Microbiological quality of roof tank water in an urban village in southeastern China

Abstract

Urban villages are unique residential neighborhoods in urban areas in China. Roof tanks are their main form of water supply, and water quality deterioration might occur in this system because of poor hygienic conditions and maintenance. In this study, water samples were seasonally collected from an urban village to investigate the influence of roof tanks as an additional water storage device on the variation in the microbial community structure and pathogenic gene markers. Water stagnation in the roof tank induced significant decreases in chlorine (p < 0.05), residual chlorine was as low as 0.02 mg/L in spring. Propidium monoazide (PMA)-qPCR revealed a one-magnitude higher level of total viable bacterial concentration in roof tank water samples (2.14 ± 1.81 × 10⁵ gene copies/mL) than that in input water samples (3.57 ± 2.90 × 10⁴ gene copies/mL, p < 0.05), especially in spring and summer. In addition, pathogenic fungi, Mycobacterium spp., and Legionella spp. were frequently detected in the roof tanks. Terminal users might be exposed to higher microbial risk induced by high abundance of Legionella gene marker. Spearman's rank correlation and redundancy analysis showed that residual chlorine was the driving force that promoted bacterial colonization and shaped the microbial community. It is worth noted that the sediment in the pipe will be agitated when the water supply is restored after the water outages, which can trigger an increase in turbidity and bacterial biomass. Overall, the findings provide practical suggestions for controlling microbiological health risks in roof tanks in urban villages.

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.