The Hilltop sewer treatment plant owned and operated by the city of Kingman handles one million gallons of sewage a day, but little of it recharges the aquifer hundreds of feet below the ground.
In fact, Jeff Corwin, wastewater superintendent for the city, told members of the Northwest Arizona Watershed Council on Wednesday afternoon that he does not know how long it takes the treated sewage to reach the groundwater.
However, he cited an hydrology report that indicates that it takes seven years for the water to reach the aquifer, believed to be at least 600 feet below the surface.
Watershed members visited the site as part of their continuing education on water-related issues.
It is located on an expanse of flat land off Route 66 northwest of the Kingman Airport Industrial Park.
Council member Earl Engelhardt, a retired manufacturing executive, said he learned a lot during the field trip.
"This is very educational in the sense that I felt wastewater treatment plants were about reprocessing water for drinking, and none of that is true," Engelhardt said.
"It likely never gets down to the aquifer" because of evaporation.
"I was just amazed that none of that water was ever used for anything" after it enters the plant, he said.
Engelhardt was among seven men who toured the sewer plant.
The Hilltop plant begins with a series of lagoons where sewage undergoes primary and secondary treatment and empties into a polishing pond.
Artificial wetlands with bulrushes neutralize harmful nitrates, and infiltration basins absorb any remaining water.
Nitrates are being monitored by a well that is 450 feet deep, Corwin said.
The well must meet drinking water standards.
"We're discharging on our own property," Corwin said, after entering the control room.
A 36-inch pipe takes treated sewage from the Hilltop section of the city and an 18-inch pipe draws sewage from the city-owned airport and industrial park.
"What basically comes in here is organic waste," he said.
Fifty percent of the flow goes into four of the eight lagoons.
He described the polishing pond as a storage area that is used for pumping out water from the lagoons.
The lagoons are designed for removing solids and reducing organic loads, Corwin said.
"We are not harming the drinking water in this valley with this treatment," he said.
Nitrate levels rarely reach one milligram per liter, Corwin said.
The water quality standard is 10 milligrams per liter.
Corwin said the city also obtains permits to burn the wetlands areas every two to three years because the thick growth can cause clogging.
He then conducted a tour in his truck of the plant site, which contains 75 acres of wetlands, 50 acres of infiltration basins and 12 acres of lagoons.
The plant recently expanded with the addition of 25 acres of wetlands, bringing the total to 75.
"Right now, there is no discharge because of planting," Corwin said.
"Everything is continuous treatment – primary and secondary in lagoons and nitrogen removal in the wetlands."
The treated sewage undergoes further treatment in the soil in the basins, which resemble empty lagoons.
"They are basins that allow for evaporation and percolation of treated effluent," Corwin said.
Corwin said no discharge has taken place so far this month because the wetlands have absorbed water initially treated in the lagoons.
During the summer, 600,000 gallons of water are lost each day from evaporation from the surface and transpiration from plants.