Monday, 31 March 2008

Simplified sewerage design

Brazilians often don’t bother to “design” simplified sewerage. Why is this? Well, it’s because they know what they’re doing. Assume one person generates 70 litres of wastewater per day. The corresponding peak flow is around 1.8 times greater – say, 130 litres per person per day, or 0.0015 litres per person per second. The minimum peak flow* used in the design of simplified sewerage in Brazil is 1.5 l/s, so you need 1000 people to be connected to the sewer before the peak flow exceeds the minimum peak flow − @ 5 persons per household (the average household size in low-income urban areas in Brazil), that’s 200 households. So a 100-mm sewer laid at a gradient of 1 in 200 (or 5‰) can serve 200 households – but you have to know how to calculate this minimum gradient (Imin) and therefore how to derive this equation (based on a minimum tractive tension of 1 Pa):

Imin = 5.64 × 10(^−3)q(^−6/13)

where q is the peak flow in l/s − so, for q = qmin = 1.5 l/s, Imin = 0.0047, which is rounded to 0.005 (5‰) or 1 in 200.

You also have to know how to be sure that a 100-mm dia. sewer can handle this minimum peak flow. So, although you can “design” very simply, you have to understand the basic hydraulic theory (details here, for example − section 2 gives the theory) – it’s not rocket science, but you can’t afford to get it wrong.

*It’s important to include this as one of the basic design parameters for simplified sewerage. The Bolivian simplified sewerage code NB688 [in Spanish] didn’t do this, so it ended up recommending greater minimum gradients than necessary (which, of course, results in higher costs) – see explanation in English here and in Spanish here.