Contents lists available at ScienceDirect Environmental Research journal homepage: www.elsevier.com/locate/envres Thermal energy recovery from chlorinated drinking water distribution systems: Effect on chlorine and microbial water and biofilm characteristics Xinyan Zhou a,b , Jawairia Imtiaz Ahmad b,c , Jan Peter van der Hoek b,d , Kejia Zhang a,* a College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, 310058, Zhejiang, China b Sanitary Engineering, Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, P.O. Box 5048, 2600GA, Delft, the Netherlands c Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Science and Technology, H-12 Sector, Islamabad, Pakistan d Waternet, Korte Ouderkerkerdijk 7, 1096 AC, Amsterdam, the Netherlands ARTICLE INFO Keywords: Cold recovery Chlorine Drinking water microbial activity Biofilm community structure Functional prediction ABSTRACT 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) en- hanced 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. 1. Introduction Fossil fuels have made up the majority part of the energy resources worldwide in the past decades (Painter, 2020). However, the extensive use of these traditional energy sources poses lots of environment issues, such as global warming (Lelieveld et al., 2019). In 2015, the United Nations Paris Climate Conference reached a consensus that the global temperature rise should be well below 2 °C and efforts should be pur- sued to limit it to 1.5 °C (Painter, 2020; Rogelj et al., 2016). Therefore, in order to achieve this target, the pursuit of new and clean low-carbon energy resources is necessary (Jiang et al., 2010). Recently energy re- covery from the water cycle has been suggested, including thermal energy from surface water, groundwater, wastewater and drinking water (Mol et al., 2011;van der Hoek, 2012a). With respect to surface water, energy recovery has already been successfully applied in prac- tice. In the Netherlands, the water from lake “Ouderkerkerplas” is used for office building cooling, and a reduction of greenhouse gas emissions of nearly 20 kton carbon dioxide (CO 2 )-equivalent/a can be achieved (van der Hoek et al., 2018). In many European countries, groundwater plays a role in the underground thermal energy storage systems and is widely used at full scale (Sanner et al., 2003). In the urban water cycle, heat recovery from wastewater via heat exchangers has been in- tensively studied (Elias Maxil, 2015; Elías-Maxil et al., 2014), and shower water has also been applied for heat recovery from wastewater in a pilot study (Deng et al., 2016). Recently, the concept of thermal energy recovery from drinking water has been proposed, and re- searchers have proven its possibility (Bloemendal et al., 2015) and explored the potential technologies in practical use (De Pasquale et al., https://doi.org/10.1016/j.envres.2020.109655 Received 23 March 2020; Received in revised form 9 April 2020; Accepted 8 May 2020 * Corresponding author. E-mail address: zhangkj@zju.edu.cn (K. Zhang). Environmental Research 187 (2020) 109655 Available online 15 May 2020 0013-9351/ © 2020 Elsevier Inc. All rights reserved. T