County specialist explains aerobic treatment units, disinfection, life cycle and maintenance

At a Valley community presentation, Zachary Burf, an environmental quality specialist for Coconino County, described the function, operation and maintenance needs of alternate on-site wastewater technologies commonly used where native soil is shallow. He said aerobic treatment units (ATUs) use oxygen injection to create an aerobic environment inside a treatment chamber; that pre-treatment allows the required vertical separation from a limiting layer to be reduced in some designs. "Instead of a common septic tank where you're going to have two chambers, this one's going to have three chambers," Burf said, describing a trash/settling chamber, an aeration chamber with a blower or agitator, and a treatment/chlorination or UV chamber. He said the aeration component requires electricity and is typically on a timer; the systems should have a non-offensive, earthy odor when operating normally. Burf explained that disinfection is achieved either with chlorination tablets designed for wastewater systems or ultraviolet (UV) bulbs; either approach is used to ensure the effluent reaching the disposal media meets treatment expectations and does not harm native soil biota. He said the county reviews chlorine-contact-time calculations during plan review and that the goal is to deliver low- or no-free-chlorine effluent to the disposal field. "It's a low chlorine content that's going to treat inside that so it has a contact time that we're going to calculate out during the plan review," he said. Performance and monitoring: Burf described typical treatment metrics: biochemical oxygen demand (BOD) and total suspended solids (TSS). He said conventional household sewage BOD is around 150 mg/L and TSS around 75 mg/L, while treated effluent from ATUs can reduce those concentrations substantially (examples from the presentation ranged from modest reductions to very low levels depending on the unit and maintenance). Burf said Arizona currently issues single-event permits for ATUs but the state is contemplating rule changes that could add routine monitoring or annual operator reports. Maintenance and lifespan: Burf said moving parts such as blower motors and UV bulbs have finite lifespans and that systems are generally designed for an expected life of 15 to 20 years for treatment components and disposal media longevity that depends on use patterns. He cautioned owners to follow manufacturer guidance for chlorination tablets and to budget for maintenance; he said some owners do not replace tablets on schedule, which reduces treatment quality. Costs and installation notes: Burf said plastic tanks are commonly used in Valley (three green lids are often visible in the field) because they are easier to transport than concrete tanks; he provided a ballpark range for materials and installation costs but cautioned field access and contractor travel can increase final prices. He advised owners to compare multiple designers and installers and to provide accurate household water-use information to designers so systems match local usage patterns.