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STP capacity calculator

Enter how many people use the building and how much water they use. The calculator gives the sewage flow, the standard plant size in KLD, the peak flow and the technology usually chosen at that size.

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Who uses the building

STP size
150 KLD
Average sewage 108 KLD from 1,000 people, plus margin, rounded up to a standard size
Peak flow
13.5 m³/h
Peak factor 3 on the average flow
Water used
135 KLD
Fresh water a day, before treatment
Treated water to reuse
97.2 KLD
For flushing, gardens and cooling towers

Usual technology at this size: MBBR or SBR plant in RCC tanks. Preliminary: the State Pollution Control Board consent sets the treated water standard, and the design is confirmed after a site visit.

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How STP capacity is calculated

Water used (KLD) = residents × litres per resident + day users × litres per day user, divided by 1,000. Sewage is about 80% of the water used. A margin of about 20% is added, and the result is rounded up to a standard plant size.

Worked example: 200 flats × 5 people × 135 litres = 135 KLD of water. 80% of that is 108 KLD of sewage, plus 20% margin is 130 KLD, so a 150 KLD plant.

Peak flow is the average hourly flow times a peak factor: 3 for up to 20,000 people, falling to 2 for large towns.

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Frequently asked questions

What does KLD mean for an STP?

Kilolitres per day: 1 KLD is 1,000 litres of sewage a day. A 100 KLD plant treats 1 lakh litres a day.

What size STP does a 100-flat society need?

About 75 KLD at 5 people per flat and 135 litres each, after a 20% margin. More if there are many staff, a clubhouse or a pool.

How much water does a person use per day for STP design?

135 litres a day for homes with sewerage is the usual norm, and 45 litres for offices and day schools. Hotels and hospitals use much more.

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