Abstract
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.
Highlights
Drinking water supply requires raw water abstraction and treatment, followed by storage, transport and distribution of finished water
The main objective of this study is to investigate the effects of temperature increase induced by cold recovery on 1) biomass within drinking water transport and distribution systems (DWDSs), quantified as adenosine triphosphate (ATP) and total cell count (TCC); 2) microbial community composition and diversity, profiled by illumina sequencing and 3) occurrence of selected opportunistic pathogens within DWDSs
Both ATP and cell counts showed comparable values among the distribution systems (DSs) (2–8 ng/l ATP, 2.0–6.5 × 105 cells/ml), indicating that the introduction of heat exchanger (HE) and increase in temperature has a minor influence on the concentration of planktonic bacteria and their activity in water
Summary
Drinking water supply requires raw water abstraction and treatment, followed by storage, transport and distribution of finished water. Drinking water supply requires energy for production and distribution purposes (e.g. overall between 4 and 13 MJ/m3 in the Netherlands) (Gerbens-Leenes, 2016). In the Netherlands, 1160 million m3 of drinking water is distributed annually by 120,000 km long DWDSs (Frijns et al, 2013; Hofman et al, 2011; Liu et al, 2017b). The temperature within these DWDSs remains in general below 10 °C during winter which offers a potential for cold recovery, and above 15 °C during summer which offers a potential for heat recovery
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