Wastewater Treatment 

Primary/Secondary Wastewater Treatment

Monterey One Water’s Regional Treatment Plant treats an average of 17 million gallons of municipal wastewater per day. When needed to meet water recycling demands, Monterey One Water can also collect additional used waters for treatment including industrial processing water, crop irrigation drainage water, and urban stormwater runoff. Learn more on our Source Waters page.

No matter its origin, all wastewater enters the Regional Treatment Plant for primary and secondary wastewater treatment. Here, the following processes are used to remove solids, pollutants, and harmful bacteria:

  • Headworks
  • Primary Clarifiers
  • Trickling Filters
  • Bioflocculation Basins
  • Secondary Clarifiers 

Take a virtual tour of the Primary/Secondary treatment technologies below.


Headworks treatment technology

Everything that goes down the drain enters our facility at the Headworks. During the conveyance process, grinders located at our pump stations act like blenders and chop up any solid items in the water. When raw sewage arrives at the Regional Treatment Plant, it is primarily water, but the large items such as rags, sticks, and toys, now ground up into smaller pieces, are removed from the water using a screen. Mechanical "rakes" keep the screens clear by removing the debris from the screen and depositing into bins that are emptied at the landfill.

Primary Clarifiers

Primary clarifiers infrastructure

Next, water enters the Primary Clarifiers where mechanical arms in these large, covered tanks slow the flow of water. Gravity then takes over and solids settle to the bottom (sludge) while fats, oils, and grease (FOG) float to the top. Both are skimmed and removed leaving cleaner water in the middle that flows to the next step.
The removed solids are decomposed naturally in anaerobic digesters to produce biosolids and methane gas. Learn more about these processes on our Resource Recovery pages.

Trickling Filters

Trickling filters infrastructure

Water is pumped to the top of these 30-foot hexagonal units, each of which is filled with a plastic material home to helpful bacteria. Rotating, mechanical arms then spray the water on the plastic media. As it trickles down, the bacteria clean the water as they feed on the organic matter in the water.

Bioflocculation Basins

Bioflocculation basin infrastructure

These open-air basins contain millions and millions of microbes which decompose the remaining "food" or organic matter in the water. To maximize biological efficiency, air is bubbled into the basins to increase the dissolved oxygen content in the water, energizing the microbes. Once full and happy, the microbes and the cleaner water flow to the next step.

Secondary Clarifiers

Secondary clarifiers infrastructe

In this final step, mechanical arms are again used to the slow the flow of water in these open-air Secondary Clarifiers. As water slows, the happy microbes settle to the middle of the tank where they are pumped back to the Bioflocculation Basins to repeat the process with new flow. The treated water that leaves this step is called secondary effluent and it is either discharged to the ocean or it becomes influent for our water projects