."

For more information, please email treatmentplantprogram@maricopa.gov.

2. Is emergency power required for collection system odor control stations?

No. However, the MCESD may require emergency power be installed on an odor control station if a loss of power would result in nuisance odors being generated. The requirement for emergency power will be determined by the MCESD as part of the review and approval process. For more information, please email treatmentreatmentplantprogram@maricopa.gov.

3. What are the design requirements for equipment and process redundancy for a wastewater treatment plant?

The requirements for equipment and process reliability and redundancy are defined within the various chapters of the Arizona Department of Environmental Quality's (ADEQ) Engineering Bulletin No. 11, "Minimum Requirements for Design, Submission of Plans and Specifications of Sewage Works." The bulletin essentially requires that 2 or more of all essential treatment units or items shall be provided. With one unit or item out of service, the remaining units or items shall meet the design capacity of the plant.

Reliability and redundancy requirements can usually be met by using redundant treatment process trains or designing a single treatment process with parallel piping and equipment which provides full redundancy (i.e. main and standby process unit(s) or equipment).

An engineering analysis should be performed when designing a new or modifying an existing wastewater treatment plant to determine what process units and types of equipment require redundancy and increased reliability. The analysis should address the impact and the criticality of process interruptions and equipment failure and assess the length of time the process or system would be out of service. For more information, please email treatmentplantprogram@maricopa.gov.

4. 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 email treatmentplantprogram@maricopa.gov.

5. What are the minimum setback requirements for a wastewater treatment plant?

Arizona Administrative Code (AAC) R18-9-B201.I specifies the minimum setback requirements for a wastewater treatment plant from the nearest adjacent property line. The specific setback distance requirements depend on no or full noise, odor, and aesthetic controls. Full noise odor and aesthetic controls by the AAC as: Full noise, odor, and aesthetic controls means that all treatment components are fully enclosed, odor scrubbers are installed on all vents, and fencing aesthetically matched to that in the area surrounding the facility.

Noise control is further defined in the Arizona Department of Environmental Quality's (ADEQ) Engineering Bulletin No. 11, "Minimum Requirements for Design, Submission of Plans and Specifications of Sewage Works," Chapter 6, Section B as: Noise control is defined as a sound level at the nearest existing property line not to exceed 50 dB on the A network of a sound level meter.

For more information, please email treatmentplantprogram@maricopa.gov.

6. What are the design requirements for venting wet wells, dry wells, basins, tanks and reservoirs?

All wet wells, dry wells, basins, tanks and reservoirs require venting. Wet and dry wells require forced ventilation and basins, tanks and reservoirs require normal ventilation.

Wet wells require 12 continuous air changes per hour or 60 intermittent air changes per hour based on the volume of the head space above the minimum sewage level. Dry wells require six continuous air changes per hour or 30 intermittent air changes per hour based on the volume of the dry well.

Wet wells and dry wells are usually vented by force draft fans or blowers in combination with odor control devices located on the ventilation discharge from the wet or dry well. National Fire Protection Association (NFPA) Section 820 specifies that all continuous ventilation systems should be fitted with flow detection devices connected to an alarm system to indicate ventilation system failure.

Basins, tanks and reservoirs are usually vented using inverted 'U' type, 'chair' or cross-flow vents using normal air currents.

In all cases, vents shall have a 16-mesh corrosion resistant screen installed to prevent the ingress of birds and insects. For more information, please email treatmentplantprogram@maricopa.gov.

7. What are the design requirements for wet well, basin, tank or reservoir overflows?

Overflows of wastewater must be contained and not allowed to spill onto the ground. If the overflow from a storage vessel is not connected to a sewer or the head works of the plant a suitable means of onsite containment must be included in the design. Typically, lined onsite emergency retention basins are used to contain an overflow.

Counter-weighted flapper valves or self-sealing valves which open and close automatically when an overflow occurs must be installed on the end of the overflow discharge pipe to control odors and prevent the ingress of birds and insects. For more information, please email treatmentplantprogram@maricopa.gov.