Technicals – Common Problem of RO, UF & NF

REVERSE OSMOSIS:

A process by which a solvent passes through a semipermeable membrane in the direction opposite to that for natural osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure.

COMMON PROBLEM OF RO, UF & NF:

The biggest problem with the Reverse Osmosis industry in India is very low level of awareness and knowledge about RO users and even sellers end. Users have not been adequately trained on membrane caring. A complete and accurate water analysis consisting of dissolved ions, turbidity, silica, sulphates, fluoride, microorganisms and organics must be practiced before the RO System design. Fouling is the most significant factors affecting the performance of any membrane resulting in increased TDS of permeate, reduction in permeate ratio or increases pressure. Fouling may be aggravated during shutdown period.

There are 4-5 type of fouling occurs in a RO System.
1)Biological & Organic fouling
2)Scaling of membrane
3)Metal Oxide fouling
4)Colloidal fouling

BIOLOGICAL & ORGANIC FOULING:

Bio fouling remains a major problem among all foulants. It is caused by microorganism and feed water containing organic matter, which becomes the food for the microbes resulting in biological attack of the membrane further it, is responsible for significant flux decrease. Humic acid is a complex organic structure that may provide a food source for bacteria so humic acid should be removed by flocculation, carbon filtration or media filtration. RO System eliminates dissolve organic matter (DOM) up to molecular weight 300 but organic substances (TOC) with low molecular weight can pass through membrane which may be carcinogenic.

Scaling of membrane:
Caused by ionic materials i.e. carbonates, silicates, sulphates, phosphate, fluoride, iron etc. Some of the soluble salts are listed below in order to their decreasing scaling problem are co3>so4 (Ca/Ba/Sr)>Si03>CaF2. Calcium carbonates scaling should be prevented/removed by lowering the feed water pH between 3.0-4.0 for 1-2 hours for non-potable water. Calcium carbonate scaling potential is determined by LSI (lengeniores saturation index) and SDI (Stiff & Davis Saturation Index).

Metal Oxide fouling:
Caused by iron, Aluminium & manganese by oxidation from ferrous ions to ferric ion and subsequently precipitated into ferric hydroxide in presence of dissolve oxygen.

Colloidal fouling:
Colloids being negatively charges material generally aluminium-silicates (Clays) and aluminium hydroxide. For raw water containing high concentration of colloidal matter coagulation and flocculation process is necessary before filtration. SDI is an onsite measurement of the suspended solid concentration in the feed water.

TIPS FOR EFFICIENT OPERATION OF RO SYSTEM:

1. Proper pre-treatment of feed water i.e. coagulation & flocculation and softening of water.

2. Reverse Osmosis must be configured with other processes like ultra filtration or micro filtration to reduce nephelometric turbidity unit (NTU) indicated higher membrane plugging possibility.

3. If the feed water is contaminated with chlorine it must be de-chlorinated by passing through activated carbon filter or dosing sodium bi sulphite because chlorine can damage polyamide membrane.

4. If ultra filtration/cartridge filter is used in pre-treatment the antiscalant should be dosed after filters.

5. Prefer acid dosing before antiscalant dosing for non-potable water Industry.

6. Antiscalant dosing device should be placed into shade to avoid algae growth.

7. Prefer at least 1-2 meter distance between acid and antiscalant dosing point.

8. Membrane should be preserved with compatible biocides if it is idle for more than 1 day to 6 months.

9. Antiscalant should be either used neat or maximum dilution ratio should be 10%.