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Cleaner Technology Projects in Denmark 1996

Advantages and Disadvantages in Connection with the Usage of Rainwater in Households

This report aims at describing the advantages and disadvantages in connection with usage of rainwater in households. The project estimates the environmental impact of the usage of rainwater compared to the situation of the water supply, the pollution from systems of discharge pipes and treatment plants. Furthermore, the health aspects, the technical aspects and the financial aspects linked to the usage of rainwater are discussed.

Usage of rainwater in households implies that less ground water has to be collected. However, ground water resources as well as consumption vary according to different parts of the country. In areas where the consumption of ground water is close to the available ground water resources, the usage of rainwater will be useful to the drinking water resources. In the metropolitan area for example, the consumption is by far exceeding the ground water resources.

A general usage of rainwater from roof surfaces could reduce the water consumption of the households by approx. 18 %.

The usage of rainwater implies that discharges of diluted wastewater from combined sewer systems (through overflow structures) to lakes and streams will be reduced. For separate rainwater systems the discharges will be reduced as well.

The discharges from treatment plants in connection with rainfalls will be reduced when part of the rainwater in the catchment area is used. Calculations in the report, carried out on the basis of a theoretically simple catchment area, show that the result will be from 1 to 10 % reduction of the total water and pollutant loads discharged. Generally, the environmental influence in relation to discharges conditioned by rainwater is dependent on the extension of the usage of rainwater.

Percolation of rainwater is having a similar environmental influence in relation to formation of the ground water and discharges conditioned by rainwater from the sewer systems. Consequently, it would be natural to combine the various potentials so that the environmental influence is as large as possible. This could for example be done by collecting and using rainwater from roof surfaces and build in overflows from the rainwater tanks to soakaways, where the excessive rainwater can percolate.

From a health point of view, there are a number of disadvantages by using rainwater from households.

Rainwater collected from roof surfaces might, among others things, contain microorganisms from animals, and moreover chemical substances from the atmosphere can be found in rainfalls. Finally, substances from the material of the roof surfaces might occur.

There is a potential risk that a mixture of rainwater and drinking water might pollute the drinking water supplies. Consequently, in theory there is a risk of illness cases as a consequence of usage of rainwater. There are, however, no examples in practice of infections of illness from rainwater systems.

It is possible to establish areas without separate rainwater systems for discharge of rainwater.

The lower degree of hardness in rainwater implies that less soap has to be used in connection with laundering.

A number of technical conditions must be considered in connection with establishment of rainwater systems: Change of piping arrangements, space requirements, corrosion of pipe and tank material, bottom sludge, formation of algae, inspection and maintenance, marking of pipe system, etc.

It will be necessary to provide the tank with drinking water in periods without rainfall events. In this connection it must be ensured that reject flow to the drinking water system does not take place.

From a socio-economic point of view, it does not pay today to establish systems for usage of rainwater in households. It is cheaper to collect ground water from other places in the country for areas with heavy loads or to carry out simple water-saving measures. In relation to the sewer systems, the costs in connection with usage of rainwater can be more than 10 times larger than for other environmental measures, for example tanks in the sewer systems or local percolation. In some cases, however, where it is very expensive to build tanks, calculated in relation to the result obtained, usage of rainwater could be a more profitable possibility.

From a private economic point of view, it normally pays to make systems for usage of rainwater in households. The repayment time with the present water rates will, under favourable conditions, be 20 years. As, however, the usage of rainwater is highly contributing to a more green character to a house or a building the usage is in practice often not conditional on profitability.

The utilisation of rainwater seems to be a solution, which can be beneficial to the environment: less ground water is collected and the discharges conditioned by rainwater from the sewer system are reduced. There are, however, certain conditions, which imply that an extended usage of rainwater could be a possibility. However, in many situations the lack of drinking water can be remedied in cheaper ways. In cases of an increased pollution of the ground water, these conditions might however change.

Furthermore, it is a fact that if we also wish to maintain the installation standard and the health and sanitary level for rainwater installations which we have in the remaining water installation, it will be difficult to make a rainwater installation today which is profitable.

Author/ institution

Carsten Rosted Petersen and Victoria Plum, Krüger A/S
Dorthe Bechmann, Statens Byggeforskningsinstitut

This report is subsidised by the National Council for Recycling and Cleaner Technology.

ISSN no. 0908-9195
ISBN no. 87-7810-566-8

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