Gympie STP Phosphorus Exceedance Investigation


Investigation into phosphorus removal at Gympie Sewage Treatment Plant (STP) to prevent the reoccurrence of costly environmental licence breaches.

Services Offered

  • Review of catchment and trade waste inputs
  • Review of STP treatment processes and effectiveness of barriers
  • Review of wasted activated sludge returns and impacts on the system
  • Review of chemical dosing systems and process philosophy for chemical dosing
  • Analysis, trending and correlation of data relating to influent, treated water and chemical dosing
  • Coordination of operational activities
  • Project management of the plant reconfigurations
  • Development of troubleshooting guides and operating procedures for the management of phosphorus
  • Training of operators on mass balance and data recording
  • Amendment and additions to monitoring schedules


Gympie STP had breached their phosphorus environmental licence limits three times in the preceding year. The operations team had been faced with challenges such as:

  • lack of plant automation and online monitoring
  • highly variable influent quality
  • lack of technical knowledge with regard to mass balance, the inter-relationship between pH and phosphorus removal and the effectiveness of chemical dosing.


CWT were able to provide the following solutions:

  • Confirmation of the causes of previous phosphorus exceedances: prior to investigation, the causes of the phosphorus exceedances were anecdotal and based on assumptions that did not have an engineering basis and were not necessarily supported by a historical data record. CWT were able to build hypothesises from data analysis, trending and correlations to enable tailor-made design solutions to be proposed.
  • Buffering of phosphorus spikes: CWT made several recommendations for the reconfiguration to mass flows through the plant to successfully buffer for periods of high-phosphorus. These included the construction of a filter belt-press return line (high in phosphorus) to the head of works (instead of the downstream Oxidation Ditch) and quarantining of trade waste receival.
  • Implemented a critical control point for phosphorus removal.
  • Changed chemical dosing philosophies and triggers to ensure system and operator response modifications are proactive and delivered in a timely manner.
  • Successfully coordinated operational activities to prevent a potential exceedance as a result of a clarifier containing high levels of phosphorus going offline.
  • Provided concept design specifications for the installation of online monitoring and chemical dosing automation.



Gympie STP is inherently more robust against phosphorus exceedances and operators are more informed through training, access to resources and online automation. To CWT’s knowledge, Gympie STP has not had another phosphorus exceedance to date.