Wastewater Treatment

The Arcata Marsh is a component of the City’s wastewater treatment facility. Arcata has turned wastewater into a resource by integrating conventional wastewater treatment with the natural treatment processes of constructed wetlands. The treatment system serves a population of 16,000, as wastewater flows through a series of ingeniously constructed ponds and wetlands. Treatment occurs naturally, by allowing the time for plants, algae and bacteria to break down the waste products. The natural processes reduce reliance on chemicals. This unique system has been the model for similar facilities worldwide. Special tours of the treatment plant are available by request.

How Arcata’s Wastewater Treatment System Works

Arcata’s sustainable wastewater treatment combines a conventional headworks system (headworks, clarifiers, digesters, and sludge drying beds) with natural treatment in oxidation ponds, treatment wetlands, and three enhancement wetlands: Allen, Gearheart, and Hauser.


Step 1. Headworks

The headworks is the first phase in treating sewage to advanced secondary standards. After raw sewage enters, sludge is separated from the effluent. The sludge is digested, dried, and composted for use on City property (Step 4). Two Archimedes screw pumps lift the sewage 15 feet. It gravity-flows through a screen to remove sticks, debris, sand, gravel, and other gritty materials, which are taken to a landfill. From the headworks, the sewage flows to the clarifiers (Step 2).

Step 2. Clarifiers

Two large tanks called clarifiers are used to separate organic solids and skim off materials from wastewater. Solids (sludge) that settle out in the bottom of the clarifiers are pumped to digesters (Step 3), while primary-treated wastewater flows to the oxidation ponds (Step 5).

Step 3. Digesters

Here, sludge is heated, stirred, and decomposed by bacteria. The digesters are anaerobic because bacteria digest sludge without using oxygen. Some sludge ingredients are converted into methane, which is used to heat the digesters.

Step 4. Sludge Drying Beds

Digested sludge, which has the consistency of pudding, is drained from digesters to covered drying beds and dried in the open air. The dried sludge is mixed with chipped wood waste and plants harvested from the enhancement marshes. The mixture is then aerobically (with oxygen) composted and used in Arcata’s natural areas and parks.


Step 5. Oxidation Ponds

Wastewater from the clarifiers gravity-flows out to 49 acres of oxidation ponds, where time and microorganisms purify it. Bacteria and other microorganisms break down (digest) the wastes, while algae provide oxygen to the bacteria. Additional oxygen is supplied mechanically by machines called aerators at times when the City wants to increase oxygen and circulation in the pond. From these “oxy” ponds, the wastewater next travels to the treatment marshes (Step 6).

Step 6. Treatment Marshes

The algae and bacteria that are important to the function of oxidation are important to remove from the wastewater before it is sent to Humboldt Bay — and their removal is the primary purpose of the treatment marshes. Thick growth of floating and submergent plants in the six treatment marshes shade the water so algae cannot grow. Algae die and settle to the bottom of the marsh, where the solids are digested anaerobically. The emergent plants (cattail and bulrush) and submergent plants (pondweed and coontail) provide habitat for bacteria and other microorganisms that feed on and further break down the compounds that are in the now-secondary treated wastewater.


Step 7. Chlorine Contact Basin

Wastewater is disinfected to kill pathogens, harmful bacteria, and viruses before discharge into Humboldt Bay (Step 9). Currently the system uses chlorine to disinfect and sulfur dioxide to remove the chlorine at two points in the system – before flowing to the enhancement wetlands (Step 8), and before discharge to Humboldt Bay (Step 9). The system is undergoing an engineering redesign that will allow disinfection to be met with ultraviolet (UV) light rather than with chlorine.

Step 8. Enhancement Wetlands

Three freshwater enhancement wetlands (Allen, Gearheart, and Hauser) were constructed to enhance the water to meet the water board’s Beneficial Uses designation. The time it takes for the water to travel through the enhancement wetlands results in nutrient and suspended solids reduction, with the added benefit of providing habitat for wildlife.


Step 9. Humboldt Bay

After the wastewater flows through the enhancement marshes, it is currently pumped back to the chlorine contact basin (Step 7), where it is chlorinated and de-chlorinated before it is discharged into Humboldt Bay. Another outcome of the engineering redesign is that the discharge location will move to an area adjacent to and connected to the recently restored McDaniel Slough tidal wetlands. The new point of discharge is expected to promote vegetation growth in the tidal wetlands and may even attract shorebirds to create another accessible and exceptional viewing location for visitors to appreciate.

For more details, download this brochure created by the Arcata Marsh Research Institute.