Next up in our Education Blog series is Population Equivalent or PE. It is the term most used to describe the size of package sewage treatment plants. It can be defined as; ‘The equivalent, in terms of a fixed population, of a varying or transient population (e.g. Restaurant or Airport) based upon a figure of 60 grams/BOD/hd/day and 200 litres/hd/day.
This means in practise that 1 person resident in a normal house is expected to produce 200 l of sewage flow containing 60 g of BOD per day. In other words 200 l of flow containing 60 g of BOD is equivalent to 1 Population Equivalent or 1 PE. So therefore a house with four people living in it would have a PE of 4 and would produce:
4 x 200 l = 800 l of flow & 4 x 60 g = 240 g of BOD.
Obviously there are lots of situations where sewage is treated at locations other than houses for example, schools, hotels, caravan parks and offices.
Figures from the Irish Environmental Protection Agency are available for all the above locations. For example 1 guest in a hotel is expected to product 250 l of flow & 75 g of BOD. So for a small hotel with up to 50 guests the design of a sewage treatment plant would be based on:
50 x 250 l = 12500 l of flow & 50 x 75 g= 3750 g of BOD. The total daily figures then need to be converted to the equivalent PE figures by dividing the flow by 200 l (flow for 1 PE) and the BOD by 60 g (BOD for 1 PE)
12500 l / 200 l = 62.5 PE & 3750 g / 60 g = 62.5 PE.
The figure for BOD should always guide the selection of package sewage treatment plant such as the BMS Blivet. The population equivalent figure also needs to be quoted in conjunction with the required effluent standard when selecting the effluent treatment plant.
Any questions about surface water or wastewater treatment please contact me.
A bit about the company;
Butler Manufacturing Services is a family owned specialist manufacturer of package (all in one) products for sewage treatment and surface water treatment established in 1986 with exports to 38+ countries worldwide including the 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/filters & vortex silt separators, chlorine contact tanks, fully fitted pumps and sumps and much more.