City Manager Ray Alley has heard for years that Holbrook’s sewer system is at capacity in some parts of town and couldn’t support additional growth. Knowing there is a lot of developable land within the city limits, as well as the potential for substantial growth in the coming years, Alley decided it was time to separate fact from conjecture and commissioned a wastewater flow study.
City engineering firm GHD, Inc., which is based in Phoenix, presented a sanitary sewer capacity analysis at Tuesday’s Holbrook City Council meeting showing where system capacity is in good shape and identifying potential trouble spots.
GHD accepted three assumptions as it evaluated the city’s wastewater flow system: development will occur in the locations city staff identified; development will follow current zoning plans; and unit flows will remain similar in the future (meaning, the typical usage per household is likely to remain consistent.)
With those assumptions in place, GHD developed a map that shows potential for system backups along Arizona Street, starting at about Eighth Avenue and extending to the wastewater treatment plant west of Holbrook. In areas where significant new residential and commercial could occur, primarily along Spurlock and Navajo Blvd. in the northern areas of the community, the sewer system is predicted to be able to support growth without stressing the system.
GHD engineer Bill Roberts explained it’s possible there are some points on the sewage system map that have been misreported, which could be providing false flow predictions, like that which shows the system is already flowing at capacity along Arizona Street. Field inspections of questionable points will confirm or correct the study’s assumptions.
The analysis projections were based on Arizona Department of Environmental Quality (ADEQ)-provided equations that set commercial use at 1,500 gallons per day, per acre; and, household use is based on the assumption of 3.2 people per household using 80 gallons of water per person, per day.
Holbrook’s wastewater treatment facility records show actual city residential use at about 60 percent of ADEQ’s 80 gallons per person prediction.
The analysis also used the city’s highest month of usage as the average flow modeled in the study. For Holbrook, July is the month of maximum flow, with 639,100 gallons of wastewater generated each day.
Roberts noted that “flowing at capacity” by ADEQ standards is a pipe that is about 75 percent full. ADEQ uses this number in consideration of the fact that rainfall or snow can cause excess water to enter wastewater pipes; if the pipes are running above 75 percent full, there is greater risk of the system being swamped when storm water enters the equation.
The wastewater study modeled what Roberts referred to as the “backbone” of the city’s sewer system, or the major sewer lines serving areas where there are already significant population centers or usage, and those where the greatest amount of growth is anticipated.
Alley recommends conducting field inspections of those points along the city sewer system where findings are questionable, and verifying or correcting the study data to assure the results are correct. For instance, there is a manhole on Eighth Avenue that is reported to be seven feet deep, yet the manhole just below that one is said to be only six feet deep. If the depths are accurate, then there is potential for backflow in the system, creating a blockage. If it turns out the depths of the manholes were misreported, then it could eliminate the concern about system backups in that area.
Once problem areas have been field checked, an undertaking that Alley said city staff can complete, the city will have a clearer picture of the true state of the sewage system.
If it turns out that there are indeed areas already at capacity or at risk of being overburdened, Alley said his recommendation would be for the city to prioritize improvement projects.
“I’ll come back to you with a game plan, and it could be an expensive game plan,” he said.
Referring to potentially high costs of upgrading a municipal sewer system, Councilman Phil Cobb asked Roberts if he knew of grant sources for such projects.
Roberts said if upgrades were required because of system overloads caused by existing flows, funding would need to come from the city or through grants; if the upgrades are needed to support new development, he said, then developers could share in the cost of improvements.
Alley said he would evaluate the study findings, then present his recommendations on how to proceed to the council during a future meeting.
