The wonderful thing about our industry is that we actually make people’s lives better. Every day around the world, there is a water specialist solving somebody’s water quality issues at a residential, commercial or industrial level. As one of the most precious commodities on our little planet, good water is in high demand; it is our job as water quality management experts to make that water cleaner, better and more useful while saving costs, preserving natural resources and easing the labor burden. One of the most frequently overlooked tools in a water specialist’s arsenal is oxygen, and more specifically, the triatomic allotrope that we know as ozone.
History and chemistry
Ozone consists of three oxygen molecules and has a very short half-life compared to its diatomic cousin that we all breathe: O2. While mankind has recognized the odor of ozone as the smell of lightning in a rain storm since the dawn of time, ozone was only isolated and then recognized as an allotrope of oxygen in the 19th century. The odor around your laser printer or photocopier is ozone gas; it has a distinct, pungent odor. Ozone’s inherent instability is what makes it so very attractive to a water dealer; it works hard oxidizing as much as it can for about a half an hour at room temperature, and then quietly fades away into safe, stable O2.
Making ozone
In nature, ozone is produced through solar radiation and lightning strikes. The water quality management industry has simply imitated nature:
Ultraviolet light – Ozone is produced by reacting oxygen with ultraviolet light in the 185 nm spectrum. While this is Mother Nature’s primary method of producing ozone, it is quite cumbersome and electricity-intensive compared to other methods.
Cold corona discharge – Also known as cold-spark corona discharge or simply corona discharge, it is by far the most common form of ozone generation in our industry, due to its relative simplicity compared to ultraviolet production methods. Simply put, corona discharge ozone production requires a high-voltage power source, anode, cathode and a suitable dielectric separator. Oxygen-rich air passes between the electrodes over a dielectric, creating an electric field, or ‘corona’ which induces O2 molecules to rearrange themselves as O3 – ozone.
Important safety notice
Ozone is an unstable and extremely powerful oxidizer. While ozone has proven itself invaluable in the water quality management industry, 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 O3 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 eighthours 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 usually have a strong influence in forming future policy and regulations. 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.
Think of ozone as a caged beast. As long as you enclose it and handle it carefully, you’re going to be safe; if you let it out of its cage or treat it without the appropriate respect and caution, it WILL hurt you. As we discuss the exciting potential for ozone in water treatment, please remember that a smart water professional will take the appropriate precautions to protect herself, her client and all associated personnel, property and infrastructure from direct ozone exposure. A good rule of thumb is that if you can smell ozone, the concentrations are high enough that you should immediately evacuate the area and properly ventilate before returning to work.
Ozone applications, is this the appropriate tool for the job?
Disinfection – Ozone is a viable alternative to chlorine; it kills significantly faster than chlorine, without creating the same volatile organic byproducts of disinfection. In my experience, ozone is best used as an adjunct to chlorine, thereby significantly reducing the amount of chlorine used while still retaining a residual active disinfectant in the water well-beyond ozone’s half-life.
CIP systems – All food, beverage and vitamin production facilities have an established need for clean in place (CIP) solutions. Ozone dramatically simplifies CIP by lowering heating energy costs, reducing the amount of cleaning chemicals required and eliminating disinfectant chemical residuals.
Cooling towers – Airborne mold, fungus and bacteria are significant concerns to building operators trying to maintaining a cooling tower. These contaminants foul heat exchangers, contribute to microbially induced corrosion, and of course, pose significant risks to human respiratory health. Properly executed ozone treatment effectively eliminates living organism and other organic challenges from the recirculating water stream which of course aid in minimizing hard water adhesions by removing the biomass.
Waterfalls, fountains and other water features – The trend towards decorative indoor humidification and ‘living walls’ has resulted in a massive increase in atmospheric water exposure in homes and buildings. Water features can become major indoor air quality (IAQ) liabilities if bacterial contamination is not properly controlled. While many water-wall manufacturers design ultraviolet disinfection into their recirculation chambers, field-experience shows that the most effective method for keeping heterotrophic bacteria (HPC) under control in this kind of application is simple ozone injection.
Wastewater remediation – Wastewater is increasingly becoming the focus of regulatory scrutiny, whether from local sewer districts or federal authorities; this is a source of significant revenue to regulatory bodies and every plant operator must be certain that BOD, COD, and TSS levels are well within compliance. Ozone and AOP are invaluable tools in attaining these goals.
Iron and manganese – The bane of rural water users worldwide, iron and manganese cause color and staining issues in water that affect residential, commercial and industrial users. Ozone effectively oxidizes ferrous iron into a ferric form that is easily filtered by a multimedia filter. It has the same effect on manganese contamination. As a side benefit, ozone is highly effective in destroying iron-reducing bacteria other ferro-bacteria that contribute to undesirable tastes, odors, and aesthetics in water.
Hydrogen sulfide (H2S) – H2S and sulfate-reducing bacteria are responsible for undesirable odors and corrosive conditions in well water around the world. Ozone can be effectively employed to kill sulfate-reducing bacteria, neutralize odors and oxidize hydrogen sulfide into an easily filterable precipitate.
Advanced oxidation processes (AOP)
AOP are an exciting extension of our industry, opening up a myriad of treatment and remediation options that harness the power of hydroxyl groups. AOP allows the savvy water specialist to address amino acids, aromatics, herbicides, pesticides, hydrocarbons, organics and volatile organic compounds in wastewater. Properly executed AOP effectively destroys contaminants by rendering them down to their constituent elements. Ozone can be a solid backbone of an effective AOP installation when installed with ultraviolet radiators and an appropriate catalyst. AOP requires a significant level of knowledge and understanding of water chemistry – counsel with your vendor or industry consultant carefully to ensure that you are deploying a viable solution. I am a firm believer in deploying in-situ pilot plants to gather sufficient operational data before investing in the capital expense of an operational AOP system.
Calculating minimum ozone demand
Determining the ozone demand of the water requires an accurate water analysis and an understanding of the amount of ozone required to react with certain contaminants. All experts have their opinions; the data below is from the criteria that I use when sizing systems. Consult with your expert and decide what you both are comfortable with before proceeding.
|
Contaminant |
Minimum Ozone Dosage per mg/L of Contaminant |
|
|
Iron (Fe) |
0.43 mg/L |
|
|
Manganese (Mn) |
0.87 mg/L |
|
|
Hydrogen sulfide (H2S) |
3.00 mg/L |
|
|
Tannins |
1.6 mg/L |
|
|
Nitrite |
2.0 mg/L |
|
|
Phenol |
2.1 mg/L |
|
|
Total organic carbon |
4.0 mg/L |
|
|
Biochemical oxygen demand |
2.0 mg/L |
|
|
Chemical oxygen demand |
2.0 mg/L |
|
|
Nitrite |
2.25 mg/L |
|
Disinfection applications 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
- Calculate the flowrate to be treated in L/hr.
Convert us customary units to metric as follows: (gpm x 60) x 3.785
For example: 10 gpm x 60 = 600 gallons per hour
600 gph x 3.785 =2,271 liters per hour
- Multiply minimum ozone demand by L/hr to calculate mg/hr minimum production rate.
- Divide by 1,000 to determine the minimum grams per hour that the ozone generator needs to produce.
Complicating factors like chelates, organic complexes, transfer efficiency, and even water temperature fluctuations will interfere with textbook ozone demand calculations, so develop an appropriate reserve capacity of 20-30 percent beyond minimum to allow for flexibility in treatment. It’s more fun to throttle down your ozone production rate than to wish that you had more to work with.
Ozone deployment – best practices
Safety first! – When designing an ozone installation, always make safety your first and foremost goal. Ozone generators should be kept cool, with an ambient temperature below 80ºF (26.6ºC). Humidity should also be kept well below one percent to prevent the formation of nitric acid within the ozone generation chambers.
After calculating the ozone demand for the application, a quality ozone generator should be selected from a reputable vendor. Consult with your vendor on whether you will use a liquid oxygen feed, oxygen concentrator, or ambient air feed, and then discuss the ambient operating temperature and relative humidity and plan for appropriate cooling and dehumidification as necessary.
Select a method for injecting ozone into the water stream. Venturi injectors are simple and relatively inexpensive, but there are also certain applications where an ozone concentration pump (compressor) is appropriate. Great strides have been made recently in the development of reliable static mixers; these should also be considered on a case by case basis.
The next step is to calculate the volume of retention required to maximize contact time. This is easy, simply multiply the time required by the operating flowrate. If you’re running at 10 gpm and require six minutes of contact time, you’ll need a total mixing/retention tank volume of at least 60 gallons.
Monitoring of ozone residuals in the downstream water is wise when working with fluctuating contaminant levels. Where budget permits, automated process monitors can be installed that have data logging capabilities as well as analog voltage outputs to signal other monitoring and automation components.
After sizing and selecting system components, safety measures and monitoring, the appropriate consideration should be given to operating procedures, minimum maintenance requirements, and operator training. When I consult with dealers or end-users on malfunctioning ozone systems, the most common problem that I see is poor maintenance and upkeep. Ozone systems, like all water treatment technologies do require periodic maintenance so plan appropriately.
Things to remember:
- Safety first – fools rush in…
- Ozone generators like lots of cool, dry air ( <-60ºF (-15.5ºC) dewpoint, which is a relative humidity of <one percecnt)
- More feed oxygen = more discharge ozone
- Ozone dissolves better in cooler water
- Ozone lasts longer in cooler water
- One ozone feed check valve is good, two is better
- Ozone destroys rubber seals and other organoplastic materials
- Ozone has a low vapor pressure
- Ozone clings to rough surfaces
- Ozone lowers the pH of water
- While ozone costs more to deploy, it is cheaper to maintain and operate than other oxidizers
Ozone has earned its rightful place in the modern water quality management pantheon. A smart water specialist would be wise to study and master ozone technologies and application methods.
Glossary
Allotrope: A different form of an element that can exhibit dramatically different properties from the other forms.
Oxidation: The loss of an electron. Substances that can cause other things to donate their electrons are known as oxidizers or oxidants.
BOD – biochemical oxygen demand: A BOD test is a way to measure how much oxygen is being consumed by microbes in a water sample. The more contaminants in the wastewater, the more oxygen is needed to sustain life. High BOD numbers indicate high concentration of contaminants in the wastewater that the microbes have eaten. Oxygen content is measured when the test starts and again at the end of five days. The difference in the oxygen content between the first day and last day is used to calculate the BOD of the water.
TSS – total suspended solids: A measure of suspended solids in a water sample. A measured amount of the water is passed through a weighing disc that captures particulate >1 micron. The amount of particulate captured in the disc is weighed after dehydration and then extrapolated to determine an indication of total suspended solids in the sampled water.
Venturi injector: The Venturi effect (named for the 18th century physicist) as applied to fluid eduction is a phenomenon whereby the rapid reduction of water pressure in one direction allows for the creation of a vacuum in another. This is the phenomenon whereby most modern water softeners ‘draw’ brine from the brine tank during regeneration.