Water is the basis of our life
Today, high population densities and overuse have put a tremendous strain on our water resources. This increases the risk of public illness and epidemics from unsafe drinking water supplies. Traditional (physical/chemical) methods of treatment such as flocculation, filtration and chlorination are often insufficient to ensure the quality and safety of drinking water. A step ahead from chemical water treatment to a more harmless and friendly technology such as ozone can be detected worldwide.
Ozone is a strong oxidant and an excellent disinfectant. Ozone is used at water plants throughout the world to address disinfection, disinfection by-products, taste and odor, color, micro-coagulation, and other water treatment needs. Ozone can effectively destroy bacteria and inactivate viruses more rapidly than any other disinfectant chemical.
Ozone reacts to oxidize a number of inorganic compounds including iron, manganese, sulfides, nitrite, arsenic, bromide ion, and iodide ion. Iron and manganese can be reduced to very low, safe levels in water supplies through ozone oxidation.
All ozone applications involve oxidative reactions, whether ozone is used for disinfection or oxidation of specific contaminants.
Ozone Water Treatment History- A Timeline for Ozonation Water Purification
- 1886: The ability of ozone to disinfect polluted water is recognized in Europe.
- 1891: Test results from Germany show that ozone is effective against bacteria.
- 1893: First full-scale application using ozone for drinking water in the Netherlands.
- 1906: Nice, France commissions first municipal ozone plant for drinking water, causing Nice city, to be called as the Birth Place of Ozone for drinking water treatment.
- 1911: Ozone Generators started in St. Petersburg, Kiev, Gorski & Moscow
- 1914: World War I – Poisonous gas research leads to the development of inexpensive chlorine gas. Interest in ozone for water begins to decline.
- 1915: At least 49 major ozone installations are on line throughout Europe.
- 1940: First Ozone Water treatment Plant in Whiting, in USA.
- 1949: Ozone Drinking Water treatment Plant in Philadelphia, PA, USA.
- 1957: Ozone is implemented for oxidation of iron and manganese in Germany.
- 1964: Spontaneous flocculation in ozone contact chambers led to France constructing an ozone plant to enhance particulate removal.
- 1965: Scotland employs ozone for color control in surface water. Switzerland uses ozone to oxidize micro pollutants such as phenolic compounds and several pesticides.
- 1970’s: French exploit use of ozone for algae control.
- 1982: USA FDA GRAS (generally regarded as safe) declaration for ozone use in bottled water.
- 1987: City of Los Angeles 600 MGD ozonation plant comes on line after 7 years of pilot testing.
- 2000: A Food Additive Petition (FAP) requesting FDA approval of ozone as an antimicrobial agent for direct contact with foods was submitted in August and was approved by the FDA in 2001.
Drinking water supplies are based on lakes and dam reservoirs, on river water and groundwater. Groundwater resources associated with hard rock geology are localized and small, so supplies come mainly from surface waters such as rivers and impounding reservoirs. A problem with water quality can be defined as a failure of a water supply to meet the minimum standards laid down in the European Council (EC) Drinking Water Directive. Council Directive 98/83/EC, the Drinking Water Directive (DWD), concerns the quality of water intended for human consumption.
The objective of the Drinking Water Directive is to protect the health of the consumers in the European Union (EU) and to make sure the water is wholesome and clean. To make sure drinking water everywhere in the EU is indeed healthy, clean and tasty, the Drinking Water Directive sets standards for the most common substances (so-called parameters) that can be found in drinking water.
Water problems can arise at four different points in the supply cycle:
- At the source,
- The treatment stage,
- During distribution to the customer’s house,
- Within the household plumbing system.
As a general rule the cleaner the raw water, the cheaper the finished water is to produce, and the safer it is to drink. So where the concentration of contaminants is excessively high then the source may be rejected due to the cost of adequate treatment.
All water sources contain natural organic matter (NOM). Concentrations (usually measured as dissolved organic carbon, DOC) differ from 0.2 to more than 10 mg L-1. NOM creates direct problems, such as odor and taste in water, but also indirect problems such as organic disinfection by-product formation, bacterial regrowth in the distribution system. To produce pure drinking water, the removal of NOM is a prior task in modern water treatment. Odor and taste production in drinking water can have several causes. Odor and taste forming compounds can be present in raw water, but they can also be formed during water treatment.
Another possibility is that the chemical oxidation leads to an unpleasant tastes and odors. Odor and taste forming compounds are often very resistant. This causes elimination to be a very intensive process.
Because of its excellent disinfection and oxidation qualities, ozone is widely used for drinking water treatment.It has been used continuously in drinking water treatment for about 100 years, beginning in Nice, France, in 1906. Since then, ozone was applied in Nice continuously, causing Nice to be called the place of birth of ozonation for drinking water treatment.
Today, Europe has about three thousand ozonation facilities and ozone is used at water treatment plants throughout the world to address disinfection,disinfection by-products (DBPs), taste and odor, color, micro-coagulation, and other water treatment needs. Ozone will effectively destroy bacteria and inactive viruses more rapidly than any other disinfectant chemical. Ozone is used at drinking water treatment plants for various reasons. The only reason that ozone has not totally replaced chlorine in municipal water treatment is its low solubility in water, and therefore its inability to provide a residual disinfection power all the way to the opposite end of the system in a municipal application. When ozone is applied as a gas for drinking water treatment, it is done primarily because of its oxidative strength. All ozone applications involve oxidative reactions, whether ozone is used for disinfection or oxidation of specific contaminants
Disinfection Treatment
Microbial contamination is the most critical risk factor in drinking water quality with the potential for widespread waterborne disease. Illness derived from chemical contamination of drinking water supplies is negligible when compared to the number due to microbial pathogens.
In the past, the primary emphasis of disinfection was to control waterborne diseases through the control of associated bacterial indicator organisms such as coliforms. With use of disinfectants, epidemic outbreaks of diseases such as cholera and typhoid have been virtually unknown for decades. Two findings made in the 1970s have resulted insignificant reevaluation of this long established disinfection practice.
The first finding was that disinfection by-products, formed in the reaction of disinfectants and certain source water organic matter, might be harmful to human health.
The second finding was the discovery of newly recognized waterborne disease-causing organisms that could not be effectively controlled by the then accepted disinfection procedures.
These new concerns require a high level of understanding of what disinfection is and how it can be accomplished with minimal side effects.
Disinfection may be defined as the inactivation of (pathogenic) microorganisms. Inactivation means that even though the pathogenic organism might physically exist in the finished water supply, it has been rendered dead, inactive or incapable of reproduction.
The major and first disinfection step normally employed in the treatment of drinking water is primary disinfection. In essence, primary disinfection is intended to completely kill pathogens present in the source water, thereby preventing the introduction of such pathogens into the water distribution network. A supplemental effect may also be accomplished by the separation and removal of pathogens in the filtration of drinking water.
Usually a secondary or final disinfection step is added to the treatment train to maintain a disinfectant residual concentration throughout the water distribution system. The role of this disinfectant residual is to provide protection against subsequent microbial intrusion or regrowth following treatment. Sufficient disinfectant is added to the flow leaving the treatment facilities to ensure that some residual is available throughout a distribution system. The basic presumption is that the specific combination of disinfectant concentration and contact time (CT) will result in a certain degree for inactivation of the target pathogen groups.
Microorganism Inactivation with Ozone
Ozone is thought to achieve disinfection largely through oxidation reactions that damage and destroy critical components of microorganisms. Although the general effects of disinfectants on bacteria and other organisms are known, Ozone is thought to exert its strong oxidizing capability to achieve disinfection by disrupting the function of the bacterial cell membrane, although enzyme and other internal cell interactions are also possible. With respect to viral disinfection, ozone is thought to achieve viral disinfection through oxidative attack of the protective protein coating;possible interactions with deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) maybe a factor as well.
The primary purpose of ozonation at many water treatment plants is to achieve disinfection log-inactivation credit for viruses, Giardia, and Cryptosporidium atregulated or above-regulated levels. The value of 1-log inactivation is the same as 90 % inactivation, 2 log is 99 %, 3 log is 99.9 %, etc.
Two types of primary disinfecting credit are considered for ozone. The first type of credit is based on measured CT as with other disinfectants. However, ozone is also thought to achieve a very rapid initial kill that does not appear to be characterized by a CT relationship. In recognition of this phenomenon, EPA criteria provide a second type of credit for at least 1 log virus inactivation and 0.5 log Giardia inactivation with in the first stage of a multistage contacting system as long as specified minimum ozone residuals are provided at the discharge from that stage.
Point of Ozone Application
Selection of the location of ozone application for meeting disinfection goals depends onsite-specific factors. In many pretreatment oxidation applications of ozone, particularly in waters with low ozone demand, it may be reasonable to provide an ozone residual over the contact time that also provides the CT required for primary disinfection requirements. However, where the ozone demand exerted in these pretreatment oxidation applications causes rapid depletion of the residual, ozone disinfection is often hampered. In such situations, ozone may have to be added at a second downstream location to accomplish disinfection.
DSINFECTION
Ozone in drinking water plants
Ozone is one of the most powerful oxidizing agents known and can be used to great advantage in the treatment of drinking water. Ozone is used for oxidation as well as primary disinfection, replacing chlorinated compounds.
Ozone provides the following advantages:
- Very effective oxidation of contaminants(organic and inorganic)
- No formation of harmful by-products such as Trihalomethanes (THM’s)
- Effective virus and cyst reduction
- Perfect improvement of colour, taste and odor
- Control of algae.
The use of ozone in water treatment plants
There are several methods of applying ozone for drinking water treatment. It is used in both single and multiple stages as in the flow diagram shown (below right). In many cases a single ozone stage is sufficient to meet primary oxidation and/or disinfection requirements.