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

(Email addresses: d.d.mara@leeds.ac.uk
and miguelpe@mafalda.univalle.edu.co)

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 *Q*_{mi} / *Q*_{ll
}= 0.10, and 6.68 per thousand for a value of this flow ratio of 0.15 (*Q*_{mi}
and *Q*_{ll }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:

*I*_{min }= 0.01 *q* ^{–2/3 }
(1)

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

*I*_{min }= 5.64 x 10^{-3} *q* ^{–6/13 }
(2)

where *I*_{min }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 (*q*_{min}) is used in these equations. If the peak daily
flow in the length of sewer under consideration is less than *q*_{min},
then *q*_{min }is usd for *q* in equations (1) and (2). The
value of *q*_{min }is chosen to represent the flow peak which results
from the flushing of a single toilet.

Originally in northeast Brasil *q*_{min }was
taken as 2.2 litres/second and this was used in equation (1), so that *I*_{min}
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 *q*_{min }of 1.5 litres/second. Applying this
to equation (2) results in an *I*_{min} 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 *q*_{min }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