Using Ozone Generators in Water Treatment Systems
Ozone can be used as a disinfectant, decolorizer, deodorizer, detoxifier, precipitant, coagulant and for removing tastes. Consequently, the use of ozone should be considered when there is a need to address any of these problems, and particularly when there is suspicion of waterborne pathogens.
Ozone production on site, via normal corona discharge, eliminates the problems transporting, storing and handling of hazardous and dangerous materials( chlorine).
For efficiency, ozone is outstanding, as it is the most powerful and effective broad spectrum microbiological control or disinfecting agent available. As an example, for the inactivation of Giardia cysts the CT (concentration & time) is about 100 times greater for free chlorine than for ozone. With viruses the CT is about six times greater for chlorine than for ozone. For Cryptosporidium, ozone is the recommended disinfectant.
Consequently, with ozone, there is less need to add high concentrations of a long lived poison to the water you will soon be drinking. When ozone is utilized, the chlorine need is greatly reduced, as it is then needed only as a way to provide a residual in the distribution system, at 0.2 mgCl2/L. This minimizes the potential for producing carcinogenic THMs and helps protect your health.
Ozone is of particular value in water treatment systems because of its ability to disinfect without adding other chemicals which may later need to be removed. As with other treatment methods, the oxidised by-products of ozonation may need to be removed by filtration or settling to provide acceptably clean clear water.
Water, being the universal solvent that it is, seldom remains pure water (H2O). Even the double distilled water used in laboratories contains dissolved traces of other substances. Ozone will decompose in water by itself (slower than in air), as well as reacting with almost anything dissolved or carried in the water.
As clean as rain water is assumed to be, it can usually be improved after ozonation and filtration.
OZONE…THE MAGIC BULLET IN FRESH FOOD RINSING
Ozonizing water for citrus washing can achieve a disinfection efficiency of 99.9 percent (of the water) with a concentration of 1 mg/l ozone and will require a retention time of only 57 seconds. On the contrary, using chlorine to achieve a 99.9 percent disinfection efficiency at the same water temperature and ph. value (15°C and 7 respectively) with 1 ppm chlorine will require a retention time of 75 minutes.
Chlorine always leaves an oxidation or disinfection by-product whereas ozone simply reverts back to oxygen.
During the oxidation or disinfection process, only one oxygen atom is used for the chemical reaction.
Another benefit of ozone is that it is generated on-site and as needed. There is no chemical to store and no residual disinfection by-products to monitor.
Ozone in water is often described as an alternative to hypochlorite as a disinfectant or sanitizer.
- Ozone solubility in water is low meaning that effective contact and mixing systems sometimes with off-gas systems need to be devised because Ozone in water, above 1 mg/litre, can liberate ozone into the air that exceeds safe levels for workers/operators (OSHA workplace maximum = 0.1 ppm).
- Significant advantages of ozone in water are that it decomposes quickly to oxygen, leaving no residues, and it has more potency against bacteria, cysts of protozoa, viruses, and fungal spores than hypochlorite.
- Ozone can oxidize many organic compounds, particularly those with phenolic rings or unsaturated bonds in their structure and can have a role in reducing pesticide residues in process water and mycotoxins in durable commodities.
Packinghouse processes where ozone in water could be applied include:
- Ozonation to sanitise packing line process water.The water in tanks where fresh fruit are dumped or floated before cleaning, sorting, and packing operations is an important site for the accumulation of pathogens that infect fruit later in storage, shipping, or marketing. Examples are blue mould of apples and pears, caused by Penicillium expansum, and green mould of citrus, caused by Penicillium digitatum. Therefore, disinfection of this water is important, and usually is accomplished with hypochlorite. Ozone can be employed in flume water in apple and pear packinghouses, and some facilities have ozonated hydrocooler water.
- Pre-conditioning of the water (to reduce particulates, BOD, turbidity, etc.) before ozonation is needed in systems where water is recycled, and this can be difficult and expensive.
A contact time of two minutes in 1mg/litre ozone will 95-100% of all fungi and none will survive in 3 minutes of contact. Spores of these pathogens die quickly in ozonated water, but fruit, soil, and other debris in the water can reduce the ozone concentration completely or to ineffective low levels.
OZONE…THE MAGIC BULLET IN WATER PURIFICATION
Common expectations of ozone in Water Treatment
- Algicide – very effective
- Amoebicide – extremely effective, kills all known
- Aquaculture aid – many uses
- Bactericide – extremely effective, kills all known
- BOD COD enhancement – quite effective, as this IS real live OXYGEN
- Coagulant – quite effective
- Contaminate remover – very selective
- Decolorizer – usually effective, organic, inorganic, simple and complex, mineral and carbon
- Deodorizer – very effective, most are removed
- Detoxifier – quite specific; some quick, others slowly, some not at all
- Disinfectant – extremely effective, most if not all
- Dissolved solids – fairly effective, inorganic and organic
- Flocculant aid – quite effective, fairly specific
- Fungicide – very effective, all known
- Hardness – slightly effective with some specific changes
- Metals – very effectively on some, manganese and iron
- Oxygenator – extremely effective, particularly boreholes
- Particulates – quite effective, inorganic and organic
- Pesticide – very effective; kills worms, flukes, fish, snails, eggs, larvae, etc.
- Precipitant – quite effective, extremely useful
- Precursor reduction – very effective, ie. THMs
- Softness – somewhat effective, assists and improves specifically
- Taste – very effective, removes or enhances most
- Turbidity – quite effective, prepares inorganics and organics for filtration
- Viricide – extremely effective, kills all known viruses
- Waste and effluent – quite effective, excellent in a wide range of uses
DIFFERENCES BETWEEN HYPOCHLORITE FRESH FOOD RINSE AND OZONE
|Microbial potency||Kills plant pathogens and microbial saprophytes effectively. Some human- pathogenic, spore-forming protozoa resistant. Maximum allowable rates under regulatory control||Kills plant pathogens and microbial saprophytes effectively, including spore-forming protozoa. Maximum rate limited by ozone solubility, difficult to exceed about 10 mg/l|
|Cost||Chemical cost low. Repeated delivery required, sometimes pH and concentration controller systems needed, minor maintenance and energy costs, chlorine storage issues||Variable: no chemical cost, but high initial capital cost for generator, usually needs filtration system when water re-used some are complex, modest maintenance and energy costs|
|Influence of pH||Efficacy diminishes as pH increases, above pH 8, pH adjustment may be needed. Chlorine gas released at very low pH (4 or less)||Potency not influenced very much by pH, but ozone decomposition increases at high pH|
|Disinfection by- products||Some regulatory concern, tri-halo compounds, particularly chloroform, of some human safety concern||Less regulatory concern, small increase in aldehydes, ketones, alcohols, and carboxylic acids created from organics|
|Worker safety issues||Chloroamines can form and produce an irritating vapor, chlorine gas systems require on-site safety measures, OSHA (TWA) limit for chlorine gas: 1 mg/l||Off-gas ozone from solutions an irritant and must be managed. MnO2 ozone destruction efficient and long-lived. OSHA (TWA) limit for ozone gas:1 mg/l|
|Persistence in water||Persists hours in clean water, reduced persistence to minutes in dirty water||Persists minutes in clean water, reduced persistence to seconds in dirty water|
|Use rates||Limited by regulation to 25 to 600 mg/l depending on application||Not limited by regulation, but Henry’s law limits theoretical maximum ozone in water to about 30 MG/L at 20°C. Most ozone systems produce 5 mg/l or less.|
|Use in warm water||Increases potency, some increase in vapors||Not practical, rapidly accelerates ozone decomposition, increases off- gassing, decreases ozone solubility|
|Influence on product quality||Little risk of injury at recommended rates. Some injury possible above 50 mg/l on tree fruits. Off-flavors on some products at high rates||In brief water applications, risk of product injury low. Stem, calyx, and leaf tissue more sensitive than fruits. Risk of injury needs more evaluation.|
|Impact on water quality||Minor negative impact: water salt concentration increases somewhat, may interfere with fermentation used to reduce Biological Oxygen Demand, some pesticides inactivated, discharge water dechlorination may be required.||Mostly positive impact: does not increase salt in water, many pesticides decomposed, Biological/Chemical Oxygen Demand may be reduced, flocculation and biodegradability of many organic compounds enhanced, precipitates iron, removes color, odors|
|Corrosiveness||High, particularly iron and mild steel damaged||Higher, particularly rubber, some plastics, yellow metals, aluminum, iron, zinc, and mild steel corroded|