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== Overview ==
== Overview of a Clarifier ==
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[[Image:Clarifier.png‎|thumb|left|Fig 1: Aerial view of Arcata Marsh clarifiers.]]The clarifier operates on the idea of slowing wastewater down so that suspended solids will settle out on the bottom of the tank. In addition to settling on the bottom of the tank, less dense oils and solids will float and settle on the top of the tank. The sludge and scum are then removed, and the water is transported onto the next stage in the wastewater treatment plant. After the entire process is completed the clarifier will have removed 40-60% of suspended solids.[[Image:Clarifier_1.jpg|right|thumb|Fig 2: Arcata's main clarifier.]]
[[Image:Clarifier.png‎|thumb|left|Fig 1: Aerial view of Arcata Marsh clarifiers. The 60 ft tank can be seen on the left, and 26 ft tank on the right.]] A clarifier operates on the idea of slowing wastewater down so that suspended solids will settle out on the bottom of the tank. Wastewater is pumped to the clarifier after large solids and grit are removed. This removal occurs in the first two stages of wastewater treatment.
 
In a clarifier the solids that settle to the bottom are called sludge. Less dense oils and solids called scum will float on the top of the tank. The sludge and scum are then removed, and the wastewater is pumped to oxidation ponds. After the entire process is completed the clarifier will have removed 40-60% of suspended solids<ref>http://ragsdaleassociates.com/WastewaterSystemOperatorsManual/Chapter%203%20-%20Primary%20Treatment.pdf</ref>.


== Arcata ==
== Arcata ==
The Arcata Wastewater Treatment Plant is equipped with two clarifiers (see Figure 1). The first is a 60 foot diameter tank that is used at all times (see Figure 2). The second is a 26 foot diameter tank provides support during big rains or can be used when the main tank is being maintenanced. both of the clarifiers are center fed clarifiers, meaning the influent (wastewater flowing into the system) is fed from the center, underneath the tank.
The Arcata Wastewater Treatment Plant is equipped with two clarifiers. Figure 1 shows both of these clarifiers. The first is a 60 foot diameter tank that is used at all times. The second is a 26 foot diameter tank that provides support during frequent winter rains. The main tank cannot process the increased volume of wastewater on its own. The smaller clarifier can also be used when the main tank is maintenanced<ref>Personal communication with Dave Couch</ref>.  
==== Center Fed Clarifiers ====
Both of the clarifiers at Arcata Marsh are center fed clarifiers<ref>http://www.cee.vt.edu/ewr/environmental/teach/wtprimer/sedimen/sedimen.html</ref>. Center fed clarifiers receive influent (wastewater entering the clarifer) from the bottom of the tank. From the bottom, the influent is pumped upward through the influent pipe in the center of the tank. This design is shown in Figure 2.


== Slowing the Water ==
== Slowing the Water ==
[[Image:Clarifier_Diagram1.jpg‎‎|left|450px|thumb|Fig 3: Diagram of the clarifier.]]
[[Image:Clarifier_Diagram1.jpg‎‎|left|490px|thumb|Fig 2: Diagram of the clarifier. Colored arrows show movement of wastewater and sludge. (Original Source: Ragsdale and Associates Training Specialists, LLC)]]
The first task of the clarifier is to slow down the wastewater it recieves from the [[Arcata Marsh headworks| headworks]](see Figure 3 for water flow).
==== Water Entering ====
==== Water Entering ====
Water first enters the tank from the influent (wastewater entering the system) pipe. A baffle slows down the velocity of the water to about 0.03-0.05 feet per second and directs the flow of water downward. The baffle is a series of concentric cylinders. This reduces turbidity significantly, allowing solids to settle out on the bottom of the circular tank.  
The first task of the clarifier is to slow down the wastewater it receives from the [[Arcata Marsh headworks| headworks]]. Water first enters the center of the tank through the influent pipe. A baffle slows down the velocity of the water to about 0.03-0.05 feet per second and directs the flow of water downward<ref>http://ragsdaleassociates.com/WastewaterSystemOperatorsManual/Chapter%203%20-%20Primary%20Treatment.pdf</ref>. The baffle is a cylinder surrounding the end of the influent pipe.  
==== Water Exiting ====  
==== Water Exiting ====  
[[Image:Clarifier_Effluent_Launder.jpg‎||thumb|Fig 4: Close up of perimeter of tank.]]
[[Image:Clarifier_Outflow.jpg‎||thumb|Fig 3: Close up of perimeter of tank. Green arrow: effluent launder, yellow arrow: weirs, red arrow: inner baffle. (Original Source: Dustin Poppendieck)]]
To ensure a consistant water flow the clarifier must also slow down the effluent (wastewater leaving the system) evenly. The perimeter of the tank is designed to accomplish this. A series of v-notched weirs along the edge of the tank slow the water as it departs. Once over the weirs the water enters the effluent ladder (a channel around the outside of the clarfier). An inner baffle that rests behind and above the weirs prevents floating solids from passing over. If water does not flow evenly after entering the tank or as it leaves over the weirs, the settling process will not occur evenly, this is also known as short-circuiting. From the effluent ladder the water flows into a main collection trough and is piped onto the oxidation ponds via the effluent pipe.
To ensure a consistent water flow the clarifier must also slow down the effluent (wastewater leaving the clarifier) evenly. The perimeter of the tank is designed to accomplish this. A series of v-notched weirs along the edge of the tank slow the water as it departs. In Figure 3 the weirs are pointed out. Once over the weirs the water enters the effluent launder (a channel around the outside of the clarfier). The wastewater can be seen flowing into the effluent launder in Figure 3.
 
Also displayed in Figure 3 is the inner baffle. The inner baffle rests behind and above the weirs. The inner baffle prevents floating solids from passing over into the effluent launder. From the effluent launder the water flows into a main collection trough and is pumped to the [[Arcata's Marsh Oxidation Ponds|oxidation ponds]] via the effluent pipe.
 
==== Hydraulic Short Circuiting ====
If water does not flow evenly throughout the clarifier, the settling process will not occur evenly. This uneven flow is known as hydraulic short-circuiting<ref>http://www.environmentalleverage.com/Primary%20Clarifiers-%20Problem%20Areas.htm</ref>.


== Settling and Removal Process ==
== Settling and Removal Process ==
[[Image:Clarifier_skimer.jpg|thumb|left|Fig 5: The skimmer arm can be seen at the top of the photo.]]
[[Image:Clarifier_skimer.jpg|thumb|left|Fig 4: The skimmer arm can be seen at the top of the photo. (Source: Dustin Popendieck)]]
Suspended solids in the tank settle out in two ways.
==== Sludge ====
==== Sludge ====
The denser solids settle on the conical bottom of the tank in the form of sludge. A large rake rotates and moves the sludge towards a hopper at the bottom of the tank. The sludge is then moved onto the [[Arcata Marsh digester|digester]](see Figure 3)
The denser solids settle on the conical bottom of the tank in the form of sludge. A large rake with a squeegee like assembly rotates and moves the sludge towards a hopper at the bottom of the tank. The sludge is then pumped to the [[Arcata Marsh digester|digester]] for treatment.
==== Scum ====  
==== Scum ====  
Lighter solids and oils settle out on the top of the tank in the form of scum. Scum is removed from the tank by a rotating skimmer arm (see Figure 5). The blade like arm collects the scum into a trough on the perimeter of the tank. Once the scum is collected it is transported to a landfill. Both the rakes and arm rotate together as they remove both sludge and scum from the tank.
Lighter solids and oils rise to the top of the tank in the form of scum. Scum is removed from the tank by a rotating skimmer arm seen in Figure 4. The blade like arm collects the scum into a trough on the perimeter of the tank. Once the scum is collected it is transported to a landfill. Both the rakes and arm rotate together as they remove both sludge and scum from the tank.
 
== Footnotes ==
<references/>


== Refrences ==
== Other References ==
# http://www.cee.vt.edu/ewr/environmental/teach/wtprimer/sedimen/sedimen.html
# All photographs courtesy of Dr. Dustin Poppendieck, Assistant Professor of Environmental Resource Engineering, Humboldt State University.
# Principles of Environmental Engineering and Science/Davis and Masten. 2nd ed
# Principles of Environmental Engineering and Science/Davis and Masten. 2nd ed
# http://www.environmentalleverage.com/Primary%20Clarifiers-%20Problem%20Areas.htm
# http://ragsdaleassociates.com/WastewaterSystemOperatorsManual/Chapter%203%20-%20Primary%20Treatment.pdf
# http://www.humboldt.edu/~ere_dept/marsh/clarifier.html
# All photographs courtesy of Dr. Dustin Poppendieck, Assistant Professor of Environmental Resource Engineering, Humboldt State University.


[[Category:Arcata Marsh]]
[[Category:Arcata Marsh]]
[[Category:Arcata marsh]]
[[Category:Engr115 Intro to Engineering]]
[[Category:Engr115 Intro to Engineering]]
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Revision as of 02:05, 23 October 2009

Overview of a Clarifier

Fig 1: Aerial view of Arcata Marsh clarifiers. The 60 ft tank can be seen on the left, and 26 ft tank on the right.

A clarifier operates on the idea of slowing wastewater down so that suspended solids will settle out on the bottom of the tank. Wastewater is pumped to the clarifier after large solids and grit are removed. This removal occurs in the first two stages of wastewater treatment.

In a clarifier the solids that settle to the bottom are called sludge. Less dense oils and solids called scum will float on the top of the tank. The sludge and scum are then removed, and the wastewater is pumped to oxidation ponds. After the entire process is completed the clarifier will have removed 40-60% of suspended solids[1].

Arcata

The Arcata Wastewater Treatment Plant is equipped with two clarifiers. Figure 1 shows both of these clarifiers. The first is a 60 foot diameter tank that is used at all times. The second is a 26 foot diameter tank that provides support during frequent winter rains. The main tank cannot process the increased volume of wastewater on its own. The smaller clarifier can also be used when the main tank is maintenanced[2].

Center Fed Clarifiers

Both of the clarifiers at Arcata Marsh are center fed clarifiers[3]. Center fed clarifiers receive influent (wastewater entering the clarifer) from the bottom of the tank. From the bottom, the influent is pumped upward through the influent pipe in the center of the tank. This design is shown in Figure 2.

Slowing the Water

Fig 2: Diagram of the clarifier. Colored arrows show movement of wastewater and sludge. (Original Source: Ragsdale and Associates Training Specialists, LLC)

Water Entering

The first task of the clarifier is to slow down the wastewater it receives from the headworks. Water first enters the center of the tank through the influent pipe. A baffle slows down the velocity of the water to about 0.03-0.05 feet per second and directs the flow of water downward[4]. The baffle is a cylinder surrounding the end of the influent pipe.

Water Exiting

Fig 3: Close up of perimeter of tank. Green arrow: effluent launder, yellow arrow: weirs, red arrow: inner baffle. (Original Source: Dustin Poppendieck)

To ensure a consistent water flow the clarifier must also slow down the effluent (wastewater leaving the clarifier) evenly. The perimeter of the tank is designed to accomplish this. A series of v-notched weirs along the edge of the tank slow the water as it departs. In Figure 3 the weirs are pointed out. Once over the weirs the water enters the effluent launder (a channel around the outside of the clarfier). The wastewater can be seen flowing into the effluent launder in Figure 3.

Also displayed in Figure 3 is the inner baffle. The inner baffle rests behind and above the weirs. The inner baffle prevents floating solids from passing over into the effluent launder. From the effluent launder the water flows into a main collection trough and is pumped to the oxidation ponds via the effluent pipe.

Hydraulic Short Circuiting

If water does not flow evenly throughout the clarifier, the settling process will not occur evenly. This uneven flow is known as hydraulic short-circuiting[5].

Settling and Removal Process

Fig 4: The skimmer arm can be seen at the top of the photo. (Source: Dustin Popendieck)

Sludge

The denser solids settle on the conical bottom of the tank in the form of sludge. A large rake with a squeegee like assembly rotates and moves the sludge towards a hopper at the bottom of the tank. The sludge is then pumped to the digester for treatment.

Scum

Lighter solids and oils rise to the top of the tank in the form of scum. Scum is removed from the tank by a rotating skimmer arm seen in Figure 4. The blade like arm collects the scum into a trough on the perimeter of the tank. Once the scum is collected it is transported to a landfill. Both the rakes and arm rotate together as they remove both sludge and scum from the tank.

Footnotes

Other References

  1. All photographs courtesy of Dr. Dustin Poppendieck, Assistant Professor of Environmental Resource Engineering, Humboldt State University.
  2. Principles of Environmental Engineering and Science/Davis and Masten. 2nd ed
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