City Nearly Doubles Capacity with DAF Pretreatment System

At a time when many water utilities are caught in the squeeze between lower capital budgets and the need for higher plant capacity, the City of Penticton, BC, nearly doubled the water treatment plant’s peak production with an advanced clarification technology. The retrofit achieved the highest benefit-to-cost ratio at lowest life cycle cost and should meet the projected water demand for the next half century without infrastructure improvements.

Penticton is a resort destination with significantly higher summer population than the 35,000 normally served by 9000 connections. The potential problem emerged in 2003 at the water plant when demand during that exceptionally hot and dry summer reached 55 ML/d (million liters per day) of the facility’s 60 ML/d capacity.

“With our original clarification process, we were at capacity,” said Brent Edge, Penticton Water Quality Supervisor.

This capacity issue arose while lingering hydraulic deficiencies existed in the drinking water distribution system and discharges of clarifier and filter backwash sludge into the sewer system sludge were eroding the capacity of the WWTP and subsequently damaging the creek environment due to its discharge water quality.

Estimates to correct these incremental shortcomings with individual projects exceeded the city’s financing capabilities. This led the municipality to contract with the Vancouver office of AECOM to study the overall situation and to develop a comprehensive -- and more affordable — solution to improve the existing infrastructure.

“The consolidation of engineering and project management responsibilities with one firm not only saved in design but likely contractor fees,” Edge said. “AECOM worked closely with the city’s engineering and operations staff to develop five conceptual retrofits — complete with capital and life-cycle operating cost estimates -- that would add the needed capacity and resolve other operational issues.”

The conceptual designs were based on one of three raw water sources or combinations thereof that would deliver more than the maximum 60 ML/d of the existing Okanagan Lake pump station and dedicated water main, Edge added. In addition to the high-quality water from Okanagan Lake, the alternative sources included a similar water quality from the city well or Penticton Creek that presented seasonal quality, and therefore, chemical treatment issues. However, all of these raw water sources were sufficient to ensure a long-term supply whose life-cycle costs were affected by their different characteristics.

The engineers modeled the five conceptual alternatives to achieve a maximum daily water production of 81 ML/d during peak demand periods. The models would all be capable of an ultimate demand of 111 ML/d through Year 2025. After assessing how to consistently produce the desired volume and water quality, the best option for meeting peak demands was increased supplemental reliance on Penticton Creek. This offered the highest benefit-to-cost ratio and lowest life cycle operating costs within the plant’s existing 60 ML/d design capacity and ultimate annual production of 7900 ML.

DAF System

The team decided to go with a dissolved air flotation (DAF) system that could overcome the creek’s raw water shortcomings. The upgrade provide an impressive 75 percent capacity increase.

The system was installed within the existing single sedimentation basin that is now two parallel high-rate DAF basins. This major component fit within the existing physical footprint and with minimal environmental impacts during construction activity. The plant has increased the effective clarification capacity from 30 to an estimated 115 ML/d without requiring construction of larger basins.

The Leopold Clari-DAF® system supplied by ITT Water & Wastewater provides the utility with the flexibility to treat both the creek and lake water at any time of year to the desired water quality levels and meet peak demands without adding new clarification capacity for the immediate future. The plant continues to operate with the existing six, single-media anthracite 4.5 x 10 x 1.8-meter filters. The lake and creek raw water now can be used exclusively or alternated to compensate for seasonal changes in water quality, plant demand and to fully benefit from different costs.

In the Clari-DAF system, the Okanagan Lake raw water produces very light floc, which allows increased clarification flow with the flotation technology. Using a dual source for raw water also uses gravity flow for creek draws instead of incurring the energy costs to lift the Lake Okanagan water from an elevation of 341 meters to 461.5 meters.

Unlike traditional sedimentation basins that settle out particulates for discharge from the bottom, a DAF system receives flocculated raw water, separates and then carries the particulate matter to the surface. Countless micro-sized air bubbles injected through diffusers at the bottom of the contactor perform this task by mixing with the coagulated solids that rise to create a bed of floc across the surface where it is skimmed off mechanically. Meanwhile, the clarified water is removed from the bottom of the basin by laterals and advances to the filtration stage.

The latest DAF technology can clarify raw water at loading rates three to four times higher than a conventional design. The Penticton plant project represents the largest DAF retrofit and largest high-rate facility in Canada and supports sustained filter loadings of double the previous rate. This should prove sufficient until 2023, verified by pilot testing and subsequent full-scale challenge tests of the filters.

Residual Handling

The WTP’s original design for handling residuals achieved lowest initial and long-term costs by sewer system discharges to the WWTP while accepting the environmental impacts of the former discharges into the creek. But treating the WTP’s residuals at the WWTP would result in higher operating costs vs. management at the WTP site. Once again, the DAF system proved inherently beneficial by generating two to three percent thicker total suspended solids that can be further processed using a relocated centrifuge that was idle for years at the complex.

Approximately four to six percent of the WTP’s waste stream derives from filter backwashes. Prior to the retrofit, the original design relied on a low-cost decant tank that overflowed into the creek. This sometimes resulted in an inadequate process and presented scheduling constraints for backwashing and the routine manual removal of accumulated solids.

This issue has been resolved by the lamella gravity thickeners which generate a high quality clarified effluent returned to the creek and a thickened sludge blended with the DAF sludge for dewatering in the centrifuge. This method, more common in Europe, was readily adapted to the Penticton plant and could become a model for other plants.

The total program cost $10.8 million and allows the plant to operate for $1.1 million in annual cost -- significantly less than a National Benchmarking Data estimate.

ITT LEOPOLD® is an exhibitor at the ACE® ‘11 event and can be found at Booth No. 1625

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