Water Journal : Water Journal September 2012-1
refereed paper membrane technology water SEPTEMBER 2012 69 Abstract Paxton Wastewater Treatment Plant (WWTP) is a 1 ML/day capacity facility situated in the Hunter Valley in New South Wales. A growing population, as well as tighter discharge requirements for the sensitive discharge environment, resulted in the need for a plant upgrade. The upgrade was delivered via an Alliance contract with Hunter Water Australia (HWA), as the contracted operator, liaising closely with the Alliance team. This paper outlines the practical issues involved in the brownfield upgrade and commissioning of the membrane bioreactor process adopted for the upgrade, as well as the problems encountered and the changes made in order to resolve them. The plant optimisation process is outlined and the performance results given. Operational and maintenance lessons learned are also discussed. Introduction Paxton WWTP was built in 1993 and originally served a dormitory town for the local mining and forestry industries. The plant had no inlet works and consisted of an intermittently decanted extended aeration (IDEA) activated sludge process that discharged into a catch pond and maturation pond prior to discharge to Congewai Creek via two artificial wetland cells. A portion of the effluent was also discharged into a two-hectare woodlot area. The NSW Government had made the commitment to sewer two local townships, and a growing population and proposed development in the area, combined with stricter discharge requirements, meant that an upgrade of Paxton WWTP was needed. During the selection process a number of options were considered. These included decommissioning the treatment plant and transferring effluent to another treatment facility. Once it was decided to upgrade Paxton WWTP, a 5-stage Bardenpho process was initially considered due to the low total nitrogen discharge requirements. After the Alliance delivery method was chosen, the cost for this process was found to exceed the allowed budget. Other process options were then considered before it was determined that a membrane bioreactor (MBR) process, combined with chemical phosphorus removal, was the most cost-effective option. The new process was designed to meet the effluent quality requirements shown in Table 1. Table 1. Concentration limits for Paxton WWTP. Analyte Percentile Limits (mg/L) 50%ile 90%ile TSS 15 10 BOD5 10 6 TN 15 8 TP 1 0.3 pH (100 %ile) 6.5--8.5 Methodology The project involved providing operational input from an operator's perspective on the design, construction, commissioning and optimisation phases of the Paxton WWTP upgrade. The initial upgrade concept design did not incorporate MBR technology. The decision to adopt MBR was made by the Alliance during the Turn-Out Cost (TOC) development phase. The upgraded process incorporated biological nitrogen removal, chemical phosphorus removal and immersed ultrafiltration membranes for solids removal. The design incorporated a high degree of automation, remote monitoring and remote control to minimise the need for operator attendance for this relatively remote site. During the upgrade the existing activated sludge plant needed to remain operational and comply with licence discharge limits, which proved to be challenging. During the commissioning and proving phases, HWA worked with the Alliance to optimise the process in terms of effluent quality and energy efficiency. HWA also worked closely with the Alliance to rectify identified defects. Alliance Delivery Method An Alliance contract model involves collective responsibility for risk, performance and outcomes (gain-sharing/ pain-sharing) and attempts to avoid a blame culture. The Alliance involved a designer, constructer and the owner/ operator. The main factors that favoured the Alliance in this instance were a major program of capital works involving multiple plant upgrades, short delivery timeframe for the upgrade program, desire to have owner/operator input to design, design flexibility, legacy documents, and learning from experiences on each upgrade. These factors are shown in Figure 1. K Jones, D Bailey, L Procter Lessons from a WWTP upgrade in the Hunter Valley MEMBRANE BIOREACTOR COMMISSIONING AND OPERATION Figure 1. Alliance delivery method characteristics.
Water Journal November 2012-1
Water Journal August 2012