FOR WATER AND WASTEWATER TREATMENT
LiAN TADBiR Engineering company supplies ozone generators for ozone disinfection applications including drinking water, cooling water, process water, aquaculture, marine parks among many other applications.
?WHAT IS OZONE DISINFECTION
One common method of disinfecting wastewater is ozonation (also known as ozone disinfection). Ozone is an unstable gas that can destroy bacteria and viruses. It is formed when oxygen molecules (O2) collide with oxygen atoms to produce ozone (O3). Ozone is generated by an electrical discharge through dry air or pure oxygen and is generated onsite because it decomposes to elemental oxygen in a short amount of time. After generation, ozone is fed into a down-flow contact chamber containing the wastewater to be disinfected. From the bottom of the contact chamber, ozone is diffused into fine bubbles that mix with the downward flowing wastewater. See Figure 1 on page 2 for a schematic of the ozonation process. Ozone disinfection is generally used at medium- to large-sized plants after at least secondary treatment. Another common use for ozone in wastewater treatment is odor control.
?HOW OZONE DISINFECTS
Ozone is a form of oxygen (O2) with the molecular formula O3. It forms when oxygen in the air is exposed to the discharge of a powerful electric current through air. In nature, it forms in the upper atmosphere when lightning passes through the air. The pungent odor often associated with passage of a thunderstorm, which leads some to exclaim how “clean” the atmosphere smells, is attributed to naturally formed ozone. Ozone is unstable and changes to O2 shortly after its formation. It is a powerful oxidant and one of the most powerful disinfectants available in water treatment.
Ozone functions as both an oxidant and disinfectant in the treatment of drinking (potable) water and wastewater. This is similar to chlorine. Chlorine and Ozone, however, operate by different mechanisms when disinfecting water. As a result, ozone and chlorine can act synergistically. Ozone’s germicidal properties are associated with its high oxidation potential. Disinfection by ozone is a direct result of bacterial cell wall disintegration, also known as lysis. This mechanism is different than that by chlorine. Although the exact chemical action of chlorine is not clear, it is believed that the chlorine residual in aqueous solution diffuses through the cell wall of the microorganisms and attacks the enzyme group which results in the destruction of the microorganism.
The graphics below illustrate the process while the photomicrograph show actual bacteria before and after ozonation:
Ozone is an unstable gas comprising of three oxygen atoms, the gas will readily degrade back to oxygen, and during this transition a free oxygen atom, or free radical form. The free oxygen radical is highly reactive and short lived, under normal conditions it will only survive for milliseconds.
Ozone is a colorless gas that has an odor similar to the smell of the air after a major thunderstorm.
Ozone has a greater disinfection effectiveness against bacteria and viruses compared to chlorination.
In addition, the oxidizing properties can also reduce the concentration of iron, manganese, sulfur and reduce or eliminate taste and odor problems. Ozone oxides the iron, manganese, and sulfur in the water to form insoluble metal oxides or elemental sulfur.
These insoluble particles are then removed by post-filtration. Organic particles and chemicals will be eliminated through either coagulation or chemical oxidation. Ozone is unstable, and it will degrade over a time frame ranging from a few seconds to 30 minutes. The rate of degradation is a function of water chemistry, pH and water temperature.
The formation of oxygen into ozone occurs with the use of energy. This process is carried out by an electric discharge field as in the CD-type ozone generators (corona discharge simulation of the lightning), or by ultraviolet radiation as in UV-type ozone generators (simulation of the ultraviolet rays from the sun). In addition to these commercial methods, ozone may also be made through electrolytic and chemical reactions. In general, an ozonation system includes passing dry, clean air through a high voltage electric discharge, i.e., corona discharge, which creates and ozone concentration of approximately 1% or 10,000 mg/L. In treating small quantities of waste, the UV ozonation is the most common while large-scale systems use either corona discharge or other bulk ozone-producing methods. Ozone test strips a must.
The raw water is then passed through a venturi throat which creates a vacuum and pulls the ozone gas into the water or the air is then bubbled up through the water being treated. Since the ozone will react with metals to create insoluble metal oxides, post filtration is required.
OZONE DISINFECTION / OZONE CONTACT TIME KINETICS
Water is a disinfected but never completely sterilized in the water treatment process. This disinfection is a two-part process that includes:
1. Removal of particulate matter by filtration. A rule of thumb is that high turbidity in the effluent is a potential health risk because viruses and bacteria can hide within the rough texture of particulates. Therefore, removal of the particulates reduces the chance of pathogenic microorganisms in the effluent. (see Particulate figure)
2. Inactivation of pathogenic microorganisms by chlorine, chlorine dioxide, ozone or other disinfectants: Contact time and kinetics are simply a measure of the inactivation due to time and concentration of the disinfectant. The USEPA has developed regulations for the minimum kill percentages (inactivation) necessary for public water to be considered potable. These regulations include a minimum disinfection of:
• Three Log (99.9%) for Giardia Lamblia Cysts
• Four Log (99.99%) for Enteric Viruses
In “water treatment terms,” 1 log inactivation is referred to as 1 credit inactivation. Different types of filtration are assigned certain removal credits. For example, conventional filtration is worth 2.5 credits for Giardia cysts. Since the EPA requires 3 log (credit) removal, an additional 0.5 credit inactivation from disinfection must be attained.
Varying degrees of disinfection can be attained by altering the type and concentration of disinfectant, as well as the time water is in contact with the disinfectant. The decision to use one type of disinfectant versus another will set the precedence for the remainder of the values needed to attain the proper disinfection. The time untreated water is exposed to the disinfectant and the concentration of that disinfectant are the main factors in the equation that will be discussed in the next section [Notice that the units of contact time are (mg/l)(min)].
Relationship Between Kill Efficiency and Contact Time
A relationship between kill efficiency and contact time was developed by Harriet Chick while she was a Fellow in the Pasteur Institute in Paris, France. The research yielded data supporting her relationship that is shown in the graph. ‘No’ represents the initial number of organisms and ‘N’ is the number or organisms at ‘Time.’ As contact time between water and disinfectant increases, the ration of N/No decreases as Chick’s Law predicts.
Factors Affecting C*t Values
• As pH increases, the value of C*t also needs to be increased. This can be explained by examining the effects of pH on free chlorine. As the pH increases, more of the weak disinfectant (OCI-) exists than the strong disinfectant (HOCI-), thus increasing the C*t value. Refer to the Table 1 below.
• The greater log removal needed, the greater the C*t needs to be, as can be seen in the table below.
Table 1: C*t for Removal of Giardia Cysts in Relation to Log Removal and pH
The strength of a disinfectant directly affects the C*t. For a weak disinfectant, the C*t will have to be higher than for a strong disinfectant. As Table 2 below shows, ozone is the strongest disinfectant, thus the C*t value required is less when compared to chlorine and chlorine dioxide.
• Different organisms have different resistances to disinfectants. If an organism has a strong resistance to a certain disinfectant, the C*t will be higher than for an organism with a weaker resistance. Refer to Table 2 below.
Table 2: C*t Values for the 99% Inactivation at 5° C of Organisms Using Various Disinfectants
* 99% inactivation at 25° C
To demonstrate the disinfection power of ozone and compare it with other oxidizing agents, Morris developed the lethality coefficient:
Lethality Coefficient= 4.6/(Ct99) where:
C = residual concentration in mg/L t99 = time in minutes for 99 percent
microorganism destruction (2-log destruction)
The table below lists parameters for disinfection by ozone for different organisms:
a – Lethality Coefficient = 4.6/(Ct99)
b – C 99:10 = concentration in mg/liter for 99 percent destruction or inactivation in 10 minutes (Morris considered these values to be valid within a factor of two)
Comparison of the values in this table with similar values obtained for chlorine is shown in the table below. These values tabulated by Morris illustrate that ozone is a more powerful germicide against all classes of organisms listed by factors of 10
to 100. The table shows values of the lethality coefficient:
Drinking Water Disinfection
Although ozone is significantly more effective than chlorine at inactivating and / or killing viruses, bacteria and cysts (e.g., Cryptosporidium and Giardia) and has been widely used in Europe for many years to treat municipal drinking water, it has not had similar acceptance in the US. Reasons include its higher cost and the fact it does not remain present long in water. US regulatory authorities have specified less expensive disinfectants such as free chlorine, chlorine dioxide or chloramines to maintain a residual capable of continuing to kill organisms throughout the distribution system.
Legislation over the past two decades, such as the Safe Drinking Water Act Amendments, Surface Water Treatment Rule, and other regulations place stricter rules on both the range and amount of disinfection needed and the concentrations of disinfection-by-products (e.g., trihalomethanes) allowed in drinking water.
This legislation is making the use of free chlorine and its derivative disinfectants less cost-effective. Security concerns since 9/11 about the presence of large tanks of compressed, liquefied chlorine located in water plants in densely populated areas may also play a role in the utilization of ozone in the future. Since it quickly inactivates or kills virtually all bacteria, cysts and viruses but leaves no long-lasting residual, ozone is the disinfectant of choice for most bottled water bottlers.
Ozone Wastewater Treatment
Since ozone quickly converts to oxygen and leaves no toxic residual, it may be more advantageous than chlorine to treat wastewater prior to discharge. Since dissolved ozone reverts to oxygen, the effluent will exert less biological oxygen demand (BOD) on the receiving stream. Ozone’s effectiveness as an oxidant often makes it the method of choice for removing color, organic chemicals and odor-causing contaminants in wastewater. In many cases, depending on the ozone contact time and concentration, it can oxidize these contaminants to water and carbon dioxide.
As examples, toxic herbicides and pesticides may be reduced to more environmentally friendly components; non-biodegradable organic compounds may be reduced to smaller biodegradable parts; proteins and carbohydrates may be lysed at double-bonded carbons to damage and destroy critical components of organisms found in the water.. Since the free hydroxyl is so highly reactive, the contact time necessary is minimal, as compared to other disinfectants.
Ozone may be combined with other oxidation processes, such as ultraviolet irradiation, hydrogen peroxide (another powerful oxidant) and proprietary catalysts to speed these oxidation process. This combination of oxidation steps is referred to as Advanced Oxidation Processes or AOP.
• Electron bombardment
• UV irradiation at < 200 m
Ozone is a highly unstable molecule with a relatively short half-life preventing it from being stored or transported, thus requiring that it be generated on-site. It can be generated from any source of gas which contains oxygen molecules. The most common sources for ozone generation are commercially prepared liquefied compressed oxygen or air in the atmosphere. Use of pure oxygen gas results in a higher efficiency of ozone generation but it increases the production cost. Using air as an oxygen source requires that the air be compressed and cleaned and dried (i.e., dehumidified). Compression of the air serves to increase the concentration of oxygen. The removal of foreign particulate such as dirt and dust is accomplished through the use of filters. Dehumidification is accomplished by lowering the dew point by refrigeration. Most ozone generators require clean, dry gas for optimal production.
Clean dry gas is passed through a chamber where the continuous discharge (arcing) from a high voltage electric current disperses electrons into the air. The electrons convert oxygen molecules to ozone molecules and oxygen atoms. The highly unstable oxygen atoms bond with hydrogen atoms in the air to form hydroxyl radicals.
It is these hydroxyl radicals that give ozone its oxidation characteristic. The gas, now containing ozone and hydroxyl radicals, is introduced into water. Ozone is not very soluble in water and is typically dissolved using a venture which relies on intimate air / water contact under very high turbulence or a diffuser which breaks the gas into very tiny bubbles which are allowed to be in contact with the water for an extended period of time.
The latter arrangement may use large aerator stones (similar in substance to those used in a small aquarium) to bubble the ozone / air mixture into a very tall column of water in a tank (“contact tank”). Fresh water is introduced at the top of the contact tank and ozonated water flows out the bottom of the tank. In all cases, the concentration of ozone imparted to the water depends on the amount of ozone present in the air after it passes through the ozone generator, the surface area presented at the gas-water interface (e.g., bubble surface) and the contact time between the gas and water.
Typically, the water flow is held constant and the amount of ozone in the system is regulated by adjusting the voltage of the current producing the electrical discharge in the generator or by adjusting the flowrate of the gas. The ozone concentration produced by a generator is typically expressed as pounds, grams or kilograms per day. Because ozone solubility in water rapidly decreases with increasing temperature, the seasonal range in feedwater temperature is a critical design parameter.
Because it is such an aggressive oxidizing agent, ozone has a deleterious effect on all living organisms. It is a severe upper respiratory irritant at very low concentrations and workplace limits have been established in the US by OSHA. Discharge limits for receiving streams and the ambient air are established by the US EPA. Ozone degradation can be accelerated by the addition of hydrogen peroxide or passing the system through an ultraviolet ozone destruct system. Activated carbon may also be effective in catalyzing ozone destruction in air or water. Also, contact time with the water or wastewater can also be lengthened to allow for further destruction.
PRIMARY ADVANTAGES TO OZONE
1. Ozone is effect over a wide pH range and rapidly reacts with bacteria, viruses, and protozoans and has stronger germicidal properties then chlorination. Has a very strong oxidizing power with a short reaction time.
2.The treatment process does not add chemicals to the water.
3. Ozone can eliminate a wide variety of inorganic, organic and microbiological problems and taste and odor problems. The microbiological agents include bacteria, viruses, and protozoans (such as Giardia and Cryptosporidium). Pathogenic and waterborne disease screening test.
DISADVANTAGES TO OZONE
1. There are higher equipment and operational costs and it may be more difficult to find professional proficient in ozone treatment and system maintenance.
2. Ozonation provides no germicidal or disinfection residual to inhibit or prevent regrowth.
3. Ozonation by-products are still being evaluated and it is possible that some by-products by be carcinogenic. These may include brominated by-products, aldehydes, ketones, and carboxylic acids. This is one reason that the post-filtration system may include an activate carbon filter.
4.The system may require pretreatment for hardness reduction or the additional of polyphosphate to prevent the formation of carbonate scale.
5.Ozone is less soluble in water, compared to chlorine, and, therefore, special mixing techniques are needed.