RAINWATER HARVESTING DESIGN

The construction of cisterns in order to collect rainwater is deeply rooted in the past. In fact, already in the Middle East in 2000 B.C. storing rainwater cisterns were used in typical middle-class dwellings in order to reuse the water for both domestic supply and private bathing facilities for the wealthy.

Nowadays, considered as an “emerging technology” rainwater harvesting is one of the most efficient solutions in order to reduce domestic water. Furthermore, this system reduces the need and demand linked with water transport systems that have negative aspects both in terms of environmental impact and costs. The water collected from rain can be used both inside and outside the house for different purposes such as for toilet flushing, laundry as well as to irrigate crops or garden.

In order to assess the building performance in terms of water consumption three main schemes can be considered as reference:

1 The BREEAM assessment scheme aims to reduce the consumption of potable water through the use of water recycling systems and water efficient components. The BREEAM Wat 01 calculator related to the water consumption assigns at a certain percentage of improvement in the water consumption a number of BREEAM credits up to a maximum of 5. A 65% improvement represents an exemplary performance. Furthermore, the BREEAM provides other detailed sections that regard water monitoring (Wat 02), water leak detection & prevention (Wat 03) and water efficient equipment (Wat 04).

2 Category 2 of the Code for Sustainable Homes establishes a scale of credits (from 1 to 5) that corresponds to the water consumption (litres/person/day). A consumption of  ≤ 80 l/p/day corresponds to 5 credits that is the maximum number of credits achievable in order to have a better performance.

3 Part G of the Building Regulations 2010 establishes that rain water and grey water can be used for WC, washing machines and irrigation, an appropriate risk assessment has to be carried out in order to avoid waste, misuse, undue consumption or contamination of wholesome water. Furthermore, the new dwellings have to achieve a water efficiency standard of 125 litres use of wholesome water per person per day.

Considering a typical household, the rainwater harvesting can reduce the water requirements by 70%- up to 85% if the rain water supplies also hot water systems.

In order to design a rainwater harvesting system main points have to be assessed.

–          Decide the demand of the water. This is linked with the average consumption of water in the house, is determined by both number of people and outdoor and indoor activities.

Indoor activities:

Consumption of a water-conserving household =   25 – 50 gallons per person per day

Outdoor activities:

It depends on the season, in summer more water is used for irrigation

–          Rainwater availability. This depends on the site characteristics in terms of climate conditions which determine the volume of rainfall along the year. The periods without rain have to be considered as well as the high-density rainfall months. The availability from every single month has to be considered in order to size the system and enable a continuous water availability.

–          Determinate the collection surface and the quantity of water that can be captured. The collection surface can be located in different points of the building in order to collect the highest quantity of rainwater. The collection surface has to be dimensioned to consider the water demand as well as the volume of rainfall.

Collection surface = 0.62 gallons per square foot per inch of rainfall 

The above data are linked with the different climate locations, an average value has been considered.

The illustrated points give a general method and first approach that can be used in the starting design process of a rainwater system.