Sierra Vista, Arizona Wastewater Treatment System for Using Reclaimed Water for Aquifer Recharge.
Wastewater Treatment Facility
The new facility employs four distinct steps to the treatment process to clean the water before it is sent to the
recharge basins. The steps are preliminary treatment, primary treatment, secondary treatment, and tertiary treatment.
While the treatment process is extremely technical in nature, a brief synopsis of each step is explained below, along
with a brief explanation of how the recharge basins work.
Preliminary Treatment: Only organic matter within the plant will be treated, so this step removes inorganic material.
The water flows through a screen that will take items such as plastics, rags, heavy paper, and rubber materials and
automatically place them in a dumpster for disposal. The “trash-free” water will then flow into a grit chamber to
separate heavy inorganic matter. The grit chamber or channel is wider than the pipeline itself and the incoming flows
are slowed, allowing material such as sand, egg shells, coffee grounds, etc., to settle to the bottom of the chamber.
The facility has two grit chambers, which are used alternately. One chamber is always active. As the inactive chamber
dries, the maintenance staff cleans the “grit” and disposes of it as a solid waste, then alternates between chambers.
The removed items will not break down in the treatment process and could cause damage to pumps and motors. At
this point, the water, free of trash and heavy inorganic matter, flows into the Primary Treatment Step.
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Primary Treatment: This step consists of four complete mix lagoons, which are heavily mixed and aerated by four, 25-
horsepower aerators. The water entering this phase of treatment is heavily laden with ammonia and organic solids
carrying bacteria or bugs that will aid in the treatment process. The goal in this phase of treatment is to begin
converting the ammonia to nitrogen and gives the bacteria, or bugs, plenty of oxygen to thrive and begin the
treatment process. The water will remain in this portion of the treatment process between 1 to 3 days. The water
leaving this step should be fully converted from ammonia to nitrogen and the oxygen demand for the bacteria should
be met. Secondary Treatment: This step consists of four partial mix lagoons, which are lightly mixed. Oxygen is
induced through the use of 8 aerators in each lagoon, ranging in size from 15 horsepower to 7.5 horsepower. During
this step the bacteria, or bugs, survive from the oxygen induced and consume the organic waste as food, as they
continue to clean the water. In each of the last two partial mix lagoons, there is an apparatus called a clarifier at the
end of each lagoon. The clarifier, again, slows the water allowing the solids to settle to the bottom of the structure and
any scum floating on the surface to be skimmed off. The solids that settle to the bottom of the clarifier contain the
bacteria or bugs that help clean the water further, so staff can return a majority of those bugs to the primary treatment
portion of the treatment process via a pump station. If it is determined that there is enough food throughout the plant
for the bugs to consume, the staff can then waste those solids to a bio-sludge wasting area. The water leaving the
clarifier, or Secondary Treatment area, should now be free of floating scum and the majority of organic suspended
solids, as it enters the tertiary treatment phase. Tertiary Treatment: This step consists of 10 wetland cells, of which 6
are planted with a mixture of bulrush and cattails. The water slowly flows through these cells creating man-made
marshlands. Since the ammonia was converted to nitrogen previously, this phase will remove the nitrogen. These
wetlands plants will use the available nitrogen within the water for their growth and will thrive from its availability. These
marsh plants will further polish the water removing all remaining suspended solids. At this point of the process, the
water is ready to be returned to the aquifer. It will be very close to drinking water standards as it exits the wetlands or
tertiary treatment area.
But then Sierra Vista allows untreated sewage septage to be dumped into the wetlands.
Recharge: The recently cleaned water will flow from the wetlands into one of the 11 rapid infiltration recharge basins.
Each one of these basins can handle 24 to 36 hours of flows. The recharge site was chosen after a soils analysis was
conducted, indicating that this area is the most conducive to allowing water to rapidly percolate into the aquifer. The
basins are used individually. As they fill, we automatically move to a dry basin, rotating between all 11 of them.
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This state-of-the-art facility should be capable of meeting the City’s wastewater treatment needs for another 20 years
or more, depending on demand. This also means 20 or more years of water recharge in the amount of nearly 2,000
acre-feet at startup to over 4,000 acre-feet at build out. However, users of the system need to do their part by
ensuring that items that go down the drain do not inhibit the natural treatment process. Household items that are
marked dangerous, hazardous, harmful, handle with care, or carry any other similar warning, should not be disposed
of through the sewer system. It is critical that users of the system follow and support the pretreatment requirements in
the City Code of Ordinances. This will extend facility life and protect our groundwater aquifer.
But then Sierra Vista has an ongoing unexplainable E. coli 0157 epidemic.