Research Undertaken to Improve Sustainability and Reduce Costs of BNR Plants

Liam Tamplin (Beca HunterH2O), Craig White (Beca HunterH2O), Jayden Ball (Beca HunterH2O), Gordon Cunningham (Queanbeyan Palerang Regional Council)

In designing the 75,000 Person Equivalent (PE) Biological Nutrient Removal (BNR) Plant for Queanbeyan Palerang Regional Council in NSW Australia we challenged some of the accepted design assumptions. In doing this we undertook four targeted research projects to remove risks, lower cost, and improve treatment performance. All projects have been designed into the new plant and the research projects are summarised below.

Enhanced Chemical Phosphorus Removal. In exploring the operation of the current Queanbeyan plant we discovered unusually efficient performance with the current chemical phosphorus removal process. We explored this further and undertook extensive jar testing and discovered a new more efficient chemical phosphorous removal approach. By co dosing calcium with ferric chloride a considerable reduction in the chemical use by up to 40% can be achieved. An additional advantage is the reduction in sludge production by up to 10% when compared to ferric chloride only dosing. Adopting this approach will save $270,000 per year (for 75,000 PE) compared to single chemical phosphorus removal.

Low Energy Hydraulic Mixing. A pilot plant was run to investigate how to use the energy in the inflowing stream to remove the need for mechanical mixing in unaerated zones. The pilot plant identified how to configure under and overflow baffles to mix sludge. We found full mixing was possible with only a low additional head loss through the reactor. This approach reduced the mixing power cost by 95% ($0.34/PE per year reduction in operating cost) compared to conventional mechanical mixing. It also eliminated the need for ongoing maintenance costs for mechanical mixers.

Enhanced Storm Treatment. Improved gravity clarification using a solids contact approach was investigated. This research identified partial bypassing of the activated sludge reactor in wet weather and recombining it with activated sludge in a gravity clarifier almost doubles the clarifier capacity without impacting UV disinfection performance. This saved significant capital cost (~ $10M) on this project and enabled high volumes of storm flow to be disinfected with smaller clarifiers.

Slow the Oxidation Ditch Down. Normal practice is to run oxidation ditches at flow velocities of at least 0.3 m/s to ensure mixing. However, slowing this velocity has other benefits such as reduced energy use, lower ammonia in cold climates, and higher biological phosphorus removal. The nutrient removal benefits come from being able to achieve a higher dissolved oxygen at lower velocities. We investigated how low the velocity could be lowered in an operating plant in Townsville and still maintain mixing. We found the velocity could be lower by up to 50% and maintain mixing for low solids concentrations which are challenging to mix. This significantly challenged well entrenched accepted design standards for oxidation ditch mixing.