Agua Latinoamerica

P.O.E. Ozone Systems: Standard Methods & Materials of Application (August 1990)

By Steve Andrews

Wicor Canada Ltd.
Minute by minute, day by day, year by year, our concerns for a healthy and safe environment for ourselves, and more importantly for our children, becomes more of a concern. We as a society have come to expect all of our unalienable rights –safe, potable water not being the least of these rights!

With the Industrial Revolution and The Baby Boom, potable water is becoming more difficult to find and, once found, our water supplies have proven to be susceptible to outside contamination in many cases.

ThePoint-Of-Entry/Point-Of-Use (POE/POU) water conditioning and purification industry has always sought to fill the void where potable or palatable water supplies could not be found. This has been done in many ways with specific water conditioning and purification equipment sold for specific contaminants or needs.

As the water conditioning and purification industry’s knowledge of chemistry and physics increased, so too did the complexity of its equipment designs increase. In order to stay competitive, the dealer must now supply more than just water softeners, iron filters, etc… Dealers must now offer systems such as ozone in addition to reverse osmosis!

Reverse osmosis by far has the lion’s share of the market, and this can be attributed to more than just performance. For over 20 years, key players in the field of reverse osmosis have been refining their system designs and promoting the application of this technology. Ozone, however, has only recently come to the forefront of POE oxidation and purification systems.

With this infant technology many dealers have achieved startling results in both a positive and negative manner. In most cases, the negative results were generated by improper application or material usage. There have been several articles written on the benefits of ozone and its application. However, there have been few articles written on the options and types of materials required when seeking a safe, dependable ozone system design. The following is a list of options and materials that have proven to be beneficial or reliable for each component within an ozone system design.

All ozone generators vary to some degree whether it be size, shape, cooling process, air preparation requirements, etc. However, through all this, the underlying fact still remains that there are basically only two ways to generate ozone. There are some characteristics pertaining to each method that one should be aware of before making a selection. There have been several articles written on the pros and cons of corona discharge and/or ultraviolet light ozone generators. With each design, there are certain options and materials that should be included or utilized to assure safety, reliability and maximum performance — they are as follows:

  1. For POE applications, all ozone generators should have air drawn through under vacuum, such as venturi nozzle injection. This will prevent possible ozone gas leakage into the home if an ozone gas injection line were to break or crack. This would not be the case if the gas lines were to break under pressure.
  2. All ozone generators should have air flow meters installed before the ozone generation chamber. This will insure proper air flow and detect potential down stream injection tubing cracks or breaks as well.
  3. Every ozone generator should have safety interlocks installed on the door of the cabinet that will shut the generator down if the door of the cabinet is opened without turning the unit off first.
  4. All ozone generators should have the following permanent labeling:
    • Manufacturer and/or sup- plier
    • Method of ozone generation
    • Serial number
    • Type of supply gas
    • Ozone production capacity in grams/hour
    • Ozone concentration in % by weight or ppm
    • Maximum positive or negative pressure of supply air
    • Electrical requirements
    • Required level of operational air flow
  5. 5. All ozone generation reaction chambers should be constructed of stainless steel material.
  6. 6. In cases of installation for disinfection purposes, all ozone generators should be installed with air gas injection tubing, gaskets, valves, sealants, etc.

The material selected should be able to withstand maximum system pressures and vacuum as well as being resistant to ozone, nitric acid (for corona discharge generators without air preparation equipment) and electrolytic corrosion. Some materials that may be used are as follows:

  1. Teflon (P.T.F.E.)
  2. 316 & 304 Stainless Steel for nonwelded applications
  3. 316L & 304L Stainless Steel for welded applications where heat sensitization can cause carbide precipitation. Tungsten-arc inert gas (TIG) welding procedures should be utilized.
  4. Kynar
  5. Schedule 80 PVC can be used but must be inspected on a regular basis and should be replaced annually.

In-Line Check Valve

(See Flow Chart)

All ozone generators are subject to premature failure

preparation equipment such as air dryers to insure consistent and reliable performance.

7. All ozone generators should have corona arc or UV indicating lights.

Ozone Injection Tubing

(See Flow Chart)

Due to the strong oxidizing power of ozone, consideration must be given to proper gas injection tubing, gaskets, valves, sealants, etc. The material selected should be able to withstand maximum system pressures and vacuum as well as being resistant to ozone, nitric acid (for corona discharge generators without air preparation equipment) and electrolytic corrosion. Some materials that may be used are as follows:

  1. Teflon (P.T.F.E.)
  2. 316 & 304 Stainless Steel for nonwelded applications
  3. 316L & 304L Stainless Steel for welded applications where heat sensitization can cause carbide precipitation. Tungsten-arc inert gas (TIG) welding procedures should be utilized.
  4. Kynar
  5. Schedule 80 PVC can be used but must be inspected on a regular basis and should be replaced annually.

All ozone generators are subject to premature failure if water is allowed to back up into the unit through the injection tubing. In all cases, precautions must be taken to prevent this from happening. In most cases, this has been accomplished through the installation of in-line check valves. It is wise, however, to utilize more than one check valve if this is the only precaution taken for prevention of water backup. When selecting a check valve, the same materials of construction recommended for ozone gas injection tubing should be utilized for check valve selection as well.

It can be very difficult to find an in-line check valve that is reliable enough to guarantee absolute prevention against water backup.

For this reason, many installations are installed with a manometer as well. The operation of a manometer is quite simple to understand and can be compared to the function of the water p-trap under your kitchen sink. Most manometers are constructed with a water trap that will prevent atmospheric air induction when under vacuum; however, when placed under positive pressure, water spillage to drain will occur before backup into the ozone generator can be induced. If the water trap is allowed to run dry, atmospheric air will enter the system under vacuum.

If proper ozone gas injection takes place, water piping will not be exposed to high ozone residual concentrations. The piping of equipment between the ozone injection point and filter should still retain some chemical resistance qualities. The following material can be added to the list previously supplied under ozone injection tubing:

  1. Ethylene-propylene ter- polymer (EDPM)
  2. Polyvinylidene Fluoride (PVDF)
  3. Hypalon (Du Pont patent) 9. Fluoropolymer (Kynar, Pennwalt patent)
  4. Ceramic
  5. Glass
  6. Schedule 40 P.V.C.
  7. “Red Polyurethane”
  8. Viton
  9. Unplasticized Polyvinyl Chloride (UPVC)

Most commercial ozone injection designs are cumbersome and too cost intensive for POE applications. Based on this and the previously stated vacuum requirements under ozone generators, venturi nozzles should be the overwhelming choice for most POE ozone applications. However, not all venturis work efficiently, so when choosing a system utilizing venturi injection, thought should be given to chemical resistance qualities and mass transfer efficiency.

The materials listed under both ozone injection tubing and water piping should be utilized as a guide for selection of venturi injectors. Proper ozone mass transfer efficiency can be achieved when gas injection is done in a 360-degree annular design, and a 1 USGPM motive water flow to 1 1/2 SCFH gas injection rate ratio is maintained within the venturi injector.

Within an ozone reactor tank the primary functions are: one, provide adequate contact time for projected reactions to take place; two, enhance mass transfer efficiency of the ozone gas; three, provide for removal of unabsorbed ozone gas. In order to achieve the desired effect, ozone reactor tanks should be designed with the following criteria in mind:

  1. Provide adequate water volume within the contact tank to facilitate completion of desired chemical reactions.
  2. Provide for dispersion of unabsorbed ozone gas bubbles in order to enhance mass transfer efficiency through greater surface contact between gas bubbles and water.
  3. Provide a definite water flow pattern through the addition of internal baffles in order to promote degassification of unabsorbed ozone gas.

Construction material is also a concern. The following item, also acceptable for ozone contact tank construction, should be added to the materials list shown previously:

16. Fiberglass Reinforced Vinyl- ester Resin (satisfactory up to 2 mg/1 of ozone)

Disinfection

In many cases, ozone systems are being sold for removal of microbiological contamination as well as inorganic and organic compounds. This has created a concern with respect to safety and reliability for the consumer utilizing POE ozone systems sold for disinfection purposes. The issue does not revolve around ozone’s ability to disinfect water, but whether the system design provides for adequate ozone residual above total contaminant demand as well as adequate contact time for disinfection to take place.

Many years ago French scientists discovered a consistent rule of thumb could be followed to assure disinfection through ozonation. This has now been established as a standard in many countries including the United States. The basis of this standard practice is the establishment of a 0.4 mg/l ozone residual for a contact time of four minutes to provide disinfection. Water supplies with this level of ozone residual (above total demand) and contact time are said to have a CT value of 1.6. CT values can be determined by multiplying the concentration of dissolved ozone residual in mg/1 (C) by the contact time in minutes (T). The following criteria should be met to assure disinfection with ozone:

  1. The ozone demand in the water must be met before establishment of ozone residual can occur for CT value purposes.
  2. A CT value of 1.6 must be established before disinfection can be assured.

Due to the low solubility of air and ozone in water, most systems will require venting of unabsorbed gases. In some cases, this gas could have up to 600 ppm of unabsorbed ozone present. This quantity of ozone gas should not be released into the local environment of the system – it should be passed through a residual ozone destructor.

Currently, there are several methods of residual ozone destruction being utilized. The most practical choice by far, for POE applications, is catalytic destruction through the utilization of activated carbon. Not all activated carbons are the same though, and one should be chosen with high micropore structure ratio. Gas velocity rates should be kept to a minimum and design should be thin as well as elongated to precipitate maximum contact time.

©2020 EIJ Company LLC, All Rights Reserved | tucson website design by Arizona Computer Guru