Wastewater Treatment

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How is your wastewater treated?

Your wastewater is collected and transported through a network of sewer lines and lift stations located throughout the city to the Easterly Wastewater Treatment Plant. The goal of the treatment process is to remove as much pollutants in the wastewater as possible to minimize the risks to public health and impact on the environment. The following process details how Vacaville’s wastewater is treated.


Primary Treatment


Primary treatment begins when pumps lift the incoming wastewater, called influent or flow, to a higher elevation to allow the rest of the treatment process to flow by gravity. The influent passes through bar screens that remove large debris such as rags, paper and leaves. Next, sand, gravel and small particulates are removed in aerated grit tanks. The removed debris and particulates are then washed and sent to the landfill.

Primary Clarifiers

In primary tanks, also known as primary clarifiers, heavy material (sludge) sinks to the bottom while lighter material (scum) floats to the top. Both materials are removed from the influent and pumped to the bio-solids (digestion) process. The influent then continues to the aeration basins to begin secondary treatment.

Secondary Treatment


Using blowers, oxygen is introduced into the influent, along with microorganisms returned from the secondary clarifiers. This secondary treatment is called “activated sludge”. The microorganisms break down suspended material that will not settle and absorb organic material in the water making it cleaner. The influent then moves to the secondary clarifiers.

Secondary Clarifiers

The flow from the aeration basins travels to the secondary clarifiers where microorganisms settle to the bottom of the tanks. A majority of the microorganisms are then returned to the aeration basins to assist in the clarification process by breaking down suspended materials. To keep a healthy microbial population in the aeration basins, a smaller amount of microorganisms is pumped to the digesters. This process of separating the solids is called “wasting”. The flow from the secondary clarifiers then travels to the contact chambers for disinfection.



Using a stronger form of chlorine, sodium hypochlorite is introduced in the contact chambers to kill any harmful bacteria in the flow so as to protect the environment when the treated wastewater is released to Old Alamo Creek. The contact chambers give the chlorine time to complete the disinfection process.


To further protect Old Alamo Creek, sodium bisulfite is then used to remove the chlorine residual. The dechlorinated water, now called effluent, is then discharged to Old Alamo Creek. The discharge is strictly monitored by plant personnel to ensure a quality effluent that complies with state-regulated limits to meet permit compliance.

Biosolids Treatment

Solids Thickening

Solids which are “wasted” from the secondary clarifiers are thickened in the Dissolved Floatation Thickener (DAF) tank. Recirculated water is saturated with air and sent through small holes to produce bubbles. This is then mixed with the incoming solids and is injected into the bottom of the tank. As the air bubbles rise to the top they capture solids, thus separating them from the water. The water overflows from the tank which is then recirculated to the beginning of the thickening process. The “float solids” are sent to the digesters for further treatment.


Solids from the DAF tank are mixed with solids pumped from the primary clarifiers (sludge and scum) and sent to tanks called digesters. The digesters are heated to 98 degrees Fahrenheit to encourage anaerobic (without air) bacterial growth which breaks down the solids by 50%. This process produces gas which is used to run some plant equipment. The digested solids are then pumped to an aerated lagoon before being dewatered.


Using a belt filtration press, biosolids are injected with polymer which allows water to separate from the solids that are then compressed to remove more water. The solids are allowed to dry out further in the sun and are finally sent to a landfill to be used as cover material. Removed water is returned to the treatment process.

Odor Control

To protect the nearby community, an odor control system has been implemented. Covers have been installed in the influent structure, grit tanks and the primary clarifiers that prevent odors from going into the atmosphere. The foul air is suctioned from these areas and is then forced up from the bottom of a mixed media bed removing the odors.