Situated some 500 km north of Winnipeg, the municipality of Barrows is home to about 300 inhabitants. Barrows has a school in the village with close to 100 students. The only available source of water supply for the municipality is an underground spring with a TDS (total dissolved solids) concentration exceeding 1900 mg/L making the water undrinkable (see Table I). At the request of John Brittain of the Ministry of Aboriginal and Northern Affairs, a reverse-osmosis membrane filtration system was designed and installed to provide the community with very good quality of drinking water. The results of system performance testing show a decrease of more than 98% in TDS, chlorides, sulfates, bicarbonates and sodium.
During pre-treatment (accumulation tank residence time ≅ 25 minutes), air is injected to trigger iron oxidation (fe++), and then the water is run through a multimedia filter, which retains suspended material and oxidized iron. Next an antiscalant is injected to chelate the calcium and magnesium (hardness). The water then enters the reverse osmosis unit, composed of 5-micron (nominal) pre-filters, each with four BWLE-440 (Filmtec) membranes, and a 10-HP Grundfos centrifugal pump. The entire unit is controlled by an automatic control system with telemeteric devices for remote data collection and operation parameter control. The system outputs 17 gpm at an operating pressure of 250 psi with a recovery rate of 75%.
In terms of the kinetics of fouling, Figure I shows pressure increase as a function of time with flow and temperature constant (17 gpm at 90C). During the first 400 hours (September 8 to November 1, 2000), pressure remained constant. Between the 400th and the 500th hour, there was a slight increase in pressure (265 psi), generally an indication of the onset of fouling. Between the 500th and the 600th hour, there was a rapid increase in pressure—from 260 psi to 320 psi. As a result, a three-stage chemical washing was performed:
- Hydrochloric acid;
- Peracetic acid;
- Sodium hydroxide.
After the washing, the system returned to its initial pressure of 250 psi. In other words, the unit must be washed every 600 hours—that is, about every 2.5 months.