Growth during the late 1990’s triggered the Colorado Department of Health and Environment, Discharge Permit requirement that planning (i.e. engineering, including preparation of plans and specifications) for plant expansion commence when 80% of plant capacity is reached and construction be initiated when 95% plant capacity is utilized. This growth results from the land use decisions of Weld County, Firestone, Frederick, Dacono, Erie and Mead, and also driven by the general economy.
This District initiated wastewater treatment engineering studies in 1999, and developed a long-range wastewater treatment plant process and construction master plan. Rather than expand the aeration lagoon treatment system from 0.5mgd to 1.5mgd through construction of two large, additional lagoons (which would consume most of the remaining original 25-acre plant site, thus making future transition to a mechanical system extremely difficult and expensive), it was concluded that the District should immediately transition to a mechanical plant. This conclusion is reinforced by the rate at which growth is occurring.
Built on 40 acres, the new wastewater treatment plant was completed in March of 2002 and shares the site with the original lagoon treatment system, which was built in 1986. In an average day, the treatment plant receives 1,000,000 gallons of raw wastewater. In a matter of 3 to 4 days, that wastewater will be treated and be sent to the St. Vrain River after passing through Ox-Bow Lake. The discharge of wastewater into receiving waters must be managed to protect public health, maintain water quality, and comply with federal and State requirements.
Our plant uses an extended air activated sludge process to treat the wastewater. Activated sludge is a biological treatment process that uses naturally occurring microorganisms to decompose the waste. We use no chemicals to treat the wastewater that is released from our plant.
When raw wastewater enters the plant it goes through a preliminary treatment process. This occurs in the Headwork’s building where the mechanical barscreen collects all the larger solids that may enter the plant such as paper towels, feminine hygiene products, latex products, cigarette butts, and anything else that may be flushed or put down the drain. The barscreen removes these items from the influent and mechanically dewaters before conveying them to a dumpster for disposal. The heavy solids, which consist of eggshells, sand and gravel, and other construction debris, are removed in the vortex grit separator. This unit is a concrete cylinder with a horizontal paddle in it. The paddle turns slowly inside of the cylinder creating enough current to keep the suspended particles from settling out, but not enough current to prevent the heavy solids from settling to the bottom of the collector. The heavy solids are then pumped from the collector to the bio-solids pond where the solids are collected until there is enough volume to have them pumped out and disposed of at either a land fill or can be land applied as fertilizer.
From the Headwork’s building the screened and settled influent is pumped to one of two aeration basins. The aeration basins hold 2.3 million gallons each and are where the secondary treatment occurs. As the influent enters the basins it is mixed with return activated sludge or RAS. RAS is the microorganisms produced in the secondary treatment process by the growth of organisms in aeration basins in the presence of dissolved oxygen. “Activated” comes from the fact that the particles are teaming with bacteria, fungi, and protozoa. When the influent is mixed with the return activated sludge, it is called “mixed liquor”. The mixed liquor is “injected” with large amounts of dissolved oxygen to accelerate the growth of the organisms. These organisms are eating machines that eat more and faster when the appropriate amounts of influent, activated sludge and dissolved oxygen are mixed, the process takes about three and a half days to treat at our current influent rate. From the aeration basins the mixed liquor gravity flows to our clarifiers where the settleable solids (RAS) and float-ables (scum) are separated from the liquid. From the clarifiers, we return the RAS to the aeration basins or waste the activated sludge (WAS) to our bio-solids pond. Through regular testing, our operators can determine how to adjust the mix to get the most efficient treatment.
The mixed liquor is now treated and the remaining liquid is called effluent. From the clarifiers the effluent gravity flows to the Operations building for the final treatment. The effluent is clear and ready for disinfection. The effluent is disinfected with high doses of ultra-violet light. There are several banks of UV light tubes assembled in the effluent channel where the effluent passes through the banks and any disease causing organisms are killed by the UV lights which is the major concern to the down stream user. The effluent is now ready to be returned to the St. Vrain River after going through Ox-Bow Lake and be used for irrigation or further treated to drinking water for downstream users. |
