What is osmotic pressure?
Osmotic pressure is the pressure generated by the diffusion of water or solution across a membrane.
If the osmotic pressure is the same either side of a membrane the water will cease to flow.
If both pressures are equal the pressure is then often referred to as hydrostatic (water flow stopping pressure).
The term osmolarity us used to describe the number of solute particles in a volume of fluid.
The term osmoles are used to describe the concentration in terms of particles e.g. a one osmolar solution contains 1 mole of osmotically active particles (molecules and ions) per litre.
Every solution has an osmotic pressure the higher the osmotic pressure the higher pressure is required in the RO unit.
What Pre-treatment Do Reverse Osmosis Units Require.
If the pre-treatment on reverse osmosis units is not correctly designed and sized fouling of the membranes will accelerate very rapidly.
There are many factors required in order to determine the pre-treatment equipment. The following are a few of the most common:
- SDI Index
A silt density index test is required on the feed water to the RO equipment. If the SDI test result is less than 3 there is no problem. If the SDI test result is more than 3 pre-treatment may be required.
- Suspended Solids
If the suspended solids are less than 2mg/l cartridge filtration is adequate. If the suspended solids are more than 2 mg/l a multi media filter and a cartridge filter or micro filtration is required.
Scaling will occur if the pH of the scalent is greater than the pH in the water. To find out if the solution will scale we use the Langellier Saturation Index. If the LSI calculation result is less than one, scaling will occur. If the LSI test result is greater than one the water is corrosive.
Scalents usually come under 3 categories
a) Carbonates and Sulphates.
b) Transitional Hydroxides
Scaling of reverse osmosis equipment is promoted by high TDS (total dissolved solids), a high concentration of scalents or a high pH causing the precipitation of CaCO2.
Scaling problems like the above can be pre-treated with ion exchange systems (Water Softeners) or dosed.
The calculations of the scaling will usually be done by the reverse osmosis unit manufacturer (Derwent Water Services) on a CAD package that is relevant to the type of membrane used.
Calcium carbonate is the most frequently found scalent in the pre-treatment of reverse osmosis equipment. This again is taken out with ion exchange resins used in a water softener.
Strontium and barium are very aggressive scalents on reverse osmosis membrane systems. Scale Inhibitors can also be used, these include
a) Chelating agents which prevent the ions from clustering by keeping it in solution but are expensive and poisonous. It is worth noting that the effluent stays on the concentrate side so it could be put down a drain or effluent pit.
b) Calgon, this stops the scale forming into crystals.
c) Plocom, this distorts the crystals causing them to break up.
Scale formation is the clustering of ions into groups of around 1000. This gives a diameter of around 0.3 microns. We call these clusters Protoneucli. These Protoneucli then group into groups which are then called neucli which in turn group to form crystals.
The most common ways of stopping scale formation on reverse osmosis membranes is as follows.
Temporary hardness LSI<2 would be a scale inhibitor or acid dosing.
Temporary hardness of LSI>2 would require a water softener or acid dosing equipment.
Permanent hardness would require a water softener or a scale inhibitor dosing system.
Silica would require Alkali (caustic dosing).
Iron and Manganese
Iron and manganese would require areation and filtration or green sand filters.
Colloids SDI>5 ultra filtration or micro filtration is required. Alsocoagulation/clarification methods will apply.
To remove organic matter we would use ultra filtration or GAC filters or flocculation. Ultra violet disinfection followed by a filter could also be considered.
Ultra filtration is the preferred method of bacteria removal. Ultra filtration would follow a UV system in extreme cases. Chemical disinfection can also be used.
Free chlorine can be removed by granulated active carbon filters or chemical dosing.