CHLORINE DIOXIDE                

Presented by N.E.M Business Solutions        

©2004

N.E.M Business Solutions        Tel / Fax  : 01823 680119     Mobile   07768 981196 

E-mail   neil@nem.org.uk

Chlorine dioxide was first produced from the reaction of potassium chlorate and hydrochloric acid by Davy in 1811. Not until the industrial-scale preparation of sodium chlorite, from which chlorine dioxide may more readily be generated however, did its widespread use occur

The most common traditional methods of generating chlorine dioxide involve mixing sodium hypochlorite (NaClO), acid (HCl2) and sodium chlorite (NaClO2) or chlorine gas (Cl2) and sodium chlorite.

NaOCl + HCl + 2NaClO2  2ClO2 + 2NaCl + NaOH

Cl2 + 2NaClO2  2ClO2 + 2NaCl

New technology has dramatically improved the onsite generation of chlorine dioxide. This new technology generates chlorine dioxide by electro catalytic and electrochemical techniques. These generators typically use only one precursor, sodium chlorite. The elimination of both chlorine and acid in the process has resulted in a much safer and simpler generation process. Some of these new generators produce chlorine dioxide directly in an aqueous solution with a concentration below 1000 mg/L. This low concentration in an aqueous solution dramatically enhances the safety of the process.

Chlorine Dioxide has the chemical formula ClO2 and is a yellow to brown coloured gas at room temperature and pressure. It is a highly reactive oxidant and for all practical areas of water disinfection, it must be generated on site using proprietary reaction and dosing equipment

By comparison: At room temperature, chlorine is a greenish-yellow poisonous gas. When added to water, however, chlorine combines with water to form hypochlorous acid that then ionises to form hypochlorite ion - 'bleach'

In general, chlorine dioxide has been found to produce fewer organic by-products with naturally occurring dissolved organic material. Chlorine dioxide is an explosive gas, but is stable in water in the absence of light and elevated temperatures. ClO2 is capable of oxidizing iron and manganese, removing colour, and lowering THM (Trihalomethanes) formation potential. It also oxidizes many organic and sulphurous compounds that cause off-tastes and odours.

 The physiological mode of inactivation of bacteria by chlorine dioxide has been attributed to a disruption of protein synthesis.  In the case of viruses, chlorine dioxide preferentially inactivates the outer protein layers, rather than nucleic acids.

It is well known that ClO2 does not react with ammonia; however, this is only one of many chemicals not affected by “The Selective Oxidiser.”

ClO2 does not react with:

acids, alkanes, alkynes, alcohols, aldehydes, aliphatic amines, ammonia, azole, carbohydrates, ethers, fats, glycol, ketones, methanol, polysaccharides, saccharides, unsaturated fatty acids and unsubstituted aromatics, among others.

Organic contaminants, such as those mentioned above, are regularly found in cooling and process water systems. The cooling system contaminants could be ammonia in a semiconductor plant, glycol in a process heat exchanger, oil in a textile air washer or a steel mill, food from a cooker or methanol from a chemical plant. Because ClO2 does not react with these contaminants, its demand is based on the microbiological loading in the water only. This demand impact is a major advantage in the areas of cost and performance.

As regards to the cost of ClO2, it is certainly more expensive on a pound for pound basis, than chlorine gas (as much as 1OX), or hypochlorite (4X). It is only when the consumption of the chlorine by the systems demand is factored in, can the economics favour the ClO2 – cases have been witnessed in heavily contaminated systems where a 1:25 replacement by ClO2 for chlorine, resulted in satisfactory treatment program where one did not exist prior. (John Murphy, Microcide Consultants).

 

A few notes on some systems that have switched to Chlorine dioxide:

Washing of cut lettuce for hamburgers for fast food chain in NZ. Processing done by Essentia Foods.

Quality requirements:

Salmonella zero

Listeria zero

E.Coli zero.

Must pass sensory evaluation test criteria (no chlorine taste).

Appearance of lettuce must be good.

TPC must be within guidelines at day 10.

TPC = Total Plate Count (microbiological surface contamination)

 

Description of old chlorine disinfection system.

 Chlorine at 100-200ppm. Dosed using sodium hypochlorite 12.5%

Terrible chlorine smell in factory with workers complaining of eye and skin irritations.

Impossible to control chlorine residual and required manual chemical addition every 15 minutes.

pH control not possible as always creeping high.

Required to dump a lot of water to maintain chlorine residual which was high cost for chilling and extra ice.

E.Coli was not always zero.

Always concerned about Listeria as Listeria not affected by chlorine at low temperatures.

TPC at day zero was inconsistent usually 1 x 105, 3 x 105 and occasional 1 x 106 counts

 

Description of new ClO2 system installed in 1996

 Chlorine dioxide at 1.0ppm in 2 stage wash. First wash stage is 8 deg.C and second wash stage is 2 deg.C.

Dosing is done automatically using Bellozon generator and automatic residual control.

No chemical smell in the factory at all.

Operators do a check on the dosing equipment every hour or so but don’t add any chemical manually.

pH is automatically controlled to 7.5.

Very little dumping of water and only chilled water is used. Chemical running cost is very low around $1.5k per year.

E.Coli is always zero.

No concerns about Listeria as ClO2 will easily kill Listeria at low temperatures.

TPC at day zero is consistent and always less than 7 x 104

 copyright 2003 Dioxide Pacific Pt  copyright 2003 Dioxide Pacific Pt

 

Tomato washing :      3 plants in Australia. Girgarre Country Foods, SPC Foods, Unilever

Tomatoes are brought to the processing factory by truck and then transported by flume to the tomato paste production area. Chlorine Dioxide is used to destroy moulds on the tomatoes and in the flume tank.

  Processing steps:

 Tomatoes are dumped from the truck onto a conveyor.

Coarse rinse with town water sprays to remove dirt and stems, leaves etc.

Tomatoes fall into flume tank (20 m3). The flume water is pumped to the sorting conveyor and back in a closed circuit with the tomatoes. Operators remove unacceptable product.

Make-up condensate water is continually added (5 m3/hr) from the tomato paste process.

Chlorine Dioxide is dosed into the flume water to maintain concentration of 0.2 - 0.4 ppm of ClO2. pH of the flume water goes to 4.0 and this is not corrected as it is acceptable.

Method of concentration control

 Bellozon CDKa3A (420 g/hr) dosing directly into flume. By-pass water is the condensate flow.

If flume water is used as by-pass then no static mixer or VA flow meter/check valve as these block up.

Control to 650mV using D1CAW0R redox controller and proportional control with industrial redox probe.

This system is only used in wet weather and occasionally during dry weather. Mould is a bad problem when there is a lot of rain during harvest.

Number of installations

 3 plants in Australia. Girgarre Country Foods, SPC Foods, Unilever.

 

Previous Treatment

 Used sodium hypochlorite and due to the high concentration of organic material in the flume water, had difficulty maintaining any free chlorine residual. This meant that moulds were not controlled and surfaces were fouled. In addition, operators would occasionally stop work due to chlorine smell in the sorting area.

 

Advantages/benefits

 Low concentration of chlorine dioxide is very effective in destruction of moulds on the tomatoes. These moulds would negatively affect the past production process if present.

Low concentration of chlorine dioxide is very effective in destruction of moulds in the flume water. If untreated, the moulds attach to surfaces of tanks and flumes and look like “meat”. Eventually, they foul screens and smell.

Chlorine dioxide effective at pH 4.0

No smell for operators

Very low running costs

No chlorinated organic by-products.

Disadvantages

 When the redox probe fails, chlorine dioxide overdoses and then smell is produced. This can be avoided by using two redox systems in parallel.

Possible build up of chlorite in the flume tank can affect the skin of operators when they handle the tomatoes i.e. hands, arms and face. If they wear gloves this can help.
copyright 2003 Dioxide Pacific Pt

 

Vegetable washing.

Vegetables of all kind are washed, cut and packed (e.g. in plastic bags).

Customers are supermarkets and fast food producers.

Previous treatment

 Usually Chlorine is used for microbiological control with concentrations varying between 100 - 200 ppm.

Problems with previous treatment

 Chlorine created a smell problem during processing in the processing hall with operators complaining of eye and skin irritation.

pH very often above 7.5 where microbiological treatment is often not effective with chlorine.

Case Study

 Parripak Food, in England - changed to Chlorine Dioxide

Batch washing

 Water change every 6-8 hrs. typical dosage: 6 ppm

Spraying, typical concentrations

Onion rings 6 ppm

Carrots 1 ppm

 

Benefits of chlorine dioxide

Shelf life increased by factor 3

Smell problem decreased significantly. Smell problem decreased significantly.