FAQs

Everyday, in every town and city, heat energy is discharged into wastewater pipes from houses, commercial buildings and industries. This thermal energy is all collected together within the wastewater network and it flows through the city within sewer pipes buried in the ground.

The amounts of energy vary from country to country and city to city, and are significant. Some examples are provided below:

• New Zealand - 30% of all energy used in houses is converted to hot water which ends up in the sewer network. 40% of all low grade heat energy (i.e. Temperatures <100degC) used within Commercial and Industrial sectors also ends up in the sewer network.

• UK - 20% of all energy used in houses is converted to hot water which ends up in the sewer network.

• China - 27% of all energy used in houses is converted to hot water which ends up in the sewer network.

When all of this energy is collected together (within the sewer network), it represents an enormous total! However because this energy is invisible, currently it is typically ignored and flushed away to be discharged into our rivers & oceans. For a low carbon sustainable future, wastewater should be identified as a large, reliable, low carbon and untapped supply of thermal energy in every town and city. This energy is contained within wastewater infrastructure which already exists and the energy is located within the urban environment where it can be used. It's available for capturing and recycling the heat energy. This concept also utilizes wastewater infrastructure for a second, and higher value purpose than just collecting and transporting used water out of the urban environment. It uses wastewater infrastructure as an energy source and energy recycling fits with the concept of a circular economy.

One of the transitions required to allow city's to decarbonise, is electrification of heating to transition off fossil fuel use. Electrification of low grade heating (less than about 80degC) can be done efficiently using heatpumps.

Wastewater provides an excellent thermal source for heatpumps, allowing wastewater source heatpumps to generate heat up to 2x more efficiently than air source heatpumps (depending upon the location and climate).

That means wastewater source heatpumps could provide the heating required for a building, district heating network or industrial purpose whilst consuming only around 1/2 of the electricity that an air source heatpump would require.

This is one of the primary benefits of wastewater heat recycling! Allowing a city to transition off fossil fuels, by using infrastructure which the city already has available (the sewer pipes), using resources the city already has (wastewater) and doing it while using about half of the electricity as air source heatpumps (thus helping manage the loads on electrical networks as a city electrifies).

Can this heat energy within wastewater be sold?

We know of at least three examples around the world where the water utility is generating income from allowing others to tap their wastewater heat. Whilst commercial details cannot be made public, this does show that there can be a business case made. Looking to the future, as the price of carbon emissions is anticipated to increase globally, the value of the zero carbon thermal energy running within wastewater networks will also increase accordingly. Wastewater heat will likely become more valuable.

There are other benefits that can also come from wastewater heat recycling:

Improved air quality: By electrifying heating systems that previously used fossil fuels, the associated air pollution from burning the fossil fuels is removed. Step by step as more fossil fuel boilers are replaced, this will improve the air quality within the city.

When wastewater is used as a cooling source -

• Reducing Urban Heat Island: Typically air conditioning systems discharge heat into the air within cities. When many many air conditioning systems are all doing this, it adds to the urban heat island issue affecting more and more cities. By using wastewater as the means to remove heat instead of discharging it into the air, this helps to reduce the urban heat island problem.

• Saving Water: Wet cooling towers use a lot of water! By using wastewater as your cooling source instead of wet cooling towers, you can save precious water as well as reduce the maintenance costs of wet cooling towers.

When wastewater is used for heating and/or cooling in Seismic regions -

• Reduce Seismic Risks: For cities in seismic regions, using wastewater as a heating and/or cooling source might allow you to remove heavy heat exchangers or cooling towers from the roof of your building. This reduces the weight of equipment on the roof of your building and reduces the seismic loads that the building must cope with in an earthquake.
  

Every city has this energy resource flowing in its wastewater networks. Some opinions say that this resource should be considered a 'public good' and the city has a responsibility to make it available at no or low cost for energy recycling. Other opinions are that the city has a valuable resource that they should maximize the value of, by selling the energy. Whatever side of the spectrum your city sits on, the energy is of little use if it is just flushed away. To allow connections to be made between supplier (the city wastewater utility) and customers (building owners etc) the energy must be made visible via a wastewater heat map.

The concept is actually quite simple.

Pass the wastewater through an external heat exchanger which is designed to work with wastewater, or pass it over heat exchanger units installed within the wastewater pipes. Extract heat out of the wastewater and the wastewater goes back into the wastewater network and continues on it's way through the sewers.

The wastewater heat is used to efficiently drive a heatpump, allowing higher temperature water to be achieved to provide heating whilst using very low amounts of electricity in the heatpump.

Generally, wastewater heat recycling is suited to projects with some scale such as larger commercial buildings, industrial processes and district heating systems for precincts, campuses or multiple houses.

Capital Costs

The capital costs for a wastewater energy installation are very project specific and cannot be estimated on a ‘typical’ basis. Apart from the system design and choice of equipment, often a large component of the capital costs can be the civil works associated with getting access to the wastewater, which may require the installation of a pump chamber. Additionally there is also provision of plant space and/or reticulating pipework to/from the building.

The following generalizations can be made however:

• Installations into new buildings are more cost effective than retrofitting into existing buildings.

• A large installation is more cost effective than a small installation. E.g. a large building such as a hospital, aquatic centre or large retail mall is normally more financially feasible than a small office building. Equally a single wastewater thermal energy centre which provides heating/cooling via a small scale district energy system to a number of buildings (in a city block or a campus) is normally more financially feasible than each building providing their own small wastewater heat recycling installation.

• Installations with long utilization periods are more feasible than projects with short utilization. For example, buildings such as hospitals, aquatic centres or some industrial facilities have a demand for heating and/or cooling all year around and so the thermal system is fully utilized for 12 months. This makes a much better business case than a building which requires heating and/or cooling only for only a few months of the year.

• If a city is planning to renew existing wastewater pipes or install new wastewater networks, that is the time to plan ahead and install access points into the wastewater network to allow the energy within the wastewater to be recycled. Perhaps the energy might not be used for some years, but if the access is already easy it is more likely to happen and it is significantly more cost effective to install access points while the wastewater network is anyway being constructed than coming back later and retrofitting access points.

Maintenance Costs

The routine maintenance needed for the specialised wastewater energy equipment varies from one equipment supplier to the next, but in general terms the amount of maintenance needed is similar to other normal commercial building HVAC plant or wastewater equipment.

Typically commercial HVAC equipment should all have a visual check weekly, a minor service every 3 or 6 months (depending on equipment supplier) and a 12 monthly annual service. It is the same for wastewater energy equipment.

Independent Cost Comparison Research

The chart below demonstrates the results of research done in Germany on investment costs for large-scale heat pump projects.

The cost of wastewater source heat pump projects are shown as being in the lowest three cost options. The bars indicate the total unit cost of investment needed to build the system, with six components of the total cost indicated by the different colours. Although the cost of wastewater heat and air source heat pump projects are similar, the efficiency of wastewater heat pumps are far superior, making them less expensive to operate. So, in this research example, it is clear that wastewater source heat pump projects are extremely good value.

Yes, wastewater can be used for cooling as well as heating.

Yes, we offer training and capacity building programmes for city authorities. Learn more.

We also have a low-cost online education course called 'Introduction to Using Wastewater for Heating/Cooling'. It is not deeply technical, and covers different aspects to present a rounded perspective for all. Discover in eight lessons how wastewater can efficiently heat and cool buildings, reduce fossil fuel reliance, and cut emissions—all in just one hour. Register Online Here.