Ozone – More Than Just a Disinfectant
Water – Disinfection
There are a number of commonly accepted methods for disinfecting drinking water to control bacteriological contamination as well as other pollutants affected by oxidation. Some of these methods/technologies, to name a few, include:
1. Chlorination (use of chlorine as the disinfectant)
2. Chloramination (use of chloramines as the disinfectant)
3. Ultraviolet radiation
4. Filtration (before or after oxidation, depending on the process)
5. Ozonation
Other than filtration, which functions mainly to improve process efficiencies, the main purpose of the above methods is disinfection. While they are all effective under a particular set of circumstances, ozone is the only one that is more universal in its potential applications as well as limiting negative health and environmental effects while having the ability to treat many of the hard to treat pollutants. Before discussing the ozone derived benefits, let’s first review background information on some of the other conventional disinfection practices and their ability to produce safe high quality drinking water.
Water – Disinfection By-products (DBP’s)
It is becoming increasingly difficult to maintain compliance with some of the newer and more restrictive permitted potable water quality standards, of which the new DBP rules are one. As of late, compliance with chlorination by-products of organic matter as it relates to total trihalomethanes (TTHM’s) formation and their associated health risks as well as providing adequate disinfection of the harder to treat pathogens (disease causing bacteria and viruses) has become a greater concern. With this said, acceptable TTHM concentration limits have been reduced from 100 to 80 parts per billion (ppb). At this same time, greater focus has been placed on some of the harder to treat organisms. But first some more details on TTHM’s and their origin and significance. TTHM’s are the result of chlorine oxidation (chlorination) of organic matter, both natural and manmade, in the water. TTHM formation favors increased time of exposure to chlorine as well as higher temperatures and pH’s. Conversely, the 5 most common haloacetic acids (HAA5) favor lower pH’s. Concerns surrounding TTHM’s stem from their potential health effects, one of which is increased cancer risks. Sources of natural organic matter are: decaying organic materials such as leaves, grass, wood, etc. Each manmade organic pollutant group contains compound(s) detrimental to human health. Some of these manmade compounds are represented by:
1. Herbicides
2. Organophosphorus insecticides
3. Polychlorinated biphenyls (PCB’s)
4. Polychlorinated dibenzo-p-dioxins
5. Phenolic compounds
6. Polycyclic aromatic hydrocarbons
7. Halogenated hydrocarbons (aliphatic and monocyclic)
8. Phthalate esters
Water – Modifications of Chlorine Disinfection Process
A number of more conventional treatment processes are used aid in removal of organic material in advance of disinfection with chlorine reduces the risk of TTHM’s. Several of those processes include microfiltration and chemical pretreatment and precipitation. These processes, in effect, remove the majority of the TTHM pre-cursors before contact with chlorine. Yet another method of controlling TTHM production does not require removal of the precursor compounds. Rather, its effectiveness is a function of disinfection with chloramines, a chlorine/ammonia compound. Chloraminesare comprised of 3 subgroups: monochloramine, dichloramine and trichloramine. The active portion of the chloramine compounds is less active than chlorine and, as in the case of TTHM pre-cursors, is less reactive with other organics as well.
Water – Ozone as an Alternate Disinfection Method
One disinfectant process that has had a history of more widespread acceptance in Europe than the United States is ozonation. At the onset, the ozone process was both unreliable and comparatively expensive when compared to the more tried and true method of chlorine disinfection. That is not the case today as recent technological improvements as well as the cost effectiveness of this process have improved significantly not to mention the “green” features of ozone (some of which are listed in the Beneficial Treatment Characteristics section below). This is a process whereby the precursors can be oxidized and filtered out of the water in advance of the chlorination. Removal of the bulk of the organic compounds leaves little if nothing for the chlorine to react with or to convert to TTHM’s.
While it has been in use for many years, largely in the role as a disinfectant for treated potable water, ozone has demonstrated abilities in the area of value added benefits. Some of these benefits include removal of a broader range of pollutants ranging from color from dyes to pesticides to personal care products etc., making it a better disinfecting agent choice in today’s regulatory environment, “green” mindset and economy. Some of the more common Beneficial Treatment Characteristics of ozone are:
1. Strongest and fastest commercially available disinfectant for water treatment, in fact it is about 3,000 times faster to purify water than chlorine (requires much shorter contact time subsequent to a smaller disinfection process unit)
2. Can treat for all microbiological contamination including: bacteria, viruses, yeast, mold, cysts and mildew
3. Can treat most organic and inorganic contaminants such as oils and metals with no chlorinated by-products
4. Can be generated onsite and does not require transport or storage (reduces health and safety risks)
5. Cannot overdose (ozone has a short process life due to its volatility and therefore offers an extremely short lived residual)
6. Effectiveness is not dependent on pH (greater process flexibility)
7. Once dissolved in water, poses no bodily irritation effects and produces no tastes or smells
8. Is less corrosive than chlorine (greater flexibility)
9. ORP measurement is an effective tool for monitoring effectiveness(functional for instantaneous and inline process control with optimal treatment occurring at about 700 mV)
Water – Ozone as an Oxidant for Other Pollutants
Yet, there are other outlying issues including endocrine disrupting chemicals (EDC’s), pharmaceuticals and personal care products (PPCP’s), etc. Many of the compounds contained in these groups have been present in our water supplies throughout much of recent history. Recent advances in technology along with increased public awareness have led to greater interest in the presence, and removal, of these compounds in finished drinking water supplies and their human health related effects. Some sources for EDC’s are surfactants, plasticizers, PCB’s, pesticides, herbicides and dioxinswhile PPCP’s include drugs (for both humans and animals), pharmaceutical residuals, hospital discharges and personal care products.
For the purpose of drinking water production, ozone does not oxidize organic matter in the same way as chlorine. Discontinued use of chlorine in favor of ozone allows for oxidation of carcinogenic precursor compounds with their precipitates being precipitated and/or filtered out before any further treatment, including chlorination. This is in contrast to disinfection/oxidation of organics with chlorine which has an undesirable carcinogenic by-product effect. In a manner similar to the TTHM pre-cursors, ozone also oxidizes a number of EDC’s and PPCP’s.
Generally speaking, if ozone will not provide the level of oxidation necessary, then all other more conventional oxidants including chlorine, acid, hydrogen peroxide and the like would not be successful either.