761 Upper Ganges Road Salt Spring Island, BC V8K 1S1 Phone: (250) 537-9902
Why This Design & Configurationadmin20122018-07-10T16:52:08+00:00
WHY THIS DESIGN & CONFIGURATION
By Ron Stepaniuk
The culmination of years of research, development and design, the Board and Management are pleased to finally be able to present the detailed design for the proposed St. Mary DAF water treatment facility.
When developing a project of the size and complexity of the St. Mary water treatment facility, there are a number of options or paths available. The NSSWD Board of Trustees elected to proceed with a traditional conservative option, simply referred to as Design, Bid and Build, which allows the Board the greatest control over what is constructed and the quality of the final operating facility. Specifically this process is intended to provide the lowest Life Cycle cost over an extended period of greater than 20 years.
The first step in this process is to engage a design consultant, by either selecting and engaging directly or issuing a request for proposal. The Board elected to engage Kerr Wood Leidal (KWL) directly, a firm with a long and proven track record in engineering design of waterworks facilities and infrastructure in BC.
The second step is to establish design criteria; what are you building and why and how will it function? If a utility is large enough to have an in-house engineering department this responsibility would usually fall to them. Since NSSWD has no in-house engineering department the design criteria were developed in direct consultation between NSSWD senior operations staff, the Board and KWL. This relationship has worked well for a number of reasons.
There were a number of fixed constraints, both positive as well as negative, that required careful and innovative solutions during the design process.
Constraints and Parameters:
NSSWD already owns the property.
Location is of adequate size and largely removed from residential properties.
Site topography allows gravity flow through the treatment process and minimizes the number of pumps required.
Potential rock fall hazard can be addressed and mitigated through design.
NSSWD is already withdrawing water from this site so no additional distribution system upgrades are necessary.
Lake depth at this location allows for a reasonably short raw water intake.
Three-phase power is in place at the site.
There is no sanitary sewer for disposal of treatment process residuals or sanitary waste.
DAF technology has been pilot tested and proven to work on St Mary Lake water.
DAF technology is currently in place and functioning on St. Mary Lake (CRD Fernwood – Highlands).
Extensive water quality data is available from NSSWD records.
Island Health has mandated the use of DAF by way of the amended operating permit.NSSWD elected to proceed with the proposed design criteria for many reasons, including:
Building Considerations included:
A long life cycle for building materials.
The concrete structure having the structural strength to support heavy piping and equipment.
That the design and construction fully meet post-disaster design requirements.
A small footprint allowing adequate access and egress and maximum usable yard area for daily operations and installation of stand-by power generator.
NSSWD’s good neighbour policy of being aesthetically acceptable and minimizing mechanical noise.
The use of underground storage tanks as they suit the topography of the site and permit the building foundation to be keyed into bedrock for seismic considerations.
Utilization of the blasted rock as structural fill throughout the site for the driveways and travelled site area, eliminating the cost and need to remove it from site.
Lessons learned during the construction, commissioning and daily operation of other island DAF plants.
The design allows for operational flexibility, including:
The treated water will meet the health and aesthetic water quality requirements of the Guidelines for Canadian Drinking Water Quality.
The intake pump station wet well enables pre-treatment to mitigate potential seasonal taste and odour concerns.
The intake pump station wet well also provides the ability for pre-treatment to oxidize soluble manganese to reduce the risk of it passing through the treatment process.
Maximum Day design flow will be equal to maximum licensed withdrawal limit.
The capture and reuse of all filter rinse water, which also promotes water conservation.
The capture and reuse of the settled filter backwash water, again promoting water conservation.
On site de-watering of DAF float and settled backwash solids reduces disposal cost.
The intake pump station wet well allows the controlled blending of recycled water (settled backwash and filter rinse) with the incoming raw water from the lake.
The intake pump station wet well provides emergency overflow storage for the clear well and backwash settling tank.
The combined expenditure for the preliminary and detailed designs is $821,000, all paid for from reserve and operating funds. It is hoped that our upcoming donor campaign will raise funds to further reduce our necessary borrowing costs and an additional portion of the District’s reserve funds will also be applied to the costs before borrowing. However, for the Borrowing Bylaw the Ministry requires that we state a maximum amount.
Throughout the design phase the project team has worked hard to reduce costs. Based on changes in design resulting from reduced plant capacity and a reduction in the overall contingency, the cost estimate for construction of the facility is approximately $8.4 million, including a 10% contingency and an allowance of up to 10% for construction phase contract administration and required quality control testing/field reviews of the construction. The final construction contract administration fees are to be negotiated based upon the duration of the construction period. Once the project is tendered and bids received, NSSWD will be in a position to proceed with the construction based upon a fixed lump sum contract.
As your representatives, the Board strongly believes this is the best design for St. Mary Lake water and the best site configuration for our plant location, and once you have an opportunity to have your questions answered we believe you will too. The Board and Management look forward to presenting the detail design and answering your questions at our Open Houses on March 12th and March 17th.