How to Design Clarifier for Primary Treatment of Water Treatment Plant?

The Design of Clarifier mechanism is very important role in primary treatment of any water treatment plant.

The primary clarifier should remove both settleable organic and floatable solids (primary sedimentation or clarification) . Poor solids removal during this step of treatment may cause organic overloading of the biological treatment processes following primary treatment.

Normally, each primary clarification unit can be expected to remove 90 to 95% of settleable solids, 40 to 60% of the total suspended solids, and 25 to 35% of biological oxygen demand (BOD).

We should consider several process control calculations may be helpful in evaluating the performance of the primary treatment process. Process control calculations are used in the sedimentation process to determine:

• Percent removal
• Hydraulic detention time
• Surface loading rate (surface settling rate)
• Weir overflow rate (weir loading rate)
• Biosolids pumping
• Percent total solids (% TS)
• BOD and SS removed (lb/day)

keep in mind that an optimally operated primary clarifier should have values in an expected range. Recall that the expected ranges of percent removal for a primary clarifier are:
• Settleable solids, 90–95%
• Suspended solids, 40–60%
• BOD, 25–35%

The expected range of

1. Hydraulic detention time for a primary clarifier is 1 to 3 hours.

2. The expected range of surface loading/settling rate for a primary clarifier is 300 to 1200 gpd/sq ft) (ballpark estimate).

3. The expected range of weir overflow rate for a primary clarifier is 10,000 to 20,000 gpd/ft.


Surface loading rate is the number of gallons of wastewater passing over 1 square foot of tank per day. This figure can be used to compare actual conditions with design. Plant designs generally use a surface-loading rate of 300 to 1200 gal/day/sq ft.

Surface loading rate (gpd/ft2) =gal day  / surface tank area (ft2)


Weir_overflow_rateA weir is a device used to measure wastewater flow See Image
Weir overflow rate (or weir loading rate) is the amount of water leaving the settling tank per linear foot of water. The result of this calculation can be compared with design. Normally, weir overflow rates of 10,000 to 20,000 gal/day/ft are used in the design of a settling tank.

Weir overflow rate (gpd/ft) =flow (gpd) / weir length (ft)


Next: >> % Calculation of Biosolids pumping, total solids (% TS) and BOD and SS removal


Author: T.P.Sivanandan

6 thoughts on “How to Design Clarifier for Primary Treatment of Water Treatment Plant?”

  1. Hello,

    Nice information; however, I am wondering is

    Weir overflow rate (gpd/ft) =flow (gpd) / weir length (ft)

    the flow for this equation is the surface loading flow


    the flow of gal day?


  2. Resp Sir,
    Please give me details design calculation of ETP/STP/WTP.
    e.g. Aeration Tank, Air quantity ? Clarifier mechhanism primary & secondary,,SDB
    each And every detail of lant design.

  3. Wether lamella clarifer can be used for water supply schemes where the requirement is more than 1 lakh liters per day

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