Case Study: Odour Risk Management at the Western Treatment Plant, one of Australia's Largest and Most Unique Wastewater Treatment Plants

The Western Treatment Plant (WTP) is one of Australia's largest and most unique wastewater treatment plants (WWTP) treating over 52% of Melbourne's sewage in a process consisting of open carriers, covered anaerobic lagoons, aerobic lagoons and activated sludge treatment. Large lagoons are also used for the storage and processing of dewatered waste activated sludge from a number of Barwon Water facilities. Changes in odor emissions from any of these sources can have a significant effect on off site odors, however reliably assessing the varying impact of these sources is much more complex than for other WWTPs due to the scale and the unique nature of the facility. In addition, the cost to implement viable odor mitigation options would be much higher than for a more conventional plant due to the large area of sources, therefore accurate assessment of impacts is critical.For these reasons, the data collection and assessment phase of the project was more comprehensive than would normally be undertaken for an odor impact assessment, as well as integrating some unique and interesting new tools and approaches including:The use of field olfactometry to measure ambient odor levels at the plant boundary and potential offsite impacts.Dispersion modeling of major sources to verify odor emission rates with field observations and data.Establishing the accuracy and reliability of using H2S as a surrogate gas that could be readily measured to ascertain odor levels from sources with diurnal and seasonal odor emissions.Confirmation of the log linear relationship between odor strength and perceived odor intensity.Objective and subjective community odor surveys over several weeks and three different seasons to ascertain odor intensity and perceptions in the community.By collecting this wide range of data it was possible to cost effectively, establish seasonal and diurnal odor variation and to cross correlate and validate assumptions and emissions used, so that more accurate dispersion modeling and impact assessments could be undertaken. It also ensured that conclusions could be validated in a number of ways providing more assurance of the necessity to undertake the proposed odor mitigation works.There were concerns that causality effects in Ausplume could result in overly conservative predictions and subsequently the implementation of more odor mitigation works than are required to mitigate the risk of odor complaints.Therefore the investigation also assessed the available odor dispersion models, (Ausplume, CALPUFF and TAPM) and their appropriateness for assessing odor impacts from this unusually large site. In particular the predictions of Ausplume were compared to the predictions of CALPUFF, community surveys and field olfactometry data to ascertain the relative accuracy of each model.This paper will present the results of this investigation including some interesting findings about the relationships between odor strength, perception (how humans interpret odor strength) and odor annoyance in the community.