Sewage Parameters 3: Population Equivalent (PE) Part 1
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 systems. In other words the maximum organic load that a sewage treatment plant can process.
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. Please note that the flow figure can vary widely from country to country from 100 l, where there are a lot of water saving measures to 300 l in countries with a lot of water use but 200 l is a good median. However, the organic component of 60 g/PE is normally fairly stable from country to country regardless of the flow.
This means in practice that 1 person resident in a normal house/dwelling 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.
In terms of concentration 60g / 200 l = 300 mg/l, would be a typical BOD and SS concentration for normal domestic sewage.
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 domestic type 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 produce 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.
Please note that the figure for BOD should always guide the selection of package sewage treatment plant such as the BMS Blivet with reference to the flow.
The population equivalent figure also needs to be quoted in conjunction with the required effluent standard when selecting the effluent treatment plant.
Next up, a few more PE calculations.
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