Laboratory studies of physical transformation processes in sewers by wastewater-grown biofilms

Changes in pipe flow capacity with time have been reported to be caused by biofilm formation, sediment accumulation and pipe deterioration. Biofilm has been demonstrated to cause increasing hydraulic roughness in natural water flows thus changing the hydraulic properties of the system. However, little work has been done in sewer pipes with heavily polluted wastewater. Sediment accumulation, deposition and erosion processes in sewers have also been reported to be influenced by microbial activity.
The thesis reports on the development of a novel method for investigating the influence of wastewater-grown biofilm on pipe flow characteristics and bed sediment stability. This work presents systematic laboratory studies of the biofilm growth under different conditions, pipe flow characterisation at different hydraulic configurations, deposit characteristics for different consolidation periods, with changes of organic matter concentration being monitored for all tests. All laboratory tests were conducted using wastewater.
The results obtained indicate that biofilm growth changes flow behaviour in pipes by decreasing flow depth, thus decreasing pipe hydraulic roughness, and increasing average flow velocity. This finding depends on the level and character of biofilm growth conditions in the pipe, as different characteristics of biofilm were obtained at different conditions. For sediment deposits, biofilm growth was observed to increase bed stability with longer consolidation phase, thus reducing bed erosion at higher shear stress. These results vary with the duration and character of the consolidation phase of the sediment bed.
The findings obtained provided a better understanding of the role of biofilm in sewer pipes and may contribute to the development of more accurate modelling of pipe flow and sediment accumulation and transport processes in sewers.