South Treatment Plant

The process

For over 40 years, the South Treatment Plant has been providing secondary-level treatment for our region. The average capacity for wet weather flow is 115 million gallons per day. The maximum capacity is 325 million gallons per day during peak storms.

Wastewater coming into the plant undergoes a series of processes, including the following:

• Preliminary treatment: where large debris like rags, paper, and leaves are removed
• Primary treatment: skimming and settling to remove sludge (heavy materials) and scum (lighter materials), which are sent onto the solids handling process
• Secondary treatment: a biological process that consumes suspended and dissolved organic material, leaving the remaining water or secondary effluent at least 85 percent cleaner than when it entered South Plant.
• Disinfection destroys most remaining pathogens, or disease-causing bacteria before the final effluent is released through an outfall pipe and diffuser into Puget Sound.

South Plant's wastewater treatment process produces valuable byproducts that can be reused within the plant and throughout the region, including:

• Biosolids, the nutrient-rich organic matter produced by treating wastewater solids, are used as a soil amendment for dryland and irrigated agriculture, forest fertilization and is composted for landscaping and gardening.
• Reclaimed water -- after disinfection, some secondary effluent undergoes advanced treatment and is reused on site in-plant processes also used off-site for landscape irrigation and to replace drinking water use in industrial processes.
• Energy recovery -- digester gas, or methane is a byproduct of the biosolids digestion process, and is captured, separated, and cleaned, then either sold to Puget Sound Energy or used on-site as an energy source for a cogeneration system and fuel cell to produce electricity and heat for plant use.