Thermal Energy Recovery from Drinking Water Distribution Systems: A study into microbial water quality and potential energy

Drinking water distribution systems (DWDSs) are intended to supply hygienically safe and biostable water for human consumption. To supply aesthetically pleasant drinking water at the customers tap, water treatment and supply requires energy for production and distribution purposes (e.g. overall between 0.47 kWh/m³ in the Netherlands). On the other hand, DWDSs also contain thermal energy as a surplus of cold or heat. Depending on the drinking water temperature within the distribution network, thermal energy can either be used for heating or cooling purposes. Thermal energy recovery potential from drinking water has been explored recently. Cold thermal energy recovery from drinking water (TED) can provide cooling for buildings and spaces with high cooling requirements as an alternative for traditional cooling and thus TED helps reduce in greenhouse gas (GHG) emissions.
The effects of increased water temperature induced by TED on the drinking water quality and biofilm development within DWDSs are not yet known. Hence this thesis was initiated with the objective to investigate the effects of TED on microbial water quality and biofilm development within DWDSs. The first part of this thesis investigated the impacts of TED at 25 ^oC on microbiological drinking water quality, using pilot distribution systems. The first study revealed that the water temperature increased to 25°C in a pilot distribution system as a result of cold recovery does not affect the bacterial water quality in the drinking water phase. However, it does affect the concentration and community composition of biofilms (Chapter 2). Hence, in the second part of this thesis, the effect of TED on biofilm was investigated extensively. In pilot scale distribution systems, both water and biofilm phases were studied with water temperatures increased to 25 ^oC and 30 ^oC after TED. It was concluded that the timeline for biofilm microbial development was influenced by temperature: the higher the temperature, the faster the microbial development of a biofilm took place. Simultaneously, higher biomass activity (ATP and cell concentration) was also observed in the water phase. In the biofilm phase, the initial faster microbial development did not lead to differences in microbial diversity and composition at the end of the experimental period (Chapter 3).
Similarly, biofilm development after TED at 25 ^oC followed for a long period of time, 99 weeks, showed that instantaneous increase in water temperature influenced the early stages of biofilm development. High temperature initiates faster growth of primary colonizers (Betaproteobacteriales, Sphingomonadaceae) (Chapter 4). Both studies univocally showed that as a result of constantly stable increased water temperature after TED, biofilms reached to a steady phase faster when compared to fluctuating drinking water temperatures in reference and control systems (Chapter 3 and 4).
After studying the microbial water quality in unchlorinated drinking water distribution systems for both water and biofilm phases, initial investigation of TED application within chlorinated networks was also performed. Compared with unchlorinated DWDSs, here chlorine dramatically reduced the biofilm biomass growth, and raised the relative abundances of the chlorine-resistant genera (i.e. Pseudomonas and Sphingomonas) in bacterial communities. As a result of TED, no significant effects were observed on chlorine decay, microbial water quality and biofilm composition during the experimental period (Chapter 5).
After extensively studying the changes in the microbial drinking water quality as a result of TED, the last part of this thesis was carried out to determine what raising the maximum temperature limit (T_max) after recovery of cold would entail in terms of energy savings, GHG emission reduction and water temperature dynamics during water transport. A full-scale TED system was used as a benchmark, where T_max is currently set at 15 °C. By raising T_max to 20, 25 and 30 °C, the retrievable cooling energy and GHG emission reduction could be increased by 250, 425 and 600%, respectively. The drinking water temperature model predicted that within a distance of 4 km after TED, water temperature resembles that of the surrounding subsurface soil. Hence, a higher T_max will substantially increase the TED potential of DWDSs while keeping the same comfort level at the customer’s tap (Chapter 6).
All of these observations indicate that increasing T_max up to 25-30 °C in TED can be safe in terms of microbiological drinking water quality. However, this is specifically the case for unchlorinated DWDSs with microbiologically stable water (AOC <10 ug C/L). More insight is required in terms of microbiological assessment of TED to further explore the potential within chlorinated systems. Further research on the effects of cold recovery on DWDSs already in operation is highly recommended. In order to get better insight on response of already developed biofilm towards increase in temperature after TED. Moreover, specific opportunistic pathogens that are sensitive to temperature increase, should be investigated thoroughly in order to provide hygienically safe water after recovery of cold from both chlorinated and unchlorinated drinking water distribution systems.