Microbiology during drinking water distribution

Drinking water in the Netherlands is distributed without any traces of disinfectants such as chlorine. The growth of microorganisms during the water’s distribution is contained by keeping the supply of nutrients in the drinking water distribution system as low as possible, that is, by striving for a biologically stable drinking water system.

Biological stability can be defined as ‘characterising a drinking water system that produces a minimum possible biological change, so that the health risks and/or customer complaints related to (microorganism) growth cannot arise.’ This research will assess whether this definition is realised in practice. Aesthetic complaints about the presence of invertebrates, odour and taste, and/or about discoloured water, do occur at some locations. Moreover, it cannot be excluded that opportunistic pathogens, which are genotypically comparable with patient strains, might be present in the distribution network.

The Joint Research Programme has traditionally always paid lots of attention to the adjustment and optimisation of water treatment in relation to biological stability, but has not focused so much on the distribution system itself. Moreover, those projects that have concerned the distribution system have mostly concentrated on the sampling and analysis of pure-water samples, of water samples collected at the kitchen tap after letting the water flow and/or of discharge water collected at the fire hydrant. Given the many variables that affect microbial growth in the distribution system, it is difficult with such measurements to identify the parameters that have the greatest impact on microbial processes occurring in the distribution system. As a result, the drinking water distribution system is still very much of a ‘black box’. The microbial processes that do occur are not understood well enough to allow for the formulation of well-founded recommendations regarding water distribution, with the aim of controlling the regrowth problem in relation to biological stability. In addition, a number of Joint Research Programme projects were more concerned with identifying processes and variables that determine the growth of microorganisms in the distribution system. One example is the Joint Research Programme on Legionella pneumophila, which was set up by Dick van der Kooij during 2002-2012. This L. pneumophila research involved considerable use of lab-scale experiments using biofilm monitors. The experimental results made it clear what conditions the water quality, temperature and pipe material needed to satisfy to prevent the growth of L. pneumophila. Consequently, this research, compared to the black-box approach, produced concrete and well-founded recommendations. With regard to future research it therefore makes sense to complement measurements made in practice with lab-scale research.

The overall objective of this project is to use a combination of measurements and lab-scale experiments to achieve an improved understanding of various microbiological processes taking place in the distribution system. This knowledge will offer the basis for (better) supported recommendations to improve biological stability during water distribution.

Microbial processes in biofilm pipe-wall/sediment

Although most biomass in the distribution system is found in the biofilm on the pipe wall or in the sediment, most studies focus primarily on the biomass present in the water. Research has however shown that bacteria that are predominantly present in the distributed water are already also present in the pure water from the pumping station. It seems therefore that active microbial processes in the distribution system occur primarily in the biofilm. Which metabolic processes occur in this biofilm in the distribution system is unknown, as is the role of this biofilm in the biological stability of the water, which substances (AOC-P17/NOX, AOC-A3, PHMOC, proteins, carbohydrates, fatty acids, etc.) are degraded and thus can cause regrowth problems, or the impact on biofilm growth of particle characteristics (few organic substances, many organic substances, iron/manganese content, etc.). Therefore, the dynamic between the biofilm on the pipe wall, the biofilm in the sediment and bacteria in the water phase is unclear. The lack of this kind of knowledge is an important reason for the difficulty of defining effective and well-founded control measures in water treatment and distribution, for the purpose of improving the biological stability of the drinking water system. The objective of this part of the project is therefore to determine the metabolic microbial processes in the pipe-wall and sediment biofilms, and to assess the role (i) of the biofilm on the biological stability of the drinking water system, and (ii) of easily-degradable and of persistent substances, particle properties, and iron on pipe-wall and sediment biofilm growth, so that more efficient control measures for treatment and distribution can be proposed and studied in a follow-up project.

Monitoring regrowth in distribution system

Growth of microorganisms in the distribution system is today routinely monitored primarily through cultures (plate count 22°C and Aeromonas) on drinking water samples, which are taken instantaneously at the kitchen tap after letting the water flow; sometimes this is complemented with ATP and cell counts. There are however a number of limitations associated with this approach. Recent Joint Research Programme research has for example demonstrated that ATP measurements and cell counts in distributed drinking water provided little information about the extent of regrowth in the distribution system at 28 separate production sites in the Netherlands; while KG22 and Aeromonas are mainly related to sediment-associated growth (Van der Wielen et al., 2016). Moreover, the presence of any dynamic in the microbial processes in the distribution system is missed when one takes instantaneous planned samples; and the building’s drinking water installation can affect the microbial quality of the drinking water at the tap, even after letting the water flow (BTO 2017.028). The influence of the drinking water installations on the microbial water quality becomes clearer when samples are also taken just before the water metre. Collection at such a sampling point also reveals how the microbial water quality changes during the transport from the service reservoirs to the water meter. The ultimate objective of this part of the project is to provide water utilities with the means of formulating an optimal and tailored monitoring strategy for regrowth in the distribution system.