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Civil engineering

Economy with water taps

Rainwater, made use of in a collection system, serves to water plants and wash cars

The scene is very well known by São Paulo residents. The summer rains bring about flooding at various points in the city, but many city districts do not have water in their taps because of  the shortage in the reservoirs. If part of this liquid, instead of running off along the gutters, were to be collected, stored and use to water plants, wash sidewalks, courtyards and cars or even for flushing toilettes, the flow of water through the gullies and streams would diminish, contributing to a reduction in flooding and inundations. Starting from this idea, researchers at the Polytechnic School of the University of Sao Paulo (USP), coordinated by professor Racine Tadeu Araújo Prado, felt themselves sufficiently motivated to project and mount a collection and usage system for rain water at the university’s Civil Construction Technical Center, which could well be made use of in various types of civil construction, with low investment cost.

One of the systems recommended, in order to eliminate the first rain water that washes off from the rooftop has two reservoirs, one small, which collects and automatically gets rid of the rain that falls in the first few minutes since it is very dirty, and one large, that is utilized to effectively store the liquid. As soon as the small one fills up, and is closed off by a buoy, similar to those used in residential water tanks, it is the larger tank that begins to receive the water. A filter placed in the guttering retains leaves, branches and other debris and prevents the blockage of the piping. If it is not possible to put the reservoirs directly on the building’s rooftop, then it is necessary to have the reservoir installation on the ground floor, which is buried, and there is another on the rooftop in order to pump up the water. The use of two reservoirs increases the capacity of supply during the dry season. As well, the energy used for pumping is very small and is not onerous to the system.

In order to analyze the quality of the water, an automatic sequential sampling collector was installed that keeps the liquid at 5°C  in order to facilitate its evaluation. This collector was specially developed for the project, which is funded by FAPESP with the objective of obtaining physical, chemical and biological parameters of the collected liquid. In order to evaluate the conditions of the collected water, from November 2003 until March of 2004, the researchers collected samples that were afterwards sent to the Adolfo Lutz Institute for analysis. One of the discoveries made is that before passing through any permeable areas, such as roofing, the rainwater is acidic, with a pH of 4.9. But as soon as it starts incorporating salts and other substances that are on the roofing it begins to turn alkaline and ends up with a pH of around 7.0, therefore within the recommendations for water drinkability of the Ministry of Health , which runs from pH 6.0 to 9.5. “But we verified some problems”, says professor Racine Prado. “Besides all of the particulate material emitted by vehicles and industry that are in the air and are deposited on rooftops, we also found the tree leaves, branches, bird droppings and small dead animals.”  All of this accumulated debris, principally after a long drought spell, comes down with the water. For this reason, one of the researchers’  recommendations for collecting, based on the results of the analyses of the samples, is to discard the water coming from the first fifteen minutes of rain, a time that is necessary for the cleaning of the roofing. For this reason it is important that the system is composed of two reservoirs.

In all of the analyses carried out, smell was absent as a parameter in all samples. In the evaluation of professor Racine Prado, the collected and analyzed data shows that it is possible to make use of rainwater, but that it is necessary to take some precautions. “We found fecal coliforms, coming from warm blooded animals such as birds, cats and rats, in 50% of the samples, as well as other bacteria that impede the water’s use for personal hygiene or for washing clothes.”  For these uses, water treatment is necessary. However, for plants, forecourts, sidewalks and cars, there are no major problems, the researcher emphasized, because normally they receive this type of water.

“Gasoline stations could also make use of this water, because human contact is very low and cars do not have major water quality demands”, says Racine Prado. In this case, it is much easier to place the reservoir underground in the forecourt of the gas station because it does not occupy an area of soil. No matter what, the researcher alerts that it is firstly necessary to make the calculation of the consumer?s needs before initiating any construction work. For a gasoline station, for example, this calculation must take into account the period of the year in which the rainwater will be in use. As well it is necessary to take into consideration that a reservoir, buried or not, demands civil construction work. For this reason he emphasized that the viability of the collection system basically depends of three factors: rainfall, the collection area and demand. If these three factors are high, the time for the recovery of the investment for some final purposes such as gasoline stations, laundries and industries, is reduced.

At USP, the rainwater captured by the collecting system was distributed to two small reservoirs, each of 2,000 liters, and used in two toilettes installed in the Civil Construction Technical Center building. When the rainwater finished, the normal water supply automatically came into action. “People didn’t perceive when it was one or the other system, since in our case the color of the water is identical”, said professor Racine Prado. The composition of rainwater, including the parameters of color and odor, vary in accordance with the geographical location of the sampling point, meteorological conditions, the presence or not of vegetation and also of pollutants.

Professor Racine Prado stated that the present legislation needs to be perfected in order to give incentive to the use of rainwater for uses that are not so noble, since that used publicly has been treated and has a high cost. The city of Sao Paulo already had a municipal law, approved by the Chamber of Councilors in January of 2002, which made the construction of reservoirs for the collection of rainwater obligatory for new buildings of more than 500 m2 of impermeable area. It is initiatives such as this and that developed at Poli-USP, which are contributing to lowering the flow that congests the canals and streams, flooding the city during every season of the heavy rains.

The Project
A study on the technical and economic viability of the use of rainwater for non-potability use in buildings (nº 02/04009-8); Modality Regular Line of Research Assistance; Coordinator Racine Tadeu Araújo Prado – Polytechnic School/USP; Investment R$ 119,650.00 (FAPESP)