Linking discolouration modelling and biofilm behaviour within drinking water distribution systems

Abstract

High quality drinking water exits modern treatment works, yet water quality degradation such as discolouration continues to occur within drinking water distribution systems (DWDS). Discolouration is observed globally, suggesting a common process despite variations in source, treatment, disinfection and network configurations. The primary cause of discolouration has been identified as mobilisation of particulate material from pipe walls and the verified Prediction of Discolouration in Distribution Systems (PODDS) model uses measurable network hydraulics to simulate this response.In this paper the cohesive properties of discolouration material are explored and it is hypothesised that in simulating the turbidity response, the PODDS model is actually describing the development and cohesive strength behaviour of biofilms. Applying this concept can therefore facilitate a rapid and simple assessment of DWDS biofilm activity. A review of the findings from PODDS studies conducted internationally is presented, focussing on the macro or observable aspects of discolouration. These are compared and contrasted with associated biofilm studies which consider discolouration material at the micro-scale. Combining the results from these (past) studies to improve the understanding of interactions between microbial ecology and discolouration are discussed with a view to DWDS operational strategies that safeguard and optimise drinking water supply.

Uncharted waters: the unintended impacts of residual chlorine on water quality and biofilms

“…3) providing direct evidence that the iron mobilised into the bulk water originated from the pipe wall. This release of entrapped metals from the pipe wall has been previously inferred based on increases in bulk water metal concentrations during the flushing of pipelines 4,5 (a trend also observed in the current study) but not conclusively determined.…”

“…Furthermore, during flushing of test 1, Medium-chlorine had a greater discolouration response than Lowchlorine, yet these regimes had similar Pre-Flush1 biofilm iron concentrations, suggesting factors other than iron concentration are influencing discolouration. Given the association between biofilms, discolouration and water quality 5 , microbiological processes are considered likely to influence the observed differences.…”

“…Globally, drinking water discolouration (evidenced by elevated turbidity, of which iron is a predominant contributor) is a leading symptom of water quality failure, commonly causes customer complaints 4 and can mask other failures, including microbial concerns. Discolouration is driven by material accumulated at the pipe wall being mobilised into the bulk water: analogous to biofilm accumulation and detachment 5 . Recurrence of discolouration highlights the need for risk mitigation, yet this can be costly and disruptive, including flushing strategies and mechanical cleaning to remove material 4 .…”

Biofilm Microbiome (Re)Growth Dynamics in Drinking Water Distribution Systems Are Impacted by Chlorine Concentration

“…The long term validity of this is not proven here as the microbiome was investigated over a relatively short time frame of 28 days. However, previous research regarding discoloration within DWDS, which is known to be strongly related to biofilm ( Husband et al, 2016 ), suggests that regeneration of material within networks is linear with time ( Husband et al, 2014 ; Cook et al, 2015 ). Hence the long term benefits from the simple mechanical cleaning and more aggressive cleaning may also be similar.…”

Whole metagenome sequencing of chlorinated drinking water distribution systems

“…The importance of microbial communities within DWDS is widely recognised, and recent research has illuminated the diversity and complexity of these communities. [1][2][3] Microorganisms in DWDS, particularly those inhabiting biofilms, govern key processes including; transformation of metals involved in corrosion, [4][5][6] degradation of disinfectant residual, 7,8 changes in water organoleptic characteristics, 9 discolouration, 10 sheltering pathogens 11 and production of toxins and virulence factors. 12,13 However, there is virtually no understanding of how microorganisms work individually or as a community, and the impacts on the quality and safety of the drinking water or on the performance of the ageing pipe infrastructure.…”