Taking the heat out

Wastewater is now the largest source of heat leakage in new buildings that are well insulated, but the heat can be recycled, says Frank Rogalla of Aqualia

It takes about 0.5 to 1kWh/m3 for water to reach a home - mostly for pumping - and a similar amount to collect, treat and return it to the environment. For an average consumption per capita of 150l/d, the water energy footprint is thus estimated at 80kWH/ pe/yr. But much of the water that enters a home is then heated, from around 10°C to 50°C for dishwashing, laundry and showering. This rise in temperature requires 47kWh/m3, and if applied to 100l/d per person, the energy footprint for water heating would be 1.7MWh/pe/yr, or 20 times higher than water supply and treatment - not counting the heat losses in many water heating systems.

But not all that energy should be lost - when wastewater is emitted from buildings, its average temperature is 15oC in the sewer on an annual basis, varying between 20-25°C in the summer and 10-15°C in the winter.

By extracting this residual energy using heat exchangers installed in the sewers, air and water could be warmed all year round, and even chilled water for air conditioning could be produced. Energy recovery from wastewater is thus very environmentally friendly, as the source of heat is permanently available and renewable, at a high temperature level, that can be exploited in the very place where it originates - with the aid of heat exchangers installed locally.

Over the last decades, enormous progress has been made worldwide in the insulation of buildings, with the demand for thermal energy for new buildings now lower by around 10-30% compared with 1980s figures. Nevertheless, for a new building, around 15-30% of the thermal energy provided to the building is lost, unused, via the sewage system.

This makes wastewater the largest source of heat leakage in buildings. According to estimates in Switzerland, approximately 6000GWh of thermal energy are lost per annum in via the sewage system, corresponding to around 7% of the total demand for thermal energy for space heating and hot water. The heat contained in wastewater is one of the last major untapped resources in the field of renewable energies. The potential of the total energy stored in the world's sewers could exceed the sum of the solar, wood and biomass energy potential.

The 5 million m3 of wastewater produced daily in Switzerland, if cooled by only 1°C, would be the equivalent of 300MWh, or the energy needed for 200,000 one-family homes. But only a fraction of the potential can be used in practice, estimated at around 10% of all buildings.

Heat recovery
Sewer or wastewater heat recovery can provide space or hot water heating to nearby residential and commercial buildings, or for industrial facilities and processes. The heat recovery can be done at the building scale, for example, GFX heat exchangers; from sewer mains such as the Swiss Rabtherm energy system and Uhrig from Germany; or from sewer pump stations or treatment plants, as achieved in Oslo, Tokyo, and Vancouver applications.

Individual solutions
The first example of such systems date back more than 30 years - for instance at the Touring Club Switzerland (TCS) in Emmen near Luzern, a wastewater heat pump has been extracting energy from the nearby sewage treatment plant to heat the office buildings since 1979. Currently more than 50 larger systems exist in Switzerland, and smaller in-house installations are estimated at around 200. In-house heat recovery from wastewater is practised in industry, swimming pools, gymnasiums, hospitals and residential buildings. The use of wastewater energy in such in-house plants is concentrated on the heating-up of water.

If quantities of wastewater are constant, such as in industrial processes, tube-bundle heat exchangers are operated in through-flow mode, whereas with non-constant wastewater flow, storage and filters systems with integrated spiral-tube heat exchangers are employed. While in-house solutions are mainly used in buildings with large quantities of wastewater, systems for single-family homes are also currently being developed. The heat pumps typically extract heat between two and four degrees, and converting it to 70°C. From 1m3 of water, or five bathtubs, about 2.3kWh can be generated.

For an economical use of the in-house solution, the minimal wastewater quantity is estimated between 8-10m3/d, so that the daily usage of about 60 people is sufficient, or 25 to 30 apartments. A home for the elderly in Glarus, Switzerland with 100 beds, has been equipped with a 30kW heat pump for wastewater heat recovery since 2004.

The heat exchanger is located in a buried, external collection pit to provide wastewater energy for the water heating and hot-water circulation. Despite heavy pollution in the wastewater, cleaning of the heat exchanger has not been necessary up to now.

Main sewer options
In sewer lines with a minimum flow of 10l/s, equivalent to about 5,000 people, a heat exchanger can be laid directly into the sewer pipe. These gutter-shaped elements are hydraulically connected in a parallel or series via an intermediate circuit to a heat pump. A precondition for this method of heat recovery is that the maximum length of medium plastic pipe that connects to the building is between 200m (built environment) and 300m (undeveloped). Not every site suits this system.

The heat exchanger elements are normally made from high-grade stainless-steel alloy, are hollow, sandwich-type elements measuring approximately 14mm thick. The newer elements have improved their heat conductivity and can now withdraw approximately 6-9kW/m2 of heat exchanger, about three times higher than the initial systems with only 2-3kW/m2 of sewer.

Because wastewater could foul the surfaces, and reduce heat transfer, Rabtherm, together with the Technical University of Zurich, developed an anti-fouling system that uses strips of copper between the sections of the exchanger elements, claiming to extend the service life of the heat exchanger to at least 50 years.

The most important precondition for the use of energy in raw wastewater is the approval of the operators of the sewers and the sewage treatment plant, and making sure that the operation of the collection system is in no way impaired. One possible risk is the cooling down of the raw wastewater, as the efficiency of biological sewage treatment (nitrification) is temperature dependent.

If the wastewater temperature falls too much, the performance of the treatment could be affected to the point of not meeting the consent values in the discharge. For this reason, every plant operator will impose limits on the amount of heat that can be withdrawn, and if wastewater temperatures are naturally already relatively low, or the treatment process is operating at its limit, the use of wastewater for heating purposes might not be possible.

In Switzerland, the biological treatment process is designed for 10°C, which is therefore the limit accepted at the end of the collection system. In addition, the total cooling should, in total, be not more than 0.5°C.

One example of an installation where wastewater provides about 70% of the total heating and cooling for the building is a medical center in Leverkusen, Germany, that employs about 300 people. At this site, the heat exchangers were set in concrete elements in the new sewer, which was completed in 2002. Operated since 2003, the system includes 120m of heat exchanger elements that extract 170kW of energy. This allows the production of 981MWh/y for heat and 545MWh/a for cooling, and reduces carbon dioxide emissions by 200 tons per year.

Total cost of the system was £416,000, of which in general the cost of the heat exchanger ranges only between 8 and 15% of the total system value, without installation factored in. The cost of operation depends on the type of heating system, but it is mainly the heat pumps that require some simple maintenance. In favourable cases, the return on investment can reach between five and six years. Several European governments offer tax incentives for the installation of sustainable energy sources, improving system economics.

Thermal uses
The energy potential of treated wastewater is much higher than that of raw wastewater, since downstream from the sewage treatment plant, the wastewater can be cooled down much more than upstream - by up to 80°C. Unfortunately, the limitation for using this large energy potential is distance, as many sewage treatment plants are located outside residential areas, or even away from larger industrial users so that no customers for the heat are available.

Ideally, the energy from wastewater can be used in the WwTP itself, e.g. for the heating the digesters or for low-temperature sludge drying. Both applications allow the use of wastewater energy at a temperature level that is interesting for heat pumps. There are, however, only few examples for the internal plant use of wastewater heat, since many facilities have large amounts of waste heat available from the use of biogas in combined heat and power units. In the future, larger WWTP could upgrade their biogas to meet natural gas quality standards and thus would be able to feed it into the public gas mains.

In Switzerland, there are 20 WwTP which use the heat of treated wastewater externally. At the main treatment pant of Zurich, Werdhölzli, a 1.5km pipe is connected to the a local energy system, providing the post distribution center of Muellingen with heat.

At completion, the project will supply yearly heat of 52,000 MWh and cooling power of 16,700 MWh - and the heat pump from wastewater will provide 60% of the heat and 92% of the cooling. Currently about half of this potential is already being operated.

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