To increase the efficiency and life cycle of a reverse osmosis system, effective pretreatment of the feed water is required. Selection of the proper pretreatment will maximize efficiency and membrane life by
- Membrane degradation
- Product flow
- Product quality (salt rejection)
- Product recovery
Fouling is the accumulation of foreign materials from feed water on the active membrane surface and/or on the feed spacer, to the point of causing operational problems.
Scaling refers to the precipitation and deposition within the system of sparingly soluble salts, including calcium carbonate, barium sulfate, calcium sulfate, strontium sulfate and calcium fluoride.
The proper treatment scheme for feed water depends on:
- Feed water source
- Feed water composition
The type of pretreatment system depends to a large extent on feed water source (i.e., well water, surface water, and municipal wastewater).
Once the feed water source has been determined, a complete and accurate analysis of the feed water should be made. It is critical in determining the proper pretreatment and R.O. system design.
Scaling of R.O. membranes may occur when sparingly soluble salts are concentrated within the element beyond their solubility limit. For example, if a reverse osmosis plant is operated at 50% recovery, the concentration in the concentrate stream will be almost double the concentration in the feed stream. As the recovery of a plant is increased, so is the risk of scaling.
The most common soluble salts encountered are CaSO4, CaCO3, and silica.
The various methods to prevent and control
scaling are described here below:
Most natural surface and ground waters are almost saturated with CaCO3. The solubility of CaCO3 depends on the pH, as can be seen from the following equation:
Ca2+ + HCO3- <-> H+ + CaCO3
By adding H+, the equilibrium can be shifted to the left side to keep calcium carbonate dissolved.
Softening with a strong acid cation exchange resin
In the ion exchange softening process, the scale-forming cations, such as Ca2+, Ba2+ and Sr2+, are removed and replaced by sodium cations.
Scale inhibitors (antiscalants) can be used to control carbonate scaling, sulfate scaling, and calcium fluoride scaling.
Lime softening can be used to remove carbonate hardness by adding hydrated lime.
Colloidal fouling of R.O. elements can seriously impair performance by lowering productivity and sometimes salt rejection. An early sign of colloidal fouling is often an increased pressure differential across the system. The source of silt or colloids in reverse osmosis feed waters is varied and often includes bacteria, clay, colloidal silica, and iron corrosion products.
Several methods or indices are proposed to predict a colloidal fouling potential of feed waters, including turbidity, Silt density index (SDI), and Modified fouling index (MFI). The SDI is the most commonly used fouling index.
Methods to prevent colloidal fouling are outlined here below:
The removal of suspended and colloidal particles by media filtration is based on their deposition on the surface of filter grains while the water flows through a bed of these grains (filter media).
Some well waters, usually brackish waters, are in a reduced state. Typically, such waters contain divalent iron and manganese, but no oxygen; therefore, they are called anoxic.
One method of handling anoxic waters is to oxidize iron and manganese by air, sodium hypochlorite or potassium permanganate (KMnO4). The hydroxides formed can then be removed by media filtration.
The efficiency of media filtration to reduce the SDI value can be markedly improved if the colloids in the raw water are coagulated and/or flocculated prior to filtration. In-line filtration can be applied to raw waters with a SDI only slightly above 5. A coagulant is injected into the raw water stream, effectively mixed, and the formed microflocs are immediately removed by media filtration.
Microfiltration or ultrafiltration membrane removes virtually all suspended matter.
A cartridge filter with an absolute pore size of less than 10 μm is the suggested minimum pretreatment required for every R.O. system.
The prevention of colloidal fouling is not only a matter of the proper pretreatment selection, but also of the system design and operation. As an extreme example, surface water could be pretreated by coagulation-flocculation and ultrafiltration. The R.O. system could then operate with a high permeate flux, and almost no cleaning would be required. If the same water, however, is just filtered with cartridge filtration, then the RO system would need much more membrane area, and more frequent cleaning and maintenance would be required.
All raw waters contain microorganisms such as bacteria, algae, fungi, and viruses. Microorganisms can be regarded as colloidal matter and removed during pretreatment as discussed previously. The difference between microorganisms and non-living particles, however, is the ability of microorganisms to reproduce and form a biofilm under favorable conditions. Biological fouling of the membranes may seriously affect the performance of the RO system. The symptoms are an increase in the differential pressure from feed to concentrate, finally leading to telescoping and mechanical damage of the membrane elements, and a decline in membrane flux.
The various methods to prevent and control biological fouling are described here below. Each method has specific advantages, but the optimum strategy is a combination of several concepts:
Chlorination / Dechlorination:
Chlorination is usually applied where biological fouling prevention is required (i.e., typically for surface waters). Chlorine is added continuously at the intake, and a reaction time of 20–30 min should be allowed. A free residual chlorine concentration of 0.5 – 1.0 mg/l should be maintained through the whole pretreatment line. Dechlorination upstream of the membranes is required to protect the membranes from oxidation.
Sodium bisulfite can be added into the feed stream (for a limited time period) during normal operation. This intermittent application is often referred to a shock treatment. Bisulfite is effective against aerobic bacteria but not against anaerobic microorganisms. Therefore, the efficiency of the shock treatment should be carefully assessed.
Biofiltration is the biological treatment of water to reduce the organic constituents that either contribute directly to organic fouling or provide carbon sources for the development of biofilms on the membrane surfaces.
Ultraviolet irradiation at 254 nm is known to have a germicidal effect.
Copper sulfate can be used to control the growth of algae.
Ozone is a strong oxidizing agent. However, it decomposes readily. A certain ozone level must be maintained to kill all microorganisms.