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Wastewater Treatment

Treatment Plant Process Diagram

The District’s wastewater treatment plant is comprised of five main process areas: Preliminary Treatment, Primary Treatment, Solids Treatment, Secondary Treatment, and Disinfection.

Preliminary Treatment: This process is used to remove typical trash like materials: rags, fabrics, plastic wrappers, personal wipes, rock, grit, eggshells and glass. These materials are incompatible with all conveyance and treatment processes and should never be knowingly deposited into the wastewater system.  

Primary Treatment: Three circular in-ground tanks called clarifiers are used to separate solids from liquid. Settleable solids and floatable material like grease are separated from the liquid portion that flows on  to the secondary treatment process. The settled and floatable solids are pumped to the solids treatment process.

Solids Treatment: Two circular above-ground tanks called digesters are used to “stabilize” solids (a.k.a. sludge) removed in the Primary and Secondary treatment processes. Waste stabilization  improves the properties of the organic waste to a less environmentally harmful form. LGVSD digesters are anaerobic, meaning there is little to no oxygen present. The digesters receive sludge from other plant processes with a solids concentration of 2 to 3 percent. The digesters are fitted with mixing and heating equipment to maintain a homogenous mix at about 98 degrees Fahrenheit. Anaerobic bacteria use the waste as an energy source for their survival and replication. The resulting “biosolids” consist primarily of bacterial cell mass created by the digestion process along with inert material. The biosolids contain useful plant nutrients and is suitable for use as fertilizer. Another biproduct of this process is the production of methane gas which is used in the District’s Biogas Energy Recovery System (BERS).

Secondary Treatment: LGVSD uses a hybrid treatment technology known by the tradename Biowheel. It is described as an integrated fixed-film activated sludge process because it provides the conditions for facultative bacteria that naturally adhere to the fixed surfaces of the rotating wheels, in addition to creating conditions to support high concentrations of suspended bacteria in the water column. By design, the rotating chambers of the wheels release trapped air below the water surface allowing micro-organisms suspended in the “activated sludge” to thrive. Activated sludge is an industry term for a secondary treatment system that utilizes and enhances natural biological activity of aerobic bacteria in a liquid filled basin to convert dissolved organic matter present in the wastewater to bacterial cell mass.  Within the same process, another class of “nitrifying” bacteria support their growth by converting ammonia to nitrate.  The subsequent removal of the bacterial cell mass in secondary clarifiers leaves a highly clarified effluent with greatly improved water quality.

In addition to the aerobic zones, each of the four aerobic basins also has a companion  anoxic basin where chemically bound oxygen (nitrate) is present, but  little to no dissolved oxygen. Nitrate produced in the aerobic basins is fed back to the anoxic basin where denitrifying bacteria convert it to nitrogen gas which is released to the atmosphere.  This “nitrification-denitrification” process removes much of the dissolved nitrogen present in the wastewater, which also contributes to improved quality of the final effluent discharged from the treatment plant.  This system wouldn’t be complete without the two in-ground secondary clarifiers to separate the solids (now consisting primarily of bacterial cell mass) from the liquids flowing from the biological treatment process. The majority of settled solids from the clarifiers  is returned to the secondary treatment process to work on incoming organic material. A small portion of the settled solids is wasted to the anaerobic digestion process to maintain a proper balance between food and micro-organisms.  The clarified effluent is sent to disinfection.

Disinfection: After secondary treatment, treated wastewater is chlorinated with industrial strength bleach, an oxidizer, to eliminate pathogenic organisms. The system consists of  an underground pipe system arranged in a serpentine layout designed to maximize disinfection contact time. De-chlorination is achieved in the final length of pipe, where a liquid reducing agent, sodium bisulfite, is injected to neutralize any residual chlorine present in the treated water before being released to Miller Creek.


Biosolids as a Resource 

Biosolids are a nutrient- and energy-rich resource that innovative wastewater utilities like the District capture and utilize. 

After biosolids are produced they are temporarily stored before they are spread onto a dedicated nine-acre area on District property, where they become part of the soil. This is a permitted, approved method of disposing of biosolids — but our goal is to treat them as a resource, not as a disposal issue. In keeping with District’s philosophy of environmental sustainability we are exploring ways to beneficially use the nutrient value of biosolids.

There are potential alternatives to disposal — for example on certain crops biosolids can be applied to reduce the use of chemical fertilizer. Another option is to compost biosolids with locally-generated green waste to produce a high-quality compost product. The District’s Board and senior staff keep a focus on implementing innovative operational improvements that will give us the ability to take full advantage of all the nutrient and energy value that biosolids have to offer.

Methane Gas as a Resource 

Another byproduct of the process is methane gas which is produced by the digesters. The District uses that to operate microturbines which generate on-site electricity, while the heat they produce goes back to the digester to support that process. The methane can also be used to fuel one of the District’s vehicles.


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