According to Moab City Engineer Rebecca Andrus, the wastewater plant currently uses drying beds to rid the solid waste of excess water. Once the waste is dried, it is sent to the landfill. However, the amount of solid waste has become too much for the drying beds to handle.
“A large contributor to the problem is that the drying beds cannot naturally dry the sludge quickly enough,” Andrus wrote in a memorandum to Moab City Manager Donna Metzler. Andrus said that one of the beds is currently full of 18 inches of sludge that is drying. “It probably won’t be dry until July.”
When the drying beds get too full the plant reaches a point where the flow begins to recirculate through the plant, Andrus said.
“Each time this happens, the flow is concentrated more and treatment becomes less effective,” the memo said, stating that a new method for drying the sludge was necessary.
The excess of waste has caused violations to the discharge permit issued to the city by the Utah Division of Water Quality for biological oxygen demand (BOD). Effluent water from the plant is discharged into the Colorado River. The city treatment plant and the state Division of Water Quality carefully monitor the chemical and bacterial levels in the discharged water.
BOD “is a water quality issue that relates to the amount of organics in the water,” Andrus said.
“The city has a permit to discharge wastewater that allows specific levels of BOD and other gauges of water quality in the effluent from the plant.”
Andrus explained that if BOD levels are too high the bacteria consuming the organics can consume the oxygen levels in the water which can be harmful to fish and other aquatic life.
Because the effluent is diluted by the Colorado River, Andrus said there hasn’t been any significant effect on the aquatic life.
“Nevertheless, the discharge permit does have its limits that we are to abide by,” she said.
Over the past few years, Andrus said the frequency of the violations has increased.
“It is my understanding that there have been 29 violations,” she said, although she noted that some of those occurrences were counted twice because the levels are measured at both weekly and monthly intervals.
“If the seven-day [level] goes over significantly enough, the monthly [level] could also violate due to that seven-day period,” she said.
Most of those violations have been minor occurrences, she said.
“The state allows a couple minor violations without repercussions as long as we can pass the fathead minnow test, which measures the effect of pollutants on aquatic organisms. The test is a much better evaluation of whether aquatic life can actually survive in our effluent,” she said, noting that the test is referred to as a WET (Whole Effluent Toxicity) test. “As far as I know, we have always passed the WET tests.”
Metzler said that the city has been seeking out a long-term solution for the problem for some time. Andrus said the city would likely only need to lease the drying machine for two months to catch up on the amount of waste that is stockpiled.
“It’s taken us two or three years to get to this point,” she said. “In the long run, the drying beds just aren’t meeting our needs, so we need to go to some sort of mechanical drying process.”
If the machine works well, the city could consider buying it, Andrus said.
“Rental for the long term just doesn’t make sense,” she said.
If the city does decide to purchase the machine, one month of the rental price would be applied to the $225,000 total purchase price. The rental cost approved by the city council is $12,500 per month with a $4,900 mobilization fee, a $2,500 start-up and training fee and a $4,900 demobilization fee.
Andrus noted that other options are available, and plant officials might determine that the belt press is not the best permanent solution.
“There are lots of different types of dewatering processes out there,” she said. “It’s kind of fascinating really.”