Water Journal : Water Journal September 2012-1
membrane technology refereed paper technical features 70 SEPTEMBER 2012 water The process of selecting the members of the Alliance involved expressions of interest, short-listing of proponents, and workshops with each short-listed proponent to assess capability and team fit. These workshops were facilitated by an independent expert. Finally, the preferred proponent was selected. The Alliance delivery method involves the development of a Total Outturn Cost (TOC), which is based on the initial/ concept design of the plant. The delivery method involves all stakeholders including representatives from the designer, constructer, owner, assets planning, infrastructure delivery, operator and maintenance groups. After the TOC is complete the detailed design is then completed followed by construction, commissioning and optimisation. In order to ensure that the Alliance was operating effectively, there were a number of oversight mechanisms. The project leadership team oversaw the operation of the project directly. The Alliance Project Management Team (APMT) oversaw the Project Leadership Team for each project, and the Alliance Leadership Group oversaw the work of the APMT. This ensured that each group was operating effectively, and it also gave the opportunity for unresolved issues to be escalated to a higher level for discussion and decision making. Alliances can result in a drain on operational resources during the design, construction and commissioning phases. HWA initially restructured the operations team to provide the extra support required and a single operational liaison was appointed for each upgrade. Regardless of the contract model, it is crucial that all issues are raised appropriately. It is important to document everything and to escalate issues to achieve progress when required. Process Selection During the TOC development for Paxton, it was found that the most cost-effective option was a packaged MBR plant combined with a chemical phosphorus removal process. This packaged plant was to be designed, constructed and commissioned by a specialist contractor and the Alliance would oversee the work conducted by this contractor. Advantages with this option included reduced design and construction costs by using a pre-designed plant. Disadvantages of this option included: • Less design flexibility; • Equipment and controls were different to other Hunter Water plants; • Modifications to the standard design were costly; • Less input into equipment selection process. Process flow diagrams of the old and new processes are shown in Figures 2 and 3. Plant Performance Plant performance data for key analytes from both before and after the upgrade is illustrated in Figures 4, 5, 6 and 7. It can be seen that there was a significant decrease in total suspended solids, biological oxygen demand and total phosphorus following the MBR upgrade. This was due to superior particulate removal provided by membranes compared to gravity clarification, and also higher aeration capacity of the MBR process. Total nitrogen levels increased following the upgrade. This was likely due to the large capacity of the blowers making turn-down difficult and resulting in over- aeration. This result also highlighted that the previous process removed nitrogen effectively. Contributing factors were likely to be the wetland cells, which provided excellent nitrogen removal via uptake by plants and microorganisms. Lessons Learned During the upgrade process the plant needed to continue operation and comply with the discharge licence conditions. This proved to be challenging. During the construction phase, the maturation pond and one of the two wetland trains was offline. This meant that all of the flow through the plant was discharged through the remaining wetland train. Normally discharge occurred by gravity flow; however, in this case flow was pumped from the effluent catch pond to the wetland cell. This resulted in hydraulic overloading and overflow from the wetland cell, which constituted a breach of the discharge licence. In response to the overflow from the wetland, the effluent was directed to the onsite woodlot for irrigation; however, the temporary irrigation system set up by the constructor resulted in an overflow from the woodlot, which again constituted a breach of the discharge licence. The abovementioned incidents highlighted the critical importance of monitoring effluent discharge during an upgrade to ensure compliance with licence requirements, even if the facility is under the control of a contractor. Screening and Grit Removal Balance Tank Bioreactor Aerobic Zone Membrane Bioreactor Bioreactor Anoxic Zone E uent discharge Aluminium Sulphate Sodium Hydroxide Citric Acid Sodium Hypochlorite Figure 2. New Paxton WWTP process flow diagram. IDAL Catch Pond Maturation Pond Wetland Cell 1A Wetland Cell 2A Wetland Cell 1A Wetland Cell 2B Wetland Cell 3A Wetland Cell 3B E uent discharge In ow Figure 3. Old Paxton WWTP process flow diagram.
Water Journal November 2012-1
Water Journal August 2012