Scale Issues within Reverse Osmosis Plant
The membrane is the core of the Reverse Osmosis plant, and deterioration of this leads to decreased output, inadequate water quality as well as elevated production costs. The majority of membranes available today are constructed with cellulose acetate or polyamide, shaped often as a tube (hollow fibre) or covered on a polysulphone support sheet (thin film composite). The membrane, or rejecting layer, permits the passing of pure water yet retains the dissolved salts.
Plant recovery rates change from as low as 10% regarding sea water toward 90% with certain low salinity brackish waters. When the water passes across the membrane area the salt concentration will increase and some moderately disolveable salts surpass their own solubility. These salts may precipitate around the membrane surface resulting in membrane fouling that might decrease output as well as enhance the product water conductivity. The 2 main problematic salts usually are calcium carbonate as well as calcium sulphate and their avoidance is vital for the membrane to function effectively.
The application of sulphuric acid in order to 'de-alkalise' the actual feed water has been the conventional method of protecting against calcium carbonate scaling. Nevertheless, sulphuric acid is dangerous to handle, boosts the sulphate content in the water and also increases the overall corrosivity in the water on each side of the membrane.
Calcium sulphate scale is often eradicated by decreasing the recovery rate, which in turn decreases the calcium and sulphate ion content level within the water. Decreasing the recovery rate is not necessarily the most effective choice though since it will mean operating the plant at a reduced productivity. The most effective way to sustain economical production would be to function with recovery rates as high as possible while simultaneously protecting against membrane scaling.
Product Requirements for Antiscalants
Preliminary lab investigation into chemical substances considered efficient antiscalants and these were matched for the operating guidelines recommended by membrane manufacturers and plant vendors.
Efficient membrane antiscalants should have the subsequent parameters:
- Safe to handle as well as safe in employ.
- Excellent scale reduction qualities.
- Membrane layer compatibility.
- Steady and efficient throughout a broad pH range.
- Compatibility with various other products.
It appeared to be realised that conventional laboratory strategies for studying scale inhibitors weren't suitable towards the Reverse osmosis process. Common threshold tests employed for fundamental product screening are not able to reproduce the rapid concentration that takes place as the water passes over the membrane surface. Ultimately, product effectiveness can only be decided within a fully operational Reverse osmosis plant.
