Critical Analysis of the Current State of Knowledge in the Field of Waste Heat Recovery in Sewage Systems

Abstract

The need for efficient use of energy and sustainable energy management and the fact that large quantities of heat are deposited in the discharged sewage have contributed to the development of research on waste heat recovery. Gray water began to be seen not just as waste, but also as an alternative source of energy. Research related to the development, improvement, and finally, the popularization of waste energy recovery devices and systems has evolved rapidly over the last two decades. Initially, technologies for gray water reuse were not widely used, which was due to the low efficiency of the current heat exchangers and the significant investment outlays that would have to be covered by potential users. Research conducted by scientists from around the world has allowed us to eliminate construction flaws, improve efficiency, and also provide information on the selection of optimal waste heat recovery technology, depending on the installation conditions and operating parameters. The ability to correctly select the device allows for effective energy collection from gray water, which improves the investment profitability. This paper reviews the research regarding issues related to waste heat recovery from gray water in sewage installations and systems. A critical analysis of the current state of knowledge was carried out with a special consideration to the technologies intended for the residential buildings.

Horizontal Shower Heat Exchanger as an Effective Domestic Hot Water Heating Alternative

“…Research shows that if the recovered energy is used to preheat water in a residential building, the return on investment can be achieved after only two years [20]. However, much depends on the type of building [21], the tap fittings used [22], the method for recovering the heat deposited in the drain water [23], and the price of the primary energy source [24]. The amount of drain water used as a heat source is also important.…”

Waste Heat Recovery Technologies Revisited with Emphasis on New Solutions, Including Heat Pipes, and Case Studies

“…Furthermore, heat could potentially also be extracted from different types of wastewater. Indicative examples that can be found in the literature concern the exploitation of: (i) the wastewater ingress within an underground transport line [18]; (ii) wastewater from livestock and agricultural farms [19]; (iii) sewer wastewater [20]; (iv) wastewater from showers [21]; and other applications involving wastewater and heat-exchanging potential.…”

Assessment of the Feasibility of Implementing Shower Heat Exchangers in Residential Buildings Based on Users’ Energy Saving Preferences

“…These are devices whose task is to transfer the waste heat deposited in the shower drain water to the tap water flowing into the shower system [60]. Depending on the method of conducting installations and the availability of space for the installation of a heat recovery system, vertical and horizontal heat exchangers as well as devices installed under the floor shower drain can be used [61]. The water preheated in the heat exchanger can be led to the domestic hot water heater and/or directly to the shower mixing valve [62].…”

“…The research showed that only about 30% of the respondents have space in their dwellings to install a vertical heat exchanger, with the answer "definitely yes" indicated by only 23% of them (Q37). As is well known, it is these devices that are most effective and are most often used [61]. The use of devices with a different design results in a reduction of the amount of recovered heat.…”