Microbiological quality of roof tank water in an urban village in southeastern China
Graphical abstract
Introduction
Over the past few decades, China has experienced rapid urbanization, and it was estimated that approximately 70% of China's population would live in urban areas by 2030 (Deng et al., 2015). Urban villages (called Cheng Zhong Cun in Chinese) refer to a special type of residential community in China. Usually, urban village was originally a real village in an urban-rural fringe area and later became surrounded by constructed urban areas during rapid urbanization. Although they are always characterized with a crowded population, lagged planning, and insufficient infrastructure, low rent and living costs are attractive to many migrants. Generally, the water supply systems in urban villages in China are connected with the municipal water supply pipe network. The remarkable feature of this water supply system is the water storage tanks, which are installed on the roofs of self-built apartment buildings in the case of water supply shortages during peak water consumption.
The simple water storage tanks on the roofs of self-built apartment buildings in urban villages are directly exposed to the natural environment. Rainwater and dust can directly enter roof tanks, causing serious exogenous microbial contamination. The warm water in the roof tank due to exposure to sunlight would accelerate the decay of disinfectant residuals (Powell et al., 2000; Miyagi et al., 2017). In addition, the mismatch between the size of roof tanks and the water demand of consumers may contribute to the low water exchange rate and long-term water stagnation (Al-Omari et al., 2008; Al-Bahry et al., 2011; Miyagi et al., 2017). Water stagnation in roof tanks can offer stable environmental conditions, facilitating a series of chemical reactions (Zhang et al., 2021b); and the important one was the decay of chlorine disinfectants. The decay of chlorine disinfectants further facilitated the regrowth of microorganisms, especially opportunistic pathogens, which are often resistant to disinfectants and can thrive under the extreme oligotrophic conditions of drinking water (Falkinham et al., 2015; Zhang et al., 2021a). The existence and regrowth of these microorganisms might cause serious problems in roof tanks in urban villages. Despite the high possibility of microbial contamination, this problem has not been taken into account by the consumers and governments. Compared with the well-managed secondary water supply systems in residential neighborhoods in China (Hu et al., 2021), roof tanks in urban villages might contain greater microbiological risks. To ensure drinking water safety in urban villages, it is necessary to evaluate the microbiological risk in roof tanks in urban villages.
This study aimed to investigate microbial contamination in roof tanks in an urban village from Xiamen, a central city located on the southeast coast of China. The specific objectives were 1) to explore the influence of roof tanks on microbial community structure and pathogenic gene markers, and 2) to identify microbial hazards in roof tanks. These results could help to provide new insight into the microbiological health risks in roof tanks in urban villages, thus providing helpful suggestions for their management.
Section snippets
Water sample collection and processing
An urban village in Xiamen in southeastern China was selected to perform this investigation. Recently, along with rapid economic growth and the significant acceleration of urbanization, a large number of migrant workers poured into Xiamen, and most of them live in urban villages where amounts of apartment buildings were built. Three apartment buildings were selected in this study, and a set of parallel water storage tanks were installed on the roof of each building to supply water to the
Physicochemical analysis of water samples
The physicochemical properties of the water samples are listed in Appendix A Tables S1 and S2. pH, DO, temperature, residual chlorine, and turbidity ranged from 7.06 to 7.77, 6.25 to 9.27 mg/L, 18.20 to 33.20°C, 0.02 to 0.39 mg/L, and 0.01 to 5.09 NTU, respectively. The concentrations of NH4+-N, NO3−-N, TN, and SO42− varied from < 0.02 to 0.08 mg/L, 1.38 to 3.00 mg/L, 1.20 to 2.98 mg/L, and 24.22 to 46.02 mg/L, respectively. The concentrations of NO2−-N were below the detection limit. The TOC
Influence of roof tanks on physicochemical water quality parameters
Water stagnation is a typical feature in roof tanks (Al-Bahry et al., 2011; Miyagi et al., 2017). Previous studies have suggested that water stagnation can affect physicochemical water quality parameters; higher pH, higher heavy metal concentrations, higher TOC concentrations, and lower residual chlorine have been reported in stagnated water in pipelines and roof tanks (Ziadat, 2005; Miyagi et al., 2017; Zhang et al., 2021b). However, only DO and residual chlorine exhibited a significant
Conclusions
This is the first study that revealed the influence of roof tanks on water quality parameters and microbial contamination in Chinese urban villages. The water quality in self-managed roof tanks was expected to deteriorate seriously due to its incomplete infrastructure and lack of management awareness of cleaning roof tanks by managers. Severe decrease in residual chlorine was observed in roof tank water samples, which induced a one magnitude higher level of total viable bacterial gene abundance
Acknowledgments
We thank Dr. Qiaoguo Tan from the College of the Environment & Ecology, Xiamen University, for his help in plotting the figures. This research was supported by the National Natural Science Foundation of China (Nos. 41861144023, U2005206), the Xiamen Municipal Bureau of Science and Technology (No. YDZX20203502000003), and the Natural Science Foundation of Fujian Province (No. 2020J05090).
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