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5.11 Case studies (Topic k)

5.11.1 The Colombian: ASAS (Solids-Free Sewerage) System

Design of the system

Criteria, parameters and methodology of the ASAS design were deducted from the observation, investigation and analysis of many different variables but mainly centred on three basic concepts: (1) the compliance of the technical criteria and parameters to social factors and behaviours, (2) the hydraulic integration of the intercepting tanks with sewers, and (3) minimum cost and simplicity as final goals.

ASAS Conceptual Premises

Social factors and behaviours.
For the conception and further development of the ASAS system, the following sociological and anthropological concepts were carefully taken into account:

Urban residential areas and small towns tend to be more homogeneous than heterogeneous in social-economical stratification and in cultural background and behaviour. This tendency increases when the social economical level decreases.

Culture is more determining in fixing living habits, including water consumption and personal and domestic hygiene. If and when the social and economic condition improves with time, basic habits stay almost unaltered. Urban communities of slightly different socio-economic levels also have the same basic habits.

In urban communities the economic situation changes very slowly within one generation (30 years), unless an external phenomenon induces change.

In urban communities of the lower levels, the average numbers of occupiers per house tends to coincide with the most frequent household. A typical household, with the average number of occupiers per house can, then, be taken as a unit that represents the community. The variations in the number of occupiers in different times of the year, which takes place in many household, does not alter the average.

The typical house in a given community are similar in type and number of plumbing fixtures. For low social-economical strata, the typical house is equipped with a shower and a pour-flush toilet (although some may have a tank water-closet), a sink and a wash trough or tray. A hand lavatory is rarely added. For other strata and non-residential users, houses include a higher number or other types of fixtures.

Conditionants for dimensioning sewers.
The following statements allow for the adoption of the discharge of the unit house as unit load and to compute the sewers design load on calculus of probability:

Since the average number of occupiers per household in low strata is greater than the number of fixtures, during peak hours all fixtures are used and produce the greatest possible discharge rate when a toilet is flushed. In upper strata, or where the average number of fixtures is greater than the average number of occupiers, a similar reasoning applies: the greatest possible discharge is related to the number of fixtures being used by the total number of occupiers.

Like all natural phenomenon, the probability that several houses producing their maximum discharge exactly at the same time is ruled by the law of probabilities. The simultaneity is conditioned by the intermittent in the use of the toilet.

Because of the reduced statistical probability in exceeding the design load, sewers can be designed flowing full, to obtain small diameters.

As a conclusion, the capacity of the ASAS system is guaranteed for the long range, more than a generation (30 years).

Conditionants for intercepting tanks.
The intercepting tanks design is based also on social concepts, and cost/efficiency criteria as follows:

It is more convenient to adopt one size of tank for the unit house, than to proportion tanks to the number of occupiers in every house because occupiers vary depending on the time of the year and over the long term. The unit tank size is adopted for the optimum period for desludging which is the one with the lowest combined costs of construction and desludging. (This optimum period was 6 years for the Cartagena, Granada and San Zenon projects and for other projects).

Since both construction and maintenance costs must be paid by subscribers, the lowest cost must correspond to monthly rates. The lowest rate corresponds to the lowest summation of monthly rates to amortize or depreciate the construction cost, and the monthly allowance to cover the desludging cost. (These estimates could involve complex financial variables. The simplest method is to work with arithmetic depreciation of construction costs. The cost of desludging is divided by the number of months of the corresponding period).

Although the actual occupancy of each house determines the real period for desludging its tank, the average period is the period of the unit house, and an average rate can be charged to all subscribers.

Possibilities and restrictions

Notwithstanding the original purpose of the system, and that it is more suitable for predominantly residential areas, the ASAS system is not exclusively used for marginal or urban residential areas of the lower social-economical strata.

Based on the ASAS technology and with proper design parameters, the system is applicable in areas of different social-economical conditions, and is accepting of commercial, industrial or institutional users. In these cases the non-residential users loads are proportional to the typical household. In larger towns, conventional sewers for commercial and industrial areas can be combined with ASAS for residential areas.

In any case, the ASAS is not recommended for dispersed homes, where low urban densities do not make it competitive with in situ systems, nor for steep areas in which access for cleaning intercepting tanks is difficult.

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