Growing up, many of us remember the colorful diagram from science class called The Water Cycle, a circular flow of blue color evaporating from the ocean up to the sky, raining down into lakes and rivers, past cows, plants and houses, and back into the sea. Today, the picture isn’t quite as simple. The current USGS diagram is more up-to-date, but where do we fit in the petroleum refineries, desalination plants, pig farms and paper mills?
Ironically, a huge obstacle to providing clean water to developing municipalities and industries is the wastewater that is produced by the sanitation, processing and agricultural infrastructure that serves those same communities. For example, municipalities often draw at least part of their water from surface sources—rivers, lakes, reservoirs, etc. These sources are subject to a variety of contaminants from the air, water and soil around them. Some of these contaminants might be metals in unhealthy concentrations, harmful parasites or bacteria such as E. coli, Cryptosporidium and Listeria.
Effective treatment of this wastewater is necessary for long-term environmental sustainability of water resources and regulatory acceptance in light of increasing public awareness and scrutiny. People are drawing water from sources that are no longer as secure as previously hoped; it is important to make sure they are using the right treatment processes.
The Environmental Protection Agency (EPA) recently released its list of best available technologies to meet the Final Long Term 1 Enhanced Surface Water Treatment rule for dealing with contaminants in water. Two of the treatments are particularly effective when applied in concert: ultraviolet radiation (UV) and ozone. Both ozone and UV have long been used for inactivation of pathogenic organisms that can thrive in water and wastewater systems.
Low-pressure mercury vapor lamps emit UV light with particular intensity peaks at 254 nm and 185 nm. The short-wave radiation is effective at breaking molecular bonds in the DNA of microorganisms. UV radiation also provides the energy necessary to spark the decomposition of ozone, which leads to the formation of two hydroxyl radicals (OH). This oxidant is the key component in the Advanced Oxidation Processes (AOP)—the combinations of ozone, UV and/or hydrogen peroxide reactions.
Ozone is a powerful oxidant with an oxidizing potential of 2.07 eV, making it the world’s most powerful commercially available disinfectant. Ozone has been used in municipal water treatment for more than 100 years, and is a crucial application step for bottled water, clean-in-place sanitation and processing of produce, meat and industrial reuse.
The ozone/UV combination effectively destroys organic contaminants largely because of the very high oxidation potential of the hydroxyl radical (2.8 eV). For ozone/UV reactions (also known as photolytic ozonation) in aqueous solution, ozone is energized and combines with water to create OH, which is stronger and less selective than either chemical oxidant. Ozone effectively reacts with the organic contaminants it can impact, with remaining residual ozone converting into two hydroxyl radicals per ozone molecule. This subjects oxidation targets to multiple attackers, dramatically changing the oxidation landscape. With the highly active and nonselective hydroxyls acting on many targets, the ability of the ozone molecules to work more effectively is enhanced. One benefit of AOPs such as ozone/UV is that with their increased oxidation levels, even stubborn organics that resist degradation can be partially oxidized to the point where they are more readily biodegradable. According to Paul Overbeck, Executive Director of the International Ozone Association (IOA), the benefits of ozone preoxidation followed by UV primary disinfection for waters with Cryptosporidium is an excellent approach for utilities to meet the EPA’s new Long Term 2 Enhanced Surface Water Treatment Rule (LT2).
The key parameters for the success of a ozone/UV system are ozone dosage, UV irradiation level and pH. For proper ozone dosing, a high dissolved ozone rate must be maintained with effective transfer of ozone gas into aqueous solution. One effective design for ozone/UV systems is the pressurized injection secondary mix UV/O3 reactor, a system that creates microbubbles, constant renewal of the gas-to-liquid mixing zone and enhanced gas solubility for better utilization of UV irradiation.(1) As pH increases, ozone will more readily be converted to hydroxyl, which increases the oxidation rate of certain contaminants like pesticides and cyanide. The balance between pH level, UV photolysis and ozone dosage must be considered in system design and control for maximum efficiency.
As noted in a relatively early application overview by Paillard et al., “For any given ozonation rate, there exists an average radiation power at which the oxidation efficiency reaches its maximum degree. Below that dose, we find dissolved residual ozone that limits the oxidation efficiency and appears to limit oxidation kinetics.
“Beyond that dose, oxidation efficiency remains stable, but the energy efficiency ratio decreases in relation to the amount of degraded substrate. The ideal radiation strength, therefore, is the lowest rate at which the dissolved ozone and the ozone in the off gases is entirely consumed.”(2)
One problematic wastewater is produces by dyes used in textile factories, and the associated sizing agents and process chemicals. Some of the dyes in this wastewater are allergenic, toxic and even mutagenic or carcinogenic. Worse, the dye components can seldom be fully removed with traditional biological and coagulation/flocculation treatments.
Color levels are measured and analyzed in American Dye Manufacturer Institute (ADMI) units. Ozone/UV treatment was able to reduce dye-finishing wastewater by 95%, from 4,000 ADMI to 200 ADMI, in just one hour. Ozone/UV also showed increased ability to affect mineralization and toxic reduction in some dyes. In addition to remarkable efficacy, ozone/UV treatment in that scenario (800 m3 per day of wastewater) would save 30% on treatment costs per month. Acidic conditions further facilitate color reduction by both UV and ozone, whose presence is more dominant in low pH conditions.(3)
Other components of some industrial wastewater are surfactants (surface-active compounds), such as emulsifiers and detergents. Their processing effectiveness and unique chemical structure (both hydrophilic and hydrophobic components) make them very hard to treat, often leading to only partial degradation and high residuals in effluent water, which has potential public health and environmental effects.
The application of ozone and UV for surfactants has shown to be more effective than biological treatments for the commonly used compounds, such as anionic surfactants. At a low pH, ozone’s electrophilic attack decomposes organic compounds, and the hydroxyl radical creates chain reactions that lead to ultimate mineralization. Ozone and UV application has also shown to be an effective pretreatment step combined with biological treatment.(4)
Phenols, another common component of wastewater, can be both toxic and hard to break down because of their benzene ring chemical structure. Halogenated aromatics, which are refractory and difficult to remove with conventional biological treatment, can cause severe pollution problems.
With ozone/UV treatment, chlorine or nitrogen elements of the benzene ring can be eliminated relatively quickly, allowing the phenol to be more completely decomposed. In their study, Ku and Su (1996) showed 99% removal of 2,4-dichlorophenol, 2-chlorophenol and 2-nitrophenol by ozone/UV in about 15 minutes.(5) In photolytic ozonation of phenols, total organic carbon removal by UV irradiation is negligible, and ozonation can reduce levels by 30%, but ozone and UV combine for 95% removal.
Studies have repeatedly shown that this combination of ozone and UV are more effective for disinfection and organic destruction than either method used in isolation. Ikemizu showed that with UV augmentation, ozone oxidation rate of organics increased 10 to 104 times.(6) Also, for compounds that are refractory to ozonation, like acids, alcohols, amino and fatty acids, ozone/UV reactions proceed at a rate 10 to 103 times faster.
Ozone and UV combined present an innovative solution to many water treatment problems. When engineered correctly, their synergistic reactions create optimum oxidizing and disinfection conditions that will break down even some of the most durable and problematic wastewater components. By working in concert and speeding up reactions, ozone/UV treatments maximize their efficiency and decrease the demand for traditional chemical and biological treatments (and their subsequent byproducts).