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.
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Door te verwarmen en te koelen met drinkwater
hoeven we minder aardgas te verstoken en kunnen we
klimaatverandering beperken. Maar is dat drinkwater
daarna nog wel goed voor onze gezondheid? Bij de TU
Delft en Waternet zochten ze het uit. ... Read more

Drinking water distribution systems (DWDSs) have been thoroughly studied, but the concept of thermal energy recovery from DWDSs is very new and has been conceptualized in the past few years. Cold recovery results in a temperature increase of the drinking water. Its effects on drinking water quality and biofilm development are unclear. Hence, we studied both bulk water and biofilm phases for 232 days in two parallel pilot scale distribution systems with two temperature settings after cold recovery, 25 °C and 30 °C, and compared these with a reference pilot system without cold recovery. In all three pilot distributions systems (DSs) our results showed an initial increase in biomass (ATP) in the biofilm phase, along with occurrence of primary colonizers (Betaproteobacteriales) and subsequently a decrease in biomass and an increasing relative abundance of other microbial groups (amoeba resisting groups; Xanthobacteraceae, Legionellales), including those responsible for EPS formation in biofilms (Sphingomonadaceae). The timeline for biofilm microbial development was different for the three pilot DSs: the higher the temperature, the faster the development took place. With respect to the water phase within the three pilot DSs, major microbial contributions came from the feed water (17–100%) and unkown sources (2–80%). Random contributions of biofilm (0–70%) were seen between day 7–77. During this time period six-fold higher ATP concentration (7–11 ng/l) and two-fold higher numbers of high nucleic acid cells (5.20–5.80 × 104 cells/ml) were also observed in the effluent water from all three pilot DSs, compared to the feed water. At the end of the experimental period the microbial composition of effluent water from three pilot DSs revealed no differences, except the presence of a biofilm related microbial group (Sphingomonadaceae), within all three DSs compared to the feed water. In the biofilm phase higher temperatures initiated the growth of primary colonizing bacteria but this did not lead to differences in microbial diversity and composition at the end of the experimental period. Hence, we propose that the microbiological water quality of DWDSs with cold recovery should be monitored more frequently during the first 2–3 months of operation. ... Read more

Drinking water distribution networks (DWDNs) have a huge potential for cold thermal energy recovery (TED). TED can provide cooling for buildings and spaces with high cooling requirements as an alternative for traditional cooling, reduce usage of electricity or fossil fuel, and thus TED helps reduce greenhouse gas (GHG) emissions. There is no research on the environmental assessment of TED systems, and no standards are available for the maximum temperature limit (Tmax) after recovery of cold. During cold recovery, the water temperature increases, and water at the customer’s tap may be warmer as a result. Previous research showed that increasing Tmax up to 30 °C is safe in terms of microbiological risks. The present research was carried out to determine what raising Tmax would entail in terms of energy savings, GHG emission reduction and water temperature dynamics during transport. For this purpose, a full-scale TED system in Amsterdam was used as a benchmark, where Tmax is currently set at 15 °C. Tmax was theoretically set at 20, 25 and 30 °C to calculate energy savings and CO2 emission reduction and for water temperature modeling during transport after cold recovery. Results showed that by raising Tmax from the current 15 °C 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 Tmax will substantially increase the TED potential of DWDN while keeping the same comfort level at the customer’s tap. ... Read more

Drinking water distribution systems (DWDSs) are used to supply hygienically safe and biologically stable water for human consumption. The potential of thermal energy recovery from drinking water has been explored recently to provide cooling for buildings. Yet, the effects of increased water temperature induced by this “cold recovery” on the water quality in DWDSs are not known. The objective of this study was to investigate the impact of cold recovery from DWDSs on the microbiological quality of drinking water. For this purpose, three pilot distribution systems were operated in parallel for 38 weeks. System 1 has an operational heat exchanger, mimicking the cold recovery system by maintaining the water temperature at 25 °C; system 2 operated with a non-operational heat exchanger and system 3 run without heat exchanger. The results showed no significant effects on drinking water quality: cell numbers and ATP concentrations remained around 3.5×105 cells/ml and 4 ng ATP/l, comparable observed operational taxonomic units (OTUs) (~470–490) and similar Shannon indices (7.7–8.9). In the system with cold recovery, a higher relative abundance of Pseudomonas spp. and Chryseobacterium spp. was observed in the drinking water microbial community, but only when the cold recovery induced temperature difference (ΔT) was higher than 9 °C. In the 38 weeks’ old biofilm, higher ATP concentration (475 vs. 89 pg/cm2), lower diversity (observed OTUs: 88 vs. ≥200) and a different bacterial community composition (e.g. higher relative abundance of Novosphingobium spp.) were detected, which did not influence water quality. No impacts were observed for the selected opportunisitic pathogens after introducing cold recovery. It is concluded that cold recovery does not affect bacterial water quality. Further investigation for a longer period is commended to understand the dynamic responses of biofilm to the increased temperature caused by cold recovery. ... Read more

Thermal energy recovery from drinking water has a high potential in the application of sustainable building and industrial cooling. However, drinking water and biofilm microbial qualities should be concerned because the elevated water temperature after cold recovery may influence the microbial activities in water and biofilm phases in drinking water distribution systems (DWDSs). In this study, the effect of cold recovery on microbial qualities was investigated in a chlorinated DWDS. The chlorine decay was slight (1.1%–15.5%) due to a short contact time (~60 s) and was not affected by the cold recovery (p > 0.05). The concentrations of cellular ATP and intact cell numbers in the bulk water were partially inactivated by the residual chlorine, with the removal rates of 10.1%–16.2% and 22.4%–29.4%, respectively. The chlorine inactivation was probably promoted by heat exchangers but was not further enhanced by higher temperatures. The higher water temperature (25 °C) enhanced the growth of biofilm biomass on pipelines. Principle coordination analysis (PCoA) showed that the biofilms on the stainless steel plates of HEs and the plastic pipe inner surfaces had totally different community compositions. Elevated temperatures favored the growth of Pseudomonas spp. and Legionella spp. in the biofilm after cold recovery. The community functional predictions revealed more abundances of five human diseases (e.g. Staphylococcis aureus infection) and beta-lactam resistance pathways in the biofilms at higher temperature. Compared with a previous study with a non-chlorinated DWDS, chlorine dramatically reduced the biofilm biomass growth but raised the relative abundances of the chlorine-resistant genera (i.e. Pseudomonas and Sphingomonas) in bacterial communities. ... Read more

Greenhouse gas (GHG) emissions contribute to climate change. The public water utility of Amsterdam wants to operate climate neutrally in 2020 to reduce its GHG emissions. Energy recovery from the water cycle has a large potential to contribute to this goal: the recovered energy is an alternative for fossil fuel and thus contributes to the reduction of GHG emissions. One of the options concerns thermal energy recovery from drinking water. In Amsterdam, drinking water is produced from surface water, resulting in high drinking water temperatures in summer and low drinking water temperatures in winter. This makes it possible to apply both cold recovery and heat recovery from drinking water. For a specific case, the effects of cold recovery from drinking water were analyzed on three decisive criteria: the effect on the GHG emissions, the financial implications, and the effect on the microbiological drinking water quality. It is shown that cold recovery from drinking water results in a 90% reduction of GHG emissions, and that it has a positive financial business case: Total Cost of Ownership reduced with 17%. The microbial drinking water quality is not affected, but biofilm formation in the drinking water pipes increased after cold recovery. ... Read more