Australia's Largest Wastewater Odour Control Facility: A Whole Life Approach

Malabar Sewage Treatment Plant (STP) serves a population of approximately 1.7 million making it the largest in Sydney. The STP is located in the suburb of Randwick in a coastal environment and is owned and operated by Sydney Water. The majority of the STP's treatment processes are located underground and ventilation is provided to ensure both a safe working atmosphere and odour containment. Its wastewater odour management system treats a maximum foul air flow of 810,000 m3/h (225 m3/s) making it the largest in Australia. Encroaching local development and an increase in odorous loads above original design thresholds are challenging the existing STP to remain regulatory compliant.MWH has recently completed a holistic odour management study of the STP to evaluate future performance and capital investment requirements. The objective of the study was to identify the most cost effective strategy for the STP to remain regulatory compliant over the next 30 years, whilst minimising capital outlay. Using a multi criteria analysis it was determined that optimising and upgrading existing assets to maximise their life was the best option followed by a replacement system.The foundation of the study has been the development of a detailed assessment of the existing ventilation (both clean air supply and foul extraction) and treatment facility (wet chemical scrubbers). Approximately 100 fans and several kilometres of ducting in sizes ranging from 0.1m to over 5m in cross section operate to ventilate the plant to the odour management facility. Working closely with Sydney Water, ventilation audits, odour characterisation, smoke and pressure testing have all been completed to confirm operational performance, identify optimisations and capabilities of the existing system to ultimately define potential savings in asset re-use.The audits have identified pinch points in the existing ventilation (excessive ventilation, no foul/clean air segregation, positive pressures) and treatment systems (packing types, heights, chemical storage and dosing capacities, recirculation flowrates). This approach allowed the maximum reuse of existing equipment to be included in the study recommendations (i.e. fans, ductwork, scrubber shells) together with replacement of dosing facilities, recirculation pumps and pipework and packing to improve installed equipment performance capability. Significant reductions in foul air flow (40% through segregation) have been identified to allow Sydney Water to consider future capital and operational costs savings come the end of the existing systems life.These improvements will achieve the required treatment capability, allowing deferment of capital expenditure for a new system. Consequently a ‘refurbishment then replacement ’ strategy has been proposed for the STP, minimising the requirement to replace existing facilities before the end of their expected life.The study has provided Sydney Water with the most cost effective strategy for maintaining regulatory compliance at Malabar STP now, and into the future.