This project aims to substantially reduce the cost of wastewater treatment by developing a technology, and the underpinning science, that will reduce sludge production, improve biological nutrient removal and enhance renewable energy production. A wastewater treatment plant should be operated to achieve satisfactory biological nutrient removal, maximum bioenergy recovery and minimum sludge production.

However, the achievement of these goals is often hindered by the low biodegradability of secondary sludge. The project team has found that free nitrous acid (FNA) was strongly biocidal to secondary sludge. Also, FNA was found to be more biocidal to ammonium oxidizing bacteria than to nitrite oxidizing bacteria (NOB).

Based on these findings, we developed a sludge pre-treatment strategy based on FNA to enhance methane production through anaerobic digestion of secondary sludge and to reduce sludge production during aerobic digestion of secondary sludge. Also, we developed an FNA-based sludge treatment strategy to selectively eliminate NOB and achieve nitrogen removal via the nitrite pathway. Collectively, the FNA-based sludge pre-treatment approach can be used to improve biological nutrient removal, enhance methane production and reduce sludge production simultaneously in a wastewater treatment plant. 


  • ARC Linkage Project


  • Sydney Water
  • City of Gold Coast

Project Outcomes


  • The bio-methane production could be enhanced by 30-60% during anaerobic digestion through pretreatment of secondary sludge by FNA at 1.5 mg HNO2-N/L.
  • The nitrite pathway could be steadily achieved with an average nitrite accumulation ratio of above 80% with part of the returned sludge treated with FNA at a level 1.35 mg N/L.
  • FNA pretretament is effective in enhancing degradation of secodnary sludge during aerobic digestion, post anaerobic digestion and post aerobic digestion.
  • FNA-based technology has the potential to transform wastewater treatment from an energy consuming process to an energy generating process.


The FNA-based technology has attracted great attention from both the academic community and industry. UniQuest Pty Ltd, the commercial arm of The University of Queensland, is currently commercialising the FNA-based technology throughout the world.

Several large international water treatment companies including Aqualia and Veolia have also shown their strong interests in this technology.


  • Huber Technology Prize (First class) from Germany in 2014



  • Wang, Q., Hao, X., Yuan, Z. (2016). Towards energy positive wastewater treatment by sludge treatment using free nitrous acid. Chemosphere, 144, 1869-1873.
  • Zhang, T., Wang, Q., Ye, L., Yuan, Z. (2016). Enhancing post anaerobic digestion of full-scale anaerobically digested sludge using free nitrous acid treatment. Journal of Industrial Microbiology & Biotechnology, 43, 713-717.
  • Zhang, T., Wang, Q., Ye, L., Yuan, Z. (2016). Effect of free nitrous acid pre-treatment on primary sludge biodegradability and its implications. Chemical Engineering Journal, 290, 31-36.
  •  Zhang, T., Wang, Q., Ye, L., Batstone, D., Yuan, Z. (2015). Combined free nitrous acid and hydrogen peroxide pre-treatment of waste activated sludge enhances methane production via organic molecule breakdown. Scientific Reports, 5, 16631, DOI: 10.1038/srep16631.
  • Wang, Q., Yuan, Z. (2015). Enhancing aerobic digestion of full-scale waste activated sludge using free nitrous acid pre-treatment. RSC Advances, 5, 19128-19134.
  • Zhang, T., Wang, Q., Khan, J., Yuan, Z. (2015). Free nitrous acid breaks down extracellular polymeric substances in waste activated sludge. RSC Advances, 5, 43312-43318.
  • Law, Y., Ye, L., Wang, Q., Hu, S., Pijuan, M., Yuan, Z. (2015). Producing free nitrous acid – a green and renewable biocidal agent – from anaerobic digester liquor. Chemical Engineering Journal, 259, 62-69.
  • Wang, Q., Jiang, G., Ye, L., Yuan, Z. (2014). Enhancing methane production from waste activated sludge using combined free nitrous acid and heat pre-treatment. Water Research, 63, 71-80.
  • Wang, Q., Ye, L., Jiang, G., Hu, S., Yuan, Z. (2014). Side-stream sludge treatment using free nitrous acid selectively eliminates nitrite oxidizing bacteria and achieves the nitrite pathway. Water Research, 55, 245-255.

Conference proceedings

  • Zhang, T., Wang, Q., Ye, L., Batstone, D, Yuan, Z. (2015). Synergistic effects of free nitrous acid (FNA) and hydrogen peroxide on methane production from waste activated sludge (WAS). Oral presentation at 3rd International Conference on Water Research, Shenzhen, China, 11-14 January, 2015.
  • Wang, Q., Ye, L., Jiang, G., Hu, S., Yuan, Z. (2014) A novel method for achieving the nitrite pathway in biological wastewater treatment. Oral presentation at IWA Specialist Conference: Global Challenges: Sustainable Wastewater Treatment and Resource Recovery, Kathmandu, Nepal, 26-30 October, 2014.
  • Wang, Q., Ye, L., Jiang, G., Yuan, Z. (2014) Sludge treatment using free nitrous acid to enhance performance of a wastewater treatment plant. Oral presentation at IWA World Water Congress and Exhibition, Lisbon, Portugal, 21-26 September, 2014.  


Project members

Professor Damien Batstone

Professor and Centre Director, ACWE
Australian Centre for Water and Environmental Biotechnology

Dr Haoran Duan

Research Fellow
Australian Centre for Water and Environmental Biotechnology

Other members

  • Qilin Wang
  • Tingting Zhang
  • Liu Ye

Industry partners

  • Kelly O'Halloran
  • Heri Bustamente