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Test for Determination of Inclined Plate Clarifier Hydraulic Loading Ratio

Use this test and calculation procedure to determine proper clarifier sizing expressed as gpm/sq.ft. of Hydraulic Loading Ratio (HLR) and Projected Surface Area (PSA).

The basic test procedure is to first determine the chemistry methodology, then develop the settling rate graph, then determine the HLR, then determine the PSA. 

  1. Determine Chemistry Methodology.
    Test coagulants and flocculants on the wastewater until a suitable chemistry methodology is found.
  1. Develop the Settling Rate Graph.
    Create a flocculated sample in a 1000 ml graduated cylinder. Use a plunger or stir stick to gently agitate the sample until a homogeneous mixture is obtained.
    Remove the stirrer. Begin timing with a stop watch. Record interface level as the solids settle in a ml vs. time format.
    Suggested timing intervals are 15 sec., 30 sec., 45 sec., 1 min., 1 min. 30 sec., 2 min., 3 min., 4 min., 5 min., 8 min., 10 min. Convert ml reading to height in feet from the bottom of cylinder. Then, plot results, height vs. time on a graph.
  1. Solve for Hydraulic Loading Ratio (HLR).
    Draw a straight line through the “rapid settling” portion of the curve.
    Example; if the bulk settling rate is 1.34’ in 20 min. Multiply by 7.481 gal/ft3 to convert to GPM/ft2. Example: 1.34’/20 min X 7.481 gal/ft3 = .52 GPM/ft2. Divide by 2 for continuous rate clarification. .52/2 = .26 GPM/ft2.
    Therefore, the HLR rate in this example is .26 GPM/ft2.
    (In general, despite calculated results, never use more than 1.0 GPM/ft2)
  1. Solve for Clarifier Projected Surface Area (PSA).
    Example flow rate of 100 GPM. Solids settling area required is 100/.26 = 385 ft2 of Projected Surface Area (PSA). Specify a clarifier with at least 385 sq. ft. of PSA.


    1. The Projected Surface Area (PSA) for an inclined plate clarifier is the sum of horizontal projected area of each plate.
    2. A typical and accepted settling rate for most metal hydroxide precipitations is 0.25 GPM/ft2.