Water Journal : Water Journal March 2011
refereed paper sewer processes water MARCH 2011 87 further investigation, as it may have cost- saving implications for networks suffering from a well developed biofilm that imparts head loss due to substantial biofilm friction. Conclusion Caustic soda washing was found to be a viable additional process, offering improved control of the elevated H2S gas levels. Of exceptional importance, this process also offers the additional benefit of control over the generation of methane gas, which the typical ferrous chloride dosing does not. For the LAP system, caustic washing has proven itself as a necessary additional step to the chemical dosing and air treatment facilities that were already in place. This procedure was progressed to a preventative measure over the summer months and saw our network safely through the warmer periods. After performing three washes over six months, a reduction in H2S of approximately 60% was still evident and CH4 levels were still well within an acceptable range. The LAP has now been converted to a recycled water main, and following the success of the caustic washing, the caustic soda dosing methodology was used as part of the cleaning process. Biofilm formation and elevated H2S and CH4 are no longer an issue, since the high quality effluent does not pose these problems to the same extent that was found when the LAP served as a sewer pressure main. Wastewater utilities that have networks similar to the LAP and can identify with these problems of biofilm-enhanced H2S and methane generation may benefit from adopting a similar caustic soda dosing methodology. In some cases, internal and external regulators are perhaps enough driving force behind a need to lower H2S emissions, especially when the existing chemical dosing and air treatment facilities are simply not sufficient. In other cases, the financial benefits associated with the Caustic Washes may also be reasonable (or at least synergic to the other benefits) where air treatment facilities will have less H2S loading, hence less frequent media changes, and the need for other chemical dosing may decrease. For any wastewater utility, maintaining customer satisfaction and public image is of equal importance to any financial bottom line. The procedure and/or dosing rig may be transferred to other similar networks as necessary, especially where a large pipe diameter and long retention time pose similar biofilm-enhanced gas generation problems. Sydney Water has several projects underway, both currently and in the near future, to provide sewer services to remote locations through the Priority Sewerage Program alliance. As these sewer networks are likely to be low- pressure sealed mains covering significant distances, caustic-shock dosing may be considered and even designed into the initial network plan as a permanent facility in order to tackle the biofilm problems. As an odour management strategy, caustic washing is relatively non-complex and financially efficient. There is no significant financial deterrent in adopting a caustic soda dosing arrangement, with low capital costs and simple equipment. As well as this, most wastewater utilities already have chemical dosing standards and guidelines developed from their existing chemical dosing requirements. In our particular situation with the LAP, when taking into consideration both the operational aspects as well as the timing aspect of the recycled water plant, Caustic washing was an excellent choice financially. Compared to the other options in Sydney Water's assessment report, the entire Caustic Wash process cost around one 20th of the air treatment facility or chemical dosing alternatives. The most important factor in the planning of a Caustic Wash on any network is the execution of a comprehensive and thorough risk assessment. For the LAP this was done at the concept stage, where all relevant internal stakeholders contributed at the initial risk assessment and were also involved in subsequent reviews. This was a major factor in the success of this project and served to identify some less obvious risks, such as ammonia generation at high pH, which were hence addressed with a risk-minimising strategy. The authors of this paper will be glad to share a little of their caustic shock-dosing experience and advice with any interested utilities, and can be contacted as detailed at the end of this paper. Acknowledgements Special thanks go to Tung Nguyen (Technical Services Manager) and Enrico Coiro (Wastewater Networks South) for their contributions in the planning and execution of this project. We would also like to thank the teams from Liverpool WWTP, Treatment Operations, Networks Operations, Technical Services, Monitoring Services and all others involved for their assistance. Footnote: This paper was originally presented at the IWA SPN6 Conference in November, 2010. The Authors Jack O'Gorman (email: jack.ogorman@ sydneywater.com.au) graduated with a BE (Hons) in Industrial Chemistry from the University of NSW in 2008. He joined Sydney Water in 2009 on the graduate program, with rotations in Networks Programs (Wastewater) and Wastewater Treatment (Lower Hawkesbury Hub). Ian Purssell (email: ian.purssell@ sydneywater.com.au) graduated from Sydney University with Bachelor Degrees in Engineering (Chemical) and Science in mid-2007. He joined Sydney Water in mid-2008 as a graduate. He undertook rotations in Networks and Treatment and is now a Production Officer at West Hornsby WWTP. Gino Iori (email: gino.iori@sydneywater. com.au) graduated with an honours degree in Chemical Engineering from the University of NSW in 1988. After working as an engineer for a large construction company (Transfield Const P/L) and an international chemical company (ICI Aust), he joined Sydney Water in 1993. He is currently a Systems Operation Officer in the Operations Division with the portfolio of Odour and Corrosion. References Gerardi, MH 2006, Wastewater Bacteria, John Wiley & Sons, Hoboken, New Jersey. Odour and Corrosion Assessment -- LAP in Malabar Network July 2009, Sydney Water internal document.
Water Journal April 2011