Leave Chemicals Behind with Ozone

Water Technology

From Volume 20, Issue 4 – April 1997Feature

Technology can be applied to multiple customer applications.

by: Beth Hamil and Allen Clawson

The ozone industry began in Europe for municipal drinking water treatment and gradually progressed to treating water for commercial swimming pools. Ozone is still used extensively in both these markets today.

Ozone didn’t get a real foothold in the United States until the introduction of ultraviolet (UV) generated systems in the late 1970s. These systems are very inexpensive compared to the much larger corona discharge (CD) systems, but are limited mostly to residential pools and spas because of their low ozone concentration and low ozone output. This does not mean that there isn’t great opportunity here; this business is probably worth $10 million per year or more.

In the last decade, CD ozone manufacturers have flourished and have significantly improved the quality, performance and design of their equipment. In addition, pilot studies have proven ozone’s superior purification and oxidation capabilities over chlorine without its negative side effects. Ozone is increasingly being used in commercial and residential pools and spas, small and large municipal drinking water and wastewater treatment systems, aquaculture, water and amusement parks, industrial wastewater and water bottling plants.

Increased environmental consciousness has accelerated the movement away from multi-chemical based water treatment and has led to a greater recognition of ozone as a strong, fast, commercially available disinfectant and oxidant. Ozone oxidation reactions occur several thousand times faster than chlorine for destroying bacteria, viruses, yeast, molds, cysts, mildew, algae and most other organic and inorganic contaminants. Ozone in appropriate doses can treat most waterborne pathogens and, unlike chlorine, leaves no harmful chlorinated byproducts in water, quickly reverting back to pure oxygen if unused. Ozone does not contaminate available water supplies, and it requires less makeup water is required to achieve equal or better operating results than chemical-based systems.

Ozone (O3) is an unstable compound generated by the exposure of oxygen molecules (O2) to ultraviolet (UV) radiation or a high energy electrical discharge. The weak bond holding ozone’s third oxygen atom is what causes the molecule to be unstable and very effective. Because of this instability, an oxidation reaction occurs upon any collision between an ozone molecule and a molecule of an oxidizable substance such as certain forms of iron and manganese or organic molecules (bacteria, viruses and some plastics and rubbers).

In an oxidation reaction, energy is transfered to the ozone molecule, leaving a stable oxygen molecule (O2) and a highly unstable oxygen atom (O1) The molecule being oxidized then bonds with the loose O1 atom, creating an oxide of the substance. Dissolved metals oxidize and are no longer soluble. The structure of an organic molecule is changed by oxidation, which often causes the whole molecule to come apart (with some help from other ozone reactions). Bacteria and virus cells are literally split apart by ozone.

Effective Ozone Use

Transfer of ozone into water is critical for effective disinfection. Only dissolved ozone is able to oxidize contaminants in water. Non-dissolved ozone “off-gases” to the surface and is lost. One of the most effective means of introducing ozone into a water stream is by Venturi injector, which uses the water stream to produce a vacuum. Ozone-containing gas is drawn into the Venturi by the vacuum and violently mixed with water, which produces very small bubbles, enabling the ozone to dissolve readily. The amount of ozone — measured in concentration of parts per million (ppm) — that can be dissolved in water before reaching a saturation limit depends mainly on four factors:

* Water temperature and pressure

* Water pH

* Concentration of ozone in carrier gas

* Mode of ozone injection into water

Ozone is highly reactive and in its gaseous form will quickly corrode most metals — such as iron, mild steel and copper — and will damage most plastics. Rubber exposed to ozone quickly hardens and cracks. Gaskets, sealing compounds and piping must be chosen with care before being used with ozone.

UV generators used on small water volumes are relatively simple and economical, but limited in output capacity. For larger installations, CD generators are required because they are capable of high output concentrations.

Most ozone generators used on small residential pools and spa pools generate ozone using UV light. Ozone is produced by irradiating ordinary air with UV light at wavelengths below 200 nanometers (nm). Longer wavelengths (around 250 nm) of UV light are more efficient at destroying ozone rather than producing it. When enough UV energy is added to an O2 molecule, it splits, freeing two O1 atoms to collide with other O2 molecules to form ozone.

The concentration of an ozone generator depends on the length and UV energy of the lamp used, the enclosure surrounding the lamp, the temperature, humidity and oxygen content of the air and the volume of air flowing through the generator. Since the construction of a given system remains fairly constant, concentration is affected most by air flow rate.

At high flow rates, air passes by the lamp quickly and does not allow the UV energy enough time to convert many oxygen molecules to ozone. At low flow rates, air passes by the lamp more slowly, more UV energy is absorbed and more oxygen converted to ozone. Ozone production, however, is much lower than in corona discharge generators. The maximum ozone concentration that can be produced using UV is less than 0.1 percent by weight. Normally, the air is caused to flow through these generators by the vacuum created by a Venturi.

Corona Discharge Systems

Using a CD system, ozone is produced by passing air through a high voltage electrical discharge. A minimum of 5,000 volts is necessary to create the corona (14,000 is a practical design maximum voltage). Air (containing 21 percent O2) or concentrated oxygen (95 percent pure oxygen) dried to a minimum of -76 F dewpoint passes through the corona, causing the O2 bond to split. Two O1 atoms are freed, which then collide with other O2 molecules to create ozone.

Ozone production can be regulated by adjusting either the applied voltage or feed-gas flow. By reducing the feed-gas flow, ozone concentration is increased, but the overall production rate decreases. Reducing the applied voltage also decreases concentration. The ozone/gas mixture discharged from the CD ozone generator normally contains from 1 percent to 3 percent (by weight) ozone when using dry air, and 3 percent to 6 percent (by weight) ozone when using high purity oxygen as the feed-gas.

Particulate matter and moisture should be removed from the feed gas as a minimum. Any contaminants in the gas stream build up quickly and affect the electrical discharge. Moisture in the feed-gas causes two serious problems: moisture will cause a significant drop in ozone production, and a small amount of nitrogen in the air converts to oxides, which then dissolve in moisture to form nitric acid. Feed-gas must be dried to below -76 F dewpoint to ensure that this does not occur. Moisture can be removed by passing the air through molecular sieves, activated alumina, silica gel or by a combination of refrigeration and desiccation.

Oxygen-fed systems are preferred for a number of reasons. First, the nature of oxygen preparation equipment ensures particulate and moisture-free feed-gas. Second, the oxygen environment increases generator efficiency by making more O2 molecules available. The clean environment created by the oxygen preparation system increases the life of internal components and significantly decreases the system’s maintenance requirements.

Oxygen is concentrated in air by passing ambient air through a molecular sieve material, which absorbs moisture and nitrogen when pressurized to just over 30 psi through a pressure swing adsorption (PSA) process. The product is approximately 80 percent to 95 percent oxygen at a relatively low flow rate and pressure. This feed-gas is then drawn through the generator under vacuum provided by a Venturi located downstream of the generator. Operating the system under vacuum rather than pressure reduces the risk of accidental exposure to ozone.

Properly Sizing Systems

Opinions vary on the best way to address ozone system and reaction tank sizing. Historically, guidelines have been vague or nonexistent regarding the application of ozone to commercial pools and spa pools. However, the U.S. Environmental Protection Agency (EPA) and Occupational Safety and Health Adminstration (OSHA) have developed a criteria for applying it to drinking water and industrial uses and for human safety issues regarding ozone off-gas. There are also established guidelines for safe use of maximum ozone levels in and around commercial pools and spa pools. Therefore, ozone system designs must address both proper sizing of equipment for disinfection and must ensure human safety.

The EPA has established a basis for a three log inactivation (99.9 percent) for Giardia lamblia cysts in drinking water at certain temperatures (32 F to 77 F) and pH values between 6 and 9. These take into account the amount of ozone residual in the water for a determined period of time without filtration. The product of concentration (C, in milligrams per liter (mg/l) or ppm) and contact time (T, in minutes) yields the CT value, which indicates the effectiveness of the disinfection process. As an example, 0.4 mg/l (ppm) ozone applied and maintained for four minutes equals a CT value of 1.6. The CT value is then applied to different organisms to determine the three log Inactivation of that organism.

For example, a CT value of .72 provides three logs of inactivation (99.9 percent) of Giardia cysts at 68 F and a CT value of 1.6 provides 99.9 inactivation at 50 F. As shown in Table 1, the higher the temperature, the faster the reaction time, and a lower CT value is required. In addition, protozoan cysts are much more resistant than vegetative forms of bacteria and viruses. Therefore, when the CT value of ozone is sufficient to inactivate the more resistant organisms, it will easily inactivate the less resistant organisms. Giardia is typically used as the benchmark.

However, testing done to inactivate Cryptosporidium shows results with a 1.11 mg/l (ppm) residual of ozone and a five minute contact time, or a CT value of 5.55 at 68 F water temperature. In situations when Crypto is present, this larger CT value should be considered.

The major differences between standards for pool water and drinking water are as follows:

* In most states, commercial pool circulation is in a closed loop, providing repeated exposure with at least four passes per day vs. a single pass as used in the EPA testing

* The demand for an oxidizer in the water increases with each bather added and the resulting environmental contamination

* Pool and spa water temperature is typically warmer, 77 to 104 F, than values in Table 1

* Disinfectants such as chlorine, bromine or hydrogen peroxide are almost always added and maintained in pools and spas

* Off-gas concerns need to be addressed to eliminate the possibility of airborne ozone where humans and equipment are present

* Pool water is filtered

Modern systems commonly employ Venturi induction in a side or slipstream to introduce ozone into water. Considering ozone’s saturation in 68 F water is approximately 30 mg/l (ppm), and dose levels are about 0.4 to 1.5 mg/l, the difference between ozone dissolution capacity and the mass of ozone provided is great. This ensures high mass transfer with no overdosing (or waste) of ozone. Given a properly sized reaction tank and six-hour minimum turnover rate, a 15 to 25 percent side or slipstream provides adequate mass transfer of ozone into solution and sufficient contact time before entering the main stream.

Water in the side or slipstream is disinfected with high ozone concentrations and then remixed with the main stream where further oxidation reactions be produced. Because the side or slipstream is diluted by a factor of at least 4 to 1 in the main stream, less ozone will enter and less ozone off-gas will occur. This system ensures that no more than 0.1 mg/l (ppm) ozone residual will be present at any time.

A properly sized Venturi injector should be used in all cases. An injector is sized by calculation depending on the water flow, inlet and outlet pressure (psi) and requirements of each ozone generator. Also, a system should have an ozone degas valve on the reaction tank, connected to an ozone destruct system, to remove undissolved ozone from the water and destroy it before it enters the atmosphere.

Beth Hamil is vice president and Allen Clawson is head of engineering for DEL Industries, San Luis Obispo, CA.