Wasted water: a source of energy for buildings?

How to take advantage from wasted heat in sewers to heat buildings?

In building science, one of the problematic focus on energy cuts. To achieve a decrease in energy expenditures, researches are made to find new ways providing energy at less expensive cost. A new source of energy is the heat coming from wasted water. Actually, water from sewers are a great source of energy since its temperature is from 11 to 20°C (55.1 to 68 °F) during a year.

Now, you are wondering how it is possible to gain energy and to cut energy expenditures thanks to that high temperature.

Using a heat pump into water gives the ability to extract calories from wasted water. Water that we reject from our laundries, showers, and sinks are at a great level of temperature in the pipes each season of the year. In order to reach the needed temperature for heating buildings, water have to be heated at least at 50°C (122 °F). Then, an interchange tool transfers the created heat into a coolant. This last supplies the heat pump. This one will have to increase the temperature to 50°C (122 °F) to produce enough for domestic heat. You can finally enjoy the heating water. This kind of pump is called hydrothermal pump. One of the drawbacks is that its efficiency relies on the proximity with source water. Nevertheless, in cities, we are not facing such distance issues since wasted water pipes are near buildings in order to minimize costs and energy losses. Besides, the advantage of such heat pump is the reversibility of the device. Actually, the pump can be used to heat buildings in winter but also to cool them in summer. There are only two conditions to install such system : a minimal rate of flow of 15 L/s and diameter of the pipes at least of 1 meter.

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To conclude, is such a system able to provide ecological and economical heat in buildings?

Firstly, since you save energy, you spend less money. This is possible in urban areas as there is a great concentration of buildings that produce continuously wasted water. It has been proved that this system will make around 25% energy-saving. But, the installation of this system is still expensive. Plus, it isn’t possible to respond to high peak demands of hot water with it. That is why this technology is more adapted to buildings with no peak in the water consumption. Another drawback of this system deals with an increase of the energy needed to treat the used water after heat has been extracted. Actually, treatment plants take advantage of this heat to sanitize wasted water. It means that they will have more difficulties in the treatment process. So we can wonder if this system is really efficient.

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