Hemodialysis wastewater is discharged to a sewer at a temperature ranging from 20 to 25 °C, which means that it maintains considerable thermal energy [5]. We have previously estimated that up to 1600 GWh per year of thermal energy is lost in dialysis units worldwide. In other words, the discharged energy could heat approximately 140,000 homes in a European country for one year, with a potential annual cost savings of 118 million euros [5].

There are two possible ways to recover heat from hemodialysis wastewater. The first is to use a heat exchanger installed in the pipe bed of the hemodialysis sewage network. The heat exchanger captures thermal energy from the wastewater, and a heat pump is used to transfer this energy to a centralized heating system. The second option is to install an external heat exchanger above ground level. The wastewater is pumped through the above-ground heat exchanger, and then returned to the sewer [15, 16] (Fig. 2).

Thermal energy recovery: A outside a sewer and B inside a sewer [16]

Which option should be chosen depends on a range of factors including existing infrastructure, costs, and the needs of the hemodialysis facility. A feasibility study can help to identify the most suitable solution in a given setting. Additionally, local regulations should be taken into account when implementing such systems [15].

Hydro-electric energy can be recovered from water flowing in reverse osmosis sewer pipes. Hydropower is a clean and renewable source of energy. It provides inexpensive electricity and produces no pollution [6]. In a previous study, we equipped our double-stage reverse osmosis system (with a flow rate of 2000 L/h, a reject flow rate of 750 L/h in the first pass and a flow rate of 1600 L/h with a reject flow rate of 600 L/h in the second pass and an operating pressure of 4 bars) with a micro hydro turbine operating on reject osmosis water pressure (Fig. 3).

System designed for hydropower generation from hemodialysis reject RO water [17]

Our device was able to generate up to 1.6 kWh of electricity per day and 487 kWh per year. This energy was used to power a 100 W light bulb or a LED TV on a daily basis. More interestingly, we were able to eliminate up to 300 kg of CO2-eq emissions per year, the equivalent of the CO2 emissions generated in a year by one home in an African country [17].