Thermal energy recovery from chlorinated drinking water distribution systems: Effect on chlorine and microbial water and biofilm characteristics

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.