Entry for June 23, 2008

When designing an ozone water treatment system, as with any engineering project, there are tradeoffs between cost and performance.  One aspect of design for such systems involves the controls.  Depending on the application, it is entirely possible to design a system with fully manual controls that will function well.  Such systems require operator attention should one of the systems components fail.  It is also to design and build a full automated ozone system that can run unattended and alert n operator should the need arise.

The cost of automation is usually the major reason why client opt for more manual controls, especially for smaller systems where the costs of controls can exceed the cost of the ozone generator.  On the other hand, if the application where ozone is being applied is critical, the use of controls to maintain proper operation, regardless of operator attention is probably justified.

The basic components of an ozone water treatment system are gas preparation, ozone generator and ozone water mixing.  Gas preparation involves the following options: compressing/drying air, concentrating oxygen from compressed air, or feeding purchased oxygen.  Gas feed is critical for proper ozone generator operation in terms of both product quality and ozone generator protection.  Moisture will not only reduce generator efficiency, but also damage the generating cell. 

For new systems, pressure swing absorption (PSA) is used for both air drying and oxygen concentration.  The process is highly reliable since it uses a solid state process with limited moving parts.  Dew point monitors can directly measure the air drying process and oxygen monitors can follow the oxygen concentration process.  These instruments are reliable and are routinely used in large installation on a regular basis.  The instruments are moderately expensive and require periodic calibration. 

An alternative is to use a pressure switch.  A critical parameter in the operation of PSA systems is the feed pressure.  If this drops below a certain level, the process will no longer work properly.  This tends to be a factor is performance problems associated with such systems.  So, a pressure switch is an inexpensive method to monitor the performance of PSA systems.  While by no means fool proof, it can catch major faults allowing for eh shut down of the system before damage is done and alerting operators to a problem.

Another technique that is also relatively inexpensive is to use a silica gel that changes color with changes in moisture content of the gas.  Both air dryers and oxygen concentrators should produce a gas with a dew point of less than -100 degrees F.  The silica materials change color from a blue to red with the presence of moisture.  While not an automated method of control, it can alert and operator to a problem.

If the quality of the feed gas is good, the operation of the ozone generator is usually good as well.  Ozone generators with good feed gas will last for a long time between servicing.  While near terms failures are not likely, they are possible.   In larger installation, monitors follow the concentration of ozone coming out of the generator.  These monitors use UV light to measure ozone.  They are very accurate, but expensive and require periodic calibration by skilled technicians.  In addition to measuring the concentration of ozone in the gas, these systems also follow the flow of the gas.  They can then calculate the production of ozone in g/h or lbs/day and compare the values to the original specifications for the generators. 

The objective of these controls is to insure that the dose of ozone in the water meets the application requirements.  While the gas phase values can predict the amount of ozone in the water, the concentration of ozone in the water is also monitored.  This can be done using a dissolved ozone monitor employing either UV or an electrochemical method.  In both cases highly accurate measurements of ozone dissolved in water can be made.  From a control point of view, one could do without the gas phase measurements since the critical parameter is the dissolved ozone concentration.  While the gas phase measurements would be highly useful tools for assessing why the concentration of ozone in the water is low or detecting the problem before it occurs, it is not a requirement and most small systems do not use gas phase monitors.

The electrochemical dissolved ozone monitors are easily calibrated on site and are relatively inexpensive.  They often come with built in control outputs that allow the monitor to control the power settings on the ozone generator thus controlling the dissolved ozone levels in the water without direct operator intervention.

Other considerations in the design of an ozone water treatment system are the handling of various systems failures.  We already mentioned the potential for failure of gas preparation when compressed air is the feed source using pressure switches.  Another potential failure is the back flow of water from the liquid side of the system to the ozone generator.  Such an occurrence is catastrophic and would result in the destruction of the generator.  As a result, protection must be put into place with back-up.

For small inexpensive systems, mechanical methods alone are popular and workable.  These include liquid traps and check valves.  Another level of protection involves liquid traps with sensors that detect the back flow and can provide a signal to a control panel to close valves, shut down generators and set off alarms.  In systems that use venturi, back flow would also change the pressure in the line from negative to positive.  This could be sensed by a pressure switch.

Leaks of ozone into closed spaces of a building can create potential hazards for people.  Although ozone is easily detected at safe levels by our sense of smell, our olfactory nerves are easily fatigued by ozone.  If the person exposed to ozone ignores the problem, they could unknowingly be expose themselves to dangerous levels.  While there are no records of anyone dying from such exposure, it is not wise to rely on our sense of smell.  The use of ambient monitors is recommended for these situations.  Most monitors come with relays that can be used to shut down the ozone generator. 

Ozone water treatment systems are designed to make ozone and mix it with water.  So the primary thing to measure and control is the amount of ozone in the water.  The next most important factor is to prevent water from backing up into the generator since this will result in the catastrophic failure of the generator.  Finally, it is important to make sure the gas feed meets specifications in terms of oxygen quality or air dryness. 

These objectives can be accomplished with simple manual methods or with sophisticated instruments and PLC.  The decision of which to use has to do with overall system cost and how critical the application is.  Spartan Environmental Technologies can supply ozone systems with a wide range of controls that can meet the specifications and budgets for a variety of applications and clients.

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Entry for June 12, 2008

Spartan suggests that you consider the specialty conference Disinfection 2009 and give special consideration to submitting an abstract for a presentation.  the conference will be held at:

Omni Hotel at CNN Center

Atlanta, Georgia

Conference: February 28 – March 3, 2009

Exhibition: February 29 – March 1, 2009

The Call for Abstracts is now available.  Please submit your abstract online by following the links on the Call for Abstracts page of the Conference website.  The deadline for abstract submittal is June 25, 2008.

The Disinfection Committees of the Water Environment Federation, American Water Works Association, the International Ozone Association, the International Water Association, and the International Ultraviolet Association, and the International Co-ordination Committee of the Water Environment Federation and the Centers for Disease Control and Prevention are sponsoring a seventh Specialty Conference to be held February 28 – March 3, 2009, in Atlanta, Georgia at the Omni Hotel and CNN Center.

This conference will provide a forum for those professionals concerned with disinfection needs and technologies and will be focused on all aspects of the disinfection of water, wastewater, reuse water and biosolids.  The conference will also have a special focus on the disinfection or other treatment of water and the treatment and disinfection of wastewater in small communities and households in developed and developing countries.

This conference is targeted for engineers, scientists, consultants, academics, health officials, wastewater and water treatment plant operators and regulators, graduate students, implementing organizations, non-governmental organizations and governmental agencies, researchers, manufacturers and their representative groups and the industrial community from North America and abroad. Technical session topics will include but are not limited to the following:

A. Disinfection Resistance

B. Emerging Pathogens

C. Modeling Disinfection

D. Risk Assessment

E. Biosolids Disinfection

F. System Sizing

G. Reclaimed Water

H. Disinfection Byproducts

I. Source Tracking

J. UV Design/Operations

K. Innovative Disinfection Methods

L. Dechlorination

M. Alternate Disinfection Systems

N. Household Wastewater Collection/Disinfection/Treatment

O. Small Community Water Collection/Disinfection

P. Small Community Wastewater Distribution/Disinfection

Q. Other

For additional information, please contact:

Technical Program/Abstracts

Nancy Bauer

1-703-684-2400 ext. 7010

nbauer@wef.org

     Attendee Registration

     Registration

     1-703-684-2441

     registration@wef.org 

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Entry for June 6, 2008

Spartan Environmental Technologies provides ozone water treatment systems for a variety of applications.  As we have mentioned in previous posts, one of the key elements of such systems is gas preparation.  Oxygen or dried air can be used to make ozone.  In this post we are going to discuss oxygen preparation systems for ozone generation.

The use of oxygen for the production of ozone permits higher rates of production for a given machine (80% more production) with higher ozone concentration (2-3% versus 6-10%).  Higher concentration ozone is beneficial since it allows for easier dissolution into water.  On the other hand, producing oxygen is more difficult than simply drying air.  This means that much more air has to be processes to make oxygen than to dry air.  The more compressed air required the larger the compressor and the more energy required.  These costs offset to some extent the benefits of oxygen inside the ozone generator.

Spartan ozone water treatment systems employ oxygen concentrators using pressure swing absorption (PSA).  This process employs two columns filled with molecular sieves, porous ceramic materials that reversibly absorb nitrogen and water from air on the surfaces. 

Compressed air (90 psi pressure and filtered to remove hydrocarbons and particulates) is fed to one of the two columns.  Under pressure nitrogen and water is absorbed on the surfaces of the molecular sieves.  As the nitrogen and water are removed, the concentration of the oxygen is increased to the 90-95% level.  This is enough to significantly increase the efficiency of an ozone generator versus the use of dried air. 

Once the column has been filled with nitrogen and water, it needs to be unloaded.  This is done by purging the column at atmospheric pressure (0 psig) with product gas from the other column that is operating.  This dry oxygen laden gas caries away the nitrogen and water captured on the molecular sieves, making them ready to repeat the process.  So while one column is producing oxygen the second column is being regenerated.

The ratio of air fed into the sieves to the amount of oxygen produced is about 11:1.  So, if we wanted to produce 260 SCFH (4 SCFM) of oxygen, we would need 45 SCFM of air.  The reason for the discrepancy with the natural ratio of oxygen to nitrogen in air (~1:5) is the amount of product gas (two thirds of total oxygen flow from the operating column) used to purge the column being regenerated.  This means that the usable product is only one third of the total flow of oxygen from the operating column.  The schematic above illustrates the process.

The only moving parts in the PSA process are the solenoid valves used to switch the flow of gas through the columns.  Thus the process is highly reliable.  Pressure switches and oxygen analyzers can follow the process and insure proper operation.

Contact Spartan if you are considering an ozone water treatment system.  Spartan supplies complete system including gas preparation, ozone generation and down stream processes.

 

 

 

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