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Impacts of temperature and hydraulic regime on discolouration and biofilm fouling in drinking water distribution systems [1]

['Katherine E. Fish', 'Civil', 'Structural Engineering', 'Sheffield Water Centre', 'The University Of Sheffield', 'Sheffield', 'United Kingdom', 'Rebecca L. Sharpe', 'Department Of The Natural', 'Built Environment']

Date: 2022-08 

Discolouration is the greatest cause of customer dissatisfaction with drinking water quality, potentially masking other failures, including microbial issues, which can impact public health and well-being. The theorised association between biofilms (complex microbial communities) and discolouration within drinking water distribution systems (DWDS) was explored, whilst studying the impact and interactions of seasonal temperature variations and hydraulic regime. Transferability of findings to operational DWDS was ensured by using a temperature controlled, full-scale distribution experimental facility. This allowed isolation of the factors of interest, with integration of physical, chemical and microbial analyses. Greater discolouration and biofilm cell accumulation was observed under warmer (summer, 16°C) temperatures compared to cooler (winter, 8°C), evidence of microbiology being an important driver in DWDS discolouration behaviour. Temperature was generally more influential upon discolouration and biofilm cell volumes than the shear stress imposed by the hydraulic regimes, which included three steady state and two varied flow patterns. However, the trends were complex, indicating interactions between the two parameters in governing microbial accumulation and discolouration. These results are important in informing sustainable management of our ageing DWDS infrastructure to deliver safe high quality drinking water. By providing new evidence that discolouration is a biofilm/microbiologically-mediated process, we can better understand the importance of targeting interventions to hotter seasons, and manipulating hydraulic conditions (which we can control), to minimise the long-term impacts of impending changing climates on water quality.

1. Introduction

The climate crisis, an increasing population, and increasing urbanisation are all driving an accelerating need to improve the resilience and sustainability of the water sector. It is critical that sustainability conserves water quality not just quantity; this is essential for society, the economy and, arguably, the environment. To protect public health and well-being, water utilities are tasked with maintaining (and improving) drinking water quality, in increasing volumes, to a growing number of customers, via an ageing distribution system exposed to new and increasing pressures (e.g. water shortages, temperature changes) [1]. Achieving this in such an uncertain socio-environmental climate requires better understanding of the interactions within the system, and how environmental changes may impact these.

Drinking water is delivered to customers through a pipe network termed the drinking water distribution system (DWDS). Various interactions and reactions occur between the pipe wall and bulk water within DWDS, which degrade the quality of the water as it is transported through the system. The largest number of drinking water quality-related consumer contacts are due to aesthetic degradation, of which discolouration is a principal example worldwide [2, 3]. Discolouration is the result of dissolved colloidal or suspended substances being present in the water column [4], often this material has accumulated within the DWDS pipes and is subsequently mobilised in volumes sufficient to cause visible discolouration [2]. OFWAT (the water services regulation authority in England and Wales) have highlighted the importance of investment in reducing discolouration, introducing penalties for companies if customer contacts regarding discolouration exceed an acceptable level in their Outcome Delivery Incentive reports [5]. Furthermore, customers rated the reduction in discoloured water as high in a “willingness to pay” survey, being prepared to pay more than a third more for this than other service improvements [6]. Even with regulatory and customer pressures, the Drinking Water Inspectorate (DWI) reported that discolouration events affected 1.81 million customers in England in 2020 (61% more than in 2019) [7]. This is a substantial increase from the 1.14 million customers in England and Wales who were estimated to have been affected by “significant, serious and major” discolouration events in 2016 [8]. This figure excludes events that were due to planned work or attributed to treatment work failure and hence captures those that were likely occurring during distribution. Reducing discolouration is an ongoing challenge likely to be intensified by global climate change, particularly temperature changes. In order to mitigate current (and future) water quality failures, water companies are required to shift from reactive to proactive management. Achieving this requires a better understanding of the processes/behaviour of water quality degradation occurring within DWDS and the impact that temperature changes have upon this, especially within the context of discolouration.

Discolouration-causing material consists of fine-sized matter originating from corrosion, chemical reactions and/or biological interactions [9]. Increases in turbidity (the measurement of discolouration) have also been associated with increases in iron and manganese concentrations [10–12], with one study reporting positive correlations of 80.58% and 71.94%, respectively, in UK district management areas [13]. This suggests an association between discolouration and metal concentrations, demonstrating that discolouration events may mask failures of other water quality parameters. Two concepts are generally proposed to describe and understand discolouration: (i) sedimentation, where behaviour is governed by the particles self-weight, as applied for rivers and sewers and with the threshold of motion described by concepts like the Shield criterion; or (ii) the idea of cohesive layers based on the observation that the particles in DWDS are generally too small and low density for their behaviour to be dominated by their self-weight. This cohesive layer concept is captured in the Prediction of Discolouration in Distribution Systems (PODDS) model, which suggests that interactions at the pipe interface lead to particles actively concentrating into attached layers at different adhesive strengths, with the minimum strength determined by the normal hydraulic regime within the pipe [14]. The attached material is then mobilised if the hydraulic forces exceed the layer strength. This PODDS modelled discolouration behaviour has been observed across many DWDS and is conserved between differences in infrastructure, water matrices and countries [12, 15, 16]. PODDS has been validated as an empirical tool by various field and laboratory studies and can accurately predict a discolouration response to changing hydraulics. However, the model provides limited understanding of the processes causing material accumulation or the effects of temperature on discolouration. Interestingly, the discolouration behaviour that PODDS predicts is analogous to the accumulation and mobilisation of microbial biofilms, which have been shown to be influenced by hydraulic regime, and governed by the cohesive properties of the biofilm [17].

Discolouration is reported to be positively correlated with temperature, such that the frequency of reported discolouration increases in the summer months [13, 18]. The accumulation of material within DWDS has also been shown to be elevated at higher temperatures [19]. Water temperature is acknowledged to differ spatially (between networks and along pipes) and temporally (i.e. seasonally); 18 countries have a regulation or guideline value for water temperature [20], although an international study on water temperature reported no clear policies are in place should drinking water temperatures exceed these values [21]. Blokker and Pieterse-Quirijns [22] found that water temperature in DWDS in the Netherlands was mostly affected by soil temperature, with annual water temperature ranges of 5–20°C being reported [19]. A data-driven modelling study of UK and Dutch systems highlighted that temperature, pipe material and bulk-water iron concentration were the three key factors correlated with, and hence influencing, discolouration material accumulation (although temperature correlations were primarily within non-chlorinated DWDS) [23]. Furthermore, in a study of 176 DMAs (supplied with a disinfectant residual) higher water temperatures were correlated with higher iron concentrations (>57% of sites) and higher manganese (>65% of sites) [13].

Despite the recorded temperature variations (and their impact on discolouration), currently only the Netherlands have strict temperature legislation for drinking water (a maximum of 25°C) [22]. The basis for this legislation is primarily the microbiological quality of bulk-water (i.e. planktonic microorganisms, not biofilm bound microorganisms), rather than discolouration, specifically the awareness that water temperature is influential in the risk of Legionella pneumophila (bacteria that causes Legionnaire’s disease) colonising DWDS [24–27] and potentially causing an outbreak. Worldwide, microbial drinking water quality is routinely assessed by quantifying coliforms: a group of Gram-negative bacteria used as indicators of the potential presence (or absence) of faecal contamination or pathogens. Changes in temperature have been found to affect the number of coliforms detected within drinking water and microbial growth potential has also been shown to increase at temperatures above a threshold of ~ 15°C [28–31]. Within DWDS microbial cells are found in both planktonic (i.e. in the water column) and biofilm (surface bound) states. DWDS biofilms are mixed-species microbial communities that develop at the pipe-water interface, adhered to the pipe wall by a matrix of extracellular polymeric substances (EPS) and contain the majority of the microbial load of a DWDS. Temperature clearly influences both discolouration and microbial quantity/activity, which suggests that these factors may be linked.

Temperature is an important determinant of drinking water quality as it affects the physical, chemical and biological processes occurring within the DWDS, including microbial growth and the rate of chlorine decay [21]. It has been suggested that the disinfection contact times within DWDS are insufficient to inactivate various microorganisms [32]. The sub-lethal doses can then exert a selective pressure which impacts the planktonic (free-living microorganisms in the bulk-water; [33, 34]) and biofilm microbiomes and cell concentrations [35, 36]. Consequently, temperature could have an indirect impact on the microbial load of chlorinated or chloraminated systems by affecting the disinfection residual concentrations, unless systems are dosed seasonally at treatment works. If biofilm behaviour is analogous to that of discolouration material accumulation/mobilisation within the DWDS, this suggests that the discolouration risk posed by biofilm would be higher in the warmer months. In a one-year study investigating discolouration risk within operational trunk mains, Sunny et al. [37] observed a positive correlation between temperature, material accumulation and total organic carbon (TOC). With respect to organics, assimilable organic carbon (AOC; the microbial available fraction of carbon) is correlated with microbial growth, which can impact water quality with higher AOC concentrations being reported to result in higher discolouration [38]. However, different target AOC concentrations are reported in the literature, with varied impacts [29, 39, 40] and different AOC behaviour being observed in differ assets of the DWDS (pipes vs. service reservoirs; [41]). Clearly, the relationships between nutrients and microbial dynamics in DWDS are complex but crucially the findings by Sunny et al [37] indicated that temperature and organic material, and therefore likely associated microbial behaviour, played a critical role in the discolouration risk posed by a DWDS. Further supporting this finding, Cook et al. [18] found that a greater number of discolouration complaints were made during higher temperatures in the UK, with fewer in the winter when water temperatures typically fall to <6°C.

Given that both microbial failures and discolouration events are more likely during warmer periods, there is a hypothesised connection between biofilms and discolouration, with temperature being an important influencing variable. Previous studies have primarily used observational field data to highlight correlations between hydraulics, temperature and discolouration. Where full-scale controlled studies have been conducted, these have considered either just [42, 43] or the impact of future extreme temperatures [44]. Seasonal temperature changes are under-explored, and their impacts have not been considered in combination with different hydraulic regimes, hence causation rather than correlation has thus far not been established at a scale representative of operational DWDS.

This study aimed to determine the impact of seasonal temperature variation (summer to winter in the UK) on the discolouration of drinking water and the quantity of biofilm bound microorganisms within DWDS, when pipes were conditioned to different hydraulic patterns. By investigating only these variables, but in combination, this study set out to determine if one parameter was more influential than another in promoting biofilm development or discolouration whilst also providing further insights into microbiology as a driver for the accumulation and release of material within DWDS.

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[1] Url: https://journals.plos.org/water/article?id=10.1371/journal.pwat.0000033

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