Wastewater Treatment by Dissolved Ozone Flotation

Introduction [1]

                    Flotation may be used in lieu of the normal clarification by solids-downward-flow sedimentation basins as well as thickening the sludge in lieu of the normal sludge gravity thickening. Water containing solids is clarified and sludges are thickened because of the solids adhering to the rising bubbles of air. The breaking of the bubbles as they emerge at the surface leaves the sludge in a thickened condition.

                  In a flotation system for solid/liquid separation, there are at least two methods by which gas bubbles can be used to increase the buoyancy of suspended solids: (a) entrapment of the bubbles in the particle structure; and (b) adhesion of the bubbles to the particle surface (see Fig. 1). In the former case, as the gas bubbles rise toward the surface, the controlled turbulence in the inlet compartment causes contact between the solids.

The floc, formed by the natural floc-forming properties of the materials or by the chemicals

that have been added, increases in size because of more contact with other solids.

Eventually, a structure is formed that does not permit rising gas bubbles to pass through or around it. [2]

Ozone

           Ozone is an oxidant. Ozone (O3), sometimes called “activated oxygen”, or “triatomic oxygen”, contains three atoms of oxygen rather than the two atoms we normally breathe. Ozone is the second most powerful oxidant in the world and can be used to destroy bacteria, viruses, and odors.

Ozone is a gas at ambient temperatures and pressures with a strong odor. Ozone can be produced as a gas from oxygen in air, or concentrated oxygen. This ozone gas can be dissolved into water, or used in the gas phase for a variety of applications.

Several investigators developed an in-depth analysis of the reactions for ozone with various organic compounds. These reactions were described by Miller et al. [4] and are summarized below:

(a) Aromatic compounds: Phenol reacts readily with ozone in aqueous solution. Oxalic and acetic acids are relatively stable to ozonation in the absence of a catalyst such as ultraviolet light or hydrogen peroxide. Cresols and xylenols undergo oxidation with ozone at faster rates than does phenol. Pyrene, phenanthrene, and naphthalene oxidize by ring rupture.Chlorobenzene reacts with ozone slower than does phenol.

(b) Aliphatic compounds: There is no evidence that ozone reacts with saturated aliphatic hydrocarbons under water- or wastewater-treatment conditions. There is no evidence that ozone oxidizes trihalomethanes. Ozone combined with ultraviolet radiation does oxidize chloroform to produce chloride ion, but no identified organic oxidation product. Unsaturated aliphatic or alicyclic compounds react with ozone.

(c) Pesticides: Ozonation of parathion and malathion produces paraoxon and malaoxon, respectively, as intermediates, which are more toxic than are the starting materials.

Continued ozonation degrades the oxons, but requires more ozone than the initial reaction. Ozonation of heptachlor produces a stable product not yet identified. Aldrin and 2,4,5-T are readily oxidized by ozone, but dieldrin, chlordane, lindane, DDT, and endosulfan are only slightly affected by ozone.

(d) Humic acids: Humic materials are resistant to ozonation, requiring long ozonation times to produce small amounts of acetic, oxalic, formic, and terephthalic acids, carbon dioxide, and phenolic compounds. Ozonation of humic materials followed by immediate chlorination (within 8 min) has been shown to reduce trihalomethane formation in some cases. Ozonized organic materials generally are more biodegradable than the starting, unoxidized compounds

 

Application of Ozone

Ozone acts both as a very strong oxidizing agent and as a very effective disinfectant.

Consequently, it has multiple uses in potable water treatment, wastewater renovation,

cooling water towers, groundwater remediation, and industrial waste treatment. The

following is a snapshot description of each of its applications [5-8].

 

Dissolved Ozone Flotation (DOF)

Conventional flotation relies on floating of suspended solids on the top of liquid by air bubbles. A better separation effect is obtained when air bubbles are very small (micro-bubbles and nano- bubbles). In traditional dissolved air flotation (DAF) systems suspended solids and oily compounds are removed by coagulation, flocculation and removing the formed sludge by flotation by applying small air bubbles to increase buoyancy. Dissolved COD remains unaffected in DAF systems. By employing DOF, high concentration of ozone is available, which means a high potential of ozone oxidation and a high volume of micro-bubbles.

Dissolved COD remains unaffected and leaves a DAF with the effluent. When ozone is added to the air bubbles, it partially decomposes to highly reactive OH radicals, which in their turn oxidize the remaining dissolved COD. DOF removes non-biodegradable COD in a high level. As it is obvious in DOF systems, water quality parameters are removed by the two mechanisms of flotation and ozone oxidation. For this the reason, DOF systems have better efficiencies compared to DAF systems. It is estimated that DAF systems will be replaced by DOF systems especially in much polluted water systems to provide a one-step reduction of water quality parameters.

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science

 

References

[1]   Nicholas P. Cheremisinoff, Handbook of Water and Wastewater   Treatment Technologies  , Butterworth-Heinemann,2002,p 62.

[2]Lawrence K. Wang,  Physicochemical treatment processes, Humana Press Inc, Totowa, New Jersey 0751,2004.

[3] M. Krofta and L. K. Wang, Flotation Engineering. 1st ed. Lenox Institute of Water Technology, Lenox, MA. Technical Manual No. Lenox/1-06-2000/368, Jan. 2000.

[4] 1. G. W. Miller, et al., An Assessment of Ozone and Chlorine Dioxide Technologies for Treatment of Municipal Water Supplies, EPA-600/2-78-147, US Environmental Protection Agency, Cincinnati, OH, 1978.

[5]. NDWC Tech brief: Ozone, Newsletter On Tap, National Drinking Water Clearinghouse (NDWC), No. 12, December, (1999).

[6]. X. Paraskeva, Y. Panagiota, Z. Graham, J. D. Nigel, Water Environment Regulation Watch, November/December, (2002).

[7]. M. R. Collins, Small Systems Water Treatment Technologies: State-of-the-Art Workshop, American Water Works Association, Denver, CO, 1998.

[8]. Tri-State (2003), Ozonation Systems, Energy Technologies, Generation and Transmission Association, http://tristate.apogee.net/et/ewtwozs.asp, November, (2003).

Recent Development in Halogenation Technology in Water Disinfection

Introduction

                Disinfection is the selective destruction of pathogenic organisms; sterilization is the complete destruction of all microorganisms. Disinfection may be considered as one of the most important processes in water and wastewater treatment. This practice used in water and wastewater treatment has resulted in the virtual disappearance of waterborne diseases.

Disinfection may be accomplished through the use of chemical agents, physical agents, mechanical means, and radiation. In wastewater treatment, the most commonly used disinfectant is chlorine; however, other halogens, ozone, and ultraviolet radiation,and organic disinfectants have been used.

  1. Recent Environmental Concerns and Regulations

       Protection of public water supplies relies heavily on the use of disinfectants. Disinfectants are used to maintain a residual in the distribution system to prevent any health problems and to maintain the water quality standards. Since the new regulation requirements, the water industry has been looking for alternative chemicals or techniques to replace chlorine. In this section, instead of studying halogenation technology, we present techniques to reduce halogenation by-products. Different techniques include (a) chlorine dioxide, (b) chloramines, (c) coagulant, (d) ozonation, (e) organic disinfectants, and (f) ultraviolet light [1–2,3–4,5–6]. To comply with the upcoming stringent law, the techniques were tested by different plants. In the past, we have used chlorine to disinfect the finished drinking water, but then it may produce trihalomethanes (THMs) and other products. These can be potential carcinogens. This includes most of the halogens, especially the chlorine [7].

           Chlorine is a major halogen used in water treatment for controlling microbial quality. Marhaba [8].described the US Environmental Protection Agency (US EPA) initiated and negotiated the rule-making process for the Disinfectant/Disinfections By Products (D/DBPs) Rule in 1992. Owing to the complexity of the problems, US EPAhad to draw on the expertise of others to prepare the rule. The regulation was proposed in two steps.

           Stage 1 of the D/DBPs Rule was proposed in 1994 and became effective in December 1998. It lowered the total THM (TTHM) maximum contaminant level (MCL) from 0.100 to 0.0800 mg/L and three other classes of DBPs. The rule also set maximum residual disinfectant levels (MRDL) for three disinfectants. To provide necessary data for stage 2 of the D/DBP regulations, the Information Collection Rule (ICR) (begun July 1, 1997, ended December 1998) was proposed in 1994 with stage 1 of the D/DBP Rule. Stage 2 was re-proposed in 2000 and required even lower MCLs for DBPs than those proposed in stage 1. The 1996 Amendments to the Safe Drinking Water Act (SDWA) require US EPAto promulgate the stage 2 Rule by May 2002. Stage 1, proposed in 1994 and promulgated in 1999, provided maximum contaminant levels (MCLs) for the sum of five haloacetic acids (HAAs) at 0.6 mg/L, BrO3 at 0.010 mg/L, and brominates trihalomethane (THMs) at 0.08 mg/L. Stage 2 MCLs of 0.040 mg/L for TTHMs and 0.020 mg/L for HAAs were proposed. Table 1 gives a summary of the proposals according to the affected parameters.

 

Table 1 Proposed Disinfectant Level on Disinfectant Residuals and DBPs

Parameter Effective Stage 1 (mg/L) Anticipated Stage 2 (mg/L)
MRDL for chlorine 4.0 4.0
MRDL for chloramines 4.0 4.0
MRDL for chlorine dioxide 0.8 0.8
MCL for TTHM 0.08 0.04
MCL for five haloacetic acids (HAAs) 0.06 0.02
MCL for bromate ion 0.01  
MCL for chlorite ion 1.0  
  1. Chlorine Dioxide

Chlorine dioxide is widely used as an alternative to chlorine for treating drinking water . Numerous chlorine dioxide generation technologies have recently been developed to improve the conversion efficiency and purity of chlorine dioxide [9]. Water utilities use chlorine dioxide for peroxidation, control of taste and odor problems, and inactivation of common pathogens. Because chlorine dioxide is an oxidizing agent that does not chlorinate, it is often used for lower THM concentrations in finished water to meet levels established by the US EPA.

  1. Chloramines

Owing to the D/DBP rule, many water utilities may be switching from chlorine to alternative disinfectants. Chloramines have become the disinfectant of choice to replace free chlorine in distribution systems because they produce fewer DBPs while controlling the re-growth of bacteria. Controlling nitrification is essential if chloramines are to be a viable alternative disinfectant scheme for distribution systems in all types of environments.

El-Shafy and Grunwald [10].  studied the formation of THMs and its species from the reaction of chlorine with humic acid substances. This has caused much attention because of their carcinogenic and dangerous health effects. They found residual chlorine in water entering the distribution pipelines was on average 0.75 mg/L and decreased with distance until it reaches zero. The low velocity and large volume of reservoirs increased the residence time and correspondingly provided conditions for more chlorine decay and accordingly an increase in THM formation. The residence time and decay of chlorine were used as good predictors for the formation of THM and Chloroform in this study.

  1. Coagulant

The evaluation of 16 sites, with optimized coagulation provide an assessment of the technique and illustrate its capabilities to meet the requirements of Disinfectants/Disinfections by-product rule (D/DBP), were done by Bell-Ajy et al. [11]  .

Jar tests were used to determine the effectiveness of optimized coagulation for the removal of organic carbon, DBP precursors, particles, and turbidity when supernatant results were compared with conventional treatment. Jar-test results indicated that optimized coagulation could enhance the removal of organic carbon and DBP precursors.

 

  1. Ozone

Ozonation is one of the alternative techniques to replace traditional chlorine . Although the use of ozone will not produce chlorinated THM, haloacetic acids or other chlorinated by products, it will react with nature organic material. Ozone and its primary reactive product, the hydroxyl free radial (OH−), are strong oxidizers.

The oxidation by-products typically include aldehydes, aldo and keto acids, carboxylic acids, and peroxide. Grosvener [12]  presented a paper providing a detailed summary of ozonation and by-product formation chemistry, effective approaches toward the control of by-product formation, and DBP precursor removal technologies. Natural organic materials (NOM), a major component of total organic materials, is a complex matrix of total organic chemicals that can be derived from partial bacterial degradation of soil, living organisms, and plant detritus.

  1. Organic Disinfectants

Wang [13] has studied the use of various organic disinfectants for water purification, swimming pool water disinfection, and sludge disinfection. The major advantage of using organic disinfectants is that organic disinfectant will not be consumed easily by the target influent water, wastewater, or sludge containing organics.

  1. Ultraviolet (UV)

Hartz [14] described the pilot study at Midway Sewer District, located south of Seattle, WA. Owing to new regulation requirements, the district commissioned an investigation of alternative methods of disinfection, a pilot study to determine the effectiveness of ultraviolet irradiation. The UV process involved subjecting the wastewater to light energy in which lamps are tuned to emit certain light frequencies . In the case of UV used for microorganism inactivation, the lamps are tuned to a specific emission wavelength, for low-pressure lamps, the frequency most effective for inactivation around 250 nm. A number of variables regarding the effectiveness of the UV systems included: (a) light intensity, (b) residence time, and (c) effluent requirements. The results were that the percentage of light transmission for this pilot trial was slightly lower than normal. It was indicated that trickling filter tended to produce a wastewater that has a lower percentage light transmission. Then, owing to the solid content contact unit following the trickling filter system, the residual turbidity is lowered and the light transmittance is slightly increased. The UV light transmission was about 62% for an unfiltered sample of the wastewater. Filtration of the wastewater sample improved the light transmission by 5%. The district has found this technique as a possible alternative.

 

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science

 

 

References

[1]. T. Governor, Water Engineering Management, February, 30–33 (1999).

[2].. W. Sung, B. Reilley-Matthews, D. K. O’Day, and K. Horrigan, JAWWA, 92, 53–63 (2000).

[3].. L. K. Wang, J. New England Water Works Association, 89, 250–270 (1975).

[4].. L. K. Wang, Water and Sewage Works, 125, 99–104, (1978).

[5].. L. K. Wang, Y. T. Hung and N. K. Shammas (eds.), Physicochemical Treatment Processes.

Humana Press, Totawa, NJ. 2004.

[6].. L. K. Wang, N. K. Shammas and Y. T. Hung (eds.), Advanced Physicochemical Treatment

Processes. Humana Press, Totawa, NJ. 2005

[7]. M. Krofta and L. K. Wang, Removal of Trihalomethane Precursors and Coliform Bacteria by Lenox Flotation-Filtration Plant, Water Quality and Public Health Conference, US Department of Commerce, National Technical Information Service, Springfield, VA, Technical Report PB83-244053, 1983, pp. 17–29.

[8]. T. F. Marhaba, Water Engineering Management, January, 30–34 (2000).

[9]  G. Gorden, JAWWA 91, 163–174 (1999).

[10]  M. A. El-Shafy and A. Grunwald, Water Research 34, 3453–3459 (2000).

[11]  K. Bell-Ajy, E. Mortezn, D. V. Ibrahim, and M. Lechevallier, JAWWA, 92, 44–53 (2000).

[12]  T. Grosvenor, Water Engineering Management, 30–39 (1999).

[13]  L. K. Wang, J. New England Water Works Association, 89, 250–270 (1975).

[14] K. Hartz, Water Engineering Management, August, 21–23 (1999).

Disinfection of Wastewater by Ozone

  1- Introduction

          New water-treatment goals for disinfection by-products (DBP) and for microbial inactivation have increased the need for new disinfection technologies. Water and wastewater systems will need to use disinfection methods that are effective for killing pathogens without forming excessive DBP [1]. Ozone is an attractive alternative. This technology has evolved and improved in recent years, thereby increasing its potential for successful application. In August 1997, the US Environmental Protection Agency (US EPA) listed ozone as a “compliance” in the requirements of the Surface Water Treatment Rule for all three sizes of drinking water systems [2]. Many of the existing facilities using chlorination may be required to upgrade their present disinfection systems to install alternative disinfection processes.

      Ozone (O3) is a molecule that can co-exist with air or high-purity oxygen, or can dissolve in water. It is a very strong oxidizing agent and a very effective disinfectant.

Ozone is a colorless gas that has an odor most often described as the smell of air after a spring electrical thunderstorm. Some people also refer to the odor as similar to the smell of watermelons. Actually, ozone owes its name to its odor. The word ozone is derived from the Greek word ozein [3]. Ozone is an extremely unstable gas. Consequently, it must be manufactured and used onsite. It is the strongest oxidant of the common oxidizing agents. Ozone is manufactured by passing air or oxygen through two electrodes with high, alternating potential difference.

         Ozone is a very strong oxidizing agent, having an oxidation potential of 2.07 V [4].

Ozone will react with many organic and inorganic compounds in water or wastewater.

These reactions are typically called “ozone demand” reactions. They are important in ozone disinfection system design because the reacted ozone is no longer available for disinfection. Waters or wastewaters that have high concentrations of organics or inorganics may require high ozone dosages to achieve disinfection. It is very important to conduct pilot plant studies on these wastewaters during ozone disinfection system design in order to determine the ozone reaction kinetics for the level of treatment prior to ozone disinfection.

 

  1. Reactions of Ozone with Organic Compounds

        Wastewater is composed mainly by organic compounds which are biodegradable and non-biodegradable. Several investigators developed an in-depth analysis of the reactions for ozone with various organic compounds. These reactions were described by Miller et al. [4], and are summarized below:

(a) Aromatic compounds: Phenol reacts readily with ozone in aqueous solution. Oxalic and acetic acids are relatively stable to ozonation in the absence of a catalyst such as ultraviolet light or hydrogen peroxide. Cresols and xylenols undergo oxidation with ozone at faster rates than does phenol. Pyrene, phenanthrene, and naphthalene oxidize by ring rupture.

Chlorobenzene reacts with ozone slower than does phenol.

(b) Aliphatic compounds: There is no evidence that ozone reacts with saturated aliphatic hydrocarbons under water- or wastewater-treatment conditions. There is no evidence that ozone oxidizes trihalomethanes. Ozone combined with ultraviolet radiation does oxidize chloroform to produce chloride ion, but no identified organic oxidation product. Unsaturated aliphatic or alicyclic compounds react with ozone.

(c) Pesticides: Ozonation of parathion and malathion produces paraoxon and malaoxon,respectively, as intermediates, which are more toxic than are the starting materials.

Continued ozonation degrades the oxons, but requires more ozone than the initial reaction.

Ozonation of heptachlor produces a stable product not yet identified. Aldrin and 2,4,5-T are readily oxidized by ozone, but dieldrin, chlordane, lindane, DDT, and endosulfan are only slightly affected by ozone.

(d) Humic acids: Humic materials are resistant to ozonation, requiring long ozonation times to produce small amounts of acetic, oxalic, formic, and terephthalic acids, carbon dioxide, and phenolic compounds. Ozonation of humic materials followed by immediate chlorination (within 8 min) has been shown to reduce trihalomethane formation in some cases. Ozonized organic materials generally are more biodegradable than the starting, unoxidized compounds.

 

  1. Advantages and Disadvantages

It is important to note that ozone, like other technologies, has its own set of advantages and disadvantages that show up in differing degrees from one location to the next [5].

Using ozone has the following advantages:

(a) Possesses strong oxidizing power and requires short reaction time, which enables the pathogens to be killed within a few seconds.

(b) Produces no taste or odor.

(c) Provides oxygen to the water after disinfecting.

(d) Requires no chemicals.

(e) Oxidizes iron and manganese.

(f ) Destroys and removes algae.

(g) Reacts with and removes all organic matter.

(h) Decays rapidly in water, avoiding any undesirable residual effects.

(i) Removes color, taste, and odor producing compounds.

(j) Aids coagulation by destabilization of certain types of turbidity.

Among the disadvantages of using ozone are the following:

(a) Toxic (toxicity is proportional to concentration and exposure time).

(b) Cost of ozonation is high compared with chlorination.

(c) Installation can be complicated.

(d) Ozone-destroying device is needed at the exhaust of the ozone reactor to prevent toxicity.

(e) May produce undesirable aldehydes and ketones by reacting with certain organics.

(f ) No residual effect is present in the distribution system, thus postchlorination may be required.

(g) Much less soluble in water than chlorine; thus, special mixing devices are necessary.

(h) It will not oxidize some refractory organics or will oxidize too slowly to be of practical

significance.

  1. APPLICATIONS OF OZONE

        Ozone acts both as a very strong oxidizing agent and as a very effective disinfectant.

Consequently, it has multiple uses in potable water treatment, wastewater renovation, cooling water towers, groundwater remediation, and industrial waste treatment. The

following is a snapshot description of each of its applications (5–8).

4.1. Disinfection against Pathogens

         Transfer of ozone into the water is the first step in meeting the disinfection objective, because ozone must be transferred and residual oxidants produced before effective disinfection will occur [9]. Once transferred, the residual oxidants, such as ozone, hydroxide, or peroxide must make contact with the organisms in order for the disinfection action to proceed. Design of an ozone system as primary treatment should be based

on simple criteria, including:

(a) Ozone contact concentrations,

(b) Competing ozone demands

(c) Minimum contact concentration-time (CT) to meet the required inactivation requirements, in combination with US EPA recommendations.

 

Ozone has been observed to be capable of inactivating Cryptosporidium and there is significant interest in this aspect of its application [10]. Similar findings have been reported for the control of cyanobacterial toxins (microcystins) under various bloom conditions [11]. Available data indicate that a significant increase in ozone dose (at least 1.5 mg/L) and CT may be required as compared with past practices. Therefore, these needs in addition to continuous monitoring and ensuring a low total organic carbon

(TOC) in the flow should be considered in planning.

4.2. Zebra Mussel Abatement

Zebra mussel (Dreissena polymorpha), has arrived in the United States by attaching itself to ships in the infested waters of its natural habitat. Subsequently, it was imported over by the ships to cause infestation in US waters. The growth of zebra mussels on raw water intake pipes decreases the capacity of water transmission to potable-water-treatment plants, cooling towers of power plants, and hatcheries. To restore the full water flow capacity, plant operators have resorted to mechanical means for removing the mussel infestation on intake pipes.

Ozonation seems to be the most promising among the chemical alternatives tested for controlling zebra mussel growth. An 11 MGD side stream ozonation process was designed for a fish culture plant in New England [12]. It involved the pumping of high concentration ozone in water solution (15–25 mg/L) into the intake of the raw water pipe and blending it into the intake water to attain a final ozone concentration of 0.1–0.3 mg/L, which is sufficient to control the zebra mussel infestation.

4.3. Iron and Manganese Removal

The standard oxidation–reduction potential and reaction rate of ozone is such that it can readily oxidize iron and manganese in groundwater and in water with low organic content. Groundwater systems that have iron levels above 0.1 mg/L may have iron complaints if ozonation or chlorination is added. Excessive doses of ozone will lead to the formation of permanganate, which gives water a pinkish color. This soluble form of manganese (Mn) corresponds to a theoretical stoichiometry of 2.20 mg O3/mg Mn.

3.4. Color Removal

Because humic substances are the primary cause of color in natural waters, it is useful to review the reactions of ozone with humic and fulvic acids. According to different authors, ozone doses of 1–3 mg O3/mg C lead to almost complete color removal. The ozone dosages to be applied in order to reach treatment goals for color can be very high.

It is interesting to note that when the ozone dosage is sufficient, the organic structure is modified such that the final chlorine demand can decrease. Konsowa [13]. found that ozone is efficient in the removal of color from textile dyeing wastewater. The rate of dye oxidation was determined to be a function of dye concentration, ozone concentration, ozone-air flow rate and pH. The products produced by the break down of the dye are nontoxic and can be removed by biological treatment.

4.5. Control of Taste and Odor

The National Secondary Drinking Water Regulations recommend that the threshold odors number (TON) be 3 or less in finished water. It has been shown that ozone can be effective in treating water for taste and odor problems, especially when the water is relatively free from radical scavengers.

It has also been observed that ozone, in combination with other downstream treatment processes, especially granular activated carbon (GAC) filtration, can greatly increase taste and odor treatment efficiency and reliability. Again, the causes of taste and odor compounds, as well as the source water to be treated, need to be carefully considered prior to designing a treatment system. Analysis and possibly pilot-scale experimentation may be required to determine the optimum choice of ozone and downstream treatment.

4.6. Elimination of Organic Chemicals

Ozone or advanced ozonation processes can remove many synthetic organic chemicals (SOC). This removal leads to the chemical transformation of these molecules into toxic or nontoxic by-products. Such transformation can theoretically lead to complete oxidation into carbon dioxide (CO2); however, this is rarely the case in water treatment. Any observable reduction in total organic carbon (TOC) is due to either a small degree of CO2 formation (for example, decarboxylation of amino acids) or the formation and loss of volatile compounds through stripping.

4.7. Control of Algae

Ozone, like any other oxidant, such as chlorine or chlorine dioxide, has a lethal effect on some algae or limits its growth. Ozone is also capable of inactivating certain zooplankton,e.g., mobile organisms, Notholca caudata. Such organisms must first be inactivated before they are removed by flocculation and filtration.

4.8. Aid in Coagulation and Destabilization of Turbidity

It is important to understand that the coagulating effects of ozone go beyond any direct oxidative effects on organic macro-pollutants. For this reason, one must be wary of studies claiming improved removal of organic matter when the data are based solely on color removal or ultraviolet (UV) absorption. Also, when studying the removal of DBP such as trihalomethanes, one must be careful to incorporate controls permitting the separate evaluation of ozone’s direct effects. Finally, the coagulating effects of ozone may not be observed with all waters. Whenever considering the use of ozone as a coagulant aid, the preozonation effects should be critically evaluated in pilot studies incorporating the proper controls.

 

5.Disinfection of Wastewater by Ozone

As mentioned above, transfer of ozone into the wastewater is the first step in meeting the disinfection objective. Once transferred, the residual oxidants must make contact with the microorganisms in order for the disinfection action to proceed. Therefore, similar requirements and kinetic relationships used for chlorine disinfectants can also be used for ozone disinfection.

5.1. Wastewater Treatment Prior to Ozonation

    The US EPA Water Engineering Research Laboratory and other researchers have evaluated several treatment plant effluents to determine the relationship between ozone dosage and total coliform reduction [14–18]. The most significant factor influencing the ozone dosage requirement to achieve a desired effluent total coliform concentration was the TCOD (total chemical oxygen demand) concentration of the effluent. For example, at five plants where the TCOD of the secondary effluent was less than 40 mg/L, a total coliform concentration of 1000 per 100 mL could be achieved with ozone dosages between 4 and 7 mg/L [18] .However, when Meckes et al. [18] evaluated a plant, which treated a significant amount of industrial waste (TCOD = 74 mg/L), a dosage greater than 12 mg/L was projected in order for the process to meet the 1000 total coliforms per 100 mL limit.

      Based on the results of the above researchers, it appears that there is no technical basis for excluding the use of ozone following any treatment scheme (primary, secondary, tertiary, or advanced). However, depending on the type of wastewater treated and/or the effluent disinfection requirement, the wastewater-treatment scheme may be an important economical consideration. A summary of the issues to consider when selecting the wastewater-treatment scheme prior to ozone disinfection is presented below:

      (a) Required Effluent Target

First: To meet the former US EPA standard of 200 fecal coliforms per 100 mL, tertiary treatment may not be necessary.

Second: To meet more stringent standards, such as 14 fecal coliforms per 100 mL, tertiary treatment should be considered.

Third: To meet a standard of 2.2 total coliforms per 100 mL, advanced treatment processes prior to the ozone disinfection process may be required.

        (b) Influent Coliform Concentration

First: Coliform removal is a function of transferred ozone dosage; thus, the influent coliform concentration will affect the amount of ozone dosage required to meet specific effluent criteria.

Second: Treatment processes that reduce the influent coliform concentration (such as filtration) will decrease the ozone dosage required to achieve a specific effluent standard.

        (c) Wastewater Quality Characteristics

First: The ozone demand of the wastewater significantly increases the ozone dosage requirements. A plant with a large industrial contribution may have a large ozone dosage requirement. Pilot testing to establish ozone dosage requirements in these plants is highly recommended.

Second: Incomplete nitrification and a high concentration of nitrite-nitrogen will significantly

increase the ozone demand and thus the ozone dosage requirement. The nitrite-nitrogen concentration preferably should be less than 0.15 mg/L to optimize disinfection performance.

 

By

Ahmed Ahmed Elserwy

Water & Environmental Consultant

Ain Shames University, Faculty of Science.

 

References

[1]   C. Gottschalk, J. A. Libra, and A. Saupe, Ozonation of Water and Waste Water: A Practical

Guide to Understanding Ozone and its Application, Wiley-VCH, New York, NY, 2000.

[2]  US EPA, Small System Treatment Technologies for Surface Water and Total Coliform Rules,US EPA Office of Ground Water and Drinking Water, Washington, DC, 1998.

[3]  US EPA, Design Manual—Municipal Wastewater Disinfection, US Environmental Protection Agency, EPA/625 1-86-021, Water Engineering Research Laboratory, Cincinnati,OH, 1986.

[4]   G. W. Miller, et al., An Assessment of Ozone and Chlorine Dioxide Technologies for Treatment of Municipal Water Supplies, EPA-600/2-78-147, US Environmental Protection Agency, Cincinnati, OH, 1978.

[5]   NDWC Tech brief: Ozone, Newsletter On Tap, National Drinking Water Clearinghouse (NDWC), No. 12, December, (1999).

[6]   X. Paraskeva, Y. Panagiota, Z. Graham, J. D. Nigel, Water Environment Regulation Watch, November/December, (2002).

[7] M. R. Collins, Small Systems Water Treatment Technologies: State-of-the-Art Workshop ,American Water Works Association, Denver, CO, 1998.

[8] Tri-State (2003), Ozonation Systems, Energy Technologies, Generation and Transmission Association, http://tristate.apogee.net/et/ewtwozs.asp, November, (2003).

[9]   S. Farooq, et al., Criteria of Design of Ozone Disinfection Plants, Forum on Ozone Disinfection, International Ozone Institute, 1976.

[10]   SNWA (2003), Water Quality and Water Treatment: Ozonation, Southern Nevada Water Authority, http://www.snwa.com/html/wq_treatment_ozonation.html, November.

[11]   S. J. Hoeger, D. R. Dietrich, and B. C. Hitzfeld Environmental Health Perspectives, 110,November (2002).

[12]   Bollyky Associates, Inc. Zebra Mussel Control by Ozone Treatment, http://www.bai-ozone.com/bai 11.htm, November, (2003).

[13]   A. H. Konsowa, Desalination, 158, 233–240, (2003).

[14]   E. L. Stover, et al., High Level Ozone Disinfection of Municipal Wastewater Effluents, US EPA Grant No. R8O4946, 1980.

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