How can I calculate the size of a primary settlement tank in a sewage treatment plant?
Thanks for the question.
When calculating the size of a primary settlement tank for domestic type sewage e.g. from a house, school, hotel, camp site or camp etc. you are looking to achieve a set upward flow velocity. In the case of a horizontal settlement tank you are generally looking to achieve an upward flow velocity of less than 0.9 m/hr. To do this you need to provide an adequate surface area.
For example, based on population for 400 Population Equivalents (PE) where 1 PE = 200 L of flow and 60 g of BOD:
Flow: 400 PE x 200 l/PE = 80,000 l/day or 3333 l/hr or 3.33 m3/hr.
Standard Upward Flow Velocity = 0.9 m/hr.
3.33 m3/hr ÷ 0.9 m/hr = 3.7 m2 is the minimum surface area required for settlement.
The settlement will often be sized on 3 x Dry Weather Flow (DWF), this would require 3 x the surface area to achieve good settlement.
3 x 3.7 m2 = 11.1 m2 is the minimum surface area required for settlement.
Standard BMS settlement tanks provide a settlement surface area of 1.64 m2 per linear metre. This means that a BMS settlement tank 6.8 m or greater in length will provide adequate settlement.
An innovation that BMS have pioneered in terms of package sewage treatment is using lamellae plate settlers to achieve settlement in a far smaller space. The lamellae plates considerably expand the available surface area for settlement meaning that the required upward flow velocity can be obtained in approx. 1/3 of the space compared to a conventional primary settlement tank design. This one of the main reasons along with the unique design of the BMS Aerotor next generation RBC, that the BMS Blivet is the most compact ‘all in one’ package sewage treatment system available.
Examples of standard BMS Horizontal Flow Settlement Tank & BMS Lamellae Plate Primary Settlement Area built in a BMS Blivet.
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