WHAT IS SEWAGE THERMAL ENERGY USE?

WHAT IS STEU?

SEWAGE THERMAL ENERGY USE (STEU)

Sewage Thermal Energy Utilization (STEU) represents a transformative approach to energy efficiency and sustainability. The term STEU was coined in the 2019 report by the American Water Works Association - Research Foundation (AWWA-RF). The concept goes beyond the traditional view of sewage as waste heat, recognizing its dual capability as a source for productive heating and cooling. STEU is a paradigm shift in acknowledging the untapped potential of thermal energy in sewage systems – energy that has been routinely disregarded in the past.

A Practical Approach to Energy Recovery

Innovative research conducted by the AWWA-RF in 2019, is built on a 2015 project by the Water Environment Research Foundation (WERF). Together these studies offer insights on the potential use of thermal energy from sewage. The core research sought to answer whether enough energy could be extracted from incoming wastewater (sewage) to power a facility's operations. Earlier academic studies had identified that sewage indeed contained sufficient energy for this purpose. However, the practical challenge was in harnessing this energy in a usable form.

The research points to this simple idea: the temperature of sewage is stable throughout the year. Sewage is warmer than the air in the winter, and cooler than the air in the summer. The graph below illustrates this - the blue line is ambient air temperature while the green line is the sewer temperature. Using the stable thermal energy in sewage we can increase efficiency and promote sustainability. Sewage “waste heat” is actually a sustainable and renewable energy resource. All we have to do to tap this resource is change our perspective and build the infrastructure.

sewage temp vs air temp

Potential Use of Thermal Energy from Sewage

Sewage is 99% water. Water has the highest heat capacity of any substance on earth, making it ideal for heat transfer. The combination of its specific heat and mass means that water's capacity for heat transfer is about 4,000 times greater than that of air. This fact brings a new perspective to the efficiency of heat pumps. While air-sourced heat pumps are highly efficient for heating and cooling in buildings, water-sourced heat pumps are even more effective due to water's superior heat transfer properties. This advantage is particularly relevant when considering sewage as the heat source. A system using sewage as a source continually renews its thermal capacity with every gallon of water that flows through it, offering a more sustainable and efficient solution.

Where Does The Heat Come From? 

In the U.S., about 18% of the energy in our homes is used to heat water, according to the Energy Information Administration. Houses that utilize a hot water heating system and appliances like dishwashers and washing machines also warm up the water. This hot water, after use, goes straight into the sewer system. Additionally, 20% to 40% of the heat used in businesses and factories ends up in sewers. So, we're essentially using energy to heat water and then sending it down the drain. We'd be making better use of the energy if we could recycle this heat.

Home Energy Consumption by End Uses
Home Energy Consumption by End Uses

A research study that analyzed energy usage in homes with different levels of energy efficiency, regardless of a home's energy efficiency determined that the energy used to heat water stays nearly the same (see below). This means that as a home becomes more energy efficient overall, the percentage of energy going into wastewater as heat actually increases. In essence, as wastewater exits these homes, it carries away all this heat energy with it.

Energy & Residential Building Type  -  Wastewater Composition & Building Type
Energy Usage & Wastewater Composition

Wastewater Networks Can Serve a Dual Purpose

Wastewater networks are not just for collecting dirty water, but also for gathering waste heat. Every wastewater network is a potential goldmine of low carbon heat energy. However, this source of thermal energy often goes unnoticed and underutilized, simply being flushed away. What makes this energy particularly valuable is its reliability, predictability, and stability. Moreover, it's conveniently located in urban areas – exactly where heating energy is most needed for buildings. The best part? Much of the necessary infrastructure, in the form of sewer networks, is already in place.

The term STEU was coined in the 2019 report. It was intended to show that the heat energy in sewage was both usable for heating and cooling. Thermal Energy is more inclusive of the concepts than just "heat". It also incorporated the term "use" to imply its usefulness not just its existence.

Dedicated to Advancing the Understanding & Application of STEU

The Sewage Thermal Energy Network (STEN) champions this innovative approach. STEN connects a diverse group of professionals – including engineers, scientists, policy experts, and civic leaders – who are dedicated to advancing the utilization of sewage thermal energy. As the leading network in this nascent field, STEN fosters collaboration and knowledge sharing to drive the adoption of STEU practices, contributing significantly to sustainable urban development and environmental stewardship. We are the premiere professional network for practitioners and policy advocates who see the potential for sewer thermal energy recycling. We facilitate collaboration between developers, municipalities, and academic researchers, fostering a network that supports public education and leads progress in this area.

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