Sewage Parameters 3: Population Equivalent (PE) Part 1
Introduction
In this installment of our Education Blog series, we’ll cover Population Equivalent (PE) — a key parameter used to determine the size and capacity of package sewage treatment systems.
What is Population Equivalent (PE)?
PE is a measure of the maximum organic load a sewage treatment plant can process, expressed in terms of a fixed population’s contribution to sewage.
Definition:
"The equivalent, in terms of a fixed population, of a varying or transient population (e.g., restaurant, airport), based on 60 g BOD/person/day and 200 litres/person/day."Flow Variability:
The 200 litres/day flow figure is a general median.
It can vary from 100 litres/day in water-efficient regions to 300 litres/day in areas with high water usage.
The organic load (60 g BOD/person/day) remains relatively consistent globally.
Practical Understanding
1 PE is defined as:
200 litres/day of sewage flow containing 60 g of BOD/day.
This equates to a typical domestic sewage BOD concentration of:
60 g/200 l=300 mg/l
Example: Household PE
A household with 4 residents:
Flow:
4×200 l=800 l/dayBOD:
4×60 g=240 g/dayPE: 4
PE for Non-Household Sewage
Domestic-type sewage is also generated at locations like schools, hotels, offices, and caravan parks. PE calculations for these settings use flow and BOD figures specific to the type of facility. 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:
Example: Hotel PE Calculation
Scenario: Small hotel with 50 guests.
Flow:
50 × 250 liters = 12,500 liters/dayBOD:
50 × 75 grams = 3,750 grams/dayConvert:
Based on Flow:
12,500 liters ÷ 200 liters/PE = 62.5 PEBased on BOD:
3,750 grams ÷ 60 grams/PE = 62.5 PE
Selection Guidance:
The BOD figure (62.5 PE) should guide the selection of the treatment plant with reference to the flow.
Important Considerations
Effluent Standard:
The PE must be specified alongside the required effluent quality standards (e.g., BOD ≤ 25 mg/l, SS ≤ 35 mg/l).Application:
PE calculations ensure the proper sizing and performance of treatment systems like the BMS Blivet, tailored to the site’s needs.
Next Topic
Stay tuned for Population Equivalent (PE) Part 2, where we’ll delve into more calculations and scenarios for different types of facilities.
Questions?
If you have specific questions or topics you’d like us to explore, feel free to contact us. We’re always here to help!
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