Turbulence accelerates the growth of drinking water biofilms

Biofilms are found at the inner surfaces of drinking water pipes and, therefore, it is essential to understand biofilm processes to control their formation. Hydrodynamics play a crucial role in shaping biofilms. Thus, knowing how biofilms form, develop and disperse under different flow conditions is critical in the successful management of these systems. Here, the development of biofilms after 4 weeks, the initial formation of biofilms within 10 h and finally, the response of already established biofilms within 24-h intervals in which the flow regime was changed, were studied using a rotating annular reactor under three different flow regimes: turbulent, transition and laminar. Using fluorescence microscopy, information about the number of microcolonies on the reactor slides, the surface area of biofilms and of extracellular polymeric substances and the biofilm structures was acquired. Gravimetric measurements were conducted to characterise the thickness and density of biofilms, and spatial statistics were used to characterise the heterogeneity and spatial correlation of biofilm structures. Contrary to the prevailing view, it was shown that turbulent flow did not correlate with a reduction in biofilms; turbulence was found to enhance both the initial formation and the development of biofilms on the accessible surfaces. Additionally, after 24-h changes of the flow regime it was indicated that biofilms responded to the quick changes of the flow regime. Overall, this work suggests that different flow conditions can cause substantial changes in biofilm morphology and growth and specifically that turbulent flow can accelerate biofilm growth in drinking water

Biofilm Growth in Drinking Water Systems under Stagnant Conditions

Safe drinking water is essential for human health and its provision in a changing climate is a global pressing problem. Research communities, governments and drinking water supplying companies are working on improving the quality of drinking water and reducing its cost. Microorganisms colonise the inner surfaces of pipes and form biofilms. In drinking water systems biofilms are problematic as they cause loss of disinfectants, harbour pathogens and affect the aesthetics of drinking water. From the engineering perspective, that leads to corrosion of the pipe’s material and reduced life of the existing infrastructure. Thus, it is imperative that we gain a deeper understanding of the growth of biofilms if we are to develop effective strategies for their removal or control. In this study we focused on the growth of biofilms in drinking water under stagnant conditions, which often occur in parts of drinking water pipes. A bioreactor was used to simulate the service lines of drinking water systems. After 4 weeks, the thickness and density of the biofilms were characterised using gravimetric measurements, and their surface area was determined using fluorescence microscopy. Also, the concentration of cells and microcolonies both in the bulk water and on the reactor surfaces was determined using fluorescence microscopy. Finally, spatial statistics were used to describe the biofilm structures that were formed on the exposed surfaces of the reactor. It was revealed that even under stagnant and oligotrophic conditions, drinking water bacteria moved from the bulk water of the reactor and attached to the available surfaces forming a high number of microcolonies. Biofilms were able to grow on the exposed surfaces of the reactor forming characteristic structures consisting of dense cell clusters. Our results revealed that even under unfavourable conditions biofilms can grow within our drinking water systems