Elsevier

Water Research

Volume 176, 1 June 2020, 115738
Water Research

Nitrifying niche differentiation in biofilms from full-scale chloraminated drinking water distribution system

https://doi.org/10.1016/j.watres.2020.115738Get rights and content

Highlights

  • Nitrifiers present in all tropical drinking water biofilms studied.

  • Two distinctive microbial clusters with high and low α-diversity were found.

  • Ammonia oxidising bacteria dominate in samples with low diversity.

  • Thaumarchaeota and Nitrospira co-aggregate in high diversity cluster.

  • Monochloramine and nitrite shape and drive the microbial community assembly.

Abstract

Tropical conditions favour the auto-decomposition of monochloramine (MCA) leading to disinfectant decay and free ammonia in drinking water distribution systems (DWDS); thus, they promote the growth of nitrifiers and the development of biofilms on the inner-pipe surface. Biofilms can adversely impact the provision of safe and biologically stable water. Moreover, there is a general lack of understanding of the role of microbial communities in DWDS in regions with warm temperatures and no distinct seasons. Here, we report a survey on biofilms from full-scale monochloraminated DWDS in a highly urbanised metropolis using next generation sequencing tools. The monitoring campaign consisted of sampling biofilms and bulk waters from 21 in-service pipes. We characterized the microbial community with emphasis on nitrifying bacteria and archaea using 16S rRNA gene amplicon sequencing and potential nitrification activity. Samples grouped into two clusters, characterized by their low (Cluster LD) and high (Cluster HD) α-diversity. Both clusters harbour microorganisms related to nitrification: i) Nitrosomonas (24.9–68.8%), an ammonia oxidising bacterium (AOB) that dominated Cluster LD, and ii) a co-aggregation of genus Nitrospira (9.8–32.5%), a nitrite oxidising bacterium (NOB), and Thaumarchaeota (1.4–10.9%), chemolithotrophic ammonia oxidising (AOA) archaea that were among the most abundant OTUs in Cluster HD. Activity tests performed with fresh biofilm samples confirmed that these two clusters represent distinctive biofilm niches performing different stages of the nitrification process. Cluster LD correlated with a high concentration of MCA, which caused dysbiosis and resulted in high unevenness of the cluster. In cluster HD, with more biomass, chemical reactions involving nitrite increased the MCA demand, releasing ammonia and allowing more nitrifiers to grow, like AOA and NOB. From this study, we conclude that an MCA residual gradient along the DWDS drives and shapes the microbial community assembly and should be considered when designing effective disinfection strategies.

Introduction

Monochloramine (MCA) has become the agent of choice in many water treatment facilities in the world to prevent microbial growth in drinking water distribution systems (DWDS) and to guarantee the water quality from source to tap. Chloramination is preferred to chlorination because a free disinfectant residual is easier to maintain and the formation of regulated disinfection by-products like trihalomethanes is reduced (Vikesland et al., 2001). Moreover, MCA is better at penetrating and effectively disinfecting biofilms (Lee et al., 2011) because it does not experience diffusion limitation (LeChevallier et al., 1990), although its efficiency will depend on the composition of the extracellular polymeric substances (EPS) in biofilms (Xue et al., 2014). Conversely, MCA has also been shown to promote nitrification. Nitrification can take place within a chloraminated DWDS due to the presence of ammonia (Baribeau and Regan, 2013), which is added to react with free chlorine and form the monochloramine (NH2Cl) disinfectant residual (Zhang et al., 2009). Hence ammonia is available as energy source for ammonia oxidising bacteria (AOB) and archaea (AOA) by oxidation to nitrite. Nitrite can react with chloramine, reinforcing its decay, and/or be oxidised to nitrate by nitrite oxidising bacteria (NOB), promoting the development of biofilms and affecting water quality in the pipeline network.

The microbial composition of biofilms developed on the inner-surface of DWDS pipes is poorly studied because of limited access to actual pipes, sampling and analysis costs, and logistical issues. Hence, most available information about microbial communities in drinking water systems derives from studies of the water phase (Bautista-de los Santos et al., 2016; El-Chakhtoura et al., 2015; Perrin et al., 2019; Pinto et al., 2012; Shaw et al., 2015), biological activated carbon and rapid sand filters (Gülay et al., 2016; Kasuga et al., 2010), and pipe biofilms developed in laboratory and pilot-scale reactors (Douterelo et al., 2016; Wang et al., 2014). The presence and role of microorganisms in full-scale systems is much less understood. Several studies have reported the occurrence of biofilms in DWDS and the major variables that affect their formation appear to be pipe material (Ren et al., 2015), water age, and water quality (Fish et al., 2016; Kooij and Wielen, 2014; Liu et al., 2016; Prest et al., 2016). Most of the available information has been obtained from using model systems (pilot- or bench-scale) and removable coupons to simulate the development of biofilms in DWDS (Douterelo et al. 2014, 2016; Gomez-Alvarez et al., 2016; Henne et al., 2012; Mi et al., 2015; Revetta et al. 2013, 2016; Wang et al., 2014), and proxies such as water meters (Ling et al., 2016; Lührig et al., 2015) with a short operation time. However informative, such systems do not necessarily reflect actual conditions in distribution pipes. Replicating real-world distribution systems at the laboratory-scale represents a considerable challenge due to differences in the surface area to volume ratio, pipe material, flow rate, and experimental duration (generally short-term). Few in situ studies have been published using next generation sequencing (NGS) to characterize the microbial community of pipe biofilms (Gomez-Smith et al., 2015; Hwang et al., 2012; Kelly et al., 2014; Liu et al., 2017; Lührig et al., 2015; Ren et al., 2015) and all of them were performed in countries with a temperate climate, seasonal changes, and disinfectant-free or chlorinated/chloraminated networks. Only one previous study on DWDS has been performed in a tropical city, namely, in Cali, Colombia (Montoya-Pachongo et al., 2018), but it used chlorine instead of MCA as a residual disinfectant. Chloramines are used as secondary disinfectant in many countries around the world including the United States, Australia, Canada, Singapore and European countries like Spain, Finland, Sweden and Great Britain. If employed in a tropical climate with high uniform temperature and no distinct seasons, the prevailing environmental conditions may expedite the decay of monochloramine by auto-decomposition producing free ammonia (Vikesland et al., 2001) and potentially trigger nitrification. More episodes of nitrification have been reported during summer or when the temperature was higher than 15 °C (Baribeau and Regan, 2013); moreover, no AOB were detected when temperatures were lower than 18 °C (Wolfe et al., 1990). The optimal temperature for nitrification in DWDS ranges from 25 to 30 °C (Zhang et al., 2009).

This study surveyed biofilms from a full-scale DWDS in a highly urbanised tropical metropolis in South East Asia, using 16S rRNA-based metabarcoding for microbial community sequencing and multivariate statistical analysis to understand the implications for the provision of biologically stable water. Biofilms were collected from pipes with long-term exposure to disinfectants, having been in service between six and 60 years, and with a range of diameters and materials. The multi-parametric approach involved cultivation-independent molecular methods and biological activity tests to characterize the microbial communities in biofilms as well as characterization of bulk water samples collected close to the biofilm sampling point. Our objectives were to i) understand the nitrification potential and determine the distributions of nitrifying AOB, AOA and NOB in DWDS biofilms and (ii) establish the relationship between nitrifying communities, microbial diversity and water quality and pipe characteristics.

Section snippets

Sampling and water treatment characteristics

The water utility relies on reservoir water, which receives inputs in the form of surface water from local catchment areas, imported (treated) water, highly-purified reclaimed water and desalinated water. Briefly, the reservoir water is pumped to the waterworks and passed through self-cleaning screens to remove particles greater than 1 m; subsequent steps include chemical treatment, filtration and disinfection. Coagulation and flocculation is achieved using aluminium sulphate as the main

Survey sampling

A total of 21 samples were collected from pipes during replacement across multiple locations. The distance to reservoir source varied from <1.0 (PB08) to 18.3 km (PB18) with a median value of 10.0 km (Table 1).

Pipes sampled had diverse sizes and ages of service. The range of diameters was from 100 to 700 mm with a median of 150 mm, while the pipe age range was from six (PB03) to 60 (PB10) years of service. The pipes were made of four types of material: Ductile Iron Cement Lined (DICL), Cast

Discussion

The microbial communities identified in this study were similar to those found in other biofilms or bulk water from DWDS with Proteobacteria as the dominant phylum, suggesting it is well-suited to surviving under potable water conditions (Liu et al., 2016). Our results also showed a high correlation of α-diversity with MCA dose. Similarly, Mi et al. (2015) found that MCA stimulated the dominance of Proteobacteria, decreasing microbial diversity in chloraminated drinking water biofilm from pilot

Conclusions

  • Samples fell into two distinct clusters with high (cluster HD) and low (cluster LD) microbial α-diversity. Nitrifiers were found in all samples with potential nitritation (cluster LD) and complete nitrification (cluster HD) activities. The cluster LD represents an uneven community exposed to the stressor MCA and the cluster HD indicates a resilient microbial community with a functionally and structurally stable assembly resulting from decreased selective pressure (lower disinfectant

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Singapore Ministry of Education and National Research Foundation through an RCE award to Singapore Centre for Environmental Life Sciences Engineering (SCELSE). The authors gratefully acknowledge input on data analysis from RohanWilliams and advice on amplicon preparation for sequencing from Daniela Drautz-Moses.

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    Present affiliation: Instituto de Investigaciones para la Industria Química (INIQUI), Consejo Nacional de Investigaciones en Ciencia y Técnica (CONICET), Universidad Nacional de Salta (UNSa), Av. Bolivia 5150, A4408FVY Salta Capital, Argentina.

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