Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
Minimum cost parameters
The minimum cost of the system obviously corresponds to the smallest possible
intercepting tanks and the smallest possible pipe diameter.
To determine that cost, iterative calculations are necessary because both tank size
and pipe diameters are directly related to rates of flow and the characteristics
of solids, but, concurrently, tank size and proportions determine the rate of flow
due to the buffering action on toilet flushings and the characteristics
of unretained solids.
Computations made in the Cartagena study, working with discreet particles of clay
or lime (the omnipresent material in the areas under study)
with specific weight of 2.65, led to a set of basic parameters for minimum cost
of tanks and sewers. The particle size to which minimum cost parameters correspond
is 0.02 mm.
Further verifications with prices in Cartagena and for other projects,
including the pilot projects of Granada and San Zenón, proved that these parameters
are still correct in spite of drastic changes in prices in Colombia
throughout the years since 1982.
The unit load for sewer design was worked out for cylindrical tanks,
which were adopted as being cheaper than rectangular tanks under the assumption
that they would be prefabricated in massive production for the South-Eastern Zone
The rate of flow of the unit load is produced by the simultaneous discharge
of all the fixtures, that is, the toilet, the kitchen sink the laundry trough
and the shower. Colombian faucets and shower valves discharge around 0.1 l/s
each for normal inhouse service heads (8-10 m water head).
For the three fixtures the compound discharge is, then, 0.3 l/s.
For the unit tank adopted for that project, (0.90 m diameter, 1.48 m length
for average occupancy of 6.8 people per house) the maximum possible discharge
increment was 0.028 l/s for tank water-closet flushings (17 liters) and 0.007 l/s
for pour-flush toilets (4 liters). The compound rates are, therefore,
0.328 and 0.307 l/s respectively.
For rectangular tanks of equivalent capacity to the cylindrical ones,
the maximum discharges are less for obvious reasons: they are wider
at liquid level and produce less over-elevation of liquid level with toilet flushings.
Since there is so little variation in maximum discharge in relation to the type
of toilet and tank shape, a conservative unit load of 0.328 l/s, rounded to 0.33 l/s,
was then adopted as a basic parameter for ASAS design.
Simultaneity equation and design loads.
For the determination of
the design load for sewers, different methods of figuring out the probability
of simultaneous discharges were considered.
In Angelo Gallizio's "Instalaciones Sanitarias", published in
Spanish in 1964 by Editorial Cientifico-Médica from Barcelona, a system to
determine water demand and drainage in large buildings was found suitable
because it takes into account three parameters to reflect more precisely the
buffering effect of the intercepting tanks:
peak duration, interval between repetitive discharges and duration of discharge.
Gallizio's equation is:
log Ar-1 - log B = log Crn
i = interval between repetitive discharges
t = duration of the discharge
h = duration of peak
A = i/t, (i, t in minutes)
B = h/i, (h, i in hours)
Crn = number of possible combinations of "r" units out of a total of
The equation was adapted under the following considerations.
Although the influence of toilet flushings
on the maximum rate of discharge from intercepting tanks is small, they
determine the actual maximum value, so the interval between uses of toilets was
taken as "y".
The maximum discharge value prevails only for an instant, but to be conservative
the approximate time for the tanks to evacuate the volume of one flushing was
adopted for "t" values.
Valves for "H" and "i" were based on observations
The curve of simultaneity
for 2 hours peaks, i = 10 minutes and t = 1 minute, is shown in figure
5.1. Tables with valves of n, r, r/n and the corresponding design flows,
have been prepared. They are not included for reasons of space.
Number of houses
Figure 5.1: Simultaneity curve for sewer design flows
Sensibility analysis with
different values for t and i, and with H varying between 1 and 2 hours produced
very close values for r/n. This means
that the discharge loads computed by this method and the capacity of sewers
dimensioned will normally not be exceeded. As may be seen, the curve tends to
be asymptotic to 23.5% values. With t = 30 seconds, it goes closer to 23%. For
values of "t" approaching one second, the curve tends to drop to 4%.
It was considered too risky to
use these low simultaneities even though the maximum discharge from
intercepting tanks really last seconds, so the more conservative values, as in
figure 3, were adopted for ASAS projects.
The monitoring of the Grananda and San Zenón pilot projects must
contribute to elucidate this question.
It might make ASAS even more economical.
Although sewers are usually
laid above water table, infiltration flows are added to the design load with
rates of 0.1 l/s/Ha for vitrified clay pipe and 0.05 l/s/Ha for plastic
pipe. Storm drains are not permitted to
be connected to tanks or sewers but 0.5 l/s/Ha are also added to design
flows. These rates are half of the
specified in Cartagena for conventional sanitary sewers.
As a fact, infiltration flows can be reduced
even more but the risk of abusive connections makes recommendable to include
Minimum velocity and slope.
The low content of fine particles in the effluents of intercepting tanks admits
velocities as low as 20 cm/s in sewers. Nevertheless, it is not
advisable to adopt a slope less than 0.1%, (equal to 1 cm fall each 10 m)
because of practical installation limitations.
This fall can be easily measured with enough precision with a hose
level. (A hose level is a garden hose of enough length, with glass or transparent
plastic tubes in both ends that is normally used in Colombia by masons and pipelayers.
The hose is filled with water. The water level in both tubes is the reference
to measure elevation falls).
This slope induces velocities
of more than 20 cm/s for 3" and greater diameters for full flowing pipes
and for unit load of 0.33 l/s. For
2" PVC pipes the minimum slope is a little less then 0.2%.
Since velocities increase when pipes are not full, minimum slopes adopted were 0.2%
for 2" pipes and 0.1% for larger pipes. Verifications with tractive
force theory proved these parameters adequate for 0.02 mm particles.
Average number of occupiers per house.
This is the basic parameter to design the capacity for sludge and scum of the
intercepting tanks. This parameter is determined dividing the
population of the area by the total number of houses.
Normally it should correspond to the number of occupiers of the
most frequent house in terms of occupancy.
It can be established with a census of population and houses or, more
easily but less accurate, with a survey.
Figures in Cartagena were 6.8 and in Granada and San Zenón, 5.76 and
5.78, rounded up to 5.8.
There is no logical reason to increase
minimum slope. As for the particle size
of 0.02 mm, it has proved adequate to produce the minimum ASAS cost in all
projects. Besides, tanks to retain that size of particles are small
enough. Only if experienced rates of
accumulation of sludge and scum are greater than the ones deducted in the
Cartagena project, tank dimensions are to be enlarged.
As may be seen, the number of free flowing
fixtures have the heaviest weight in determining the unit load.
For projects with different number and type
of fixtures, this must be taken into account.
These basic parameters were also adopted
for the Granada and San Zenon projects and the other projects designed by the
consultant because general conditions were not as severe but similar to those
of the Cartagena projects.
As a conclusion, it can be
stated that basic parameters of ASAS makes this technology applicable for areas
with any socio-economic level and that the numeric values set for the Cartagena
project are applicable in places with more or less similar conditions to those
in Cartagena, Granada and San Zenón.