Water Journal : Water Journal September 2012-1
refereed paper rural pipeline management water SEPTEMBER 2012 75 • Biological infestations Various organisms can exist within a pipeline network. A common nuisance organism is Plumatella, a Bryozoan that forms colonies seasonally in pipelines, shearing off to foul downstream meters and even decomposing causing taste and odour problems (Figure 3). In 2009, when the WMP was being constructed and the NMP had been operating for several years, a study into the effects of pipe fouling was completed. The study drew upon experience with the NMP as well as information collected specifically for the study of the WMP. This work complemented the ongoing research project underway between Victoria University and GWMWater, focusing on Bryozoa in the NMP where at least five different species have been identified (Mitra et al., 2012). The WMP had been operating for only a short period at the time of this study, meaning that rather than relying on operational experience, a risk assessment approach was adopted to identify the likely types of fouling and the possible consequences of their development under different scenarios. Key findings included: • Development of sediment and/or iron and manganese fouling downstream of Lake Bellfield was a risk due to occasional storm-event high turbidity and typical seasonal stratification of the reservoir. The risk is exacerbated as the Lake Bellfield outlet tower can only extract water from the bottom of the dam. Figure 2 shows examples of iron-manganese slime (Tasmanian Hydropower example) and sediment attachment (Victorian large regional transfer mains example) that can occur on the walls of the pipeline. • Bryozoa were identified within the WMP. The Bryozoa Plumatella and Fredericella are known nuisance organism in parts of the NMP, which draws water from the Murray as opposed to from Lake Bellfield and Taylors Lake. • A relatively small increase in pipe roughness, due to manganese/iron/ sediment and/or Bryozoa fouling, may over time cause significant increases in pumping requirements across the network, especially given the flat topography of the network, where most pumping energy is used to overcome friction rather than for static lift. Managing pipe fouling in order to maintain low pipe friction conditions is important to keep energy use down. Examples from elsewhere of pipeline fouling due to biofilms and sediments suggest the following: • Eppalock Pipeline Iron and manganese slime build-up prior to pigging was in the range of 500 to 2000 gDS/m2. After a 30- week pumping season period the friction headloss was observed to increase to approximately 200% of that of a clean pipe (i.e. a pigged main). Draining and flushing of the pipe restores the friction headloss to a level equivalent to approximately 130% of a pigged main. • Otways Pipeline Build-up over 15 to 20 years of biological infestation removed by pigging has increased hydraulic capacity by about 7%. • Recent Major Regional Transfer Pipeline Sediment build-up had occurred, possibly reducing the hydraulic capacity of the pipeline by about 10%. The sediment build-up was measured to be up to about 40 gDS/m2 at the inlet end of the pipeline. • Winneke-Preston Pipeline Manganese type biofilms can be managed by scouring with flow rates that produce shear stresses at the wall in excess of about 9.7 N/m2. Figure 4 summarises what flow rate is needed for different diameter pipes to achieve this condition. Manganese on pipe walls Sediment on pipe walls Figure 2. Example of iron and manganese slime and sediment build-up. Figure 3. Example of Bryozoa found in the Wimmera Mallee Pipeline (WMP). Figure 4. Scour velocity and flow rate for removal of iron and manganese slimes for different pipe diameters (based on achieving a shear stress of > 9.7 N/m2).
Water Journal November 2012-1
Water Journal August 2012