Water Journal : Water Journal March 2011
industry news regular features 24 MARCH 2011 water W2W Alliance Achieves Sustainable Odour Control When it comes to real estate, a wastewater treatment plant (WWTP) in close proximity to residential areas would be a little too close to home for most people because of odour emissions and other concerns. In Western Australia, however, major advances in odour management and control systems have significantly reduced odour emissions and eliminated resident complaints at the state's largest WWTP. As part of the W2W Alliance (W2WA), Sinclair Knight Merz (SKM) has provided engineering support in a five-year $352 million program to upgrade Perth's three largest WWTPs at Beenyup, Subiaco and Woodman Point. Woodman Point WWTP, the state's largest wastewater treatment plant, serves a population of 600,000 people living south of Perth's Swan River. The plant currently treats 120 ML/d of wastewater and has a rated hydraulic capacity of 160 ML/d. The population is expected to grow at a rapid rate, with the current plant capacity reached in 2015. An ultimate capacity of 320 ML/d is projected at full development. Woodman Point WWTP was originally developed in 1966 as a primary treatment facility. At that time residential areas were distant from the plant and the surrounding land uses were rural, public reserves and industrial. In response to development in the catchment, the plant was upgraded and expanded several times. From 2000--2002, it was upgraded to provide full secondary treatment, sludge digestion and odour management. In 2003, comprehensive odour modelling predicted that elevated odour levels occurred beyond the existing buffer, although few odour complaints had been received. A subsequent telephone survey of residents close to the plant found that the majority were annoyed by the odour. To ensure that local residents were not unduly impacted, the environmental regulator directed that a further 50 per cent reduction in odour emissions be achieved by the end of 2008. W2WA consequently undertook a program of work to upgrade the Woodman Point WWTP and install best practice odour control. Odour control improvements included covering the Sequencing Batch Reactor (SBR) bioselector (anoxic zone), upgrading existing odour containment structures, increasing extraction rates throughout the treatment plant, decommissioning the existing scrubbers, constructing a new odour treatment system and installing a 50-metre-high discharge stack. Stage 1 improvements also incorporated a sludge amplification component, which included a new sludge loadout enclosure, ferric chloride dosing of sludge and high- temperature waste gas flares to reduce odour emissions. Odour Control System Design Chemical scrubbers had been installed at Woodman Point in 2002 and in the other Perth metropolitan WWTPs as the Water Corporation's preferred technology for odour removal. They provide effective and reliable treatment, and operations staff are familiar with their operation and maintenance requirements. However, with the high hydrogen sulphide (H2S) loads predicted at Woodman Point, chemical scrubbers would consume a large quantity of chemicals and be expensive to operate. Biotrickling filters, combined with downstream chemical scrubbers, were determined to be the optimum combination of a robust and reliable system at the lowest capital and operating cost. Incorporation of high-efficiency biotrickling filters was critical to ensuring a more sustainable approach to odour management, by eliminating chemicals and moving to a treated effluent water source, compared with traditional chemical scrubbing technology alone. The downstream chemical scrubbers treat the low-odour Sequencing Batch Reactor (SBR) bioselector airstream and biotrickling filter discharge, providing a target odour discharge of less than 1,000 odour units (OU) and less than 100 ppb H2S. Based on comprehensive odour modelling, a 50-metre-high discharge stack was required to achieve effective dispersion of any remaining odours. System Reliability & Performance The target reliability and availability of the odour treatment system is 99.9 per cent, equating to a downtime of just eight hours and 45 minutes a year. In order to achieve this strict requirement, the design incorporated numerous elements to maintain continuous operation of the treatment systems and fans. Careful consideration was given to the critical service requirements for electricity and water, to ensure standby systems were generally available for all mechanical and electrical equipment.
Water Journal April 2011