Effluent heat recovery systems turn unused wastewater into commodity

The U.N.’s Intergovernmental Panel on Climate Change recently released a report detailing the global impact of a 2.7-degree-Fahrenheit rise in temperatures above preindustrial levels, which could occur as early as 2030. The report is extensive, covering the effects on humankind and the wider natural world, prospective pathways to limiting temperature rise and global steps that encourage sustainable development. With the residential and commercial sectors consuming approximately 39 percent of total U.S. energy use last year, confronting this challenge lies squarely in front of owners and engineers.

Developing designs that take advantage of every energy-efficient opportunity requires a broad view of available and developing technologies. One technology that has not been widely adapted, but has the potential to provide unique energy efficiency solutions, is effluent heat recovery. Effluent heat recovery systems, also referred to as wastewater or sewer water heat recovery systems, use effluent leaving the building as an energy source. The U.S. Department of Energy estimates that the equivalent of 350 billion kilowatt-hours worth of hot water is sent down the drain each year. This waste represents a vast untapped source of energy.

These heat recovery systems draw effluent back into the building. The solids are separated and returned while the remaining fluid is filtered and passed through a heat exchanger, where it serves as a heat source, or sink, for a heat pump system. The temperature of the effluent allows the heat pump cycle to operate in either direction, providing heating or cooling as required.

The amount of effluent leaving a building does not typically meet the volume requirements of a heat recovery system. There are two options to address this deficiency: install an underground storage tank that provides enough volume for intermittent use or connect the building to a sewer main or water treatment plant.

Connecting to a sewer main allows for a wide range of scalability, because the effluent flows available upstream of the connection could be used at downstream locations. If an owner has a large group of buildings served by a campus sewer main, the combined effluent flows may be used to serve the needs of several buildings without drawing additional effluent beyond what is created. When additional flow is required, cities and other municipalities controlling wastewater conveyance and treatment must consent to allow additional effluent to be drawn away from the mains. This logistical hurdle is the largest challenge to the implementation of these systems. Often, authorities that have jurisdiction do not have policies regarding the use of effluent in heat recovery applications. Bringing all parties together very early in project development to discuss designs is crucial to avoid late pitfalls.

With the demand for unique and encompassing energy efficient designs on the rise, using previously underexploited energy sources, like effluent flows, will become the rule rather than the outlier.

For more information about effluent heat recovery systems, contact Joe Fehrenbacher, P.E., at jfehrenbacher@hanson-inc.com.

Posted on October 23, 2018