CHLORINE DISINFECTION OF PRODUCE WASHWATER
WHAT IS CHLORINE?
Chlorine for disinfection is available in three different physical phases; solid (calcium hypochlorite Ca(OCl)2), liquid (sodium hypochlorite NaOCl or common bleach), and gas (chlorine Cl2). It can be used in any form as a disinfectant for washers, flumes, dump tanks, and hydrocoolers. However, chlorine gas is extremely concentrated and is not recommended for use in the produce industry.
The solid phase (calcium hypochlorite) is less corrosive than liquid bleach (sodium hypochlorite) and will cause less damage to the produce and equipment. The disadvantage of the solid is that it is less stable at normal storage conditions.
HOW DOES CHLORINE WORK?
All types of chlorine will form hypochlorous acid (HClO) when added to water. Hypochlorous acid is the active ingredient of chlorine solutions. It undergoes oxidation to affect the reproduction and metabolism of microorganisms. This results in disinfected water through a reduction of pathogens.
A microorganism is any yeast, mold, bacteria, protozoa, or virus. A pathogen is a disease or injury causing microorganism. Pathogens of interest in the fresh produce industry include E. coli, Cyclospora, Salmonella, and Hepatitis. A cyst is a protective coating for a dormant microorganism, and an ova is a female germ that is capable of reproducing.
HOW EFFECTIVE IS CHLORINE?
Hypochlorous acid is effective against a variety of pathogens, but has some limitations. For water that has undergone sedimentation and filtration, chlorination will be about 90% effective against viruses. Cysts and ova are immune to chlorine treatments and must be removed by coagulation/sedimentation and filtration. A 5 micron filter will significantly reduce spores and cysts.
WHAT CONDITIONS ARE BEST?
Although hypochlorus acid readily reacts with pathogens in water, correct use is essential for maximum effectiveness and safety. In order for chlorine to be effective, the treated water must be filtered before treatment to reduce the amount of organic material.
The effectiveness of chlorine is controlled by pH, temperature, contact time, and dose. Neutral pH (6.5 to 7.5) produces the maximum amount of hypochlorous acid. If the pH of the water is too low (pH < 6.0), chlorine will escape as a gas, decreasing effectiveness and increasing equipment corrosion. If chlorine is added to an alkaline water (pH > 8.5), the amount of hypochlorous acid formed will be greatly reduced, and the water will not be disinfected. Bleach (liquid chlorine) and calcium hypochlorite (solid chlorine) both increase the solution's pH. pH test kits are available from most chlorine equipment distributors. When the pH is too low, the water can be buffered with lime (CaO). If the pH is too high, it can be brought down with sulfuric acid (H2SO4). Both of these chemicals are inexpensive and readily available.
Temperature is another important factor for water disinfection with chlorine. Warm temperatures are desired to increase the capabilities of chlorine, but temperatures that are too high may damage the produce. In general, every 18 0F drop in water temperature will double the required contact time.
Concentration of chlorine solution is a major controlling factor in water sanitation. Example concentrations can be obtained from the chart at the end of this article. The United States Food and Drug Administration (USFDA) released a draft document in April, 1998 that stated typical contact times of 1 to 2 minutes and concentrations of 50 to 200 ppm.2 The commonly recommended concentration is 100 - 120 ppm.1,4,5 The effectiveness of chlorine at those concentration will be affected by the contact time, the amount of suspended solids, and types of pathogens in the treated water.
WHERE IS CHLORINE ADDED?
A variety of processes may be used for chlorination in the packing line. Liquid or solid phases may be added to dump tanks or hydrocooler water as needed to maintain chlorine concentrations. The manual mode may not result in adequate concentrations over time. Automatic injection equipment is available to help maintain chlorine concentrations. It is important to avoid any "dead flow" areas. Proper circulation is a must for water sanitation.
A mixing or contact tank circulates the water and provides the required contact time for the chlorine/water solution. Flow through the mixing tank must be such that the dump tank or hydrocooling water is circulated every 20 minutes. It is important to filter this water before treatment to help maintain the effectiveness of the added chlorine.
Water to be recycled for washing produce should be filtered before treating to help maintain the effectiveness of the added chlorine. A mixing chamber should be used to provide the required contact time for the chlorine/water solution.
Solutions formed by chlorine and water are highly corrosive to all common metals. Chlorine does not react with gold, silver, platinum, or other specialized alloys. Chlorine resistant materials also include stainless steel, hard rubber, polyvinyl chloride (PVC), and certain other plastics.
HOW DO I CONTROL MY DOSAGE?
It is important to have a method for monitoring and controlling the amount of chlorine entering the water system. Manual control requires less maintenance and operator education than automatic control. It should only be used when water flow and organic content are constant.
Many companies distribute automatic controls for packing lines that require variable disinfectant concentrations.* The concentration of chlorine in the mixing tank must be checked with an accurate test kit at least once a day and after any large increase in organic loading.
Because of the corrosiveness of chlorine, inspections of the entire water system are needed. Leaks must be attended to immediately. All water system connections must be checked at least every six months.
Employee training on the use of chlorine is provided by most distributors and the Chlorine Institute. (See contact information at the end of this document.)
IS CHLORINE A HEALTH HAZARD?
Chlorine gas is labeled as a toxin on the MSDS (Material Safety Data Sheet, available from any company that sells a chemical). Chlorine is also an irritant to the respiratory system, skin, and mucous membrane. In the liquid form, it will burn skin and eyes on contact. When products of chlorine come in contact with organic material, trihalomethanes (THMs) are formed. THMs are known to cause cancer over long term exposure.
The Occupational Safety Hazards Association (OSHA) requires the maximum long-term ( > 8 hours/day) human exposure level to chlorine in air to be 0.5 ppm. On a short-term ( < 15 minute) basis, the maximum level for chlorine exposure is 1 ppm in air. Concentrations in air above 5 ppm in air will cause choking, coughing, and skin and eye irritation.
Chlorine must be handled safely. Liquid chlorine (sodium hypochlorite) is readily vaporized to gas. It must be handled in a manner that prevents the accumulation of high concentrations in the air. Monitors can be purchased to measure the chlorine concentrations in air. Automatic metering devices greatly reduce chlorine gas in the air.
Only trained personnel should be in charge of storing and handling chlorine. The Chlorine Institute and most equipment manufacturers will provide this educational material. In general, chlorine solutions should be handled with protective clothing and glasses. Shower and eyewash stations should be near the handling area when mixing chlorine with water or when handling solid hypochlorite.
HOW DO I IDENTIFY CHLORINE?
All phases of chlorine are soluble in water and have a distinct swimming pool smell. The liquid (sodium hypochlorite) is a clear-amber color. The solid form (calcium hypochlorite) of chlorine looks like a salt block. The gas form of chlorine has a yellow green color and will settle to the ground.
DISPOSAL OF TREATED WATER?
The Michigan Department of Environmental Quality (DEQ) requires effluent concentration of chlorine to be less than 0.038 mg/L (ppm) for water treatment plants. Free chlorine residuals produce byproducts that kill biota in the receiving environment and are harmful to humans. In case of a spill for chlorine products, contact the MiDEQ hotline at 800-292-4706.
Chlorine residual will be greatly reduced or eliminated by proper use of lagoons, sulfur compounds, or activated carbon.
Lagoon storage of effluent containing chlorine will reduce the chlorine content. After a 2 to 5 day period, the activity of chlorine is significantly decreased. This process is easy and effective, but will requires a properly sized lagoon.
Both sulfur dioxide and sulfite salts react with free chlorine and combined chlorine. In general, the concentration of sulfur dioxide or sulfite salts must be equal to the concentration of chlorine in the water. If excess sulfite is present in the effluent, the dissolved oxygen in receiving waters will be reduced. Sulfur dioxide is classified as an irritant and is also corrosive. It must be stored in the same type of containers that are used for storing hypochlorite. Proper safety training is usually provided by the distributor or can be obtained from the Compressed Gas Association.*
Granular and powdered carbon is also used to dechlorinate water. Carbon may also filter other waste materials depending on particle size of the carbon, pH of the water, and suspended solids of the water. The proper dose of carbon is usually 30 to 40 mg of carbon per liter of water.
WHAT WILL I NEED TO BUY?
Chlorine is inexpensive and readily available (several companies that distribute automatic controls and chemicals are listed at the end of article). Chlorine for commercial use is stored as sodium hypochlorite (common bleach) at 9.0 % solution. Chlorine for household use is stored as sodium hypochlorite (common bleach) at 5.25% solution. It can also be purchased as a solid (calcium hypochlorite) in large quantities. Pool and spa dealers sell calcium hypochlorite. Chlorine must be stored in tightly closed containers away from heat and direct sunlight, or alcohol.
Mixers should be purchased to obtain the correct circulation and contact time. Each type of chlorine uses different methods of mixing the chlorine with water. To avoid overdosing and off-gassing, monitors are a necessity. Proper operation and maintenance procedures must be followed for safety. Equipment should be checked frequently for failure. Since chlorine is corrosive, spare equipment parts must be available on-site. All diaphragms and gaskets must be replaced at least every two years.
WHERE CAN I BUY CHLORINE AND EQUIPMENT?
Chlorine and equipment can be obtained from the following companies. These companies were chosen only for reference. This article is not intended to be an advertisement nor provide a product recommendation. There are many companies that sell similar products that these companies offer. For a list of general chemical companies visit the web at the Chemical Companies' Websites:
AFC International, Inc.