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 base 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.

An example of a standard horizontal sewage settlement tank

A bit about the company

BMS is a quality manufacturer of package products for wastewater and stormwater treatment. BMS started in 1986 and has exported products to over 50 countries around the world.  It is a third generation family business with huge expertise in wastewater and stormwater.
BMS also manufacture a full range of products for surface water and wastewater treatment including our pioneering next generation RBC system the Aerotor, which forms part of our world leading Blivet package sewage treatment plant, a ‘one stop shop’ for all surface water products from design to installation including attenuation, interceptors, holding tanks (incl. chemical resistant), hydrobrakes, rainwater harvesting tanks & vortex silt/oil/debris separators, fully fitted pumping stations, our plug & play vehicle wash water recycling system; the Recyclone and much more.  Please see our enclosed Company & Product Profile for a full summary or View our full product/services catalogue at this link

BMS also can provide a Free Design Service for Sewage Treatment, Pump Stations or Surface Water and a full O&M, Repair, Advice and Testing service for existing installations.  We also can do free CPD Presentations on Sewage Treatment/Surface Water Management at any location.  Let us know when suits.

We would be delighted to discuss any requirements you might have, and we are available to meet you at your convenience.  Please do not hesitate to contact us.