Dear Suresh Vanik

Yes I think you can use Antiscalant in RO membrane cleaning. You should carefully select the materials. Antiscalants made of polyacrylic acid are susceptible to the formation of a foulant that settles on the membrane surface if there are high levels of iron. This foulant will increase feed pressure requirements, but typically can be cleaned using a low pH cleaning.

The primary fouling risk in untreated water treatment systems is scale formation that occurs when the solubility of any low solubility salt is exceeded. The   scaling Mechanism at the membrane surface is due to a concentration gradient that occurs as product water continuously passes through the membrane,leaving behind an ever-increasing level of dissolved and suspended solids . These concentrate

at the membrane-separating surface creating a boundary layer, an effect known as concentration polarisation. Within this boundary layer, salts may precipitate and suspended solids can start to deposit on the membrane surface and within the spacers, leading to scaling and fouling.Many natural waters will deposit calcium carbonate on the membrane surface if untreated. Calcium sulphate is another common scale. The scaling usually begin in the latter stages of the plant. This results in increased pressure drop and the need for greater feed pressure to maintain constant product water output.Scale forms in three stages:

1. Ions in solution concentrate as the water passes along the membrane, particularly in the boundary layer near the separating surface. The ions start

clustering as a proto-nuclei of up to 1000 atoms, a reaction that is readily reversible.

2. As the proto-nuclei grows, the ions start ordering themselves and a regular-shaped nuclei develops.This stage is also reversible; but as the nuclei grows, reversibility is less likely.The  Most widely used scale inhibitors are based on an

extensive range of organic compounds. These work by three closely related mechanisms that interfere with one or more of the stages of crystal growth:

1. Threshold Effect — These inhibitors retard the precipitation of salts that have exceeded their solubility products, e.g., phosphonate-based products.

2. Crystal Distortion Effect — These inhibitors distort normal crystal growth and produce an irregular crystal structure with poor scale forming ability, e.g., polyacrylic acid [CH2CHCOOH]n with molecular weights in the 1,500–2,500 range.

3. Dispersancy — Dispersants work by placing a surface charge on the crystal. Comparable charges cause the crystals to repel one other and are dispersed

into the water bulk.Numerous polymers are commercially available, the

most common of which is polyacrylic acid. These polymers function as crystal distortion agents; but at higher molecular weight, they also exhibit dispersancy

properties. Although it is debatable how effective dispersancy and crystal distortion properties are within a tightly packed membrane element, many of these polymeric substances can exhibit good threshold properties against calcium carbonate and calcium sulphate so that scale inhibition occurs by more than one mechanism.

Phosphonates, as typified by the stable C-P bond,overcome the tendency of hydrolysis to the O-P chain, which is typical of polyphosphates such as SHMP.

Phosphonates are widely used in water treatment formulations as scale and corrosion inhibitors and iron sequestrants. In membrane systems, they act as

‘super-threshold’ agents. These products have the ability to hold highly supersaturated solutions in a stable condition during the finite time it takes the water to exit the membrane system.

The function of RO Antiscalant is different as above so Antiscalant is not used in place of Cleaning chemical 

Dear Suresh Vanik,

All reverse osmosis (RO) feed waters are different in ionic composition. As a result, different types of scale may deposit on the membranes if the correct antiscalant is not used. Selecting the right antiscalant and cleaning chemicals will optimise operating efficiency and help to prevent plant failure.

Below a list of some of the most common types of scales that were identified on RO membranes during autopsy procedures and the appropriate chemicals to use.

Always a membrane autopsy is recommended  to ensure deposits are correctly identified. 

Scale Species Information Antiscalant Cleaners

Calcium Carbonate is the most common scale found in membranes. Appears as an off white powder. Easily cleaned with a strong acid product.Genesys LFGenesol 37 and 38

 Silica - can be present on a membrane as colloids, aluminosilicates on the lead membrane elements, or as scale on the final elements. Both forms are difficult to remove, being dependent on pH and temperature.Genesys SIGenesol 40 or 703, depending on colloidal or silica scale

 Calcium sulphate (gypsum) - crystalline structure with very sharp blade-like features, which cut the membrane surface. Common occurrence in areas where there is a natural sulphate content in the underlying geology. This makes it very difficult to clean.Genesys CASGenesol 50, 34 and 703

Difficult to clean - ask us for advice on cleaning protocol design

 Calcium phosphate - appears in many forms. Commonly found in RO membrane systems fed with waste water or agricultural leachate, due to the high phosphate loading.Genesys PHOGenesol 37 and 38

 Magnesium hydroxide (brucite) - precipitates at high pH, above 9.5, in membrane systems. Can be found in second pass elevated pH brackish water boron rejection plants, where pH levels are increased above 9.5 with sodium hydroxide to aid boron rejection. Relatively rare.Genesys MGGenesol 37 and 38

Barium sulphate (barite) - appears as white crystals, which can take on-blade like formations. Very difficult to clean effectively.Genesys BSGenesol 50 and 34

Difficult to clean - ask us for advice on cleaning protocol design

 Strontium sulphate (celestine) - appears as crystals or as needles, which can damage the membrane surface. Very difficult to remove.Genesys CASGenesol 50 and 34

Difficult to clean - ask us for advice on cleaning protocol design.

To view the full publication with figures illustrating the scale species, please use the following link:

http://www.genesysro.com/ro-membrane-antiscalant-cleaning-chemicals.php

Hoping this will be helpful,

Rafik

Thanks all of you for details and your views regarding question. I appreciate for that.

Hi Suresh,

Anti-scalant is normally use in RO system OPERATION. In RO cleaning it is not necessary to use antiscalant because you are using RO product water to clean the membrane. In RO cleaning you are using low pressure water so potential of scaling is not there. Go to www.membranes.com and look for Technical Service Bulletin TSB107 for RO cleaning procedure. There is a list of generic chemical shown in the bulletin to help you.