Assessing the impact of biofouling on the hydraulic efficiency of pipelines

Pipeline distribution systems account for the vast majority of the physical infrastructure in the water industry. Their effective management represents the primary challenge to the industry, from both an operational and public health standpoint. Biofouling has a ubiquitous presence within these systems, and it can significantly impede their efficiency, through an increase in boundary shear caused by characteristic changes in surface roughness dynamics. Nonetheless, conventional pipeline design practices fail to take into account such effects, partially because research findings that could contribute to upgraded and optimised design practices appear inconsistent in the literature. The overall aim of this study was to improve the current scientific understanding of biofouling within water and wastewater pipelines; for the purpose of instigating a step-change in pipeline design theory by incorporating biofouling, thereby enabling future pipelines to be as sustainable as possible. The nature of the problem, necessitated the need for a multidisciplinary approach, based upon engineering and microbiological principles and techniques. The primary focus of this study was to investigate the impact of biofouling on surface roughness, mean flow structure and sediment transport within wastewater systems. To this effect biofilms were incubated with a synthetic wastewater on a High Density Polyethylene pipe, within a purpose built pipeline facility for 20 days, at three steady-state flow regimes, including the average freestream velocities of 0.60, 0.75 and 1.00 m/s. The physico-chemical properties of the synthetic wastewater were purposely designed to be equivalent to the properties associated with actual wastewater found within typical European sewers. The impact of biofouling on flow hydrodynamics was comprehensively identified using a series of static pressure tappings and a traversable Pitot probe. Molecular and image analysis was also undertaken to support the observations derived from the aforementioned measurements, particularly with regards to the structural composition and mechanical stability of the biofouled surfaces. The study has confirmed that the presence of a low-form gelatinous biofilm can cause a significant increase in frictional resistance and equivalent roughness, with increases in friction factor of up to 85% measured over the non-fouled values. The reported increases in frictional resistance resulted in a reduction in flow rate of up to 22% and increased the pipe’s self-cleansing requirements. The structural distribution of a biofilm was shown to play a key role in its overall frictional capacity and strength, which in turn was found to be a function of the biofilms conditioning shear. In particular, it was found that a biofilm conditioned at higher shear will have less of an impact on a pipe’s overall frictional resistance, although, will be stronger and more difficult to remove than a biofilm conditioned at lower shear. The biofilm’s impact on frictional resistance was found to be further compounded by the fact that traditional frictional relationships and their derivatives are not applicable to biofouled surfaces in their current manifestation. In particular, the von Kármán constant, which is an integral aspect of the Colebrook-White equation is non-universal and dependent on Reynolds Number for biofouled surfaces. It was found that the most suitable manner to deal with the dynamic and case-specific nature of a biofouled surface was to quantify it using a series of dynamic roughness expressions, the formulation of which were the culmination of this study, and should be the focus for further research. The influence of different plastic based pipe materials and flow regimes on biofilm development within drinking water distribution systems was also briefly investigated using a series of flow cell bioreactors and molecular analysis techniques. Keywords: Biofilm; biofouling; pipe; hydraulic efficiency; equivalent roughness; von Kármán constant; Colebrook-White equation; drainage network; wastewater; drinking water