Wastewater flows from the collection system to the Wastewater Treatment Plant. It is designed to treat the wastewater to high water quality standards. The effluent is delivered to a holding pond where the water is reclaimed for use on the Pine Butte Golf Course, and the disposal of sludge to a sludge storage lagoon. The treatment plant is an oxidation ditch (or Paasveer ditch) which is an extended aeration, activated sludge process with excess sludge treated by aerobic digestion and applied to drying beds or sent to the sludge storage lagoon.

Raw sewage from the city flows to the site through two outfall lines to two distribution vaults. In emergencies, sewage can be diverted to the lagoons at the distribution vaults. Normally, however, the sewage from the two outfalls merge at a manhole and flows to the head-works through a 18" reinforced concrete pipe.

The wastewater enters the headworks through an open shear gate (or stop gate).  At periods of very high flow, the wastewater can spill over the weir in the curtain wall and flow through the bar screen in the opposite channel. Any grit that accumulates in the headworks can be removed by isolating one channel at a time with the shear gates.

From the headworks the sewage drops into the oxidation ditch for aeration and mixing by means of a mechanical brush rotor for a detention period of 24 hours. The rotor forces the sewage around the ditch and mixes it with the micro-organisms (or "bugs") which oxidize the biodegradable matter in the sewage. The rotor also beats air into the liquor to supply the dissolved oxygen required by the bugs.

The mixed liquor from the oxidation ditch flows to the clarifier, enters the inlet trough along the perimeter of the basin and flows underneath an aluminum skirt and inward to the weirs near the center. During the normal 3-hour detention time, the solids (or "bugs") settle to the bottom and are scraped toward a hopper by a rotating arm assembly. The settled sludge is pumped from the bottom of the clarifier to the oxidation ditch by the return sludge pumps. Scum which accumulates in the inlet trough can flow to the scum trough by gravity, by manual scraping, or by installing water sprays along the inlet trough. The scum collected by the rotating scum pipe is returned to the oxidation ditch for further processing. Although most of the settled solids are returned to the oxidation ditch, they can (when necessary) be wasted to the aerobic digester by means of an airlift pump located at the digester.

Clear water from the clarifier flows to the control building where the flow is metered. It is then directed to a Ultraviolet Disinfection System to disinfect the wastewater.

Solids that are removed from the activated sludge system flow to the digester and are pumped into the digester till it is ultimately disposed of in the sludge storage lagoon. It is continuously aerated there to keep it from becoming too anaerobic or odiferous.

The Wastewater Treatment Plant was designed for an average daily flow of 600,000 gallons and is operating now at around 200,000 gallons per day.


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City Council
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