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Arcata Marsh clarifiers

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Figure 1. The 60ft diameter primary clarifier tank at the Arcata plant.

After passing through the Headworks where large solids, objects and particles are removed, the wastewater moves on to the primary clarifiers. Primary clarifiers, also known as sedimentation tanks, are used in wastewater treatment plants to remove suspended solids from the wastewater being processed. The Wastewater Treatment Plant uses two clarifiers: one 26 foot diameter tank and one 60 foot diameter tank (Figure 1). The 26ft diameter clarifier is used as a backup to the 60 foot diameter clarifier. During a large rainstorm, a much larger volume of water is processed by the clarifiers. If only the 60 foot clarifier was used, the retention time of the wastewater would be too short and the effluent would still contain large amounts of solid waste. The Arcata Wastewater Treatment Plant has the second, smaller clarifier to supplement the larger clarifier during such an event. Also, the 26 foot clarifier can be used independently while the 60 foot clarifier is being repaired.


File:Clarifier Drawing (800pix).gif
Figure 2. [1] Cross-sectional diagram of a primary clarifier.
Figure 1. The influent well in the center of the clarifier.


Wastewater enters the clarifier through the influent well (Figure 1) in the center of the tank. The influent well contains a series of baffles which reduce the velocity of the influent as it enters the clarifier; this is to prevent short-circuiting[2]. Short-circuiting can occur when the water inside the clarifier gets disturbed and mixes, creating lower retention times and allowing solid waste to exit the clarifier along with the effluent.

Figure 3. The surface skimmer.


Once the influent enters the tank, it's velocity begins to decrease. Once the wastewater's velocity decreases to 0.01-0.03 feet/second[2], suspended solids begin to settle out of the water; the denser solids (sludge) settle to the bottom of the tank and less dense solids (scum) and oils float to the surface. In the 60 foot clarifier at the Arcata Wastewater Treatment Plant water spends roughly 3.8 hours[3] in the tank allowing the solid waste to settle out. After 3.8 hours; up to 95% of the settleable solids, 60% of the suspended solids, and 40% of the BOD will have been removed from the wastewater[2].


The floating scum is skimmed off of the surface by a large rotating arm called the surface skimmer (Figure 3). Once collected by the surface skimmer the scum is deposited into the scum trough and is transported to a storage container alongside the clarifier (Figure 4). Once the storage container is filled, the scum will be disposed of in a landfill.

Figure 4. Floating solids and oils are deposited into this dumpster after being skimmed off of the surface.


The sludge on the bottom of the tank gets collected by rotating collector arms which moves the sludge to the center of the tank. Once the sludge collects at the bottom of the tank it gets pumped to the digester to be processed and converted into biogas and compost.


Figure 5. Effluent flowing through weirs and exiting through the effluent pipe.

After spending 3.8 hours in the clarifier the effluent, water free of suspended solids, flows through a baffle and then through weirs at the edge of the tank. The baffle, which is essentially a wall partially submerged in the wastewater to prevent surface scum from flowing through the weirs with the effluent. The weirs, which have notches cut into them at regular intervals around the entire edge of the tank, allows the effluent to flow out of the tank at a constant even flow[2]. Because the flow through the weirs is the same around the whole tank, the effluent and the wastewater still in the tank is not disturbed, minimizing mixing and an increase in turbidity. After the effluent passes through the weirs, it flows into the effluent pipe and is transported to the oxidation ponds for further treatment.

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  2. 2.0 2.1 2.2 2.3
  3. [1]

Engr115 Intro to Engineering