Research on the microbial effects of ‘thermal energy from drinking water’ on water quality

In the Climate Agreement the Dutch government made commitments on the reduction of the emission of CO₂. These commitments are being carried out in the energy transition and, in view of the transition’s urgency, every available sustainable source of heating and cooling capacity needs to be considered. The drinking water utilities are therefore also being increasingly approached and asked whether they want to offer heating and cooling capacity from their water infrastructure: ‘Thermal Energy from Drinking Water’ (TED). This explains the importance of knowing more about the effects TED has on drinking water quality.

The container with the test installation, consisting of five flow cells, to study the effects of TED.

Aquathermia or TED is not a new technology. It has already been in use in the Netherlands for about 18 years, including for collective heat supply, as in the case of the EVA-Lanxmeer district in Culemborg. When TED is used for heat recovery, the drinking water cools down. Depending on the situation, TED can also be a source of sustainable cold, in which case the drinking water warms up. An example of this is the existing TED system at Sanquin in Amsterdam (Waternet). Here, during the winter season cold is ‘extracted’ and stored in a geothermal energy system and then used for cooling in the summer. Our current knowledge is not adequate to determine the temperature to which drinking water can be warmed up in TED applications without endangering drinking water safety.

Existing research gives limited picture

Earlier research showed that TED can be safely implemented – thus without risks to drinking water safety – when certain conditions are met, such as the use of certified materials, application of double-walled heat exchangers, and operation up to the relatively low temperature of 15 °C. In the ‘Elaboration of heat-cold recovery from drinking water’  (WKD) project, within the Joint Research Programme with the water utilities in 2013-2017, KWR carried out microbiological research into the effects of TED. TU Delft and Waternet also researched TED and published their findings in 2020. But none of this research offers a full answer to these two questions:

1. What is the temperature limit to which the drinking water can be warmed up in the winter for the recovery of cold capacity?
2. Can a TED heat exchanger be safely installed for heat recovery at a location where the drinking water in the pipe network in the summer becomes relatively warm?

Unique test installation

In the Aquathermia theme within the WarmingUP research collective, KWR is therefore now investigating the effects of TED. Drinking water utility PWN has made a site available for a test installation. Drinking water is taken at the site from a PWN transport main and collected in a separate system via a ‘break tank’, so that there is no risk of an exchange of substances with the PWN drinking water. The test installation offers the possibility of studying the conditions (i.e., the temperature of the water or of the heat exchanger) under which microorganisms can develop in the biofilm on the RVS and EPDM in a heat exchanger.

In the test installation the water is run through ‘flow cells’. These cells replicate conditions that also apply in the heat exchanger. The incoming and outgoing temperatures of the pilot water can be adjusted to test different temperature scenarios in practice. The installation also allows for the testing of different materials.

One of the five ‘flow cells’ in which water flows and is warmed up. Small sheets of RVS and EPDM are placed in the flow cell to study biofilm formation.

Applying results

The researchers want to learn more about what is and what is not possible with TED from the results of the research, which are expected over the course of 2022. They want to be in a position to make recommendations about how TED can best be implemented to contribute to the energy transition without impairing the quality of drinking water. The test installation at PWN now simulates a situation with cold recovery in the winter, and is set up in such a way that it is also expected to reveal the extent to which the application of TED at locations with (overly) warm drinking water in the summer could lead to undesirable growth of microorganisms in the heat exchanger.