Reverse Osmosis System

Type: Cross Flow Filtration System

Purpose: Removal of TDS, TOC impurity from Feed water to the system.

Process Description

Reverse Osmosis membranes are used to remove contaminants that are less than 1 nm nominal diameter. Reverse Osmosis Water Systems typically remove 90% to 99% of ionic contamination, most organic contamination, and nearly all particulate contamination from water. Reverse Osmosis removal of non-ionic contaminants with molecular weights 300 Dalton and for particles, including colloids and microorganisms. Dissolved gases are not removed (eg. CO2). During reverse osmosis, pretreated water is pumped past the input surface of an RO membrane under pressure (typically 4–15 bar, 60–220 psi) in cross-flow fashion.

The membranes in Reverse Osmosis Water Systems are typically thin film composite (polyamide). They are stable over a wide pH range, but can be damaged by oxidizing agents such as chlorine, present in municipal water.

Pretreatment of the feed water with microporous depth filters, softener and activated carbon, Pre dosing systems for removal of oxidizing compound, Anti-scalent for lowering the scaling frequency of membrane & pH Correction chemical for lowering or increase the required pH values is usually required to protect the membrane from large particulates, hardness and free chlorine. Typically 75%-90% of the feed water passes through the membrane as permeate and the rest exits the membrane as concentrate that contains most of the salts, organics, and essentially all of the particulates.

Working Principle

The purest form of membrane technology is reverse osmosis. This process removes 95% or more of the salts dissolved in the incoming water. Almost all organic molecules with a molecular mass of more than 100 g/mol are retained by the membrane. Only the water molecules can pass through the membrane, and these form the product output.
A synthetic semi permeable membrane is used to separate water from dissolved impurities.

When a semi permeable membrane separates a dilute and concentrated solution of salts, due to osmosis, the water from the dilute solution side passes through the membrane to the concentrated side till osmotic equilibrium is attained. Now, if the pressure is applied and increased gradually on the concentrated side, the flow of water continues to reduce till the applied pressure is equal to the osmotic pressure. Any pressure in excess of the osmotic pressure reverses the direction of flow of water and water from the concentrated side enters the dilute side. This process is called the Reverse Osmosis.

Limiting conditions on feed water to Ro unit
Turbidity NTU Nil
Free Clorine PPM Nil
Heavy metal like Iron, Manganese PPM Nil
Oil & Grease PPM Nil
TOC – PPM < 3.0
Synthetic organic compound (SOC) have generally more adverse effects on RO/NF membrane compared with natural organic matter
Temperature oC < 40
SDI < 4

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