By Lawrence Henke
Future research will need to look at the effect of ozone in standardized conditions that would be possible in a water or wastewater treatment facility. It may be necessary to address increasingly more complex and varied water conditions as scarcity and environmental pressures on water use grow.
Ozone has long had a role in the disinfection of water; its ability to inactivate or kill microbes such as bacteria, protozoa and viruses has been well studied and applied for decades. Of equal and perhaps more importance, however, is its ability to oxidize high molecular weight substances in water.
One of the more striking consequences of modern living with respect to water treatment has been the explosive proliferation of chemicals found in water supplies. Considerable effort has been made on the more obvious water pollutants, with pesticides, insecticides, herbicides and other agricultural chemicals leading the list.
But in more recent years, more potentially dangerous chemicals have been found in our waters, many of which have escaped through the wastewater treatment systems. Some are medical sub- stances such as antibiotics, birth-control medications, analgesics or psychoactive drugs like antidepressants.
In some instances, improvements in analytical capabilities have allowed formerly undetected levels to be determined, even to the levels of parts per trillion. In other instances, pollutants have been identified because they have been sought.1
Others include licit and illicit pharmaceuticals, both human and veterinary, which have been found in surface waters and consequently find their way into our drinking water. This poses a new and dangerous hazard, as well as a new challenge for water treatment.
More surprising is the presence of seemingly innocent chemicals that we use daily. Cosmetics, perfumes, sunscreens, soaps and other common products can have biological consequences. Some of the more well known of these are the endocrine disruptors, but of equal concern now are benign drugs such as caffeine, ibuprofen, aspirin, DEET and even addictive substances such as cocaine.
Drugs are almost exclusively organic chemicals, some very intricate compounds of a high molecular weight and complex chemistry and some of which combine both organic and inorganic chemicals, others organic only. Many do not easily lend themselves to natural decomposition. Some are available for reintroduction into the human body and can be detrimental, even in diluted form; others affect animals or can be accumulated through the food chain in water.
Organic substances are proteins, carbohydrates and lipids (fats) composed of carbon along with nitrogen, hydrogen and oxygen. These ultimately decompose to carbon dioxide, water and with trace elements such as iron, manganese, calcium, sodium and other more complex substances. The problem is time; natural reactions are slow. When hastened by oxidative processes, the reactions are faster, but can introduce intervening molecules that can themselves be hazardous.
The US EPA has offered three contaminant candidate lists that identify some of the chemicals considered to be hazardous in drinking water systems. There is a lengthy and thorough process used in identifying the items on these three lists, which include: the frequency of occurrence, the level of the substance in the environment and the levels of health impact.
Some of the chemicals noted have been on all three lists while some have been added and others discarded, either through studies that suggest a limited exposure or by an improvement in water treatment technologies and procedures. Among the several efforts to remedy these situations has been oxidation by advanced oxidation processes (AOP). Associated with many AOP technologies is ozone, either alone or in combination with hydrogen peroxide and in many cases, followed by carbon adsorption tanks.
Examining each and every chemical for ozone’s effect is an immense task, if not impossible. Since some studies have been focused on representative substances—partly because of the ability of ozone to react with water to form the hydroxyl radical (*OH)—ozone shows great promise for the future.
It would be nice to have a single approach to the destruction of organics in water, but the problem is complicated by the presence of more than just one substance in almost all cases. Moreover, the presence of a substance at one time does not necessarily mean it will be present at another time.
Several studies of effluents from industrial sources or hospitals, along with samples of many surface sources after wastewater treatment plant processing, have presented at various levels of contamination. Groundwater sources are not immune to intrusion by organic contaminants; many VOCs, solvents, agricultural products and pharmaceuticals can travel some distance in subsurface rock.2
Many experiments have been conducted using ozone as an oxidizer, often using hydrogen peroxide or ultraviolet light in combination to generate the hydroxyl radical that is more powerful than ozone alone. There has been no standard test range for either the ozone dose in mg/L or the time in mg/L/min. or the detention time following exposure. Other variable control factors include pH, water temperature and the presence of competitive ions such as iron or manganese and alkalinity (carbonate is a hydroxyl radical scavenger).
Because of this, ozone has a relatively short half-life in water, again depending on pH, temperature and water composition. Its effects must be used quickly. The hydroxyl radical is even more reactive and it will be consumed almost immediately.
The molecular structure of the target substance is also of importance. Those with ring structures are affected differently than those with long hydrocarbon chain structures. The position of atoms in the structure and the polarity—whether negative or positive—of smaller sub units will determine how the molecule is destroyed.
In some cases, the products of oxidation are themselves more toxic than the original, large molecule, so testing after the process is necessary. One study involving oxytetracycline (OTC), when ozonated for five to 30 minutes, yielded products more hazardous than OTC itself. Oxidation is commonly followed by an adsorptive carbon column.
Some studies have counterintuitive results. The dose of hydrogen peroxide, for example, is important when added to ozone; higher doses are, in many cases, less effective than lower HO for the production of the hydroxyl radical. This is because of interfering chemical reactions, somewhat beyond the scope of this discussion.
Another important factor is pH. Formation of the hydroxyl radical is favored at higher pH levels, while the effect of ozone alone is favored at lower pH levels. Studies are often conducted at three pH levels (three, seven and nine) to determine the best level for a given substance.
Detention time following ozonation is critical to its effectiveness. In some studies, contact of up to 120 minutes or more is necessary to break down the bonds that hold the molecule together; in other cases, times as short as five minutes will suffice.
The result is that although there have been many studies on the effect of ozone alone or with hydrogen peroxide or UV to form the hydroxyl radical, each study has had its own parameters and few share the same water conditions. Thus it is difficult to predict with certainty the behavior of ozone against a specific contaminant.
Future research will need to examine the effect of ozone in standardized conditions that would be possible in a water or wastewater treatment facility. It may be necessary to address increasingly more complex and varied water conditions as scarcity and environmental pressures on water use grow.
- Richardson, S. D. “Water Analysis: Emerging Contaminants and Other Issues,” Analytical Chemistry, 2009, In Press.
- Barnes, K.K., Kolpin, D.W., Focazio, M.J., Furlong, E.T., Meyer, M.T., Zaugg, S.D., Haack, S.K., Barber, L.B., and Thurman, E.M. 2008 Water- quality data for pharmaceuticals and other organic wastewater contaminants in ground water and in untreated drinking water sources in the United States, 2000–01: U.S. Geological Survey Open-File Report 2008–1293, 7 p. plus tables.
About the author
Larry Henke is an independent water treatment consultant in the Minneapolis, MN area, serving industrial filter and disinfection applications and small non-community and community public drinking water systems. A graduate of the University of Minnesota, he has more than 20 years of experience in the water treatment industry. Henke is a member of the American Water Works Association and the National Ground Water Association, as well as WC&P’s Technical Review Committee. He can be reached at (612) 599-7477 or [email protected].