By Water Quality Association – Ozone Task Force
As there are a number of factors inhibiting ozone’s ability to become dissolved in water, there are also factors that can inhibit ozone’s ability to remain dissolved in water. The final segment of this three-part series will discus the decomposition of ozone and how the effect of ozone can be measured or ozone itself can be measured in water.
With the main aspects of ozone generation and the means of applying ozone gas addressed, another important step in designing an ozone system will be discussed, calculating the required dosage of ozone for the application. Finally, with the impurities oxidized and precipitated from the water, the last step of the process is filtration.
Decomposition of ozone
Ozone that reacts with inorganic, organic and microbial matter becomes part of the reaction product: for example, MnO2 or acetone. Some of the oxygen atoms are also released, combining to form molecular oxygen dissolved in the water.
Residual ozone in air or water after instantaneous oxidation reactions will continue to react with slow reaction rate contaminants while the unstable ozone molecule naturally decomposes to oxygen. The rate of ozone decomposition is affected by the environmental conditions including temperature, pH and level of reactive material still available.
Typical dissolved ozone half-life is 20 minutes at pH 7.0 and 20°C. The decomposition rate of ozone in aqueous phase increases with increasing pH and temperature. Comparatively, the decomposition rate of gaseous phase ozone is much longer and is dependent upon temperature and reactive airborne materials.
When ozone is either consumed or decomposed it will revert back to its elemental state of oxygen or O2. Ozone and oxygen gas that does not remain dissolved in the water, and is not consumed through oxidation, maybe vented from the reaction vessel to an ozone destruct system. The ozone destruct maybe designed with heated catalyst material, the material most commonly used is manganese dioxide.
The heat and catalyst reverts the ozone gas back into oxygen where it can then be released back into the ambient air. A well design ozone system will be sized so that the ozone will be completely consumed or revert back into oxygen prior to be delivered to the service plumbing loop, for consumption.
Measuring ozone in water
Ozone can be measured in the water using two primary technologies, colorimetric method test kits or electronic meters. Colormetric test kits are typically less expensive than electronic meters, but due to variations in test methods and other potential variables when field-testing between samples, they can be less accurate.
Colorimetric method – dissolved ozone
Chemical test kits utilize ampoules of vacuum-sealed reagent to draw up a sample when the ampoule tip is snapped off. After mixing, the filled ampoule is placed in the cell holder of the photometer supplied. The obtained absorbance value is converted to ppm with the included calibration charts. The two primary chemical schemes utilized are Don, I understand your point but it will be redundant to the following pragraph.and Indigo trisulfonate.
The DDPD chemistry employs a methyl-substituted form of the diethyl phenylene diamine (DPD) reagent. Potassium iodide is added to the sample before analysis. Ozone reacts with the iodide to liberate iodine. The iodine then reacts with the reagent to give a blue-violet color. Various free halogens and halogenating agents produce color with the reagent. Chromate (below 25 ppm) in test samples will not interfere with results. Results are expressed as ppm (mg/L) of ozone.
The indigo trisulfonate reagent, which reacts instantly and quantitatively with ozone, bleaches the blue color in direct proportion to the amount of ozone present. Malonic acid is included in the formulation to prevent interference from chlorine. Results are expressed as ppm (mg/L) of ozone.
Electronic method – dissolved ozone
Electronic monitors or controllers use a membrane-covered amperometric sensor. The sensor consists of a gas-permeable membrane stretched tightly over a gold cathode. A silver anode and electrolyte solution complete the internal circuit. During operation, ozone diffuses from the sample through the membrane.
Once inside the sensor, the ozone reacts with the electrolyte solution to form an intermediate compound. A polarizing voltage applied to the cathode completely reduces this intermediate compound. The reduction produces a current between the cathode and the anode, which the analyzer measures. The current is directly proportional to the rate at which ozone diffuses through the membrane into the sensor, which is ultimately proportional to the concentration of ozone in the sample.
An advantage to the electronic method is that it measures the sample in real time and allows for control of the ozone generator as well as measuring ppm (mg/L) of ozone. The disadvantage is capital expense and maintenance.
Electronic method – ORP
ORP stands for Oxidation-Reduction Potential. In practical terms, an ORP probe is a voltmeter, measuring the voltage across a circuit formed by a reference electrode constructed of silver wire (in effect, the negative pole of the circuit) and a measuring electrode constructed of a platinum band (the positive pole), with the analysis water in between.
The reference electrode is surrounded by salt (electrolyte) solution that produces a small amount of voltage. The voltage produced by the reference electrode is constant and stable, so that it can form a reference, which the voltage generated by the platinum measuring electrode to which the oxidizers in the water may be compared. The difference in voltage between the two electrodes is what is actually measured by the meter. (Note: High or low pH can alter ORP readings; optimal accuracy requires pH levels within 6.5 – 8.0.)
Calculating ozone generator capacity
No one private point of entry (POE) or small public water project can be considered equal. Water quality will differ from aquifer to aquifer, and some of these supplies maybe considered ’problem water‘ applications. Ozone cannot just be applied to a problem water application; one must first look at the water itself, via a water analysis and make a list of water treatment goals.
A water analysis will provide a list of contaminants found in a water sample, list a maximum contamination level (MCL) and the actual level of the contaminant found, typically expressed in ppm (mg/L). Water treatment goals should be addressed based on the water analysis, in this case, iron, manganese and sulfide ion.
To determine the appropriate quantity of ozone or ‘ozone demand’ required, one must calculate the theoretical stoichiometry for the application. Stoichiometry simply put is the math or accounting behind chemistry. The sample calculation provided in Figure 1 shows given ozone dosage requirements per mg/L of contaminant; by entering the actual contaminant load in mg/L (from the water analysis) and gpm flow rate of the system, the amount of ozone required can be calculated. Often, a safety factor for an unknown load is added to the calculation to ensure there is enough ozone for the particular application. .
Fundamental aspects of filtration
This section reviews the principles of filtration and its use for private POE and public small system applications for the removal of these contaminants.
Given that backwashing granular media filters (inert, oxidizing, catalytic) are primarily used for the removal of such oxidized inorganic contaminants, it is important to understand the fluid pathways between media granules within these filters. They measure from 35 to 55 microns in diameter, at a minimum, 3,500 times larger than the precipitate they are designed to remove.
Even with significant agglomeration, it is unlikely that particles such as iron precipitates, will achieve a combined size to cause entrapment within such a large fluid pathway. This is possible, however, with the addition of polymer chemistry, which is outside the scope of this paper.
Filtration mechanisms employed by backwashing media filters are categorized as straining and non-straining. Straining mechanisms are defined as size exclusion, where the size of a particle to be removed is larger than the fluid pathways (or interstitial spaces) within a filter medium. While straining serves as the primary mechanism of bag and cartridge filters, its effect is minimal for the removal of oxidized iron and manganese with granular media filters.
These contaminants are primarily removed from influent water streams by non-straining mechanisms, where a particulate must be transported onto media granules via gravitational forces, flow diffusion gradients and inertial effects of momentum. In addition, as minutely sized particulates closely approach the surface of a media granule, attachment may occur provided that surface chemical interactions are favorable. The resultant effect of non-straining mechanisms is demonstrated in Figure 2 where it can be seen that filtered particulates are deposited on media granules themselves while the fluid pathway between them remains open.
For backwashing filters to function, particles attached to the surface of filter grains must be removed periodically. Accordingly, while strong particle attachment forces are desirable during filtration cycles, the opposite is desired during filter media cleaning cycles.
Thus, it is important to understand that media within a properly designed backwashing filter will both collect particles as well as release them. Unfortunately, both occur during the course of a filter cycle.
Once a filter has been backwashed it will yield the least amount of particulate removal, conversely as a filter bed begins to build up the particulate mater will fill in to the contours of the media allowing less space for the particles to flow through due to smaller fluid pathways, thus capturing more particulate over time. Once the filter bed begins to reach capacity the particles become unstable due to hydraulic shear forces and can cause detachment, allowing them to flow through with the process water flow.
Filter media selection
Throughout the past 30 years, many filter media have been marketed to the residential water conditioning industry that, in essence, defied various aspects of filtration physics, as understood within the scientific community. While most of these mystery media (typically advertised as being proprietary), are no longer here, the scientific community and the application guidelines specified by them for media selection and application for backwashing filters, hasn’t changed much.
This distinction is made in response to a commonly heard inference that if ozone technology is used as pretreatment, filtration physics change. Unfortunately, this is not true.
The limited amount of information provided above (and the understanding that backwashing media filters do not remove particles by physical straining) challenges the theories of some filter technology mechanisms that are currently being marketed, such as depth or multimedia filtration. Coarser media is used on the influent versus the effluent of filter bed construction.
It has been suggested that larger particles are strained near the top of the bed and smaller particles are strained near the bottom, thus utilizing the full depth of the filter bed. In some cases, particle removal may be enhanced by this type of graded filter bed design, though successful operation may be primarily attributable to causing changes in hydrodynamic forces, such as pressures and flow rates throughout the bed versus straining or size exclusion.
An inert (or non-oxidizing non-catalytic) filter media is recommended for removal of the particles. The most common is 45/55-mesh size silica sand.
An ozone system’s success and longevity can often be gauged by three main factors: engineering of the system, installation of the system and preventative maintenance performed through the years. The first factor is critical; an improperly engineered ozone system can lead to the system’s inability to achieve the water treatment goals required.
To determine water treatment goals, one must understand issues regarding the water, which often can begin with the senses: smell, taste, feel and sight. This information can be useful when determining the appropriate actions to take; however, a water analysis is always the best starting point.
Every component of the engineering process is important; if the correct method of producing ozone for the application is not chosen, nor an analysis provided, nor the calculations made, the system can be undersized or oversized. If the incorrect method of mass transfer chosen, the ozone may not be effective,which could lead to greater than normal amounts of ozone to off-gas (or in other wasted ozone).
It is always typical to have some ozone off-gas as it is better to lean to the high side when sizing a system, therefore it is always a good idea to have an ozone destruct system. Finally, without proper post filtration, which may cause the precipitated contaminants to be carried by the water to the point of use.
It is the intention of the WQA – Ozone Task Force to utilize the combined knowledge of its colleagues to help promote ozone and the information surrounding its use to better inform both water treatment specialists and consumers alike. This three-part series has outlined the fundamental aspects and steps required when considering an ozone system for private point-of-entry (POE) and small public water treatment applications and provide technical guidance to those considering, or currently employing, ozone for the purpose of oxidation.
For more information please contact either the International Ozone Association (IOA), which can be found at www.io3a.org and email at [email protected] or ClearWater Tech LLC, which maybe found at www.cwtozone.com and email at [email protected].