Feature

Reuse & Recycle of Treated Industrial Effluent Using Chlorine Dioxide

Posted on 11 February, 2010 | Tags: Water Management

Water management is one of the key areas that the industries work on not only to reduce the water footprint, but reuse and recycle of water in the industries substantially adds to the profit margins. Chlorine dioxide because of its excellent oxidation properties disinfects water for reuse in various applications.

Growing industrialization and ever growing population has resulted in rampant growth of Water treatment industry. But there are few challenges too that the industry is facing in terms of availability and quality of water.

• Water Availability - The sources of usable water are limited whereas the demand of water has increased manifolds. This has resulted in water scarcity of usable water thus putting tremendous strain on both the government bodies as well the industry to put in efforts to optimize usage of water for industrial applications and make available potable water for the communities.
• Water Quality - Rampant use/ misuse of water is not only resulting in depletion of fresh water resources but also pollution of surface water and ground water in many areas. As a matter of fact, polluted water has entered our homes and is a cause of illnesses. Deterioration of water quality is leading to an impact on the entire community in specific localities causing an epidemic and illness. This phenomenon is observed especially during the onset of monsoons.

Water is the lifeline for industries as well as the communities and enormity of above challenges is haunting both the public and private enterprises. The physicochemical properties of the polluted water need to be treated in order to make it compatible for reuse in industrial applications and also disinfection of the potable water. The physical properties like the suspended particles or particulate matter can be easily treated with mechanical separation techniques like clarifiers, filters etc. However, it is the chemical properties especially the microbiological growth, which needs unique approach. There have been halogen based technologies like chlorine which are historically used for disinfection of the water. Also in industrial applications non-oxidizing biocides are also used to address the biodiversity of the microbial population. However, both chlorine and non-oxidizing biocides have some limitations. Chlorine Dioxide (ClO2) in such situations becomes a unique proposition as it not only addresses the limitations of chlorine but also makes it possible for reuse of the treated effluent in industrial applications and addresses the concern of E-coli (coliform bacteria) in potable water disinfection. Unique properties of ClO2 make it a superior bactericide technology enabling the private and public enterprises in conserving / optimizing the available water and also improving the quality of potable water.

Chlorine Dioxide (ClO2)
ClO2 is a gas that is yellow to greenish yellow in color and is a multipurpose oxidant/ biocide. This was developed in 1930s by research and development wing of Mathieson Alkali Works in USA. Chlorine atom has a valency of +4 and exhibits unique properties due to high level of oxidation. Experiments revealed that

• ClO2 Would bleach cellulosic material to higher brightness levels as compared to hypochlorite or hydrogen peroxide at lower residual levels
• This gas exhibited much better bactericidal properties even at different conditions of pH and temperature.
• In the gas form it could not be safely compressed or liquefied
• In the solution form in water it could be safely handled even at its maximum solubility

Because of these unique properties it found immediate application in the Textile and Pulp and Paper Industries. The use of ClO2 as disinfectant in pulp and paper industry has been known since many decades as early as 1930s. It has also been used for municipal water disinfection

since 1940's in the US and European countries. It was initially used to destroy odor and control the taste in the potable water supply by Public Health Department of New York city. It has since been successfully used commercially by many municipal water establishments for potable water disinfection. However, it has taken really a long time to understand the unique features and its potential in industrial applications for recycling and reuse of treated effluent. 

The unique properties of ClO2

• Unlike chlorine, ClO2 does not dissociate in water. It remains as a true gas and is quite soluble in water up to 2900 ppm. At lower temperatures the solubility can be further increased.
• It has to be generated at site as it can not be produced and shipped in bulk due to its sensitivity to pressure and temperature. Hence, it needs unique generators complete with safety features to be installed at the site where it has to be used.
  It is 2 - 3 times more effective than chlorine as a biocide and hence can be used at much lower residuals.
• It is pH independent. As it is a dissolved gas, it is not affected pH unlike chlorine which dissociates to OCl ions.
• It disinfects by oxidation. It is a highly powerful and effective oxidizer.
• It has biofilm penetration capability and hence can remove and oxidize the biofilms which harbor the anaerobic bacteria.
• It can destroy phenolic/organic odor.
• Does not form Trihalomethanes (THM) like chlorine as it does not react with organics.  

Chlorine Dioxide is pH independent
The following dissociation figure shows how Chlorine and Bromine effectivity reduces as the pH increases. ClO2 however remains as such even at alkaline pH and hence retains its effectivity as biocide at any pH.

ClO2 an Effective Biofilm Remover
ClO2 can penetrate through the cell interstices and disrupt the DNA there by killing the microorganisms, whereas Chlorine can not penetrate through the slime layer.

Generation of Chlorine Dioxide
ClO2 has to be generated at site just before its intended use. There are several methods generation which differ on account of reaction yield, number of reactants required and complexity of generators. The following three methods are used broadly for generation of chlorine dioxide.

• Reaction of chlorine dissolved in water with sodium chlorite
• Reaction of sodium hypochlorite and hydrogen chloride (HCl) with sodium chlorite
• Reaction of HCl with sodium chlorite

The complexity of reactions in each method results in different efficiency of ClO2 generation and varying yields. The chlorine method is by far the most efficient method.

1. Reaction of chlorine dissolved in water with sodium chlorite

2NaClO2 +  Cl2      →  2ClO2 +  2NaCl
Sodium Chlorine Chlorine Sodium
Chlorite  Dioxide Chloride
This method is the most efficient method and results in the yield of around 90% ClO2 generation. This method is also known as "Single Pump Method".

The generator used for generation is known as "GENEROX", which is a patented ClO2 generation equipment developed by Ashland Water Technolgies, USA. This equipment incorporates the safety features required for safe generation and use of ClO2. The chlorinated water supply at a specified concentration is used to react with sodium chlorite across a packed reactor column.

2. Reaction of sodium hypochlorite and HCl in water with sodium chlorite

2NaClO2+ NaOCl+2HCl→  2ClO2+   3NaCl+    H2O
Sodium       Sodium  Hydrochloric Chlorine Sodium  Water
Chlorite    Hypochlorite Acid Dioxide Chloride

This method is used whenever chlorinated water generated through chlorine gas can not be made available due to either safety or environmental reasons. This method uses the liquid sodium hypochlorite and hydrochloric acid to form hypochlorous acid necessary for generation of chlorine dioxide when reacted with sodium chlorite.  This method is also known as " Three Pump Method".

3. Reaction of Acid with Sodium Chlorite :

5NaClO2+ 4HCl  →   4ClO2 + 5NaCl +  2H2O
Sodium  Hydrochloric  Chlorine Sodium  Water
Chlorite  Acid   Dioxide  Chloride

This method is also known as "Acid Method" as no chlorine source is used. This is the least efficient method. The "Generox" equipment is similar to the above one shown in figure 4.

Applications of Chlorine Dioxide :
ClO2 finds application in various industries due to the unique properties discussed above. Some of the areas are as follows;

• Cooling Towers : As a primary biocide especially in contaminated systems like fertilizer, refinery and petrochemical complexes.
• Waste Water Treatment : In effluent treatment plants in almost all the industries especially where water recycle is in practice. Some of the applications are;
  Chemical oxidation
  Phenol destruction
  Disinfection
  Odor control 
• Sea water macrofoulant
• Legionella control
• Drinking water and raw water disinfectant
• Pulp and Paper mills
• Food and Beverage industry disinfectant

Case Studies:
Two case studies are presented here where ClO2  has been used. First case study has been carried out in an industry where chlorine dioxide has been instrumental in improvement of the process and water recycle and the other one is municipal case study where use of ClO2 helped in improving the quality of water for human consumption.

CS -1 : Recycle and reuse of treated effluent in a petrochemical complex

The plant effluent being treated through the Low Total Solids (LTDS) stream effluent treatment plant was thought to be recycled to cooling tower as make up water. This would reduce the fresh water being used as make up by almost 200m3/hr. The treated effluent had two limitations; ORP was low and Chemical Oxygen Demand (COD) was high. The Oxidation Reduction Potential (ORP level) of the treated effluent was below 250mV and the COD was higher than 50 ppm.

The use of this treated effluent as make up in cooling tower caused the upset cooling water quality which deteriorated in terms of high Total Suspended Solids (TSS), enhanced microbiological load, increased the chlorine demand on the tower and substantially increased the mild steel corrosion rates to 3 - 5 MPY. This had an adverse impact on plant efficiency as the heat transfer coefficient reduced. 

After studying the system, it was thought to implement ClO2 treatment at the LTDS reservoir to increase the ORP and reduce the COD load. After implementation of the ClO2 dosing system at the LTDS reservoir, the ORP of the treated water increased to more than 450 mV consistently and the COD reduced to less than 30 ppm.   Post ClO2, the use of treated water as make up to cooling tower improved the microbiological control, reduced the chlorine demand and improved the corrosion control to 1- 2 MPY. Thus ClO2 helped in recycling a substantial amount of treated effluent to cooling tower as make up and reducing the consumption of fresh water. The following figure shows the schematic flow of the treated effluent and installation of ClO2 generating system.  

CS 2 :  Application of ClO2 in drinking water to improve the water quality
The Municipal corporation water supply department in the Western part of India was facing the poor water quality problem of the drinking water. The water had a slight yellow color and a distinct odor. This also led to complaints about illnesses from the public. The analysis of the water showed that it contained coliform bacteria and high suspended solids.

The water supply system was using chlorine gas at two stages one before the clarifier and other at the clarified water reservoir to address the microbial issue. However, enhanced chlorine dosing did not help matters.
The municipal committee on the recommendation of Chembond Ashland team decided to implement ClO2 treatment at both the stages. After implementation of ClO2 dosing, the water quality substantially improved in terms of appearance, odor and microbial contamination.  The following table illustrates the pre ClO2 and post ClO2 water quality difference;

The difference in the water quality was also visible to the naked eye.