Conserving water in good weather and bad
Systems for using rainwater and grey water can easily be combined
Virtually all public buildings consume vast quantities of water for flushing toilets, cleaning purposes or watering green space. To date, precious and expensive drinking water has typically been used in all three cases. Grey water and rainwater – which is almost free of salt and limestone – represent a practical alternative. For buildings with large roof areas in particular, a professional rainwater management system offers considerable cost-cutting potential.
Summary
Systems for recycling rainwater
Underground tanks/rainwater cisterns
A rainwater cistern resembles a conventional water tank but is buried in the ground and completely sealed from contaminants. The tanks are made of concrete or plastic and normally installed at a depth of 80 to 100 centimetres to protect against frost. Cisterns are the ideal solution for watering landscaped areas and flushing toilets: rainwater no longer just trickles away without being used and many hundreds of litres of drinking water are saved over the course of a year.
The major benefits of underground tanks are that storage in a cool, dark place prevents algae and germs from forming and such tanks do not take up space inside the building. It is advisable to carry out the earthworks needed for underground tanks at the same time as excavating the foundations or laying pipework, thereby keeping additional costs low.
Indoor tanks
If a rainwater harvesting system is to be retrofitted into an existing building, it is usually simpler to install plastic tanks in a basement area. In such cases, the overall cost is normally much lower because no excavation work is needed and the connections are easier to install. A big advantage of locating the rainwater harvesting system in a basement is good access to all pipes, meaning that any leaks or faults will be noticed quickly and can be resolved more easily. A combination of outdoor tank and indoor system is also an option.
Rainwater infiltration
Combining a rainwater tank with a rainwater infiltration system has a positive impact on the environment as well as offering technical benefits. While harvesting rainwater helps conserve water resources, infiltration of the overflow facilitates independence from the mains water system. Joint planning enables savings to be made on building costs, and sewer charges are lowered. The most common forms of rainwater infiltration system are troughs, trenches, surface infiltration and the combined trough-trench system, which is only used for ground with impermeable layers.
Green roofs
Green or "living" roofs absorb rainwater and go well with rainwater recycling systems, allowing such systems to be smaller and boosting their cost-effectiveness. Excess water can be reused for watering and in certain circumstances for flushing toilets or can be discharged to groundwater via an infiltration system. Subsidies are available for green roof initiatives, plus discounts on sewer charges if a certain amount of water retention can be demonstrated. Extensive roofs covered in a light layer of vegetation hold approximately 50% of rainwater, while intensive roofs can retain almost all precipitation.
- In order to collect and drain rainwater, it is accumulated on the roof. A control valve with a float located in a collection cylinder controls the suction process. Using hydrostatic pressure, the rainwater passes to a collection tank via a system of hoses and pipes.
Flat green roofs are multitalented
- Water stays in its natural cycle
- Lower sewer charges
- Improved microclimate in the surrounding area
- Heat insulation in winter
- Cooling effect on the indoor climate in summer
- Increased sound insulation
- Aesthetic design element
- Flood prevention
Using grey water
“Grey water” is relatively clean wastewater from activities such as hand washing or showering which can be treated for non-potable use or for use as process water. In office buildings it is derived mainly from washbasins and dishwashers. For companies with more than 150 employees, harvesting and reusing grey water makes financial sense by delivering considerable cost savings over time.
Grey water is generally treated using a membrane biofilter. Although the resulting water is not suitable for drinking, it is hygienically clean and can be used for watering landscaped areas, cleaning purposes and flushing toilets. The advantage over rainwater is that treated process water is available at all times. Ideally, grey water and rainwater systems should be combined, topping up with water from the mains supply as required.
