Ozone for drinking and wastewater treatment

The increasing demand for safe water necessitates the treatment and reuse of wastewater as a means of minimizing water requirements and water disposal. Effluent discharges contain variety of stubborn chemicals resistant to degradation by normal oxidants and dissolved oxygen. Ozone as an oxidant is preferred due its green chemistry of starting materials and products. This mini review covers concisely the scope, benefits and limitations of ozone initiated oxidations as tool for oxidative degradation and disinfection in water treatment and for conversion to value added products in industrial processes.

As a consequence of population growth and industrial development, an increasing number of biologically resistant organic pollutants are produced and discharged into the environment, causing various problems in drinking and wastewater systems as well as in respect to human health.

In addition, the diminishing availability of water resources is a challenge as the reuse of municipal and industrial wastewaters and the recovery of potential pollutants used in industrial processes become more serious.

Natural waters contain many impurities that completely dissolve in water, substances which are insoluble in water and substances that form suspensions in water. Natural waters are also biologically contaminated by bacteria, algae, protozoa and other organisms. Oxidation technology is a highly effective method of eliminating micro-organisms and toxic substances in water and wastewater.

Drinking water is considered to be safe for human consumption, if it is free from disease causing micro-organisms, turbidity, colour, odour, and from any objectionable taste. Oxidative water treatment methods include chlorination, hypochlorite, iodination, use of ultra violet light and ozonation. When applied in its elemental form or as hypochlorite, chlorine is the standard of disinfection against which all others are compared.

Depending on the pH of the water and on the presence of ammonia, the chlorine may take the form of HOCl, OClÉ, Cl2 or chloramines. Iodine is capable of inactivating enteroviruses in water under controlled conditions.

Studies by Olivieri et al. on the comparative modes of action of the halogens on a small bacterial virus indicated that iodine reacts with the coat protein where as chlorine probably inactivates the viral nucleic acid. Iodination may only be used in emergency situations as high levels of iodine can be toxic to humans. Ultra-Violet radiation (UV) kills the vegetative forms of bacteria, spores, protozoa and viruses. Wavelengths from 200 to 295 nm have the strongest oxidation effect.

Used alone, the disadvantage of this method is the high cost and the possibility of subsequent infection.

Ozone is one of the most effective oxidant which has been widely applied in water and wastewater treatment therefore ozonation is preferred to chlorination and is frequently adopted to remove herbicides from drinking water. It not only decontaminates water but also gives it a pleasant taste, lowers its colour and kills off odour produced by oxidation and mineralization of organic impurities.

Ozonation has some advantages over chlorination, as can be prepared in situ, it does not add to its chemical pollution and no excess reagent removal need. Ozonation of halogenated organic compounds in aqueous media has been studied by many researchers and it was found to be one of the effective processes for the treatment of several chlorinated organic compounds in water.

The formation of organic by-products of chlorination and ozonation processes is well established. The most common ozonation by-products are aldehydes as well and short-chained carboxylic acids. The oxidation potential of ozone (2.07V) is 1.52 times higher than that of chlorine. It is this high oxidation potential that allows ozone to degrade most organic compounds.

The oxidation potentials of strong oxidizing agents are compared in Table 1.Ozone is a stronger oxidizing agent than the normally used reagents such as peroxides, chlorine and hypochlorous acid.

Species    Oxidation Potential/eV
Fluorine                   3.06
Hydroxyl radical      2.80
Nascent oxygen      2.42
Ozone                    2.07
Hydrogen peroxide 1.77
Perhydroxyl radical 1.70
Hypochlorous acid  1.49
Chlorine                  1.36

Generation & properties of ozone
Ozone can be produced several ways viz. Electrolytic ozone generation, photochemical ozone generation, radiochemical ozone generation and ozone generation by corona discharge method. The most widely used method of ozone generation is ultra violet (UV) radiation and the corona discharge method.

Ozone created with UV radiation has limited uses while corona discharge ozone generators provide a wider range of applications. The ozone containing air flow is normally calibrated either by iodimetric/ titration methods or photometric methods.

Corona discharge method
The corona discharge method is the most popular type of ozone generation for most industrial and personal uses as it is considered to be the most effective and economical means of controlled ozone production. It stimulates the production of lightning where energy is used to convert oxygen into ozone. The process is carried out by passing clean, dry oxygen gas through two electrodes separated by a dielectric and a discharge gap. Voltage is applied to the electrodes, causing an electron flow across the discharge gap. These electrons provide the energy to disassociate the stable oxygen molecules, leading to the formation of ozone. Where ozone is used in the water treatment industry, the corona discharge method of production is used almost exclusively.

Ultra-violet radiation
Ozone readily absorbs UV radiation at 254 nm therefore for efficient ozone photolysis, UV lamps must have a maximum radiation output at 254 nm. At this wavelength, H2O2 is produced as an intermediate which decomposes into •OH radicals. Many organic contaminants absorb UV energy in the range of 200-300 nm and decompose due to direct photolysis or become excited and more reactive with chemical oxidants. Common low pressure mercury lamps generate over 80 per cent of their UV energy at this wavelength. According to Takahashi the use of the O3/UV system can achieve complete mineralization of organic compounds with short molecular chains.

Properties of ozone Physical properties of ozone
Ozone is a naturally occurring molecule that exists in the atmosphere. Ozone is a highly corrosive and toxic gas which exists as a gas at room temperature. The gas is colourless with a characteristic odour readily detectable at concentrations as low as 0.02 to 0.05 ppm (by volume) which is below concentrations of health concern. Ozone has a very short lifetime in acid or neutral solutions, but in alkaline solutions there can be a half-life of several hours.

Solubility of ozone
Some organic solvents have a higher capacity of ozone solubilisation than water. The ozonation of halogenated organic compounds in organic solvents demonstrated the higher rate of decomposition than in water. Dissolved ozone is more stable at lower temperatures and its solubility decreases with increasing temperature.

Chemistry of ozone
Ozone is a molecule composed of three oxygen (O3) atoms. It is thermodynamically unstable and readily reverts back to oxygen (O2). The single oxygen atom will combine with any molecule that is available to form another stable molecule. In an ideal situation, this monatomic molecule will react with another single oxygen atom and destabilize as O2. Due to its structure, molecular ozone can react as a dipole, an electrophilic or a nucleophilic agent. As a result of its high reactivity, ozone is very unstable in water.

The half-life time of molecular ozone varies from a few seconds up to few minutes and depends on pH, water temperature and concentration of organic and inorganic compounds in water. In 1977, Hoigné and Bader described the reaction of ozone in aqueous solution towards other compounds in two ways, by direct reaction or by indirect reaction with radical species formed in ozone decomposition. (i) Direct oxidation of compounds by molecular ozone [O3(aq)] and (ii) Oxidation of compounds by hydroxyl free radicals produced during the decomposition of ozone.

The ozonation process at acidic pH mainly takes place through the direct oxidation reaction with molecular ozone which reacts selectively with specific functional groups. Electrophilic attack by ozone molecules may occur at atoms with a negative charge density or at electron rich parts of the organic molecule like C-C double bonds.

At alkaline conditions, ozone decays mostly into hydroxyl radicals and by chain reactions to other radicals, which cause an unspecific radical reaction with organic substances. Ozone is unstable in aqueous solution and decomposes spontaneously through  a complex mechanism that involves the formation of •OH radicals.

Ozone can oxidize a variety of compounds directly or via radical species formed during its initiation reaction (1.7), propagation reactions (1.9–1.13) and termination reactions (1.14) and (1.15), all of them resulting in a generation of free radicals. The fundamental role played by the hydroxide ions in initiating the ozone decomposition process in water is well known.

In aqueous solution, the decomposition of ozone is strongly affected by the pH; the higher the pH, the higher the decomposition of ozone into more reactive species that participate in the ozonation process. Ozone exists as molecular ozone in acidic pH and in alkaline pH it decomposes into secondary oxidants such as •OH, HO2,HO3 • and HO4 • with •OH being an important one since it has the highest oxidation potential of 2.8 V.

Wastewater & drinking water
Wastewater reuse has become an attractive option for protecting the environment and extending available water resources while recycling municipal wastewater is one of the ways to resolve part of the shortage of potable water and industrial water. Decomposition of organic substances by ozonation is one of the most promising processes in water and wastewater treatments and is used for the removal of odorous compounds, hazardous chemicals like pesticides and chlorinated organic carbons.

It is also used in combination with biological activated carbon to remove natural organic matter as a precursor of disinfection by-products in drinking water treatment. Water is processed by conventional waterworks and then the effluent is successively treated using microfiltration and reverse osmosis. Although the water is already of a high-grade quality after these processes, concerns arise from the fact that trace organic substances in the treated water may still be potentially hazardous to human beings.

In order to meet extremely stringent quality standards further treatment needs be applied before they are used for direct potable water. With the development of large scale ozone generators and lower operating costs, there has been increasing interest in using ozone to remove compounds that are difficult or too expensive to remove by other methods. In some cases, ozone treatment alone adequately degrades contaminants to meet water quality standards.

Ozonation is also a practical option in the treatment of drinking water. Ozone is extensively used in drinking water treatment throughout the world mainly for disinfection and oxidation a combination of both. Ozone is a very selective and powerful oxidant able to achieve disinfection with less contact time and concentration than all weaker disinfectants, such as chlorine, chlorine dioxide, and monochloramine. Disinfection and oxidation can be achieved simultaneously if ozone reactions are responsible for the oxidation. However, if ozone-resistant compounds have to be oxidized, ozone has to be transformed into •OH radicals thus decreasing the disinfection efficiency. Therefore, optimization of disinfection and oxidation requires careful evaluation of the overall process.

In drinking water, the problem of by-product formation has become even more prominent since the recognition of the importance of micro-organisms such as Cryptosporidium parvum oocysts (C. parvum), which are more resistant against disinfection thus requiring higher ozone exposures and in turn leads to more by-product formation, leading to the undesired formation of disinfection by-products. Herbicides are applied to soils repeatedly and because of their rather slow degradation their occurrence in ground waters as well as surface water has become more and more frequent.

In the last few years, research efforts have been underway to develop powerful oxidation methods for achieving an efficient degradation of such herbicides in aqueous medium to try to avoid its dangerous accumulation in the aquatic environment and toxicity towards humans and animals. The harmful nature of such organochlorines is due to their persistence, toxicity and a capacity to accumulate in living tissues.

These properties make organochlorines arguably the most damaging group of chemicals to which natural systems can be exposed. The immediate concern is the potential toxicity of these compounds to aquatic organisms and humans through drinking water or the consumption of vegetable crops irrigated by polluted water.

During ozonation, intermediates that retain the aromatic ring of the pollutant are formed. Successive degradation of such intermediates, by reaction with ozone and •OH radicals, leads to aromatic and aliphatic C±C and C±Cl bond scission, and to the formation of low molecular weight compounds such as aldehydes and simple organic acids. In ideal cases, oxidation results in manageable quantities of mineral salts, carbon dioxide and clean water.

Typically, ozonation rarely produces complete mineralisation to CO2 and H2O, but leads to partially oxidised products such as organic acids, aldehydes and ketones, where oxygen is introduced into many of the carbonaceous sites within the product’s molecules. Although ozonation does not remove pollutants to a high extent from water, it leads to decolorisation of water, changes in the structure of the pollutant and the formation of organic compounds of lower molecular weight, which are thought to biodegrade more easily than the initial substances.

Although molecular ozone is a very strong oxidizing agent, it reacts selectively with aromatic organic compounds or atoms that carry a negative charge. Many compounds such as chlorinated organics do not have any strong nucleophilic sites and are not readily oxidized by ozone. Also, the use of ozone can sometimes generate oxidation sub products that are more toxic than the original contaminant. Therefore, advanced oxidation processes need to be employed. The aim of such processes is to increase the production of hydroxyl radicals by enhancing the decomposition of ozone.

Advanced Oxidation Processes (AOPs)
Advanced oxidation processes (AOPs) are processes which make use of the •OH radicals as the oxidant by exploiting their high reactivity, making them suitable for the oxidation of ozone-resistant compounds such as pesticides, aromatic compounds and chlorinated solvents such as tri- and tetrachloroethane in drinking water treatment. AOPs are able to completely oxidize recalcitrant compounds, making them less harmful and forming easily biodegradable products.

One limitation of ozone treatment is that it functions most effectively at a pH range of 9 to 12 due to the hydroxide ions initiating ozone decomposition. The oxidation of ozone-resistant compounds requires the conversion of ozone into •OH radicals therefore the high pH is required to allow the ozone and hydroxide ion to form a hydroxyl radical. The mechanism of attack of ozone to the organic molecule has been carefully described by Hoigne and co-workers. These authors found that for acid or neutral values especially in the case of aromatic compounds, molecular ozone acts as direct oxidizing  agent of the organic matter, while for basic pH a reaction between ozone and the hydroxide ion exists.

This gives rise to the generation of the hydroxyl radical which is a well known oxidizing agent present in a series of processes known as advanced oxidation technologies (AOTs). The hydroxyl radical is one of the most reactive free radicals and one of the strongest oxidants .

A variety of organic substances are subject to degradation by hydroxyl radicals. The hydroxyl free radicals are among the most reactive oxidizing agents in water approaching the diffusion control rates for solutes such as aromatic hydrocarbons, unsaturated compounds, aliphatic alcohols, and formic acid.

When hydroxyl radicals are formed the reaction is 1000 to 1000 000 times quicker, and therefore more efficient in many applications. For complete ozone consumption of a water, the •OH radical yield is almost independent of the rate of the ozone decomposition. Therefore, the main advantage of ozone-based AOPs is a shorter reaction time which allows the application of higher ozone dosages without causing excess ozone concentrations at the outlet of a reactor. Ozone-based AOPs are most commonly applied in drinking-water treatment because conventional treatment schemes including an ozonation step can be easily retrofitted for these processes.

Conventional ozonation processes can be transformed into an AOP by increasing the reaction time after ozone addition, increase the pH, or add hydrogen peroxide. The first two possibilities can be costly, while the addition of hydrogen peroxide is a cheap solution which is most commonly applied in drinking-water treatment.

Ozone decomposition is initiated by H2O2 through the formation of an OH radical and superoxide which further reacts with ozone. This reaction sequence yields one OH radical per decomposed ozone molecule. The main advantage of the AOP O3/H2O2 lies in the acceleration of the ozone transformation process.

Activated carbon
Activated carbon acts not only as the adsorbent but also as a catalyst in promoting ozone oxidation. Activated carbon accelerates the transformation of ozone with •OH radical generation. The hydroxyl radicals formed are not bound to the surface of carbon and they are free to react in the aqueous phase. Activated carbon is therefore an initiator of the radical-type chain reaction that transforms O3 into in the •OH aqueous phase. Beltran et al. also found that the presence of activated carbon during the ozonation process accelerates the ozone decomposition reactions.

Catalytic ozonation
In general, the AOPs when applied in the right place, give a good opportunity to reduce the concentration of the contaminant from several hundred ppm to few ppb. Early work performed by Paillard et al.,showed that catalytic ozonation, which they termed the “Catazone process”, was very efficient in reducing the total organic carbon concentration in water.

Previous studies have shown that ozone in combination with homogeneous or heterogeneous catalyst is effective to remove contaminants from water. The ozone/catalyst system was more efficient than oxidation with ozone at high pH values. Among the principal existing AOPs, the heterogeneous catalytic ozonation developed recently is known as the most promising process for industrial effluent treatment because of its low cost and easy operation.

Among the two possible mechanisms of catalysts, one possibility is the catalyst would behave only as an adsorbent with ozone and the hydroxyl radical as the oxidant species. In the second mechanism the catalyst would react with both ozone and adsorbed organics. The organic radical species would be then easily desorbed from catalyst and subsequently oxidized by •OH or O3 either in bulk solution. Depending on the type of catalyst and organic molecule, either the adsorption or diffusion of organics at the surface of the catalysts, or elevated concentrations of •OH radicals produced at the solid–liquid interface, were considered to be mainly responsible for the improvement of ozonation induced by the presence of catalysts.

The efficiency of the catalytic ozonation strongly depends, amongst other factors, on the nature of the catalyst and its surface properties as well as the pH of the solution that influences the properties of the surface active sites and ozone decomposition reactions in aqueous solutions.

Ozone is extensively used as oxidant in organic conversions, in water and wastewater treatment due to its oxidation and disinfection capabilities. In the water field, ozone is always produced diluted in either oxygen or air and it is not feasible to produce the gas in pure form (i.e. 100% O3). In order to improve the performance of ozonation , ozone-based advanced oxidation processes (AOPs) such as O3/UV, O3/H2O2, O3/OH!, O3/UV/TiO2 and homogeneous or heterogeneous catalytic ozonation have been studied in several workers.

The aim of many AOPs processes is to increase the production of hydroxyl radicals (COH), which are highly reactive species, through enhancement of ozone decomposition. To improve the selectivity and use of molecular ozone, new approaches that combined ozone with a third medium were attempted. Thus, high ozone concentrations can be achieved in the aqueous phase, which would enable faster oxidation rates. Better and effective ozone utilization can also be expected.

Ozone can be adsorbed at high concentrations from an oxygen or air stream on to a silica-based adsorbent. Which leads to enhanced bulk reaction rates. The pollutants present also would be oxidized faster. Further, the reactions at the liquid solid interface enhance the kinetics and selectivity. Commercially available polydi-methyl-siloxane (PDMS) dissolves ozone ten times higher than water.

Again, the reaction region can be either in the aqueous phase or in the PDMS phase, depending on the whether the pollutant is soluble in PDMS or in both phases with potentially useful results. The advanced oxidation processes (AOPs) are of high relevance. Ozone adsorbed on high silica zeolite was observed to be more stable than ozone existing in bulk water.

The use of heterogeneous catalysts in the liquid phase offers several advantages over homogeneous ones such as ease of recovery and recycling, atom utility, and enhanced stability. Catalysts based on high support area inorganic support materials have better thermal stability. Such materials have attracted a lot of interest as solid catalysts and reagents in liquid phase organic oxidations.

Those form the basis of some new industrial catalysts, which are used as replacement for toxic and corrosive traditional reagents.

Studies by Ernst et al. revealed that the catalyst exhibited different adsorption abilities to different model compounds and that the adsorption of organic model substances on the catalyst’s surface.

Rich, P.R.et.al. in their study reported the efficiency of  supports as CeO2 > alumina > NaY zeolite > ZrO2 > TiO2 reported that while Al2O3, SiO2, SiO2-Al2O3, TiO2 have marginal catalytic activity for decomposition of ozone, the efficiency of oxides as support materials for metal ion catalyst loading for ozone decomposition as SiO2 ñ Al2O3 ñ SiO2-Al2O3 ñ TiO2. Catalytic efficiency of other metals stand alone for ozone initiated oxidations was Pt > Pd > Ag > Ru @ Rh @ Ir > Ni > Cd > Mn > Fe > Cu > Zn @ Zr. Co, Y, Mo, Ti and Au were reported to have little activity.

The characteristic properties such as of acidity, shape selectivity and thermal stability of the zeolite catalysts are important factors that enable them to be used for highly selective synthesis in the fields of organic chemistry, fine chemicals and chemical intermediates. The numerous modifications of zeolites in respect of the number and strength of acid centres, isomorphous substitution and doping with metals provide an opportunity of employing catalysts that are tailored to suit the reactions.

Earlier we have reported the ability of 0.5% metal (Pd, V and Ni) loaded on ã-alumina and uranyl (UO2 2+) loaded silica, microporous Na-Y and ZSM- 5 supports,, mesoporous Al-MCM-41 molecular sieve materials and silica for catalysed oxidations of saturated hydrocarbons using ozone as oxidant, i.e. The ‘Catazone’ process for functionalization of long chained hydrocarbons.

The current communication gives an insight into the aptness of metal (Pd, V, Ni and U) loaded microporous zeolite-Y materials as the heterogeneous catalysts in the catalytic oxidation of higher hydrocarbons using ozone as oxidant at moderate reaction conditions.

Summarizing the mini review, we intend to include some of interesting results from our laboratories related to value added conversions of saturated longchained hydrocarbons and disinfection of gram negative and gram positive bacteria. The ozone initiated functionalization of saturated long chained hydrocarbons was successfully completed at room temperature and atmospheric pressure conditions.

Further, the ozone facilitated oxidation of textile dyes in aqueous systems was investigated taking the water soluble, toxic anionic azo dye, amaranth as an example.

The depletion kinetics of the dye was studied in detail. With excess concentration of ozone and other reagents and low [dye], reaction followed pseudo first-order kinetics with respect to the dye. Added neutral salts had marginal effect on the reaction rate and the variation of pH from 7 to 2 and 7 to 12 exerted marginal increases in the reaction rate suggesting molecular ozone possibly is the reactive species in oxidation of dye. The reaction order with respect ozone was near unity and it varied slightly with pH and flow rate variations. The overall second-order rate constant for the reaction was(105 ± 4) M-1 min-1.

The kinetics of ozone initiated disinfection of gram negative bacteria Escherichia coli  and Pseudomonas Aeruginosa  and the gram positive bacteria, Bacillus subtilis  were investigated as a function of ozone concentration, ozonation duration and flow rates in semi batch reactors. The rate of disinfection of all the three microbes followed pseudo first-order kinetics with respect to the microbe count and first-order with respect to ozone concentration.

The inactivation was faster at lower pH than at basic pH. Molecular ozone was found more effective in disinfection than hydroxyl radicals. Ozonation of natural waters spiked with microbes has significantly decreased the BOD levels of the control and microbe contaminated waters.

Conclusions
In conclusion, ozone can be effectively used in wastewater treatment. It significantly improves the water quality including the microbial disinfection and oxidative degradation of refractory organics. Ozone has a high germicidal effectiveness against both the Gram-negative and Gram-positive strains of microbes. In addition to the water treatment, the scope of ozone utilization in Advanced Oxidation Processed in obtaining value-added products is gainful as reactions can be achieved at moderate temperature and pressure conditions,which is an added advantage.