Tube settlers use multiple tubular channels sloped at an angle of 60° and adjacent to each other, which combine to form an increased effective settling area. This provides for a particle settling depth that is significantly less than the settling depth of a conventional clarifier, reducing settling times.

Tube settlers capture the settleable fine floc that escapes the clarification zone beneath the tube settlers and allows the larger floc to travel to the tank bottom in a more settleable form. The tube settler’s channel collects solids into a compact mass which promotes the solids to slide down the tube channel.

Tube settlers offer an inexpensive method of upgrading existing water treatment plant clarifiers and sedimentation basins to improve performance. They can also reduce the tankage/footprint required in new installations or improve the performance of existing settling basins by reducing the solids loading on downstream filters.

Made of lightweight PVC, tube settlers can be easily supported with minimal structures that often incorporate the effluent trough supports. They are available in a variety of module sizes and tube lengths to fit any tank geometry, with custom design and engineering offered by the manufacturer.

The advantages of tube settlers can be applied to new or existing clarifiers/basins of any size:

• Clarifiers/basins equipped with tube settlers can operate at 2 to 4 times the normal rate of clarifiers/basins without tube settlers.

• It is possible to cut coagulant dosage by up to half while maintaining a lower influent turbidity to the treatment plant filters.

• Less filter backwashing equates to significant operating cost savings for both water and electricity.

• New installations using tube settlers can be designed smaller because of increased flow capability.

• Flow of existing water treatment plants can be increased through the addition of tube settlers.

• Tube settlers increase allowable flow capacity by expanding settling capacity and increasing the solids removal rate in settling tanks.

According to Tubular Settlers-A Technical Review, from the Journal AWWA, 331:335 (June 1978), written by Mr. Roderick M. Willis, there are three basic requirements essential for successful performance of tube settlers.

1. There must be laminar (or viscous) flow conditions within the tubes at the maximum flow rate required. Laminar flow is essential so that each slowly-settling floc particle within a tube maintains a steady descent to the collecting surface of the tube and is not intermittently swept upward by turbulent currents within the tube.

2. The residence time within each tube must be ample so that a floc particle entering at the extreme upper edge of the tube will have sufficient time to settle to the collecting surface a vertical distance below. (Once the particle reaches the collecting surface, the coalescing tendency between particles creates a steady sludge formation).

3. The velocity of flow through the tubes must not exceed a critical maximum that would cause the settled sludge to lose stability and be swept out of the tube in the direction of normal flow. As a corollary, the volume of the tube must be ample to allow either accumulation or a continual discharge backward of all sludge, without critically changing the normal flow rate through the tube

Size, Shape, and Configuration
In addition to system design criteria, size, shape, and configuration need to be evaluated when choosing a tube settler module design. The vertical settling height within a tube should be kept as short as possible, within the restraints of cost and plugging potential, to minimize the settling distance for the particles. A V-groove base should be provided to allow for the rapid accumulation of solids into a compact mass, which slides continuously down the tube.

When selecting a tube settler, careful consideration should also be given to the vertical height of the modules. There are several different size tube settlers, each having a specific application rate. (See Design Application Rate chart) Tube settlers with a vertical height of 24” (610mm) and a tube length of 28” (711mm) are the most commonly used size. 3’ (914mm) and 4’ (1220mm) tube heights, because of longer residence time, are advantageous in many applications as well, such as high flow/high turbidity applications and where existing basins need improved settling capabilities to increase the plants total capacity.

Tube settlers manufactured with the tubes aligned in the same direction avoid the formation of crossing points that crossflow tubes promote. Many manufacturers compromise tube settler flow-through with “criss-crossing” tubes in order to achieve module rigidity. Crossing points can re-suspend the floc particles, affecting process performance. Tube modules that have flow in only one direction are easier to clean and have less chance for plugging, particularly where ends of modules meet.

Also, the tube module design should incorporate features that would prevent gaps along the installed modules. These gaps allow short-circuiting of unclarified water to pass around the tube settlers. A module design which allows nesting with adjacent modules is ideal because it maximizes available area and increases the total module strength.

Material of Construction
Tube settler modules should be constructed of evenly-spaced sheets of PVC which are solvent-welded to form a durable bond between sheets and channels. The PVC material is inert and resistant to deterioration from naturally-occurring constituents in water or wastewater.

Material and finished modules to be used for potable water treatment plant should be Certified to ANSI/NSF-61 Standard for drinking water.

Any material used for either water or wastewater should include an ultraviolet radiation inhibitor.

Some tube settler modules are constructed of ABS, which is a highly flammable material. A spark from welding, drilling, or a cigarette, etc. could ignite the ABS tube settlers, causing injury to personnel and damage to the tube settlers and surrounding structures.

PVC modules (unlike ABS and other materials) have a specific gravity considerably greater than water and will not float. ABS modules have a specific gravity only slightly higher than water and require a tie-down system. This results in greater costs for material and installation. A tube settler module constructed of PVC will not require any hold-down system or clips.

The mechanical properties of PVC exceed those of ABS, resulting in a more structurally sound installation. The most important mechanical property of PVC, flexural modulus, is 30% greater then that of ABS. PVC is denser, has greater tensile strength, higher impact strength, and is much less flammable then ABS.

Tube Settler System design is based on these three criteria:

• Flow -- gpm (m/hr) : Required hydraulic flow capacity through the basin

• Area -- ft2 (m2) : Plan area of tank for tube settlers.

• Design Application Rate: Flow/Area of 1.5 to 3.5 gpm/ft2 (3.66 to 8.56 m/hr)

Tube settlers handle maximum application rates from 2.5 to 4.5 gpm/ft2 (6.11 to 11.00 m/hr). The recommended application rate for design purposes is 1.5 to 3.5 gpm/ft2 (3.66 to 8.56 m/hr). This design application rate should be verified in accordance with local design standards for allowable flows, application rates, etc. Consideration of the influent water chemistry, settleability, and basin hydraulics should also play a role in selecting the proper application rate.

Support System Design
Tube settler module support systems should have a bearing surface of approximately 2” (50mm). Bearing surfaces of more than 2” (50mm) will cause blockage of the tubes that are in contact with the structural members. Supports less than 2” (50mm) wide can create loading forces that could crush the modules and reduce their effectiveness. Tubular supporting structures are not recommended because they create point loads, which can severely damage the tube settlers.

The support system can be made of stainless steel, painted carbon steel, or aluminum.

Supports for the modules must be located a minimum of 1’-0” (305mm) from the end of each module for modules less than 8’-0” (2439mm) in length and 1’-6” (458mm) for modules greater than 8’-0” (2439mm) in length.

The support system must be designed with consideration of both live loads (human traffic) and dead loads (Dead loads include the weight of the PVC tube settlers, floc build-up, troughs, baffles, protective surface grating, etc.). The dry weight of PVC tube settlers is approximately 1.75 to 2.00 lbs/ft3. The support system should be designed in accordance with a maximum 8’-0” unsupported span of the tube settler. This design will support both dead and live loads with an adequate safety factor. Some manufacturers will design based on a 10’-0” (3048mm) unsupported span that limits operator access and can be potentially dangerous.

In rectangular tanks, the supports should span the full width of the tank. If this is not possible, intermediate support columns may be used. However, the support system should not impact any sludge-collecting device operations or other basin operations. Often concrete center columns are available for use in attachment of the tube settler support system.
Circular clarifiers can use supports that span from the outside diameter to the center well, creating pie shape tube settler areas. If complete coverage is not required for circular clarifiers, cantilevered support frames can be attached to the outer wall to provide the required coverage area.

Brentwood provides complete engineered systems, including tube settlers, supports, baffles, troughs & weirs, and protective surface grating. The advantages of a Brentwood installation are:

• Single source responsibility. Avoid problems with coordination of engineering, installation, and pricing of different components.

• Save money by purchasing an economical packaged system.

• Exclusive products and features like AccuGrid Protective Surface Grating and integrated structural ribs provide unique benefits to the tube settler system.

• Extensive engineering experience in both plastics design and water treatment technologies are utilized in every system design.

Project Customization
Brentwood tube settler modules can be “custom fit” for either rectangular or circular tanks. Standard lengths available are 6’ (1829mm), 8’ (2439mm) , 10’ (3048mm) , or 12’ (3658mm), with standard widths of 1’ (305mm) or 2’ (610mm). Other sizes are available upon request.

Every tube settler system is different due to project requirements, tank configuration, etc. Please contact us to provide detailed budget pricing, schematic system layouts, and specifications for your particular system design.

*Does not include new inlet piping, flocculation chamber, troughs, weirs, outlet piping. Includes only site grading, excavation, concrete tank

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