Sediment Formation in Reverse Osmosis Membranes

Precipitation occurs at the membrane surface if the concentrations in the slightly dissolved raw water are too high. The slightly soluble salts causing the precipitation problem are given below.

CaCO3 > CaSO4 > Silica > SrCO3 > BaSO4 > SrSO4 > CaF2 > CaSiO3 > MgSiO3 > MgSiO3 >Ca3(PO4)2 > Fe(OH)2

High resolution from CaSO4, BaSO4 and SrSO4. CaSO4 deposits ocur move frequently in the natural water resources because the Ca++ ion is present in excess of Ba++ and Sr++ ions. But since it is difficult to dissolve the BaSO4 and SrSO4 salts oen by one, precautions must be taken against these two compounds.

The most calcium carbonate (CaCO3) precipitate is observed. This is because; CaCO3 is a fast dissolving compound and high amounts of free Ca++ ions are found in natural water resources. CaCO3, SrCO3, BaCO3 precipitates can be avoided by using acid dosing, antiscalant dosing, softening system and reducing the recovery rate.



CaSO4, BaSO4, SrSO4 and CaF2 removal is solved by the same methods as CaCO3 removal. But the acid dosing method  used for CaCO3 removal results further sulfate precipitation since it reduces pH. Thus, the removal of the sulphated compounds is not acid dosing.

What is the Silt Density Index (SDI)?

The Silt Density Index (SDI) test is performed to determine the potential for contamination of the feed water on the surface of the reverse osmosis membrane.

This test is not done in the laboratory environment. The SDI test is applied daily to water samples taken from the cartridge filter outlet. The pretreatment system and raw water properties may change the test density.


 Membrane choice is made taking into consideration the raw water source and raw water characterization.

The silt density index is also considered to be one of the standard factors in the selection of reverse osmosis processes and the application of membrane processes. The SDI value is a measure of the drop in filtration rate from a membrane to the membrane in standard conditions.

In other words, the amount of suspended solids, bacteria and colloidal substances present in the water to be treated.


Reverse osmosis membrane produced from 2 different raw materials is used in the market. These materials include cellulose acetate  and aromatic polyamides. Both  materials have different advantages and disadvantages.


*Cellulose Acetate Membranes: Cellulose acetate has higher flux and needs smaller area. It is more resistant to chlorine at the same time. If chlorine is used as a bactericide then a chlorine remover is not needed.


*Polyamides Membranes: Polyamide membranes can operate at higher temperatures (35 o C), less resistance to chlorine, no bacteria formation on the membrane surface. Finally, it operates at a wider pH range.

What is the Reverse Osmosis?

 Reverse Osmosis is a membrane filtration process used to supply pure water and drinking water, which seperates all undesired minerals in water from water.

These systems operate cross flow, not the filtration process. Because the pores that allow water to pass through the membrane are extremely small. (About 1 mm to about 2.000.000 hole diameter)

Only water molecules and some very small inorganic molecules can pass through such a small pore. Other molecules are expelled from the system in concentrated water phase.