Water Treatment Process


Constructed in 1968, the City of Bellingham’s Water Treatment Plant (WTP) has a capacity of 24 million gallons a day (MGD) of treated water with one filter off line for backwashing. Over the years, the plant has been updated to include computer controls and the latest in water treatment technology.  The WTP uses what is known as an in-line or contact filtration process in which a chemical that aids or enhances the coagulation process is added just prior to filtration. This type of treatment is preferred for high-quality source water like that found in Bellingham.

Lake Whatcom serves as the drinking water source for Bellingham with additional water rights coming from the Middle Fork of the Nooksack River via the Deming Glacier on Mt. Baker.  Lake water travels two-thirds the length of Lake Whatcom by gravity were it enters a submerged intake, then into a gate-house and finally a screen house.  In the screen house, fish and debris are removed and chlorine is added. 

Chlorine helps with the coagulation process and prevents algae growth in the pipes carrying  unfiltered water. 

  After screening, a pipeline conveys the water to the WTP.

At the WTP, the 66-inch diameter pipeline bifurcates, or divides, into two, 48-inch diameter pipelines that lead to two rapid mixing basins.  Water flows from the basins into a common filter applied water channel for distribution to the filters.  These large filters consist of anthracite coal, silica sand, and an under drain system.

Following gravity filtration through the multimedia filters, the filtered water flows into a 1 million gallon reservoir located inside the plant called the clear well. The water then flows into a 16 million gallon reservoir for disinfection and distribution into the water system.  The reservoir provides contact time with the chlorine prior to distribution. Alum and a cationic polymer that cause microscopic impurities in the water to clump together are added at the WTP. These larger particles are more easily removed by filtration.

Finally, soda ash is added to raise the pH of the treated water to help protect the 410 miles of water mains and our water customer’s plumbing.  Powdered, activated carbon is occasionally added for taste and odor control.  Last, water quality tests are performed every day before the water leaves the filtration plant. 

The City of Bellingham operates a state-certified laboratory at the WTP.  Extensive water quality monitoring is conducted of the treated water at the plant, and throughout the distribution system. Treated water quality is monitored continually by on-line instrumentation and regularly analyzed for more than one hundred sixty (160) constituents.  There are roughly ninety (90) designated sampling stations that are used to monitor water quality throughout the city each month.

For more information about treated water quality and monitoring programs, please contact the Technical Supervisor at the Operations Division of the Public Works Department.


Particulate impurities in water result from land erosion, pickup of minerals and the decay of plant material.  Additional impurities can be added by airborne contamination, industrial discharges and by animal wastes. For these reasons, surface water sources are likely to contain suspended and dissolved organic (plant or animal origin) and inorganic (mineral) materials and biological forms, such as bacteria and plankton.

These particulates (commonly called suspended solids) cover a broad size range.  Larger-sized particles such as sand and heavy silts can be removed from water by slowing down the flow to allow for simple gravity settling.  These particles are often called SETTLEABLE SOLIDS.

Settling of larger-sized particles occurs naturally when surface water is stored for a sufficient period of time in a reservoir or lake.  Smaller-sized particles, such as bacteria, fine clays and silts, do not readily settle.  Water treatment is required to produce larger particles that are settleable.  These smaller particles are often called NONSETTLEABLE SOLIDS or COLLOIDAL MATTER.

The purpose of the coagulation and flocculation is to remove particulate impurities, especially non-settleable solids and color, from the water being treated.  Non-settleable particles in water are removed by the use of COAGULATING chemicals.  The mixing of the coagulant chemical and the raw water is commonly referred to as flash mixing.  The primary purpose of the flash mix process is to rapidly mix and equally distribute the coagulant chemical throughout the water.  The reaction between the colloidal matter with the coagulating chemical occurs within seconds, and the first results are the formation of very small floc particles.  When pieces of floc clump together, they form larger, heavier floc which can settle out and/or be removed by filtration.

Plant intake area where chemical coagulation takes place

Inside the WTP there are six granular media filters with a total surface area of 3,360 square feet. Each filter has two cells, which are controlled by one rate-of-flow effluent control valve. Each cell is backwashed separately using a pumped backwash system. Filter media is considered “mixed” because there are different constituents in place which aid in removing contaminants. 

In Bellingham, our filter media consist of 31 inches of anthracite and 11 inches of silica sand. This filter media is supported by AWI stainless steel under drains.

Each filter cell has three wash water troughs and two rotary arm surface wash sweeps. The sweeps are used during the cleaning of the filter to help remove trapped solids within the filter media. The filters are also equipped with a filter-to-waste system.  This system is used to remove the higher filter effluent turbidities which will occur during the first few minutes at the beginning of a filter run.  Filtering-to-waste for this period makes sure that only the cleanest water is delivered to customers.

The WTP produces drinking water continuously, and with the help of many storage reservoirs stationed throughout the city.  The reservoirs store twice the daily average demand for the City and are used to supply the peak water demands during the day.  The storage reservoirs are refilled at night when the water demand is lower.  Typically, on a warm summer day the water demand goes up in the afternoon and drops off around midnight.  The production rates at the plant average between 8 million gallons per day during the winter months and up to 20 million gallons per day in the summer.

Seasonal fluctuations in water use are a large reason why water conservation is so important.  Each year, City customers consumed 67% of the available water stored in Lake Whatcom.  Water stored in Lake Whatcom is used most during the summer when the water demand is the greatest and there is less rainfall to help replace the water used.  The good news is that City water customers are doing their part to conserve. 

In the late 1990’s, 34% of the total water used in a year was consumed in the summer months. Today, that percentage has dropped to 30%, mainly due to water conservation.  Visit the water conservation webpage for information and tips.


One of the plant filters.  Each filter is 560 sq ft. The coagulated water from Lake Whatcom flows into each filter and then passes through the media.

The purpose of the filtration process is the removal of the particulate impurities and floc from the water being treated.  Floc removal is accomplished by contact with the media grains (sand, coal or other granular substance) throughout the depths of the filter.  After initial coating or conditioning of the media surfaces with floc at the beginning of the filtration cycle, subsequent applications of floc will build upon the material previously deposited on the media surface.  Over a period of time (several hours) the floc material accumulates in the filter media bed to the point where the impurities start to break through causing higher turbidity on the filter effluent, or clog up the filter to a point where the flow is reduced significantly.  At this point the filter must be cleaned by backwashing.

Backwashing is the process of reversing the flow of water through the filter media to remove the entrapped solids.  The backwash flow rate is usually 10 times higher then the filtration rate in order to expand, or fluidize, the media in order to release the entrapped solids.  This process may use two to four percent of the process water to clean the filters.

Filter unit design, filter media type and thickness play a role in determining filter removal efficiency.  Dual media filters have lighter, larger diameter grains in the top layer of the filter which stop the larger particles.  Smaller particles are usually stopped further down in the filter media. The larger grain size in the anthracite coal layer of a dual media filter permits greater depth penetration of solids into the anthracite.  This coal layer provides larger solids storage volume in the filter.  The sand layer below the anthracite is used as a protective barrier against breakthrough.

Administration and Control

State certified plant operators are on duty 24 hours a day to monitor and control the treatment process. The plant is fully automated and has a state-of-the-art computer system which provides operators with accurate and up-to-date information.

The City has a number of educational material available on the topic of water quality and water treatment.

Tours for individuals or community groups can be arranged by contacting the Chief Operator in the Operations Division of  the Public Works Department.

Public Education

Over the last several years, the City of Bellingham has developed and implemented an extensive and highly successful public education program focusing on the City’s water supply. The program targets schools, civic groups, drinking water customers, watershed residents and other community interests, with the goal of increasing public awareness of water resources, water quality, and water conservation issues.