A number of Quebec municipalities are confronted with feedwater problems of a bacteriological or physicochemical nature hardness, manganese, iron, dissolved solids) in the production of their drinking water. In many cases, these water sources do not meet government quality standards for drinking water. Membrane filtration preceded by the appropriate pretreatments can be a solution.
Following talks between the municipality, BPR Groupe-Conseil, and H2O Innovation, agreement was reached on a pilot project using nanofiltration membrane filtration. The choice of this type of membrane was based on the physicochemical properties of the water to be treated and the quality of water required by the municipality.
The pilot study would serve to confirm the choice of this type of membrane and identify the required pretreatment processes. The study was carried out over a 50 day period in the summer of 1998. A complete chemical analysis of the water was required in order to configure the pretreatment and ensure optimal membrane performance while keeping in mind system maintenance and operating costs.
Treatment sequences (table 1)
- Pumping of feedwater (artesian wells);
- Antiscalant dosing;
- Filtration (nanofiltration unit);
- Accumulation of treated water.
The use of an antiscalant allowed us to reduce clogging due to scaling substances and water hardness. This optimized membrane performance and lifespan.
The main parameters were the following:
- Permeate recovery rate (treated water);
- Operating pressure;
- Flow of treated water;
- Maximum membrane flux;
- Maximum recovery per membrane;
- Amount of desired drinking water;
- Frequency of membrane washing.
Computerized simulations were used to guide us throughout the pilot project. Three complete raw and treated water analyses were carried out on days 1, 30, and 50 of the project to obtain a more comprehensive range of possible variables in the quality of the raw and treated water.
A comparative table (table 2) of the findings of laboratory analyses obtained during the pilot project revealed that the desired water quality objectives had been attained, expressed in terms of the following parameters: chlorides, conductivity, total hardness, manganese, total dissolved solids, and total solids. Figures 1 to 6 chart the progress of the above-mentioned parameters during the study. They also show that each of the critical raw water parameters attained values below provincial standards for drinking water quality.
The results of the silt density index (SDI) analysis performed on raw water samples revealed that the water contained an SDI of less than 0.1, which translates into a very low membrane-clogging potential. These findings led us to conclude that specific pretreatments such as sand or multimedia filtering were unnecessary. A dose of antiscalant and the use of a 5 micron prefilter upstream from the nanofiltration system proved sufficient.