Preliminary Treatment/ Screening
Preliminary treatment involves screening out objects such as personal hygiene products, wipes and other debris. Trash that is removed from the system is taken to the landfill. Each year, almost 200 tons are removed. After the flow passes through the screening systems, it is ready for the primary treatment process.
In the primary treatment phase, settling of the wastewater by gravity is used to efficiently remove more than half of the solid material coming into the plant. These solids settle to the bottom of the tank and are transferred for further treatment during Solids Processing.
Secondary Treatment/Aeration Tanks and Clarifiers
Secondary treatment creates ideal food and oxygen conditions that allow bacteria and other microscopic organisms to work quickly and efficiently to digest dissolved waste, just as they would normally in the natural environment but on a much larger scale. At the end of secondary treatment, nearly all waste has been removed from the water.
Disinfection is the final step in the liquids treatment process. Instead of chlorine, which was typically used in past decades, the plant uses ultraviolet lamps. This is a safer process than chemical disinfection. Water passes through chambers containing ultraviolet lamps that kill any remaining disease causing bacteria and pathogens.
Following treatment, effluent (treated wastewater) from the plant is transported by pipeline and discharged to the Columbia River via an underwater diffuser, an underwater mechanism that dilutes and mixes effluent into the river. The State of Washington’s Department of Ecology closely monitors this process to ensure that all effluent meets strict environmental standards. The diffuser is periodically inspected by a diver to ensure efficient operations.
All solids collected during the main treatment process are further processed by mixing and heating for one month in large tanks called digesters. This process is called anaerobic digestion, where microorganisms further stabilize the organic material in a similar process to composting. This treatment meets state and federal standards and results in a final product called biosolids, which have beneficial agricultural uses.