Challenge

The Northern Village of Ile a La Crosse, Saskatchewan, Canada, operates a surface water treatment plant for its drinking water supply. The water source is Lac Ile a La Crosse in northern Saskatchewan, with the corresponding cold water temperatures. The inlet turbidity is typically less than 5 NTU, but the raw water total organic carbon (TOC) can be as high as 12 mg/L. Prior to this year, the system included two conventional packaged treatment units consisting of flocculation, tube settling and granular media filtration. The filter run lengths were short, producing excessive wastewater and low system production efficiency. The high TOC was not properly addressed, which resulted in trihalomethane (THM) concentrations out of compliance with regulatory standards. In addition, during high demand periods in the summer, the water was sometimes out of compliance for turbidity, resulting in advisories to boil the water. The existing package treatment systems were also near the end of their useful life.

Solution

AECOM, the consultant hired to design a new water treatment system, ultimately picked the Trident® HS package treatment process from Siemens Water Technologies, which consists of two-stage clarification and mixed media filtration in a single tank configuration.

The system process consists of solids contacting with external sludge recirculation prior to tube settling for removing settling solids, followed by upflow passage through a bed of buoyant media in the Adsorption Clarifier®, and mixed media filtration for final polishing. All three treatment stages are combined in a single packaged treatment system. This multi-barrier approach allows the system to dampen variable contaminant loads while increasing system net production with a smaller footprint.

Chemical doses are auto-controlled by a PLC via an inlet flow meter. Plant personnel can adjust set points on the control system’s Operator Interface Terminal (OIT) to ensure proper operation.

Results

Put on-line in April, 2008, the new flocculation, settling, and filtration system was designed for a flow rate of 56.8 m³/hr (250 gpm) using a single tank package treatment system. The raw water turbidity has been between 1 and 4 NTU. The raw water TOC has ranged between 7 and 12 mg/L. The system consistently produces treated water with a turbidity of 0.15 NTU or less. The THM concentration in the treated water is about 78 µg/L. Before the plant was placed online, the province began to consider lowering the compliance levels for THMs from 100 µg/L to 80 µg/L. The system is already operating in compliance with the new level. In addition, the filter run length is three times longer than with the previous system.