De-chlorination

UV Systems for De-chlorination Applications

The purpose of de-chlorination is the removal of free chlorine and combined chlorine compounds from potable water, as supplied by municipal water supplies.  Chlorine will oxidize the surface of thin film composite polyamide membranes, causing the membrane to lose its ability to repel or reject salts.  As a result, the membranes used in Reverse Osmosis (RO) or Electro-deionisation (EDI) systems will exhibit shorter lifetimes when exposed to chlorine in the water they are treating and membrane manufacturers will usually specify an upper limit to ensure a suitable life of the membrane.  As a result, it is usual to use a method of dechlorination upstream of the RO or EDI system. 

Traditionally, two methods are commonly employed:-

1: Dosing with Sodium Meta-Bisulphite

This is widely used by many companies, but it is not attractive to process users because:

  • It is another chemical added to process;
  • There is dosing equipment to maintain;
  • There is a potentially hazardous operation to consider;
  • Sodium meta-bisulphite can scale RO membranes;
  • Sodium sulphate can be formed, which is a stimulant to sulphate reducing bacteria (SRB’s), with odour and taste implications.

 

2: Filtration through Granulated Activated Carbon

Granulated Activated Carbon (GAC) is highly porous with a very large surface area.  Certain contaminants accumulate on the surface of the activated carbon by a process of adsorption.  Many organic compounds can be adsorbed by activated carbon and it is also effective for the removal of chlorine.

However, the activated carbon filters used in many water treatment devices can themselves become a source of contamination.  Over time, the activated carbon granules can become saturated with chemical contaminants, resulting in the release of these compounds into the finished water, possibly in even higher concentrations than in the source water.  In addition, there are usually high pressure head losses over the GAC bed and there is a regeneration / replacement cost.  However, the major problem associated with carbon in any form is bacterial contamination.  Wet activated carbon, richly infused with trapped organic matter, provides an ideal breeding ground for bacteria.  High bacterial levels occur when the carbon is fully saturated and then left to stand (e.g., overnight). As the water temperature inside the carbon cartridge rises, bacteria breeding escalates.

 

UV Provides a Chemical-Free Alternitive

De-chlorination by treatment with Medium Pressure UV avoids all the pitfalls associated with both sodium meta-bisulphite and GAC.  At high doses, UV is very effective at removing free chlorine from water.  Other benefits of using UV for this purpose are: the water receives a high UV disinfection dose; there is a degree of TOC destruction; it eliminates labour and the safety hazard of mixing sodium metabisulphite; it eliminates the risk of introducing micro-organisms onto reverse osmosis membranes (via sodium metabisulphite injection); there is an overall improved water quality at point-of-use.

 

Mechanism of UV Treatment: -

Chlorine gas dissolves in water to form hydrochloric acid and hypochlorous acid

Cl2  +  H2O    à    HCl  +  HOCl

HOCl  àß   H+   +   OCl-

HOCl and OCl- are the dominant forms of Cl2 in aqueous solution at a pH of 5 – 9.  Both these chemical species absorb UV within a wavelength range of 250 - 350 nm with the HOCl peak at 292 nm and ClO- between 270 and 310 nm.  This absorption characteristic is such that medium pressure lamps have a distinct advantage over low pressure lamps.   Photolysis of HOCl and OCl- proceeds as follows:

HOCl + hn → •OH + Cl•

ClO- + hn  → O• + Cl•

The hydroxyl radical (•OH) reacts with dissolved organic matter (DOM), and the chlorine radical (Cl•) is reduced to the chloride ion by DOM.  The UV dose required is at least a magnitude higher than that required for UV disinfection.