Residential Ozone Applications
By Greg Reyneke, MWS
Ozone is a highly effective technology for addressing numerous waterborne contaminants in commercial, industrial and recreational water usage environments. There are, however, relatively few dealers that use it on a consistent basis. This is usually because the dealer lacks the knowledge or confidence to recommend the technology. Ozone can be slightly intimidating to start using, since it is inherently more complex than a typical softener. As with softeners, though, if you know enough to get started and have a decent support network, you will be just fine. The key to success is to leverage the resources that you already have. Hopefully, you are already a member of WQA and your regional WQA, as well as enrolled in the MEP program. There, excellent information about ozone and problem water contaminants can be found.
Ozone consists of three oxygen molecules and is highly unstable. This inherent instability is what makes it so very attractive for treating waterborne contaminants. It works hard oxidizing as much as it can for slightly less than a half-hour at room temperature and then degrades into safe, stable O2. This makes it very attractive for numerous oxidative applications. Ozone is naturally produced through solar radiation and lightning strikes. In the water-treatment industry, we simply imitate nature.
Ultraviolet light. Ozone is produced by reacting oxygen with ultraviolet light in the 185-nm spectrum. UV ozone generation has a lower up-front cost and is not very sensitive to ambient humidity, making it ideal for air-handling applications. (I prefer not to use it for residential water projects, since the UV lamp upkeep and sensitive ballasts make it cumbersome and electricity-intensive compared to other methods.)
Cold corona discharge. Also known as cold-spark corona discharge or simply corona discharge, this is my preference for residential applications. Simply put, corona discharge ozone production requires a high-voltage power source, anode, cathode and a suitable dielectric separator. Air passes between the electrodes over a dielectric, creating an electric field, or ‘corona,’ which induces O2molecules to rearrange themselves as ozone (O3). Corona discharge produces a much higher concentration of ozone than UV, but is sensitive to ambient humidity. Always strive to maintain a dew point lower than -30°C (32°F) for optimum performance.
For residential applications, it is rare to see liquid oxygen feeds (or even oxygen concentrators), as you would in commercial and industrial applications, which keeps costs down, makes for a simpler installation and keeps maintenance costs reasonable. There may be times, however, that they are necessary, so be sure to discuss the options with your equipment vendor(s).
Iron, as one of the most abundant elements in the Earth’s crust, is commonly found in well-water supplies. While iron generally poses no health threat to humans and animals, it is a stain-causing and sediment-forming nuisance that feeds numerous strains of bacteria. Iron can exist in both soluble and insoluble forms. The ferrous iron (II) form is highly unstable and quickly develops insoluble iron (III) hydroxide when exposed to oxygen at elevated pH levels. Anaerobic groundwater can hold significant amounts. US EPA’s secondary MCL for iron is 0.3 mg/L; anything above 0.1 mg/L will typically cause staining over time. Iron-reducing bacteria (IRB) feeds on iron in water and presents its own set of unique challenges that can complicate your treatment options.
Manganese is far less abundant than iron, but still quite common and easy to recognize from telltale black staining. US EPA’s secondary MCL for manganese is 0.050 mg/L, with many states having an action level as high as 0.5 mg/L. The consumption of elevated levels of manganese has been linked to neurological disorders. Bathing and showering in water containing manganese has not been shown to pose an appreciable health risk to adults, but there is very little data on its effect on developing bodies. Therefore, it is prudent for pregnant women, babies and young children not to drink water containing manganese.
Hydrogen sulfide’s infamous rotten-egg smell distinguishes it from iron, manganese and most other contaminants that are found in well water. In addition to its unpleasant odor, hydrogen sulfide concentrations as low as 1.0 ppm are quite corrosive and even very low levels can tarnish silverware and stain porcelain fixtures. It is important to evaluate the water holistically when addressing hydrogen sulfide, since higher pH ranges (7-12) will include other species of sulfur (like sulfide or bisulfide) that do not have odor. Hydrogen sulfide can enter water from geothermal activity, decay of organic matter and, to further complicate issues, various strains of sulfur bacteria that commonly exist in groundwater supplies. They eat sulfur and their metabolic byproducts contain hydrogen sulfide gas. Sulfur-oxidizing bacteria will convert sulfide into sulfate, while producing a black slimy biomass.
The key to establishing an effective water treatment plan is to understand the nature of the problem, evaluate potential treatment options and then ensure it can be purchased, installed and maintained affordably.
Site survey and water test(s)
When performing a site survey, you should determine the following information at a bare minimum to ensure that you are able to provide a smart solution:
- Type of water source
- Type of storage and capacity (if any)
- Floats and other controls in use
- Peak water flowrate available to the water treatment system
- Drainage facilities
- Pipe size and material of influent water supply
- Peak flow demand of the facility being treated
- Space available for the system
- Electrical power available for system
- Dimensions of doorways and other entry ways
- Daily water consumption habits
- Client’s water quality concerns to be addressed
- Water chemistry from a recent test
The biggest failures that I have witnessed and perpetrated in treating problem water have resulted from inadequate, incorrect or inaccurate testing. If any parameters seem strange or outside of the norm for what you have seen in the area before, don’t be afraid to test multiple times: “measure twice, cut once” is wise counsel.
You should always test for the following, using industry-standard test methods:
It is smart to also test for non-pathogenic bacteria with a Biological Activity Reaction Test (BART) to understand the overall situation better and anticipate interfering biological factors. BART is a simple, safe and effective method for predicting the population size and activity of specific groups of bacteria. Aerobic organisms grow around the ball, while anaerobic organisms will grow lower down in the water column. Results are determined by observing the rate of change of color/opacity (as well as variations in color on some tests) as the microorganisms at the bottom of the water column consume the nutrient mixture. A BART test usually takes two to eight days of incubation at room temperature, giving you the flexibility of performing the test without having to send a refrigerated sample to a testing laboratory.
BART kits contain specific sanitary nutrients in the base of a tube where the water being tested will be poured and a ball that will float at the top to restrict the amount of oxygen entering the water column. It is wise to test for slime-forming, iron-related and sulfate-reducing bacteria each time. Accurate water chemistry information will help you to calculate the amount of ozone required to provide appropriate treatment. Here are some helpful minimum dosage requirements used by our team (see Table 1):
Disinfection applications will require a minimum ozone dosage of 0.5mg/L in addition to any other established ozone demand.
Calculating minimum ozone generator production rate:
Grams of ozone per hour = water flow (liters/hour) x ozone demand
1. Calculate the flowrate to be treated in L/hr.
Convert from US customary units to metric as follows: (gpm x 60) x 3.785
For example: 1 gpm x 60 = 60 gallons per hour
60 gph x 3.785 = 227 liters per hour
2. Multiply minimum ozone demand by L/hr to calculate mg/hr minimum production rate.
3. Divide by 1,000 to determine the minimum grams per hour that the ozone generator needs to produce.
Complicating factors will interfere with textbook ozone demand calculations. Best practice is to include a reserve capacity of 30 percent beyond minimum to allow for flexibility in treatment. It’s better to throttle down your ozone production rate than to wish that you had more available. The next step is to calculate the volume of retention required to maximize contact time. You simply multiply the time required by the operating flowrate. If you are operating at 10 gpm (37.8 L/m) and require six minutes of contact time, you’ll need a total mixing/retention tank volume of at least 60 gallons (227 liters). Strive for a minimum contact time of one minute and maximum of 30 minutes, which is the half-life of ozone at room temperature (approximately 77°F/25°C).
Another perspective is to consider the dosage rate. Dosage rate factors time and ozone concentration to maximize oxidation and minimize cost. For example, a dosage of 0.5 ppm for two minutes is the same as 1 ppm for one minute and 2 ppm for 30 seconds. This dosage rate is important when evaluating the type of ozone injection.
Equipment selection and system design
Once you know what you’re dealing with and the challenges involved, you can begin the process of selecting an ozone injection configuration. Typical residential ozone installations are either atmospheric or in-line designs.
Atmospheric ozone injection
Atmospheric ozone injection can be quite cost-effective, especially if the homeowner is already planning on storing water and repressurizing it. A significant benefit of working with an outdoor atmospheric tank is that lower ozone dosages can be very effective due to increased contact time and the tank can safely vent gases and unused ozone to atmosphere. Some dealers will introduce the raw water into the tank through an aerating head to further assist in the oxidative process. Mixing nozzles can also be used to provide as much contact and movement as possible. A significant disadvantage of the atmospheric tank is that soluble precipitates will form at the bottom of the tank, necessitating periodic cleanouts. Further, if the tank installation does not include sanitary venting and protection of animal/insect ingress, it can become a source of bacterial contamination. An atmospheric configuration typically incorporates the following components, assuming that pump and fill controls are already in place:
Air pump. Sized to generate the pressure and flow needed to deliver the ozone to the desired point. The air will be pushed through an optional dryer, through the generator and then all the way to the tank where it will rise to the top and plunge down to the bottom (where it exits the diffuser), so take this into account and size accordingly.
Optional air dryer. The ozone installation will usually include an air dryer if ambient humidity is less than 20 percent. Air dryers can be passive-desiccant or active-heating dryers. Consult with your vendor to ensure that the pumped air is dry enough before entering the generator
Ozone generator. The generator is sized according to manufacturer’s guidelines along with your calculations. Be sure to discuss each application with your vendor to help minimize potential problems or complications.
Distribution tubing. One of the most common mistakes made in ozone installations is to use the incorrect tubing after the ozone generator. Remember that ozone is a very strong oxidizer and it can destroy vinyl and polyethylene fittings/tubing. Stick with proven tubing, like PVDF, FEP and PTFE. Always use the style and type of fitting that the tubing manufacturer recommends for the application. For example, most PVDF tubing manufacturers only advise barbed fittings, where PTFE can be used with barbed or compression fittings. Keep tubing shielded from direct sunlight/heat and keep the run as short as possible to minimize degradation.
Ozone diffuser. The diffuser introduces ozone to the water. This is not a place to be cheap; use a diffuser made of materials compatible with the water you are treating and select one that generates extremely small bubbles to enhance contact.
The most common failures that I see in atmospheric tank applications is where the O3 dosage is too low and you are then inadvertently promoting the growth of slime-forming aerobic bacteria. Remember that in warmer states, the water in an atmospheric tank can get quite hot, so your ozone will not be active for as long as it would in a northern climate.
In-line ozone injection
If there is no space for atmospheric injection, or the ambient temperature makes it unfeasible, an in-line injection design is appropriate. In-line injection requires a faster rate of ozone production and suitably designed injection, mixing, contact and ozone destruction. The preparatory steps are exactly the same as for atmospheric injection, with an air pump, air dryer, ozone generator and distribution tubing. The difference comes in with prefiltration, injection and contact times:
Prefiltration. Protect your injection system, mixer(s) and contact tank(s) from sediment and foreign material with a prefilter in the 30- to 50-micron range.
Backflow prevention. While check valves are important on all ozone installations, it is critically important to incorporate at least one ozone-compatible check valve and a Hartford Loop on a direct injection system after the ozone injector to prevent contamination of the ozone injection loop with water, as well as backflow of water into the ozone generator.
Injection. The injector type and location are important for ensuring that you inject as much ozone as possible into solution. Size the Venturi injector according to manufacturer’s specifications and plan on installing it so that it can be easily accessed for periodic maintenance and cleaning.
Static mixer. Static mixers allow for significantly improved contact between the ozone and the solution to be treated, and are well worth the added expense.
Ozone contact tank. A properly sized contact tank further ensures an appropriate ozone dosage while allowing off-gassing to an ozone destruct system and then to atmosphere.
Ozone destruct. Unused ozone in the contact tank indicates that there is sufficient ozone to fully oxidize the contaminants or more commonly insufficient contact time in solution because the contact tank is undersized. The contact vent can be routed to an ultraviolet or catalytic ozone destructor that will force it to revert back to the oxygen that is found in normal ambient air.
Dealing with the byproducts
Once the target contaminants have been oxidized into insoluble precipitates, it’s time to take out the trash. This can be effected with cartridges, bag filters or even a self-backwashing multimedia filter. Budget, required flowrate and the amount of contaminants to be removed will help to make the right decision. Think about the size of particulate to be removed and bench-test to establish the appropriate filter pore size that you will need.
It is also very important to plan for redundancy by installing parallel identical filters, or step-wise series filters. A successful approach is a self-backwashing multimedia depth filter followed by a one-micron post-filter, which ensures maximum removal of precipitates, while retaining the ability to properly clarify the water. You will minimize the potential for bacterial contamination of the post filters through good design, safe handling and disinfection procedures. It is sometimes appropriate to also soften the water at this point and then disinfect with 254-nm UV to provide peace of mind to the end user.
Installation and follow-up testing
Even the best-designed systems will fail if improperly installed. Make sure that you follow industry best practices while adhering to prevailing local codes and manufacturers’ guidelines. Locate equipment away from excessive humidity, direct rain, direct sunlight and freezing conditions. Dry, clean, cool equipment will provide the very best results.
Don’t plan on forgetting about your client once you’ve been paid. Instead, plan on returning within 30 days after installation to retest the influent and effluent waters and confirm that everything is working as promised. Discuss a sensible preventive maintenance plan with your client to help ensure that the system is properly maintained and that you are able to best serve them. Plan on visiting your client at least annually to maintain the system and ensure that their water is what they expect it to be.
Ozone safety notice
Ozone is an unstable and extremely powerful oxidizer. It can be deadly to humans and other animals. Concentrated ozone smells similar to chlorine and not surprisingly, they both act similarly against living organisms. Airborne ozone can cause:
- Decreases in lung function
- Aggravation of asthma
- Throat irritation and cough
- Chest pain
- Shortness of breath
- Susceptibility to pulmonary infections
- Damage to eyes and mucosal membranes
- Damage to skin
The US Occupational Safety and Health Administration (OSHA) guidelines for ozone in the workplace are based on time-weighted averages. Ozone levels should not exceed 0.1 ppm (parts per million) for each eight-hour-per-day period of exposure doing light work. The OSHA website cites several American Conference of Governmental Industrial Hygienists (ACGIH) guidelines for ozone in the workplace:
- 0.2 ppm for no more than two hours exposure
- 0.1 ppm for every eight hours per day exposure doing light work
- 0.08 ppm for every eight hours per day exposure doing moderate work
- 0.05 ppm for every eight hours per day exposure doing heavy work
Unlike OSHA, National Institute for Occupational Safety and Health (NIOSH) safety and health standards are not enforceable under US law, but they do have a strong influence in forming future policy and regulations, as well as being sensible. The NIOSH recommended exposure limit for ozone is 0.1 ppm. According to NIOSH, ozone levels of five ppm or higher are considered immediately dangerous to life and/or health. A good rule of thumb is that if you can smell ozone, then you should evacuate the area and properly ventilate before returning to work.
Ozone requires a little planning and effort, but the benefits can be dramatic. Learn more and take the plunge!
About the author
Greg Reyneke, Managing Director at Red Fox Advisors, has two decades of experience in the management and growth of water treatment dealerships. His expertise spans the full gamut of residential, commercial and industrial applications, including wastewater treatment. In addition, Reyneke also consults on water conservation and reuse methods, including rainwater harvesting, aquatic ecosystems, greywater reuse and water-efficient design. He is a member of the WC&P Technical Review Committee, currently serves as President of the PWQA Board of Directors and chairs the Technical and Education Committee. You can follow him on his blog at www.gregknowswater.com