Dechlorination of water through activated carbon technology

From swimming pools and drinking water to process water production!

ORGANOSORB® activated carbon Chlorination is a common method to maintain water, such as drinking water and swimming pool water, biologically safe. It is used to destroy pathogenic micro-organisms, to oxidise taste/odour-forming compounds, and to form a disinfectant residual. The use of chlorine in these applications can lead to the formation of disinfection by-products (DBP) such as the trihalomethanes (THMs), haloacetic acids. In addition to toxicity of THMs in drinking water, they can give a taste in beer or soft drinks in the brewing and bottling industry. ORGANOSORB® activated carbon is used for removing disinfection by-products.

Swimming pools: removing chloramines with activated carbon

In public swimming pools chloramines (combined chlorine) can be formed when free chlorine reacts with compounds containing nitrogen. These chloramines can cause poor air quality and irritation of the eyes of the bathers.  In some swimming pools considerable amounts of nitrogen compounds are continually introduced into the pool water by bathers through urine, perspiration etc. The Urea will slowly hydrolyse to ammonia in the pool water and react with the hypochlorite to form chloramines.  The combined chlorine, which originates from the reaction of hypochlorite and ammonia, can be removed with ORGANOSORB®activated carbon.

Industrial demand for dechlorinated water

In certain applications the residual chlorine must be removed prior to the use of the water. In the petrochemicals’ and power generation industries, there is always a demand for demineralised water for process and boiler feeds. Production of process water such as demineralised water is generally achieved by Ion Exchange Resins (IER) or membrane processes such as Reverse Osmoses (RO). Ion Exchange Resins (IER) or membrane can be damaged by oxidation with residual free chlorine. ORGANOSORB® granular activated carbon is often installed before the membranes or ion exchange resins for dechlorination and the reduction of organic compounds that may foul these processes. The dechlorination reaction rate depends on the nature of the free chlorine. In order of increasing reaction rate:

  1. Chlorine gas
  2. Hypochlorites
  3. Chloramines and dichloramine
  4. Chlorine dioxide

Activated carbon technology as an efficient dechlorination method

Chlorine can be added as chlorine gas or as sodium or calcium hypochlorite. In drinking water purification plants it is added to the purified drinking water to ensure a residual concentration of 0.1ppm when it reaches the consumer. Some bottling plants super chlorinate the water to over 10ppm. Activated carbon technology is a well-known and very efficient method of dechlorinating water. The mechanism of the dechlorination reaction is a combination of hydrolysis of free chlorine to the hypochlorite ion and catalytic decomposition of the hypochlorite ion on the carbon surface with the result that the free chlorine is transformed to the chloride ion. Dechlorination mechanism of activated carbon:

  • Hydrolysis of free chlorine in water

Cl2                +       H2O      →     HOCl                         +      HCl Chlorine      +       water    →     hypochloric acid     +      Hydrochloric acid

  • Hydrochloric acid dissociation in water

HOCl         →          H+           +             OCl- Low pH                   High pH

  • Catalytic destruction of hypochloric acid

HOCl + C*   →    HCl + C*O *Activated carbon catalytic site

Chlorine half length value as measure of the dechlorination reaction rate

The dechlorination reaction rate is often expressed and measured as the chlorine half length value (dechlorination half length). The dechlorination half length represents the granular activated carbon bed depth required to reduce the chlorine concentration by 50% under the defined test conditions. Different methods are used to evaluate the dechlorination performance of activated carbon at the laboratory scale.

Free chlorine concentration as function of SCT

Following graph visualises the impact of the dechlorination half length on the free chlorine concentration as function of the superficial contact time (SCT). The shorter the dechlorination half length, the faster the free chlorine breakdown reaction is. Table 1 summarises different parameters influencing the dechlorination half length.

Table 1: parameters influencing the dechlorination half length
Parameter Impact
Activated carbon particle size The smaller the mean particle diameter the smaller the dechlorination half length
pH of the water Higher pH increases the dechlorination half length
Temperature The higher the temperature the shorter the dechlorination half length
Organic matter Higher dissolved organics will increase the dechlorination half length
Backwashing One of the dechlorination side reactions is the formation of carbon dioxide (CO2). Regular backwash

> Read more on how breakpoint chlorination works and chloramine removal in water

DESOTEC’s complete water dechlorination solutions

DESOTEC activated carbon ORGANOSORB® types 9 CO and 11 CO are coconut based steam activated carbons that have excellent dechlorination properties combined with superior hardness and highly microporous structure for disinfection by-product removal. Since kinetics are important, finer products (0.425-1.7 mm) are selected unless lower pressure drop is required. A particle size of 0.6-2.36 mm should be selected in that case. When a fresh activated carbon water filter is put on stream, it may give initially a pH rise to a value between 9 and 12, with the final value depending on the water source. Generally we can say that the softer the water, the higher the increase and extent of the pH rise. Using ORGANOSORB® 11 CO can, in certain cases, reduce the pH peak and thereby decrease the amount of time required to put an activated carbon filter into service. > Feel free to contact us on any kind of dechlorination demand!

Organosorb Dechlorination at a yoghurt factory

Dechlorination at a yoghurt factory