Water Quality and Treatment

Sunset at the Metropolitan Weymouth Water Treatment Plant

Protecting Water Quality from Source to Tap

Safe Drinking Water is Our Mission

If water is life, then there is nothing more important in our daily lives than safe and sustainable water supplies. And there is nothing more important to Metropolitan than safeguarding the quality of drinking water we provide to our 26 member agencies and the communities they serve. To ensure we meet this responsibility, every year we test our water for more than 400 constituents and perform nearly 250,000 water quality tests on samples gathered from throughout our vast distribution system. We invest in the latest and best technology at our Water Quality Laboratory in La Verne and each of our five smaller treatment plant laboratories. And we rely on the expertise of our highly skilled staff from disciplines across the sciences, many of whom are leaders in their fields of research and regulatory compliance.

Metropolitan’s water meets or surpasses all state and federal regulatory requirements.

Our staff oversees treatment operations that enable us to ensure water quality remains well-within state and federal guidelines. We go beyond the minimum requirements and conduct investigations that develop and optimize advanced analytical contaminant detection methods that provide additional operational support and understanding of water quality contaminants that are not regulated.

Testing water quality using a Florence Flask in a laboratory
Ozone disinfectant systems at the Weymouth Water Treatment Plant

The Mechanics of Treating Water

Before water pours from a tap, it passes through one of our five water treatment plants, which together can disinfect more than 2 billion gallons of water daily, using a five-step treatment process. All of our facilities use ozone as the primary disinfectant. Ozone destroys a wider range of micro-organisms and allows Metropolitan to keep pace with stringent regulations that limit the levels of drinking water disinfection byproducts in water. The result is historically low levels of disinfection byproducts systemwide. Ozone disinfection also provides increased protection from pathogens and improves the taste and smell of water.

Learn more about ozone disinfection here.

For the water treatment process, untreated water starts with the addition of acid, ammonia, and chlorine. Following this, it undergoes an ozone disinfectant process and then is mixed with alum, ferric chloride, and polymer. It then undergoes flocculation and then sedimentation. After passing through a filter aid, it is then filtered and treated with fluoride, ammonia, chlorine, and caustic soda. Finally, it arrives at the finished water resivoir for transfer to member agencies.

Treatment Steps


As water enters the treatment plant, the first step is Disinfection and, depending on the source water quality, it may be necessary to add various chemicals to optimize this process. Water is disinfected using ozone primarily, which kills microorganisms, including pathogens such as viruses, including SARS-CoV-2, and protozoa such as Cryptosporidium and Giardia.


As water flows through the ozone contactors, hydrogen peroxide may be added for taste and odor control. If there are any interruptions in the ozone process, chlorine can also be used as a back-up disinfectant and is readily available.


The second step in the treatment process is Coagulation, where chemical coagulants such as alum (aluminum sulfate) or ferric chloride and polymer are injected into the water and mixed rapidly using flash jet mixers. The coagulants help unwanted particles stick together, making larger particles that are more easily removed. Water then flows into the mixing and settling basins, where large mechanical mixers or flocculators gently agitate the water.

The third step is Flocculation as water further mixes with the coagulant chemicals added in the previous step, allowing time for larger suspended particles in the water to bind together and form “floc.”


The fourth step is Sedimentation. In this process, the floc particles, which are much heavier than the surrounding water, settle to the bottom of the basin, forming a layer of material that is later removed. In the fifth step, Filtration, filter aid is applied to settled water from the sedimentation basins to help remove particles.


Filters consist of layers of anthracite coal, sand, and gravel filter media. As water passes through, the filters remove smaller particles from the water as well as larger particles that did not settle during the sedimentation process.

Once the treatment process is completed, chlorine and ammonia are added to the water to form chloramines and maintain a disinfectant residual in the distribution system. This ensures water quality is maintained as supplies travel through the distribution system. Caustic soda also is added as a corrosion control measure to adjust the pH level of the water and protect pipes and plumbing fixtures. Fluoride is added to help prevent dental caries as recommended by the U.S. Department of Health and Human Services. Treated water is temporarily stored in finished water reservoirs and distributed to member agency connections.

Metropolitan's Treatment Plants

Aerial view of the Weymouth Water Treatment Plant
Water treatment pools at the Robert B. Diemer Water Treatment Plant
Aerial view of water treatment pools at the Joseph Jensen Water Treatment Plant
Aerial view of the Robert A. Skinner Water Treatment Plant facilities
Aerial view of the Henry J. Mills Water Treatment Plant facilities
A lab technician examines a water sample gathered from a water treatment plant

Supporting Innovation with Science

Metropolitan, in partnership with Los Angeles County Sanitation Districts, recently began operating the Pure Water Southern California Demonstration Plant, a demonstration facility in Carson, to test and study a new treatment train to recycle water for potable reuse in the future. The facility takes treated wastewater and purifies it using a three-step process: membrane bioreactors, reverse osmosis, and UV light with advanced oxidation. An onsite lab allows engineers and scientists to test samples from each purification process for numerous water quality factors. These tests help monitor the effectiveness of each step in the process, optimize operations at the facility, and ensure the purified supplies meets water quality standards. Water samples also are analyzed at Metropolitan’s Water Quality Laboratory, Los Angeles County Sanitation Districts’ laboratories, and contract laboratories.

Should the project move to full-scale after the demonstration phase of study, the purified wastewater would be delivered through 60 miles of new pipelines to the region's groundwater basins, as well as for some local industrial use. A full-scale facility would produce up to 150 million gallons of water daily, enough to serve more than 500,000 homes. As the state develops regulations for direct potable reuse, the water also could potentially undergo additional treatment and be blended with Metropolitan's existing supplies at two of its treatment plants that provide drinking water to the region.

Responding to Water Quality Challenges & Concerns

PFAS (Per-and Polyfluoroalkyl Substances) 

For decades, manufacturers have widely used chemicals known as PFAS in everyday industrial and household products such as firefighting foam, clothing, non-stick cookware, personal care products, and paints. Known for their resistance to heat, oils, stains and water, concern is growing about the use of these chemicals because of their possible health effects in high concentrations. Because they don’t easily break down, they’re often referred to as “forever chemicals.” 

The family of PFAS includes over 12,000 chemicals, some of which have been found in some water supplies across the country. Since 2013, Metropolitan has monitored for PFAS, including the two most common and most studied PFAS – perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). We have infrequently detected four PFAS compounds at low concentrations just above the detection level, including one-time detecting very low levels of PFOS in a source water lake that Metropolitan uses infrequently.

In March 2023, the U.S. Environmental Protection Agency announced proposed drinking water standards for six PFAS.

Metropolitan continues to ensure the region has a safe drinking water supply by voluntarily monitoring our source and treated water for PFAS and supporting our member agencies as they assess whether PFOA and/or PFOS are present in their supplies. Metropolitan is prepared to handle any increased demands for imported water that may result from the loss of affected local supplies.

Below are two resources aimed at helping public understanding of PFAS: a fact sheet as well as a list of frequently asked questions and answers.




Cyanotoxins are produced from blooms of cyanobacteria — also known as blue-green algae — in lakes and reservoirs. In high concentrations, they can be harmful to humans and animals, especially when ingested. As blue-green algae blooms have become more common with climate change, cyanotoxins are a growing concern locally and globally. They are not currently regulated, but state and federal regulators do recommend certain responses based on the amount of toxins being released by a bloom.


Cyanotoxins are largely a recreational issue, with officials recommending people and animals avoid swimming in or eating fish from cyanotoxin-affected water. Drinking water agencies like Metropolitan can avoid using water affected by a bloom and treatment ensures drinking water is safe.


Metropolitan has an extensive algae and cyanotoxin monitoring program and has detected cyanotoxins in its source waters during significant bloom events. Continuously refining its methods for cyanotoxin monitoring and detection, Metropolitan is ready to comply with evolving regulations. On February 4, 2021, California initiated the process for developing notification and response levels for four cyanotoxins.

A lab technician examines blooms of cyanobacteria from water samples

Source Water Protection

Protecting water quality starts at the source. This requires careful management of watershed lands. Metropolitan imports water from two places — the Colorado River through the Colorado River Aqueduct, which we own and operate, and Northern California via our participation in the State Water Project. Each source has its individual challenges from constituents that enter the water. Some are natural and come from the environment, and others come from cities and farms. Some constituents such as total organic carbon and bromide in source waters can potentially form harmful contaminants when in contact with disinfectants such as chlorine or ozone during the disinfection process. Certain industrial processes — like dry cleaning, fireworks and rocket fuel manufacturing — have left constituents in the environment, as has the use of certain fertilizers and pesticides. Many of these chemicals have since been banned from use. When these chemicals make their way into California’s water supplies, specialized treatment is required, or the supplies must be abandoned.

Metropolitan partners with state and local water agencies in support of programs that protect water at its source and on its path to treatment. For example, we partner with Central Arizona Project and Southern Nevada Water Authority on initiatives to address water quality challenges on the Colorado River, including salinity, invasive species and industrial contaminants, as part of the Lower Colorado River Water Quality Partnership. Metropolitan also provides funding for projects to clean up groundwater contamination.

Managing Salinity


When Metropolitan first began delivering desalted Colorado River water to Southern California cities in 1941, the “softened” water was touted for its ability to reduce laundry suds. Today salinity control remains a high priority. Colorado River water has the highest level of salinity of all of Metropolitan’s sources of supply (averaging approximately 630 mg/L since 1976). Metropolitan’s board in 1999 approved a Salinity Management Policy that set of goal of reaching salinity concentrations in delivered water of less than 500 mg/L when hydrological conditions allow.


Why is the river so salty? Many of the salts are indigenous and date to prehistoric marine environments that left deposits of saline sediments. Salts erode easily, dissolve and get transported in the river system. Metropolitan works with the Colorado River Basin Salinity Control Forum, an organization of the seven Colorado River Basin states of Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming to coordinate ways to prevent salts from moving into the river system. Measures include better irrigation practices and rangeland management. Since the 1970s, the salt load has been reduced by 20 percent.

The start of State Water Project deliveries to Southern California in the 1970s gave Metropolitan an additional tool of blending less salty (250 to 325 mg/L) SWP supplies with Colorado River water. Like the Colorado River, SWP salinity can vary greatly depending on hydrologic conditions.


High salinity levels can also occur in groundwater (referred to as brackish water). The desalination of brackish groundwater and other local supplies enhances the continued supply reliability of the region by maximizing local groundwater resources. For this reason, since 1991 Metropolitan funding has helped recover a little more than 1 million acre-feet of groundwater, cleansing it of salts, metals, nitrates, viruses and bacteria and creating another source of water that lessens our dependence on imported supplies.


In addition to desalinating brackish groundwater, Metropolitan has explored the development of seawater desalination since the 1950s. Since 2014, we have offered financial incentives as part of the Local Resources Program; two projects are in the environmental and permitting stage, while a third is undergoing environmental review.

A close-up view of a group of quagga mussels

Controlling Quagga Mussels


From the first discovery of quagga mussels in Lake Mead in January 2007 and their subsequent spread into parts of Metropolitan’s water system, the district has devoted countless hours and tens of millions of dollars toward containing and managing the invasive pest. Spread by commercial ships and inadequately cleaned and dried recreational boats, quagga and other associated mussels had already wreaked havoc in other parts of the country before arriving in the west.


The tiny mussels rapidly reproduce, clogging pipes, water pumps and siphons, and growing on the walls of canals and other submerged infrastructure.

Soon after their discovery in Lake Mead, Metropolitan created a Quagga Mussel Control Plan. Crews constructed continuous chlorination facilities at Copper Basin Reservoir, Lake Mathews and Lake Skinner all of which take Colorado River water and, therefore, have quaggas. Each year, during Colorado River Aqueduct shutdowns, Metropolitan crews dry up (desiccate) quagga mussels and remove them from the system, clean tunnels, and chlorinate strategic sites (sometimes using a mobile chlorinator). Divers perform underwater maintenance, stripping layers of quagga mussels from intake structures.

Recent inspections have demonstrated that the combined use of chlorine and regularly scheduled shutdowns effectively control mussel infestation along the length of the CRA and downstream systems.


We have been able to avoid infestation at our Diamond Valley Lake storage reservoir because CRA deliveries into the reservoir stopped prior to the arrival of quagga mussels in our CRA system. Our success also is due to a rigorous boater education campaign that enlists our Diamond Valley Lake marina operators to enforce launch requirements for visiting boats, and interview boaters to ensure requirements listed here  have been followed.

Partnership For Safe Drinking Water

A voluntary effort by six of the nation’s most prominent drinking water organizations, the Partnership for Safe Water offers tools that allow operators, managers and administrators to improve water quality beyond proposed regulatory levels.

Since Metropolitan joined the partnership in 1996, all of its water treatment plants and distribution system have received awards under the program. Our Diemer, Jensen and Weymouth plants have been honored for achieving the highest possible levels of individual filter turbidity performance, while Metropolitan’s treated water distribution system has achieved the highest level of optimization and demonstrated commitment to continuous improvement.