Disinfection byproducts

EBMUD takes steps to stem formation of disinfection byproducts and maintain high quality drinking water.


Our treatment and distribution systems are regulated by the United States Environmental Protection Agency and the State of California Division of Drinking Water, under the state and federal Safe Drinking Water Acts.

EBMUD drinking water quality remains in full compliance with all state and federal regulations. 

Municipal water agencies that use surface water protect public health by filtering and disinfecting all water delivered to our customers. These processes reduce viruses, Cryptosporidium, E. coli, Giardia, and other water-borne pathogens that can cause illnesses.

Filtration: Coagulants are added, and then the water passes through filters of anthracite and sand to remove particles like dust, parasites, bacteria, and viruses.
Disinfection: A disinfectant (chlorine) is added to kill any remaining parasites, bacteria, and viruses, and to protect the water from germs when it is piped to homes and businesses. Disinfectants are added before filtration and/or after, depending on the water treatment plant.
Corrosion Control: After filtration and disinfection, the water chemistry is adjusted to minimize corrosion, particularly of metal pipes and plumbing components.

When chlorine is used for disinfection, disinfection byproducts such as trihalomethanes (THMs) will form. Naturally-occurring organic matter in the water reacts with chlorine to form THMs. THMs are present to some degree in all chlorinated drinking water systems and are regulated so they don’t approach levels that may present a risk to public health. 

Natural organic matter in water originates from leaves, grass, and other plant material that gets washed into the reservoirs. As the seasons change, the amount of organic matter in the water changes, and therefore the amount of THMs that form when the water is chlorinated also varies seasonally.

EBMUD has begun work on several large capital projects at our treatment plants and in our reservoirs that will help us better control formation of disinfection byproducts. These projects will allow us to use less chlorine, remove some of the natural organic matter, more precisely adjust the water chemistry to control disinfection byproduct formation while simultaneously disinfecting the water.  

Trihalomethanes, or THMs, are byproducts of the process used to disinfect water during the treatment process. They are formed when natural organic material, such as the decaying vegetation commonly found in lakes and reservoirs, reacts with chlorine used to treat the water.

Yes. THMs are found to varying degrees in all chlorinated drinking water. They form when chlorine reacts with naturally-occurring organic matter in water. Chlorine is necessary for disinfection and required by regulations to protect public health. It has been used to disinfect water for almost a century due to its effectiveness at killing bacteria and viruses in water, reducing the incidence of intestinal illness and other health problems caused by waterborne pathogens.

Yes, the water is safe to drink. EBMUD drinking water quality continues to meet or surpass every state and federal public health requirement. The EPA has set regulatory limits on THM concentrations to be protective of public health for all potential health risks. EBMUD drinking water is below the regulatory limits.

If you have a specific medical condition that you are concerned about, please consult your doctor for recommendations.

To address this challenge, the District has increased water quality sampling, implemented operational changes and a new flushing program. EBMUD is testing THM removal technologies through a pilot aeration system at the Lafayette Water Treatment Plant. In addition, infrastructure upgrades designed to lower THMs are already underway.

The District is currently flushing pipes in areas most affected by these conditions. Flushing removes sediment and mineral deposits, and helps maintain a chlorine residual within the distribution system to ensure the delivery of high quality water to customers. Customers in Orinda and Berkeley have been notified via Nextdoor about flushing in their neighborhoods.

This past November, EBMUD configured a pilot water treatment facility to test new treatment processes, doses and configurations. EBMUD is using this testing facility to evaluate different coagulants and filter configurations to address these water quality challenges.

EBMUD is also considering accelerating plans that are already underway, including pretreatment and disinfection modifications at our in-line water treatment plants. Our five-year capital improvement plans include reliability and improvement projects at several inline water treatment plants, which will reduce disinfection byproducts significantly.

Ultimately, major changes to treatment processes may be required to deal with changes due to reduced water consumption, higher temperatures and higher concentrations of natural organic carbon in raw water. These changes are significant, and would require a major investment by EBMUD.

California’s historic drought, followed by record-setting rainfall, continues to have impacts on drinking water systems throughout the San Francisco Bay Area. The East Bay Municipal Utility District is experiencing water quality and treatment challenges due to reduced water consumption, higher temperatures, and higher concentrations of natural organic carbon in our untreated ‘raw’ water from Pardee Reservoir in the Sierra foothills.

Reduced water consumption – a tremendous benefit for EBMUD’s water supplies and future water needs – has reduced flows and water use in our distribution systems. Reduced use can result in increased water age – the amount of time water remains in our pipes and storage tanks.

Water age, along with water chemistry changes, can affect the residual disinfection used to protect the distribution system – the treatment that removes water borne pathogens from our drinking water. Flushing moves water through the distribution system, removes sediment and mineral deposits that can accumulate in water pipes, and helps maintain our high water quality throughout the drinking water system.

EBMUD tests for THMs as part of routine testing in our distribution system throughout the year. Samples are taken from 16 different locations each quarter. Individual numbers can vary widely due to many factors. In 2019 the system average was 44 parts per billion (ppb), under the state and federal limit of 80 ppb.

We regularly report these levels to the State Water Resources Control Board Division of Drinking Water, as well as in our Annual Water Quality Report (AWQR) available to the public.

Studies show that some people with long-term exposure to drinking water containing THMs in excess of state and federal limits may have an increased risk of getting cancer. This is not the case with EBMUD drinking water, which continues to meet all state and federal guidelines. 

Yes, it is safe to shower to bathe, and wash dishes.

Pregnant women may wish to consult their physician for advice and drink water according to their doctor's recommendations. 

Planned capital improvements to make water treatment plants more resilient to changes in source water quality include:

  •  Improved removal of organic material at the Walnut Creek and Orinda water treatment plants 
  •  A project to improve water quality in San Pablo Reservoir
  •  Disinfection improvements at Orinda Water Treatment Plant

These are some of the significant investments EBMUD is making to ensure we continue to provide reliable, high quality water to our customers.

EBMUD meets or surpasses all state and federal drinking water requirements and thus water purification filters are unnecessary. However, there are filters that remove THMs. If you choose to filter your water, make sure you purchase a device that has been tested and certified, and follow manufacturer guidelines for filter replacement to avoid creating additional water quality problems.

Frequently asked questions in Spanish and Chinese

Additional resources 

Questions on tap water quality: