Water Treatment Steps

Water being cleaned inside of treatment plant.
Photo of basin in Water Treatment Plant by Paul Conrad


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, alum and a cationic polymer are used at the WTP for this process. The mixing of the coagulant chemical and the raw water is commonly referred to as flash mixing and happens in two separate areas of the prefiltration process. Alum is added at the flash mixer as the water enters the DAF and Cationic Polymer is added after the water enters the flume. 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 become easily removed during the pre-treatment process and as the water passes through the multimedia filters.


In November of 2018, the WTP process was upgraded with a pre-treatment Dissolved Air Flotation (DAF) system. After alum is added to the water, it then can travel between one of the two DAF basins where dissolved air floats the floc clumps to the surface. The floc is removed from the water through a desludging process, the sludge that is removed from the water is sent to the Wastewater Treatment Plant for processing.


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 combination of air scour and 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. The filter media is supported by AWI stainless steel under drains.

Each filter is connected to an air scour system that uses an air blower to blow air into the bottom of the filter that floats all the trapped particles in the filter back to the top to be collected and sent though the backwash system to the wastewater plant for processing.

The purpose of the filtration process is the removal of remaining particulate impurities and floc from the water being treated that was not already removed during the DAF pretreatment process. Floc removal is accomplished by contact with the media grains (sand and anthracite) 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. Well before this point the filter must be cleaned by backwashing.

Backwashing is the process of using air scour and reversing the flow of water through the filter media to remove the entrapped solids. The backwash flow rate is usually 10 times higher than the filtration rate in order to expand, or fluidize, the media to release the entrapped solids. This process uses less than one percent of the process water to clean the filters.

Filter unit design, filter media type and size all 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.


Sodium hypochlorite, a form of chlorine, is used to sanitize the water, ensuring there are no living organisms that could carry disease. At the pretreatment stage of the water treatment process, chlorine is added at 0.02-0.10 mg/l to reduce algae growth within the WTP system. For sanitization, after the water has gone through the dual media filters, chlorine is added to a residual of 0.42-0.52 mg/l and stays in contact with the water for 24 hours to ensure sanitization before the water leaves the WTP. As the water leaves the CT reservoir it will have a chlorine residual of 0.24-0.30 mg/l so more chlorine is added as the water enters the distribution system to ensure residual chlorine levels, required by law, are measurable at 95% of the water distribution system sampling sites. This chlorine level translates to 0.75 to 0.85 mg/L depending on the season.

As part of the 2018 upgrade, the City now produces chlorine on-demand through electrolysis. This is safer for plant operators and the community because gas chlorine is no longer stored or used on site.

The brine enters the electrolysis generator, the positive anode and negative cathode breaks the bonds of the saltwater creating 0.8% sodium hypochlorite, which is used for disinfection.


The WTP produces drinking water continuously, and with the help of many water 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 throughout the day. The storage reservoirs levels are constantly monitored by Plant Operators and refilled when the water demands are 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 14 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 consume 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.

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 educational materials 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.