Product Info.


HEI Inclined Plate Clarifiers Use Gravity & Innovative Engineering

A gravity clarifier is the most economical method of removing solids from liquid because natural gravity is the source of energy and it is free. A clarifier simply provides a non-turbulent zone where heavier than liquid solids, suspended by turbulence, are given sufficient time to settle to a quiescent surface. The HEI inclined plate clarifiers are compact units with multiple layers of settling area utilizing less than 25% of the floor space required by conventional clarifiers.

Principle of Horizontal Clarifier

A particle carried forward by the velocity of the liquid flow must settle at a rate that allows it to reach the bottom before passing through the clarifier. Thus, particles beginning at a point “a” must traverse some route lying between ab and ab’ in order to avoid being carried over the outlet.

If V is the horizontal velocity of the liquid, S the solids particle vertical settling velocity, L the length of the settling device, and D its depth, then particles entering at point A will settle to the bottom of the device only if V does not exceed: S(L/D)

Since Vmax / S = L / D then, Vmax = S (L / D)

Therefore, the velocity at which a horizontal clarifying device may be operated successfully is directly proportional to its length and inversely proportional to its depth.

This analysis applies to multiple horizontal plate units also. The spacing between plates is usually a few inches as opposed to a depth of several feet in a horizontal tank; therefore, “settling-out” times are dramatically reduced. The flow must be non-turbulent to prevent settled solids from being reentrained within the moving liquid. Small plate spacing and a large surface area permits laminar flow at higher velocities than large horizontal tanks would allow.

Horizontal clarifying devices become self-flushing if they are inclined at an angle which exceeds the angle of repose of the settled solids. In such cases, flow enters the lower end of the device where settling particles move to the floor eventually sliding back out the entrance. Clear effluent leaves the top of the device.

However, when the device is inclined, the furthest settling particles no longer fall through distance D but some longer distance D’. This new longer settling distance D’ is related to D by the relation: D = D’ cos Ø.

Theta “Ø” is the angle, the device is inclined to the horizontal plane. Thus settling distance is increased by the factor: 1/cos Ø In the case where Ø = 60º, 1/cos Ø = 2.

The maxi mum settling distance is twice the distance between the plates. It is apparent then that the lower the angle of inclination, the smaller the settling distance. However, the angle of inclination must exceed the angle of repose of the solids to be separated. The previous equation may be modified to express the cosine of an inclined plate clarifying system as:

Vmax = L / (D / cosØ) (s) = L·cosØ / D (s)

Inclined Plate Clarifier

A reduction of the required floor space is acquired by diminishing the separation between the horizontal plates to a few inches and stacking the settling surfaces. Inclining the plates to provide self flushing, 45º for heavy particles and 60º for light particles, reduces the available horizontal projected area (effective settling area) by a factor equivalent to the cosine of the angle. The surface area diagram (below) graphically compares the floor space requirements of an HEI inclined plate clarifier with the equivalent horizontal projected settling area.

Settling Rate

The settling rate for a specific solids should be determined by standard laboratory tests. Light particles, such as metal hydroxides, usually require a design parameter of 0.25 gallons per minute per square foot of horizontal projected area. These low density solids require the inclined plates to be set at a 60º angle to induce the particles to slide down the plate. Heavier particles (such as sand that easily flow) will readily slide from plates set at a 45º angle.

Maximum flow rate of an inclined plate clarifier is based on the flow rate per unit of a horizontally projected surface area. Retention time in the clarifier is not a design criteria. However, attaining optimum performance requires the prudent design to recognize several additional, very important factors.

Inlet Plenum

An inlet plenum must be provided to uniformly distribute the influent to the inclined plate compartments. Laminar flow must be established as the flow enters the plate area. The hydraulic momentum of the incoming liquid must be dissipated to prevent channeling. The HEI design does not use orifices which may clog with heavy suspended solids.

Outlet Area

The outlet area must be designed to force uniform flow from all plate compartments and also over the entire width of the plates. For example, wide plates (4 foot and over) with side outlets do not utilize the center section and must be proportionately decreased. A poorly designed outlet can result in one 50-60% plate utilization. The HEI clarifier utilizes an orifice type weir with orifices placed on each side of the plate to force uniform flow from each compartment.

Equalization Compartment

The HEI inclined plate clarifier provides equalized flow between two or more plate packs. After installation, clarifiers will often settle which creates channeling and turbulence through one of the plate packs. A prudent design prohibits channeling.

Solids Removal

Solids sliding off the plates must be provided with a sufficiently large compartment to insure adequate capacity for the accumulated solids. Turbulence and channeling are avoided by continuously removing the solids which will disrupt the flow pattern if allowed to build up and contact the inclined plate.

There are essentially only two designs of sludge storage compartments in general use. The conventional design is an inverted cone or pyramid with angles to match the expected angle of repose of the solids to be collected.

Due to the hydrostatic head present before and during sludge removal, two adverse conditions tend to be created. The sludge which accumulates between draw downs will compact, changing its angle of repose. With the draw off pipe open the hydrostatic head will cause the more fluid supernatent to create a channel (rat hole) from the top sludge layer to the outlet. The result is too much liquid and not enough sludge removed.

The preferred design is a compartment with almost a flat bottom and a top driven motorized rake which will break up compacted sludge and direct the sludge to the center discharge point preventing the “rat hole” phenomenon. This design also allows for the maximum amount of sludge storage below the plates for a given height (three time as much as a cone bottom design).

HEI Inclined Plate Clarifier with Motorized Sludge Rake

Sludge Thickening

For most applications there is insufficient volume below the inclined plates to provide adequate storage time to attain sludge thickening or compaction. A secondary tank is required to provide sufficient storage time to accumulate and thicken the collected solids. Laboratory studies must be performed on each sludge to determine thickening rate. Usually sludges must be retained in non-turbulent condition for 4-24 hours to reach an optimum concentration. A typical well-flocculated clarifier influent may contain 300-500 ppm suspended solids. The solids will settle to a volume of approximately 10% of the initial volume (0.3-0.5%). Hence, a 10% underflow is required to remove the accumulated solids. The sludge accumulator or sludge thickener must have the capacity to store the accumulated solids for at least 24 hours. The filter press or other compaction device must be sufficiently large to continuously compact the collected solids. Contact your HEI technical representative to assist you in the sizing of your liquid solids separation and solids compaction system.


HEI inclined plate clarifiers are constructed of 1/4″ ASTM A36 structural carbon steel. The inclined plates are fiber reinforced plastic with spacers and brackets fabricated of 304 stainless steel and polyvinyl chloride plastic. Units will be fabricated entirely of stainless steel on request. All carbon steel surfaces are media blasted to SSPC SP-6 finish and coated on the interior with high build polyester epoxy and the exterior with acid resistant epoxy. The units will be coated with FRP on request.

Manufacturing Specifications

  1. Self supporting – suitable for foundation mounting
  2. Fabricated of 1/4″ A36 steel
  3. Lifting lugs provided
  4. Surfaces are sand blasted to SSPC SP-6 and painted with acid resistant epoxy paint.
  5. FRP inclined plates are separated by plastic I-beams set on a 60 degree angle.
  6. Design settling rate of .25 gpm/sq. ft. horizontal projected area.

Optional Equipment

  • Flash mix & flocculate sections with mixers
  • Acid resistant FRP coating on interior and/or exterior
  • All stainless steel construction
  • Pumping systems
  • Automatic sludge removal system
  • Oil skimmers
  • Covers
  • Ladders and platforms
  • Seismic zone certification

Installed Dimensions HEI Inclined Plate Clarifier

Slant Plate Clarifier With Inverted Pyramid Bottom Slant Plate Clarifier With Motorized Rake
Model No. Flow Rate L W H Dia. of
Model No. Flow Rate L W H Dia. of
110/60/SA 25 8’11″ 5’11″ 11’8″ 12′ 110/60/MA 25 8’11″ 5’7″ 10’5″ 12′
150/60/SA 35 10’1″ 7’5″ 12’10″ 14′ 150/60/MA 35 10’1″ 7’1″ 10’5″ 14′
250/60/SA` 60 10’10″ 8’10″ 13’7″ 18′ 250/60/MA 60 11’11″ 9’0″ 10’5″ 18′
220/60/SB 55 10’3″ 5’11″ 13’10″ 17′ 220/60/MB 55 10’3″ 5’7″ 12’7″ 17′
300/60/SB 75 11’2″ 7’5″ 15’2″ 20′ 300/60/MB 75 11’2″ 7’1″ 12’7″ 20′
500/60/SB 125 13’5″ 8’4″ 15’8″ 25′ 500/60/MB 125 13’4″ 9′ 12’7″ 25′
1000/60/SB 250 14’4″ 11’4″ 18’4″ 35′ 1000/60/MB 250 15’5″ 11′ 12’11″ 35′
440/60/SC 110 10’11″ 5’11″ 18′ 23′ 440/60/MC 110 10’11″ 5’7″ 16’10″ 23′
600/60/SC 150 12’7″ 7’5″ 20’2″ 27′ 600/60/MC 150 11’10″ 7’5″ 17′ 27′
1000/60/SC 250 13’5″ 8’4″ 21′ 36′ 1000/60/MC 250 13’11″ 9′ 19’1″ 36′
2000/60/SC 300 17’3″ 11’4″ 19’4″ 50′ 2000/60/MC 300 16’10″ 11’1″ 16’6″ 50′
2900/60/SC 725 19’2″ 11’11″ 23’6″ 60′ 2900/60/MC 725 19’3″ 12’3″ 23′ 60′
4200/60/SC 1050 22’2″ 16’5″ 23’9″ 73′ 4200/60/MC 1050 22’1″ 15’11″ 21′ 73′


  1. HEI Model No. (110/60/SA) typically indicates: 110 sq. ft. horizontal projected area. 60 indicates the angle of inclination of the plates, “m” indicates motorized sludge rake, “s” indicates sloped bottom, A indicates the length of the settling plate, A is 30″ long, B is 60″ long, C is 120″ long.
  2. Flash mix and flocculation sections for 2,000 sq. ft. and greater size clarifiers are shipped separately.
  3. Clarifiers supplied with 45° inclined plates will have 40% more horizontal projected area for the equivalent outside dimensions.
  4. Dimensions given are approximate.
  5. HEI will custom fabricate clarifiers to accommodate reduced floor space or ceiling heights.
  6. Dimensions of units having 1,000 sq. ft. or less include flash floc sections.

© Hoffland Environmental Inc, 2012