Dissolved Air Floatation (DAF) Treatment Plant will Greatly Improve Treated Water Quality
North Salt Spring Waterworks new DAF treatment plant on St. Mary Lake will greatly improve treated drinking water quality from St. Mary. Specifically mandated to the District by Island Health, DAF technology has a proven history of effectively treating water that faces challenges such as frequent algal blooms and turbidity.
Any water treatment process for water purification selected in BC must be able to comply with the BC Drinking Water Protection Act, and meet Island Health’s 4-3-2-1 Policy for treatment of surface water. In addition, the structure housing the treatment equipment must be able to function after a disaster, such as a major earthquake, and therefore must meet the post-disaster and regulatory requirements of the BC Building Code.
As well as meeting post-disaster building standards, in the District’s case the facilities must be adaptable to the constraints of the District’s existing Tripp Road site. The site is sloping with a narrow access. Additionally a geotechnical report confirmed that the west side is exposed to a potential “rock fall” hazard. This means the facility requires a small “footprint” which, when compared to other treatment plant options, the Dissolved Air Flotation (DAF) process can accommodate. The main building has been specifically designed with the site constraints in mind and will have a footprint of only 2,500 square feet.
A wet-well housing the raw water pumps and intake pipes must be situated near the shoreline. Working in such close proximity to the lake will also entail meeting the appropriate setbacks from the lake’s edge and complying with Ministry of Transportation, Ministry of Environment and Department of Fisheries and Oceans regulations as well as any local bylaws.
The main building will be a concrete structure, partially underground, and will house two DAF “trains” and all ancillary equipment such as pumps, pipes and electrical equipment. A small office, lab and bathroom as well as chlorine-generating equipment and waste de-watering equipment will be housed on the main floor. Finished water and process waste will be stored under the main floor for reuse, disposal or delivery to the distribution system.
Cutaway of a typical DAF Unit
How does a Dissolved Air Floatation water treatment plant work?
A Dissolved Air Flotation (DAF) treatment plant has seven (7) distinct steps. Initially, raw water is drawn from the lake and delivered via low lift vertical turbine pumps from the “wet-well” to the main process building. This is where the seven step treatment process begins.
Step #1 – Coagulation: As water enters the flash mix chamber a chemical coagulant is added to the water while it is being rapidly or “flash” mixed.
Step #2 – Flocculation: The flow is slowed down in the two flocculation chambers where large paddles slowly mix the water. The coagulant helps dissolved and suspended material to clump and bind together creating small particles known as “floc”.
Step #3 – Dissolved Air Floatation: The water passes into the DAF chamber (tank) where a stream of air-saturated water is pumped through a series of diffusers on the bottom of the DAF tank. As the air-saturated water exits the diffusers, the change in pressure allows the air to be released as millions of tiny bubbles that rise to the surface of the DAF tank. The rising air bubbles lift the “floc” to the surface where it is skimmed off by a rake sweeping across the surface of the tank. The waste skimmed off the top is called “float”.
Some other treatment processes allow “floc” to settle and be removed from the tank bottom but algal cells are semi-buoyant and float more readily than settle. One of the benefits of using a DAF process is that floatation works very well at removing the algal cells that are common in St. Mary Lake. Another benefit is that floatation reduces the amount of chemical required to form the floc. In addition, the DAF process removes cells gently. This means that if there is a toxic bloom in the lake, there is less likelihood of treatment processes damaging the cells and causing “intracellular” toxins to be released. Instead, those toxins remain inside the cells and are removed as part of the waste product skimmed off the top.
Step #4 – Dual Media Filter: The filters consist of sand and anthracite (carbon) of graduated sizes. By this stage in the process, most removal of undesirable materials has already taken place. The main purpose of the filters is to polish or finish the water; however, filters also act as an additional barrier in the event of a failure in any of the preceding process steps.
Step #5 – Ultra Violet Disinfection: Ultra violet light delivered at 40 millijoules/cm2 has proven effective at inactivating Cryptosporidium and Giardia so UV will be used as the primary disinfection process. Because UV light does not continue to disinfect as the water moves through the distribution system, a secondary disinfection process is also needed.
Step #6 -Chlorine Disinfection: Chlorine will be generated on-site and added at a dose sufficient to maintain effective residual chlorine levels throughout the distribution system. Residual chlorine levels must be maintained to prevent re-growth of bacteria within the distribution system. The amount of chlorine needed for disinfection will be considerably less than what the current process uses.
Step #7 – Containment: The finished water will be stored in a “clear-well” located under the main building until it is pumped to the main reservoirs and distribution system. Process wastewater (from backwashing and rinsing filters) will be collected in tanks. “Backwash” and “rinse” water will be returned to the inlet of the plant and fully recycled. DAF plants are almost universally designed to allow 10% of the flow to come from recycled process water.
The “float” (waste skimmed off the DAF tank) will be collected, dewatered and disposed of by transport to either Hartland Landfill or Burgoyne Liquid Waste Facility depending on the moisture content.