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A Note on the Bolivian Condominial Sewerage Standard (Norma Boliviana NB 688, December 2001)

by D. Duncan Mara and Miguel R. Peña Varon

School of Civil Engineering
University of Leeds
Leeds LS2 9JT, UK

The Bolivian authorities are to be congratulated on producing a national standard for condominial (i.e. simplified) sewerage.  We hope that this will serve as a model for other countries, and not only those in Latin America.

However, in Section 4 of Chapter II of NB 688, and in particular subsection 4.4 “Criterios de Diseño de Redes de Alcantarillado” (pages 29-34), there appears to be an implicit departure from current Brazilian practice in condominial sewerage design.

Tables II.4 and II.5 give the minimum gradient for a 100 mm diameter sewer as 7.98 per thousand for Qmi / Qll  = 0.10, and 6.68 per thousand for a value of this flow ratio of 0.15 (Qmi and Qll are the peak flows at the beginning and at the end of the design period, respectively.)  These gradients can be written as 1 in 125 and 1 in 150, respectively.  Current Brasilian practice, at least that of CAESB (the water and sewerage company for Brasilia and the Federal District), is to use a minimum gradient of 1 in 200 (i.e. 5 per thousand) for a 100 mm diameter sewer.  Original practice in northeast Brazil was to use a minimum gradient of 1 in 167 (6 per thousand) for this size of sewer.

How can these differences in Brazilian and Bolivian practices be explained?

In Brazil a minimum peak flow is used, but this is not mentioned in the Bolivian standard.  The governing equations are:

(a)    for a minimum self-cleansing velocity of 0.5 m/s:

Imin = 0.01 q –2/3                                                                                (1)

(b)   for a minimum tractive tension of 1 Pa:

Imin = 5.64 x 10-3 q –6/13                                                                           (2)

where Imin is the minimum sewer gradient (m/m) and q is the peak daily flow (litres/second).

Both equations incorporate a Manning’s n of 0.013, and equation (2) is for a proportional depth of flow (d/D, or h/D as in the Bolivian standard) of 0.20, i.e. initial flow conditions in the sewer.

In Brazilian practice the minimum peak daily flow value (qmin) is used in these equations.  If the peak daily flow in the length of sewer under consideration is less than qmin, then qmin is usd for q in equations (1) and (2).  The value of qmin is chosen to represent the flow peak which results from the flushing of a single toilet.

Originally in northeast Brasil qmin was taken as 2.2 litres/second and this was used in equation (1), so that Imin was calculated as 6 per thousand (i.e. 1 in 167).  Current Brazilian practice (as recommended in the national sewerage standard, Norma Brasileira NB 9649, 1986) is to use a qmin of 1.5 litres/second.  Applying this to equation (2) results in an Imin of  4.67 x 10-3, or 1 in 214.  CAESB has rounded this to 1 in 200, or 5 per thousand.

(Details of all these equations and calculations are given in Low-cost Urban Sanitation, by Mara, published by John Wiley & Sons, 1996.  See also Simplified Sewerage: Design Guidelines, by Bakalian and others, World Bank, 1994; and PC-based Simplified Sewer Design, by Mara and others, University of Leeds, 2001).

Thus, in comparison with current Brazilian practice, the Bolivian standard for condominial sewerage design is too conservative; specifically, its lack of consideration of a qmin value results in unnecessarily steeper minimum gradients.  This not only increases costs (especially in flat areas), but also reduces the number of people that can be served by each size of sewer.

1 February 2002