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Aerial shot of WWTP The Wastewater Treatment Plant provides treatment of all wastewater collected through the sanitary sewer system.

Mission: Protect public health, the environment, and the waters of the State of California by providing appropriate treatment processes to all wastewater collected within our service area while monitoring and ensuring water discharged meets or exceeds State and Federal requirements.


  • Operate and maintain the City of Merced’s Wastewater Treatment Plant in a reliable, safe, productive, and cost-efficient manner.
  • Maintain compliance with State Water Resources Control Board, National Pollutant Discharge Elimination System (NPDES), and Clean Water Act permit requirements.
  • Promote safety in the workplace through increased employee involvement, as well as taking full advantage of the industry’s educational and technical programs, which are in compliance with Cal- OSHA and Fed- OSHA regulations.

The City of Merced Wastewater Treatment Plant is State of the Art

The Merced Wastewater Treatment Facility (WWTF) has gone through many changes over the years, adding equipment and processes in order to keep ahead of Merced’s needs and California EPA guidelines. The WWTP recently finished two major upgrades (Phase IV and Phase V) to improve the quality of the treated water, referred to as plant effluent, and to improve the quality of biosolids and methods of treatment.

The first step to treat wastewater, once it arrives at the treatment plant, is to screen out plastics and other inorganic matter. Then grit, which is comprised of coffee grinds, dirt, eggshells and other inorganic materials, are removed. At the completion of the Phase IV project, an entirely new headworks structure was created to accomplish these tasks. The headworks is comprised of Influent flow monitoring, four influent pumps, two Parkson perforated plate screens, two Pista grit separation cyclones, and a grit classifier. The structure is also used by operators to divert flow to other processes in the plant.

The next step in treating wastewater is to remove all solids that will settle out or float to the top. This is accomplished in large circular tanks, called primary clarifiers, that slow the rate in which the water moves allowing the organic materials to fall out. In Phase V a third primary clarifier was added. The new clarifier allows for greater solids removal, which in turn allows other downstream processes to function more efficiently.

After primary clarification, the wastewater flows to aeration basins where a sophisticated biological treatment is utilized. This biological system simultaneously nitrifies and denitrifies for ammonia removal, while also removing a majority of the suspended solids not removed in primary clarification. These solids are converted into more microorganisms. Generally 1 ½ pounds of organisms are produced for every pound of solids destroyed.

After nitrification and denitrification, the wastewater is disinfected. Previously, the WWTP would add chlorine to disinfect, then calcium thiosulfate to dechlorinate before returning the water to Hartley Slough and eventually the San Joaquin River. During Phase IV construction, ultra violet disinfection was added to disinfect the water without adding harmful chemicals that could affect the receiving waters.

During primary clarification, solids (sludge) are pumped from the bottom of the primary clarifiers and the top of the clarifiers (scum) to the anaerobic digesters. These digesters are mixed, heated, and using an anaerobic biological process; breaks down the solids. Before Phase V, sludge would then flow to a secondary digester where it could settle, and then be pumped to drying beds where it would sit for up to six months while it dries. Phase V added upgrades to the digesters and the flow of sludge. First, we gained the ability to capture and utilize the methane produced by the anaerobic process to heat the digesters instead of using costly diesel or natural gas. Second, the sludge now flows to a solids holding tank and is dewatered with centrifuges, then dried in active solar dryers. These are basically large green houses with concrete floors and mechanical moles that turn over the sludge to help it dry faster. The sludge is now dry in as little as two weeks. This new process prevents the possibility of ground water contamination from the drying beds and produces a cleaner and more thoroughly treated sludge that can be used to fertilize feed crops.

The Merced Wastewater Treatment Plant is now one of the most advanced facilities in the state. It is capable of treating up to 12 million gallons of influent a day, which is important for courting large manufacturers to come to our city. Our active solar dryers are the largest in North America and our solids handling serves as a model for other treatment plants in California. The Merced WWTP is something that all Mercedians should be proud of.

To schedule a tour of the WWTP, please call (209) 385-6892.