From Volume 21, Issue 5 – May 1998
Designs incorporate use of bubble diffuser or venturi injectors.
by: Philip C. Olsen
Two transfer methods are common for point-of-entry (POE) ozone applications today.
One incorporates a bubble diffuser installed near the bottom of a water column (see figure 1). As the bubble diffuser is supplied with pressurized ozone gas, small bubbles are created and ozone is transferred into solution as they rise to the top of the water column.
The other method uses a venturi eductor to pull ozone gas into a pressurized stream of water (see figure 2). As water flows through a venturi, it creates a vacuum from which ozone gas is pulled into the water stream. A violent gas/water mixing area is created, transferring ozone into solution.
While ozone will oxidize inorganics such as iron and manganese very quickly, it is slower to react with other organic and microbiological contaminants. If ozone is introduced via a bubble diffuser, the water column is sized to satisfy whatever retention time is needed with a specified concentration of ozone. If ozone is introduced via a venturi, a separate tank is needed to satisfy time/concentration requirements.
In each case, hydrodynamic flow characteristics must be considered in tank design to decrease the possibility of untreated water circumventing from tank inlet to outlet sooner than desired. The following are 10 factors to take into consideration when designing systems using either gas transfer method:
1. Construction materials
Gas delivery lines, associated fittings/valves and the transfer device itself (bubble diffuser/contactor or venturi) must be constructed of materials resistant to ozone gas. These materials include :
Stainless Steel (316L & 304L) Kel-F 2800
Teflon Kal-Rez Halar Chem-Rez Kynar Gortex
Teflon Ceramic Hypalon
2. Dissolved ozone in water
Because ozone is only partially soluble in water, concentrations are always significantly lower than what is contained in ozone gas. For instance, while an ozone generator producing a concentration of 1 percent by weight is equivalent to 6,250 parts per million (ppm) by volume, maximum solubility in water at this concentration is only 3.53 ppm in water (at 25 degrees Celsius.).
In reality, dissolved residual ozone concentrations above 1 ppm are rare in POE applications. Accordingly, a greater number of materials can be exposed to ozone in solution. In addition to the materials listed above, PVC, CPVC, and high-density polyethylene are commonly used for ozonated water lines and storage/retention tanks.
The objective of many POE ozone installations is to oxidize and precipitate inorganics such as iron and manganese previous to filtration. Iron or manganese oxides become problematic as they may begin to accumulate in the venturi and contact (retention) tanks prior to filtration.
If a venturi is exposed to raw water, an inspection/maintenance program is recommended to assure ongoing system performance. For any tank installed prior to filtration, the water outlet must be located at the lowest possible point inside the tank to prevent accumulation of precipitates.
4. Raw water pressure fluctuations
Water supplied from a well/pump system is controlled by a pressure tank and switch. Pressure fluctuations are inherent in this type of system and will cause variations in the amount of O3 delivered through a venturi into the raw water stream.
When a venturi is placed in-line between the well pump and pressure tank/switch the vacuum produced can change dramatically between the systems cut-in pressure and cut-out pressure. Because air flow into the generator inherently changes the applied ozone, the dosage delivered to the water will change..
5. Pressure/flow reduction previous to filtration
POE installations typically use an existing well pump to backwash media filters installed downstream of the ozone system. Most backwashing medias require a flow rate of 18 square feet at 30 psi minimum for backwash.
For a 10-inch diameter tank, this equates to 10 gallons per minute (gpm). Many homes have a well/pump system capable of supplying this volume at 30 psi. When a venturi is installed between the well pump and the filtration system, this flow rate can be seriously compromised.
In reviewing injector performance charts, the inlet pressure to a venturi would have to be 80 psi for the venturi to create a vacuum while delivering 10 gpm to a pressure tank set at 40 to 60 psi. However, very few homes have a well/pump system capable of supplying 10 gpm at 80 psi. Because of this, the installation of POE ozone systems have typically led to reduced flow rates available for filters to backwash and have caused filters to fail.
6. Pressurized systems vs. non-pressurized systems
If ozone is to be vented/destructed from a pressurized storage/retention tank, an automatic air/ozone release system must be used (see figure 8).
In non-pressurized systems, an atmospheric storage/retention tank is used and venting is caused by changes in water level. While this involves a more straightforward process when compared to venting a pressurized tank, one problem that does arise is sometimes the tank is not air-tight.
Measures should be taken to assure ozone gas will not vent from the tank at any other location than through the venting/ozone destruct system.
If ozone demand – based on feedwater characteristics – has been met, a residual amount of dissolved ozone will remain in the water. If a residual does not exist, ozone demand probably has not been met. Measurement of an ozone residual verifies the operation of the ozone system.
8. Elimination of residual ozone
Ozone is very corrosive to materials commonly found in plumbing distribution systems and should be removed before it enters the plumbing distribution system. Ozone will also oxidize ion exchange media used in water softeners and membranes used in reverse osmosis drinking water systems.
Because of this, any residual ozone in solution should be eliminated previous entering these water treatment system components and the plumbing distribution system. Typically, UV irradiation or activated carbon is used for this.
9. Natural decomposition of ozone
Over time, ozone is reduced to oxygen. While the rate of this reduction is commonly referred to as a half-life of 20 minutes, the actual rate of ozone depletion varies based on water quality characteristics and temperature. Retention time should not serve as the means for post- treatment elimination of residual ozone in solution.
If ozone is used to oxidize iron, manganese or, in some cases, high levels of hydrogen sulfide, precipitates will be created requiring filtration. The most logical technology for this task includes the installation of backwashing filters.
Many ozone systems have – based upon the method of ozone gas injection used – significantly reduced the flow rate originally required to backwash filters installed downstream. This ultimately causes the failure of these filters.
Refer to the table for sizing and backwash criteria for backwashing filters. While this table is based on the use of 45/55 silica sand as the filtration media, it can be used as a guideline. Other medias are available that may require lower backwash flow requirements.
A dealer can design and install a very functional ozone system, but regardless how well it works, if adequate attention is not given to the design of the filtration system, the end result will produce a dissatisfied customer requiring the removal of the complete water treatment system.
Philip C. Olsen is president of Cartwright, Olsen and Associates, Cedar, MN.