Salmon Creek Treatment Plant

1. 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. Additionally, odorous gases from wastewater are pumped through the new biotrickling filter to reduce odors. After the flow passes through the screening systems, it is ready for the primary treatment process.

2. Primary Treatment

In this phase, gravity causes the solid waste to settle to the bottom of the tank, allowing for efficient removal of more than half of the solid material coming into the plant. These solids are transferred to the solids process for further treatment. Aluminum covers help reduce the escape of odors.

3. Secondary Treatment

Secondary treatment creates ideal food and oxygen conditions that allow bacteria and other microscopic organisms to quickly and efficiently digest dissolved waste. This process is similar to what happens in a natural environment but on a much larger scale. At the end of secondary treatment, nearly all waste has been removed from the water.

4. Disinfection

Disinfection is the final step in the liquids treatment process. Water passes through chambers containing ultraviolet lamps that kill any remaining disease-causing bacteria and pathogens. The plant uses ultraviolet lamps, a safer option than chemical disinfection. Washington State Department of Ecology closely monitors the treatment process to ensure all treated wastewater meets strict environmental standards.

5. Discharge

Following treatment, effluent (treated wastewater) from the plant is transported by pipeline and released into the Columbia River through a diffuser, which dilutes the effluent as it mixes into the river water. The diffuser is periodically inspected by a diver to ensure it is not damaged in any way.

6. Solids Processing

All solids collected during the primary and secondary 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 process similar to composting. This treatment meets state and federal standards and produces biosolids, which have beneficial agricultural uses. Additionally, two carbon adsorbers collect and remove odorous air from this process.

7. Biosolids Recycling

Biosolids can be used as a nutrient-rich fertilizer that reduces soil erosion and provides micronutrients and nitrogen to the soil. Biosolids removed from the plant are regularly applied to farmlands in accordance with federal and state requirements. During dry weather, the biosolids are taken to farms in Lewis County, near Centralia. During the remainder of the year, biosolids go to other farms near Goldendale, where the weather and agricultural practices can support a nearly year-round operation.

8. Monitoring & Quality Control

Plant operation requires constant monitoring and quality control testing. Liquids and solids are carefully monitored by certified operations staff who test and evaluate the liquids and solids at each stage of the process to ensure effective treatment and compliance with strict regulatory requirements.

In the bathroom

• Only flush toilet paper, everything else goes in the trash.
• Don’t flush prescription drugs or other medications.
• When shopping:
  • Choose products with the EWG VERIFIED™ mark.
  • Avoid products with plastic micro-beads, synthetic fragrances, parabens, or phthalates.

In the kitchen

• Put fats, oils, grease, and grit in the trash—not down the sink.
• Cook at lower temperatures if using older non-stick pans.
• When shopping:
  • Look for products with the EPA Safer Choice label.
  • Choose stainless steel, cast-iron, or cookware with a ceramic coating.
  • Avoid products with “PTFE” or “FLOURO.”

In the garden

• Use pesticides carefully and as a last resort.
• If you must, spot-spray on pests; don’t spray pesticides over large areas.
• Dispose of pesticides at your local household hazardous waste facility.
• Scoop, bag, and trash pet waste.
• When shopping:
  • Choose neem tree oil as a natural pesticide.