Arcata Wastewater Treatment Plant Processes
Treatment Processes References

The Arcata Wastewater Plant Process Overview

The Arcata Wastewater Treatment Plant uses several treatment stages to clean the city's effluent to permitted levels for release to Humboldt Bay. The primary goal of wastewater treatment is to remove suspended solids and Biochemical Oxygen Demand or (BOD). BOD is an indirect measure of the organic material present in the wastewater (Davis & Matsen, 2004).

Wastewater flowing through the plant encounters four major types of treatment. Each type of treatment is achieved through multiple process (Figure 1).

  • Primary Treatment: The main objective is to remove large suspended solids material (dark blue in Figure 1).
  • Secondary Treatment: The main objective is to degrade organic material (dark green in Figure 1).
  • Tertiary Treatment: The main objective is to remove excess nutrients, such as nitrogen and phosphorus (light green in Figure 1).
  • Disinfection: The main objective is to eliminate pathogens from the discharged waters (grey in Figure 1).

Diagram of the steps of treatment at the Arcata Wastewater Treatment Plant.

Figure 1. The treatment processes at the Arcata Wastewater Treatment Plant.

Primary Treatment

Primary treatment, dark blue in Figure 1, includes the headworks, clarifier, and digesters. The headworks removes sediment and other inorganic material by using bar screens and grit separators (U.S. Environmental Protection Agency, 1993), and elevates the water to allow for gravity flow to the following stages. The wastewater then flows to the clarifier which is used to settle out suspended solids. The solids from this process are sent to the digesters where they are used to produce compost and methane. The compost is used as fertilizer in the city parks (Couch, 2008) and the methane is used to produce heat for the digestion process (U.S. Environmental Protection Agency, 1993).

Secondary Treatment

Secondary treatment, pictured in dark green in Figure 2, begins as effluent from the primary clarifier is pumped to the oxidation ponds. Algae growing in the ponds add oxygen to the water, allowing microorganisms to flourish. The microorganisms, through natural processes, remove up to 50% of the BOD from the wastewater (U.S. Environmental Protection Agency, 1993). Additionally, the slow movement of the water allows for further settling of solids that were not removed during primary treatment.

Secondary treatment continues as the effluent reaches the four treatment wetlands. As the water flows through the six acres of shaded wetland the algae that were important to the oxidation ponds settle out of the wastewater (U.S. Environmental Protection Agency, 1993). Microorganisms living on the roots of native plants, pictured in Figure 2, continue to reduce the BOD and breakdown the settled algae.

Microorganisms on the roots of plants breakdown pollutants in wastewater

Figure 2. Microorganisms on the roots of plants breakdown pollutants in wastewater.

Tertiary Treatment

Some of the water from the treatment wetlands is chlorinated and dechlorinated before being send to the enhancement marshes (Figure 3). The enhancement marshes are open to the public for recreation and provide some tertiary treatment for the city's wastewater. The enhancement marshes continue to remove BOD from the water, as well as reducing the nitrogen and phosphorous content. As the water flows out of the marshes it completes a circle and is again disinfected. Some of this water will be dechlorinated and released to Humboldt Bay and some returned to the enhancement marshes to continue the cycle.

The Enhancement Marshes at Arcata Marsh and Wildlife Sanctuary provides tertiary treatment and a recreation area for the community.

Figure 3. The enhancement marshes at Arcata Marsh and Wildlife Sanctuary provide tertiary treatment and a recreation area for the community.

Disinfection

After leaving the treatment wetlands the effluent is mixed with water returning from the enhancement marshes. This mixed effluent is chlorinated to remove bacteria and pathogens. A portion of this water flows to the enhancement marshes, while the rest is dechlorinated for release to Humboldt Bay.