At Irvine Ranch Water District's Michelson Water Reclamation Plant tertiary, or advanced, treatment is used to reclaim wastewater. This method concentrates and speeds up the process used by nature to clean water and to biodegrade wastes.
The steps in the tertiary process include:
Preliminary Treatment
Raw wastewater enters the plant at the headworks, where it receives a rough mechanical screening. Bar rake screens trap large debris and screen the wastewater. The debris and screenings from the mechanical bar screens are then conveyed to grinders, which then return the screenings to the wastewater coming into the plant.
Primary Clarification
The water leaves the headworks and flows into the primary clarifiers, settling basins where the water is left undisturbed for a few hours. Most of the solids, called sludge or biosolids, settle to the bottom of the primary clarification tanks. The sludge is pumped through underground pipes to the County Sanitation Districts of Orange County, where it undergoes further treatment and is eventually composted for fertilizer. The clarified water, now called primary effluent, flows to the next stage of treatment.
Flow Equalization
Primary effluent can either be sent to the next step in the treatment process directly, or it can be diverted to the Flow Equalization Basins. These basins, which hold up to five million gallons of primary effluent, are storage areas which allow the biological nutrient removal process to be operated at a constant flow rate.
Biological Nutrient Removal (Activated Sludge Process)
The heart of the treatment process takes place in this step. This is the biological step in the treatment process occurs, using naturally occurring microbes to decompose the organic material in the wastewater. Primary effluent first enters two baffled zones mixed by submerged propellers. The removal of nitrogen (denitrification) occurs here because the microbes utilize nitrates as a source of oxygen and the nitrogen gas is released into the atmosphere. Methanol is added as necessary to provide a supplemental carbon source to promote denitrification.
Although the organisms that conduct this portion of the treatment are the same organisms that can be found in nature in ponds, lakes and streams, they exist in much higher concentration in the controlled atmosphere of a reclamation plant. To allow these organisms to live in such high concentration, air must be pumped into the oxic zone of these tanks, hence the name aeration basin. Ambient oxygen--the same air we breathe--is pumped into the bottom of the 30-foot deep tanks using a fine bubbler aeration system to evenly diffuse the oxygen across the bottom surface of the tanks. Nitrification takes place in this portion of the tank, when nitrogen in the form of ammonia is converted to nitrate. Water remains in the aeration basins approximately five hours.
Secondary clarification
When the microbes have completed the job of consuming the waste material and removal of nitrogen, the water flows to the secondary clarification tank. Here, more of the solids in the water settle out. Water remains in these tanks for about two hours The flow leaving the clarifiers (secondary effluent) is very clean, with about 90 percent of all the contaminants removed. A small amount of aluminum sulfate (alum) is introduced as the water leaves the secondary clarifiers. Alum is a coagulant, which makes any remaining particles in the water cling together for easier removal in the next step. When the wastewater leaves this area, secondary treatment has been completed. The remaining steps are part of tertiary, or advanced treatment.
Filtration
Filtration is the first part of tertiary treatment. Secondary effluent is pumped to the top of the two-story dual media filter, where the water is allowed to trickle through layers of anthracite coal and sand. This mimics the way nature cleans water, as rain falls to the ground to soak into underground aquifers. When the water leaves the filters, more than 99 percent of the contaminants have been removed and the water is sparkling clear.
Disinfection
The final step in treatment is the addition of chlorine. The chlorine kills any bacteria and virus remaining in the water that could be harmful to people or to the environment. The water remains in this tank for a minimum of two hours. At this step in the process the water also receives chemical treatment to ensure proper pH balance.
Distribution
The entire tertiary reclamation process takes only 12 to 16 hours at Michelson Water Reclamation Plant. Michelson's capacity is 15 million gallons per day and the average flow is now approximately 11 million gallons per day. When the finished product (final effluent) leaves the Michelson plant it is ready to immediately be used in the community. During high demand periods the water flows directly from the plant and into some 250 miles of reclaimed water pipelines for reuse in landscape and agricultural irrigation and other uses. During cooler months when demand is lower, reclaimed water is sent to storage reservoirs such as Rattlesnake Reservoir, Sand Canyon Reservoir or several constructed tank-type reservoirs throughout the community. During winter months, some reclaimed water is also sent to the Orange County Water District's Green Acres Project which provides reclaimed water to areas outside IRWD boundaries.
Links:
How reclaimed water is used
Michelson Water Reclamation Plant Virtual Tour
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