Secondary Clarifiers Modification Cost of 0.6-Million Achieves 36-Million Cost Savings for Calgary's 500 ML/d Bonnybrook BNR WWTP

At the 500 ML/d Bonnybrook Wastewater Treatment Plant, Secondary B, consisting of activated sludge reactors 5 – 8, underwent a major retrofit to the biological nutrient removal technology in 1997. Subsequently, a full-scale study was undertaken in February 2000 to determine Reactor 5's treatment capacity during Calgary's winter months. The study indicated Reactor 5's treatment capacity had to be derated from its 57,500 m3/d design capacity to 37,000 m3/d. To recover the 82,000 m3/d capacity loss for Secondary B, Reid Crowther and Partners Ltd. recommended a number of physical and operational modifications to Reactor 5's three secondary clarifiers. Two recommended physical modifications were completed on November 2000.In January 2001, a full-scale comparison study was undertaken to assess the positive impacts on Reactor 5's performance. The study results confirmed the total success of the modification project in terms of the performance improvement and resultant cost savings for Secondary B. This paper details the project's success in the recovery of the 36% capacity loss, in the production of an excellent-quality effluent, and in the creation of CAD 36-million capital cost savings, as well as CAD 360,000 savings in annual operation and maintenance costs for The City of Calgary.