Entry for October 31, 2007

Todays posting has to do with the use of ozone in color removal from drinking water. In our next posting we will cover another important application for color removal in textile wastewater.

Color is an important concern for drinking water treatment plant operators since it is responsible for a significant number of customer complaints about water quality.  In addition, many color problems are associated with fulvic or humic acids.  These compounds are also precursors to the formation of chlorinated organics in drinking water.  So their control is important for more than aesthetic purposes.

Color in water is due to either the presence of particulate matter which scatters light, apparent color, or dissolved species such as fulvic or humic acid, true color.  Filtration with membranes having 0.45 micron pores can effectively treat problems associated with particulates, apparent color.  Even in situations with dissolved organics, true color, conventional treatment processes (coagulation, flocculation, sedimentation and filtration) can remove a substantial portion of color.  Activated carbon can be used, but depending on the level of color, activated carbons can have a short life time in this service.

Ozone is an effective oxidizer of color and won’t create chlorinated organic compounds.  The use of ozone will decrease the final amount of chlorine required.  For fulvic and humic substances ozone doses of 1-3 mg ozone/mg of carbon can affect nearly complete removal of color.1,2 Other studies have shown that 1 mg ozone/l can remove 10 color units (CU).3

Rakness 4 provides successful examples of ozone treatment of highly colored water.  In one case, 7-10 mg/l of ozone with 10 minute hydraulic detention time (HDT) reduced water with 60-80 CU to 5-8 CU consistently.  In another case, 18 mg of ozone/l with a contact time of 10 minutes HDT reduced water with 135-140 CU and a small amount of hydrogen sulfide to 5-8 CU.

At any dose there appears to be a threshold limit to the amount of color that can be removed using.  If the threshold color limit is acceptable to the customers, further treatment may not be necessary.  If additional treatment is required, ozone should be used as part of a treatment train to remove virtually all of the color.  The combination of ozone with activated carbon has been shown to provide overall removal of color (90-95%).5

The following are some design considerations for the use of ozone in color removal.6 If only direct filtration is employed, pre ozonation should be used.  If conventional treatment is employed (settling and filtration) the point of ozone feed should be tested before settling, before filtration and two step ozonation.  The reaction of ozone with color appears to have two phases, a fast phase and a slow phase.  The ozone dose should be designed to satisfy the first fast reaction phase to most efficiently use ozone.  Roughly half of the ozone dose to reach the threshold limit will produce 60-70% of the color removal.  Other methods can then be used to further reduce the color.  As with most ozone applications, dynamic pilot studies should be used versus laboratory data for designing an ozonation system for color removal.

Spartan supplies a wide range of ozonation equipment which has been used for color removal applications.  You can learn more about Spartan and our products at our website: www.SpartanWaterTreatment.com.

1.  Watts, C.D. 1985. Organic By-Products of Ozonation of Humic and Fulvic Acids. Proc. Intl. Conf. on the Role of Ozone in Water and Wastewater Treatment, Edited by R. Perry and A. E. McIntyre. Selper Ltd., London

2.  Killops, S. D. 1986. Volatile Ozonation Products of Aqueous Humic Material. WTR. Res., 20:2:153

3.  Flogstad,H. & Odegaard, H.O. 1985. Treatment of Humic Waters by Ozone. Ozone Sci. Engineering., 7:2:121

4.  Rakness, K.L. 2005. Ozone in Drinking Water Treatment: Process Design, Operation and Optimization. AWWA Denver, CO

5.  Klimkina, N.V. 1987. An Evaluation of Treatment of colored Waters Using Oxidizing Sorption Method. Gigiena Sanitaria, 1:16

6.  Langlais, B., Reckhow, D. A. & Brink, D.R. eds. 1991. Ozone in Water Treatment: Applications and Engineering. Chelsea, MI: Lewis Publishers, Inc.

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Entry for October 29, 2007

Continuing our discussion of applications for ozone in water treatment, we will briefly cover the topic of hydrogen sulfide.  Hydrogen sulfide is found in ground water around the world.  It is perceived as the odor of rotten eggs and can be smelled in air with a concentration as low as 0.001 ppm.  It can also cause water to have an unpleasant taste.  It can also be corrosive  and result in teh discoloration of water fixtures, silverware and clothing. Aeration can be used to strip some of the sulfide out of the water, but this simply turns a water issue into and air pollution issue.

Ozone on the other hand quickly reacts with sulfides to convert them to sulfates.  The first reaction is to sulfur to sulfite to sulfate.  Without sufficient oxidation, solid sulfur can be formed as a colloidal suspension. 

According to the stoichiometry of the ozone sulfide reaction, a ratio of 3 mg of ozone to 1 mg of sulfide to sulfate is required.  In practice, the actual ratio is adjusted to 4:1.  This ratio is chosen to insure a small ozone residual which will insure complete oxidation of the sulfide.  In addition, a small excess will allow for some variability in the in coming sulfide concentration.

As noted above, sulfide can be stripped from the water by aeration.  If fine bubble diffusers are employed to mix the water and ozone care should be taken in the location for the diffusers in a multi chamber contact vessel.  The ozone should only be introduced in the first chamber to prevent stripping the sulfide.  If a catalytic ozone destroyer is used, the stripped sulfide could poison the catalyst. 

If you have water contaminated with hydrogen sulfide contact Spartan Environmental Technologies about an ozone based solution.

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Entry for October 25, 2007

We indicated in our last note that we would start a series of postings on applications for ozone in water treatment.  The first application we are going to consider is iron and manganese removal.  Iron and manganese are primarily a nuisance in water treatment since water contaminated by these species can stain water fixtures, and clothing that is washed with this water.  In some industrial applications the presence of these materials can affect the operation of a process or the quality of a product.

There are numerous methods available for treating iron in water including simple aeration.  Aeration is an option assuming the Fe is not complexed with organic materials or the reaction has to take place under acidic conditions.   Manganese, complexed or not, cannot be oxidized by aeration. Chlorine can also be used for oxidation of iron and manganese, but the use of chlorine can result in the formation of THM if organic material is present in the water. 

In cases where standard oxidants will not work or will cause problems downstream, ozone is may be a good choice.   Application of ozone for iron and manganese removal depends on a variety of factors.  We provide some base line information on the conditions and amounts of ozone required below.  Pilot testing will define the exact amount of ozone required and the type of ozone generator equipment needed.

Ozone oxidizes iron from Fe (II) to Fe (III).  Fe (III) hydrolyzes to Fe (OH)3 which precipitates to a solid form which can be filtered.  It should be noted in most treatment methods including those using air or chlorine, that filtration is an important step in the process.  The oxidation reaction requires 0.43 mg of ozone per mg of Fe (II).  Excess ozone can be used without negative effect.  Fe oxidizes in the pH range of 6-9.  As soon as ozone is applied to water bearing ozone, it will be immediately oxidized.  The water will turn a cloudy brown color.

Ozone oxidizes Mn (II) to MnO2 (Mn IV) which is insoluble and can be filtered out of the water.  The oxidation reaction requires 0.88 mg of ozone per mg of Mn (II).  Excess ozone beyond this ratio will form soluble Mn (VII), permanganate.  This will be clear as the water will turn pink.  If oxidizable organic material is present in the water and there is sufficient contact time, permanganate will be reduced back to MnO2 (Mn (IV)).  Manganese oxidation is most effective around a pH of 8.

In general, when organic materials are present in water, more ozone will be required than the amount shown above since ozone will at least partially oxidize these materials.  As we noted filtration is an important part of the process.  The nature of the precipitated iron or manganese will vary depending on the nature of the organics present, pH and other factors.  So selection of the filter requires some analysis.

It is important to note that if ozone is replacing another oxidant that at start-up ozone there might be stripping of deposits of iron and manganese existing in the piping and on pieces of equipment found in the water treatment plant.  During the break in period, therefore, iron and manganese may remain high until these deposits are removed.

Ozone is most often used in applications where some of its other benefits can be used.  Besides removing iron and manganese ozone will act as a micro flocculent to improve filtration, it is a potent disinfectant that may reduce the amount of other disinfectants required downstream and it can remove taste, odor and color from the water.

Ozone use is not indicated in all situations.  If more than 100 micrograms of bromide ion are present the formation of bromated might be possible.  With water temperature above 105 degree F ozone will decompose prematurely.  Pilot testing should be conducted before ozone is selected.

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Entry for October 23, 2007

In the last set of postings to this blog, we have discussed how ozone is made and used in water treatment. We will now start a series of postings on applications of ozone in water treatment. We begin here with a brief overview of overview of ozone water treatment applications.

In Europe, ozone has been used in the treatment of drinking water for over 100 years. Since that time it has found uses in a wide variety of other applications food processing, beverages, textiles, aquaculture, aquariums, pools, water features, industrial wastewater, municipal wastewater, cooling towers among many others.

Ozone has a number of benefits in water treatment which have created the applications noted above including disinfection, flocculation, oxidation and increasing dissolved oxygen levels in water. Ozone often is employed when multiple water treatment challenges are present at the same time. Like other water treatment technologies, such as filtration, it is another tool in the tool box for engineers and operators to use improve water quality. In fact, in drinking water, ozone is being used in conjunction with other water treatment technologies to create multiple barriers to pathogen reaching the public.

Ozone is a powerful and fast acting disinfectant that has been proven to inactivate viruses, bacteria and protozoa. In treating giardia lamblia for example it is 1,000 time more effective than chlorine. It is also less sensitive to the pH of water with respect to its disinfection capabilities.

Ozone is also a powerful oxidant that can be used to remove hydrogen sulfide, iron and manganese, color, taste, odor and disinfection byproduct precursors from the water.

Ozone has also been shown to act like a micro flocculant. Many water treatment plants that have switched to ozone have seen a reduction in the chemicals used for coagulation and flocculation. These reductions have been in the range of 20-50%.

In subsequent postings we will look at the various applications for ozone in water treatment in more detail.

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Entry for October 20, 2007

Spartan Environmental Technologies is an active member of the International Ozone Association. We pass onto our readers the following call for papers:

2008 Annual Conference

Orlando, Florida

International Ozone Association – Pan American Group

24-27 August 2008

CALL FOR PAPERS

The International Ozone Association – Pan American Group requests “Abstracts” for both Oral and Poster Presentations for its 2008 Annual Conference and Exposition to be held at Disney’s Coronado Springs Resort in Orlando, Florida 24-27 August 2008

The conference will provide current technical, process and operational information to engineers, scientists, and end users of ozone and active oxygen species.

Topics can include:

• Aquatic Animal Life Support

• Microorganism Inactivation

• Ultrapure Water

• Chemical and Biochemical Reactions

• Food, Beverage and Agricultural

• Biofiltration

• Spa/Pool/Aquarium

• Bromate Formation and Control

• Industrial Applications

• Ozone Generation

• Wastewater Treatment

• Contactor Design

• Advanced Oxidation

• Air treatment & Bldg. Remediation

• Emerging water & wastewater contaminants

Abstracts (up to 500 words) are due by April 1, 2008

Abstracts should be e-mailed to Conference Program Chair at abstracts@io3a.org.Abstracts also will be accepted by mail.

Hotel and registration information will be available through our website www.io3a.org and the PAG office.

General Information

Paul Overbeck (Executive Director), IOA Pan American Group

PO Box 28873

Scottsdale, AZ 85255, USA

480-529-3787 • 480-473-9068 (fax)

info@io3a.org • http://www.io3a.org

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Entry for October 18, 2007

In our post of October 16th, we discussed factors affecting the driving force to move ozone from the gas phase, where it is created, into the liquid phase, where it is needed for water treatment applications.  These included factors such as gas concentration, bubble size and the Gas volume/Liquid volume ratio.  In this posting, we will discuss some other factors such as temperature, pH and ozone demand of the water.

Ozone solubility increases with decreasing water temperature.  This can be expressed by the following equation: In Ha = 22.3 – 4030/T where Ha = apparent Henry’s constant, atm/O3 molar fraction in the liquid.

There is also an effect of pH on the solubility of ozone in water as expressed in the following formula:

Ha = 3.84 x 10-7 x [OH]0.035 exp(-2428/T).  Apparently, the experimental data shows considerable scatter.  It should also be noted that at elevated pH ozone decomposes more readily which can make determining molecular ozone solubility difficult.

Ozone demand effects transfer of ozone into the water by consuming ozone and thus shifting the equilibrium.  This is not a solubility issue but does effect the design of the ozone mixing system.  If the ozone is consumed as it enters solution, it is much easier to transfer the gas to the liquid.  In some water ozone transfer efficiency can effectively reach 100% if the compounds in the water readily react with ozone.

The factors affecting the transfer of ozone into water are fairly complicated except in idea systems.  As a result, lab and pilot evaluations are almost always needed to assess the design of an overall ozone water treatment system from the total amount of ozone needed to the best method for transferring it into solution given a specific treatment objective.

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Entry for October 16, 2007

Spartan Environmental Technologies supplies advanced oxidation processes for industrial water and wastewater treatment applications. We thought that our readers might find teh following conference work shop of interest in order to learn more about this area of water treatment.

Prof. Dion Dionysiou would like to inform you of a Sunday Workshop

(SUN5) on “Advanced Oxidation Technologies in Water Treatment:

Fundamentals and Applications” to be held at the 2007 AWWA Water Quality Technology Conference on November 4, Charlotte, North Carolina. The workshop is formulated by internationally known speakers who will cover a variety of AOTs and their applications on the elimination of several classes of organic contaminants in drinking water. for more information, please contact:

Dionysios (Dion) D. Dionysiou, Ph.D.

Associate Professor of Environmental Engineering Drinking Water, Water Supply, Quality and Treatment and Nanotechnology Laboratories Department of Civil and Environmental Engineering

765 Baldwin Hall

University of Cincinnati

Cincinnati, OH 45221-0071

Phone (513) 556-0724

Email: dionysios.d.dionysiou@uc.ed

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Entry for October 16, 2007

In the last few postings we discussed in general the subsystems of a ozone water treatment system. In this posting, we will focus in a little more detail about the factors that influence the dissolution of ozone gas into water.

There are a number of factors that create a driving force for ozone to enter solution. One of the most important factors is the concentration of ozone in the gas phase. As we mentioned in earlier postings, ozone concentration in the gas phase typically varies from 2-12% depending on the way it is produced. Henry’s Law states: At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Essentially, the higher the concentration in the gas phase the higher the concentration can be in the liquid phase.

Another factor that will influence the transfer of ozone from the liquid to gas phase is the size of the gas bubbles. The smaller the bubbles, the easier the transfer from the gas phase into the liquid phase due to the ratio of surface area to volume. The bubble size can be affected by the mechanical method of mixing. A venturi can create small bubbles because of this mechanical action. Fine bubble diffusers that are placed at sufficient water depth can create small bubbles. As the depth of the water increases, the pressure of the water column compresses the gas in the bubble making it smaller.

The ratio of the gas volume to the liquid volume (G/L) also will influence the transfer of ozone from the gas to liquid phase. The smaller the ratio the higher the ozone transfer efficiency. In practice this ratio is typically in the range of 0.1 to 1.0.

The amount of time the bubbles are in contact with the water will also increase the transfer of ozone into the liquid phase. In practice this is affected by the positioning of the degas stage. As we discussed in one of the preceding posts, most of the gas fed to the water is either oxygen or nitrogen and will not dissolve into the water. The gas has thus to be removed. In some applications, this is done immediately after injection which means that the ozone has a short time to dissolve into the water. In other applications the dissolving is allowed to occur in a contact vessel where the ozone has substantially more time to dissolve. As a consequence the G/L ratio can be larger while achieving the same ozone transfer efficiency.

In subsequent postings we will further discuss factors associated with ozone transfer. If you have questions regarding this posting or about an application for ozone please contact Spartan Environmental Technologies.

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Entry for October 11, 2007

We have recently been posting a discussion on the major components for a ozone water treatment system.  In this posting we will discuss some considerations for instrumentation for such a system.  You can also find additional detail on this type of instrumentation at www.spartanwatertreatment.com/ozone-water-treatment-system-instrumentation.html.

As we noted in our posting regarding gas preparation systems, the gas feed to an ozone generator must be dry, > -70 degrees C dew point.  Dew point monitors are available for this application and are a very good idea to be included in a system.  For small systems where the cost of the monitor might not be justified, the use of a pressure switch may be an alternative.  In PSA type dryers, pressure is a major factor in their proper operation.  If the pressure drops below a certain level, the dryer will not work properly and dew point may become out of specification.  So the pressure switch will shut down the system before this occurs.

Measurement of the amount of ozone produced can be achieved by a high concentration monitor and mass flow meter.  These components can be relatively expensive, but for large systems they are easily justified.  Smaller systems will often rely on the production curves from the ozone generator manufacturers and standard flow meters.  This will be back-up by measurements of the dissolved ozone in solution.  If the dissolved ozone concentration is at the design level at the appropriate point of the production curve, it can reasonably be assumed that the unit is working properly.  The situation becomes difficult to analyze if the dissolved ozone values do not meet expectation.  Then it is more difficult to determine where the problem lies.

Ozone generators often come with controls that allow the power output to be varied with an input signal (4-20 mA) from another device.  For example, a dissolved ozone monitor can send such a signal as can an ORP monitor.  Both provide a measure of the amount of ozone in the water.  This process can also be handled manually by measuring ozone residual with a test kit.  These kits rely on the Indigo Blue method and the use of a color wheel or colorimeter.

The last piece of instrumentation that should be employed is an ambient ozone monitor.  This device measures the amount of ozone that is found the environment around the ozone water treatment system.  This is an important safety device especially for ozone systems that are located in a building.  These units can be integrated with the ozone generator should a leak be detected above a safe level.

Spartan Environmental Technologies can provide complete ozone water treatment systems including the necessary instrumentation for a variety of applications.

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Entry for October 9, 2007

In recent postings we have discussed the components of an ozone water treatment system including gas preparation and the ozone generator.  We will now discuss mixing of the ozone with the water.

As noted in the previous posting, ozone is prepared in air or oxygen at a concentration of 2-10% by weight.  Ozone is solubility in water is limited by its rather low concentration in the feed gas.  A good ozone water contacting device, however, can achieve greater than 95% transfer efficiency under the proper conditions.  The ozone transfer efficiency is simply the amount of the gas that ends up in the water.

Since most of the gas is inert, provisions also have to be made for letting this gas escape from system.  As noted above, some small percentage of ozone will remain in the gas phase.  Depending on the amount of ozone remaining in the gas and the location of the vent, it may be necessary to destroy this remaining amount of ozone.

In terms of the options for mixing ozone and water there are two main approaches with several minor options.  In practice, fine bubble diffusers and venturi injectors are the most common methods employed for the contacting of the ozonated gas and water.  You can learn more about these methods at www.spartanwatertreatment.com/ozone-mixing-contacting.html

Fine bubble diffusers employ porous ceramic structures to form relatively small bubbles.  To achieve high ozone transfer efficiency the diffusers need to be at a sufficient depth to give the bubbles enough time to diffuse their ozone into the water.  Typically these depths are 16-20 feet.  An advantage of this approach is that besides being simple without any moving parts, the normal pressure of an ozone generator is sufficient to push the gas through the diffusers.  A disadvantage of this approach is that the water from the mixing chamber must be pressurized again, pumped, onto the next process or distribution system.

A venturi injector relies on a device called a venturi which creates a vacuum using the flow of the water to be mixed (see http://en.wikipedia.org/wiki/Venturi_effect). The advantages of this approach is that very small bubbles are created in the venturi aiding mixing, a very small space is required and the liquid remains under pressure during the process.  The disadvantages are that you need to add pumping energy to overcome the pressure drop of the venturi, back flow of water to the ozone generator is more complicated to prevent and you may still need a tank to provide contact time between the water and the dissolved ozone. Venturi injection is often done in a side stream to the main flow where the side stream is remixed eventually with the main flow.

Other methods employed include a novel device called a GasTran system that using a spinning porous disk and various packed bed designs.

Once the gas has been mixed with the water, the gases that did not dissolve in the water must be disengaged.  In the simple bubble contactors that operate under atmospheric pressure, the gas is simply vented from the tank.  In the pressurized systems, special degas devices must be used.  One class of device is a hydro cyclone.  When combined with a gas relief valve, the system can remain under pressure while venting the excess gas.

In both systems, the vented gas may need to pass through an ozone destroyer to remove traces of ozone that did not dissolve into the water.

The final design of the mixing system will depend on many factors. Spartan Environmental Technologies as a systems integrator can help you decide which approach makes the most sense for a given application.

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