Background
The ozone/cavitation process has proven beneficial for effective treatment of potable water source waters. Although these successes have largely been for disinfection purposes, the ozone process has afforded owners of this type of treatment system several ancillary benefits such as removal of taste and odor source contaminants as well as natural and manmade organics, including pesticides and herbicides. This process is capable of oxidizing a large portion of refractory type pollutants often found in both water and wastewater. Removal of these pollutants in either case is beneficial as often times the potable water plants draw water out of rivers, lakes, reservoirs, etc. containing treated water from a wastewater treatment plant. Although the functionality of some of the more conventional methods may have merit they may not be a viable due to, among other things; regulatory requirements, economics and/or available space. Due to more recent advancements in water and wastewater treatment technologies, these refractory type pollutants can, and often times are, more cost effectively removed through less conventional methods. One of the more efficient technologies in this regard is the cavitation/ozone process.
Cavitation
First of all, as the name implies, there are two complementary technologies that are the basis for this treatment process, cavitation and ozone. They work in tandem to disrupt, oxidize and then further disrupt and oxidize pollutants contained in the liquid simultaneously. Energy added to a liquid first causes small bubbles to be created and upon reaching the critical point collapse radiating energy. Energy provided by the cavitation process produces extremely high localized temperatures and pressures, on the order of 11,100oC and 29,000 psi respectively. These conditions make it possible to further some oxidation reactions as well as allowing some oxidation reactions to occur that would ordinarily be impossible.
Cavitation and its role in various water and wastewater treatment processes have become increasingly relevant. Cavitation improves ozone treatment primarily through its ability to enable more efficient breakdown of pollutants subsequent to more efficient ozone application and treatment. Demonstrating the effectiveness of this ozone/cavitation process hybrid technology, when compared to ozone as a standalone technology, is the improvement on effective treatment capacity by a factor of three to one. When considering minimal cavitation related energy cost demands in relation to the cost of ozone treatment system and onsite ozone production, the ozone/cavitation process becomes a far more viable alternative to other existing technologies.
Ozonation
Ozone and its use, in many cases, do not require shipment of chemicals onsite. The obvious exception to this would be receipt of liquid oxygen. Until recently the primary role of ozone in water and wastewater treatment has been that of disinfectant. It is interesting to note that, pound for pound, ozone has about ten times the disinfection capabilities of chlorine. Use of ozone for wastewater disinfection can be particularly beneficial from a cost standpoint for systems with restrictive bacteriological and chlorine limits. Systems with these more restrictive bacteriological requirements have correspondingly high chlorine demands for disinfection and de-chlorination demands to comply with the required near zero chlorine residual concentrations at the point of discharge. These increased chemical requirements have proportionately increased chlorine and de-chlorinating chemical costs. Several of the past ozone issues, questionable reliability and high cost point, have since been resolved. Those improvements along with escalating costs to meet the recent more restrictive effluent water quality standards have helped make ozone based disinfection a more viable alternative when compared to many of the more common technologies.
Impact on Water Treatment
Ozone as a method of water treatment has been around for a number of years with varying levels of use. It was first used for disinfection purposes in a Nether lands water treatment plant near the end of the 19th century. By 1915, there were a large number of other drinking water plant installations, though they were mainly located in Europe. It wasn’t until 1987 that the first large scale ozone treatment system was installed at a water treatment plant in the United States. While ozone for bacteriological treatment of potable water sources in the United States has found minimal acceptance so far, further consideration of many of the micro-pollutants (natural and manmade) found in drinking water sources may soon make this method a main stay for surface water treatment. A brief listing of a much broader group of contaminants is:
1. Pharmaceutical and Personal Care Products
2. Endocrine Disrupting Compounds
3. Trace Organic Chemicals
4. Phthalate Esters
5. Bisphenol A
6. Nonylphenol Ethoxylates
7. Fragrance Materials
8. Estrogens
9. Pesticides
10. Ultra Violet Blockers
The aforementioned pollutants are largely manmade in origin with many of them linked to plastics, pesticides, pharmaceuticals, detergents and/or surfactants and are not effectively removed below the “no effect” concentrations through conventional wastewater treatment processes. This inability leads to many of these receiving waters becoming a source of concern for water quality as it relates to drinking water safety. Even when considering the level of dilution and natural degradation of the various pollutants, a sizeable fraction of these pollutants still find their way into the drinking water supply. Drought type conditions with lower than normal water levels only serve to exasperate this problem. While many of the more conventional water treatment processes are able to remove much of the pollutants of concern through filtration and/or oxidation, that’s only half of the story. In the case of oxidation, the resulting disinfection byproducts may include compounds such as total trihalomethanes or haloacetic acids, each of which have their own associated health risks.
This need to provide better control over pollutants entering the drinking water system leads to the necessity for a higher level of treatment for drinking water plants sourcing water from surface water supplies. In the case of chlorinated byproducts, removal or oxidation of the precursor compound prior to chlorination would serve to mitigate many of the health issues. With regard to Giardia Lamblia and Cryptosporidium, oxidation with ozone is an effective means of controlling concentrations of these cysts and improving treated water quality.
Impact on Wastewater Treatment
While ozone as a method of water treatment has been around for many years, it lacks the same history and credibility with wastewater treatment. While ozonation for disinfection of treated wastewater saw a window of acceptance during the 70’s and 80’s, this was short lived. At that time, ozone treatment technology had not yet matured to a level of functional reliability and with increasing energy costs it lost momentum as an emerging disinfecting technology of choice. More recently as NPDES permits have become more restrictive, chemical costs going the way they usually do, up, and the need to better control or eliminate use of residuals producing chemicals has increased. All the while, ozone treatment technology has seen significant improvements in reliability, effectiveness and equipment costs. A combination of these factors has led to resurgence in consideration of ozone use as a disinfectant.
While ozone treatment holds value as a disinfectant, it’s beginning to gain traction for use in primary wastewater treatment, with a measureable benefit being a reduction in pollutant loading to the treatment system and a corresponding reduction in aeration. Reduction of pollutant loading to the treatment system results in attenuation of aeration requirements which is significant due to its relationship with one of the biggest line item costs associated with operating a biological treatment system, electrical power. This, in addition, to the ability to oxidize some of the more difficult pollutants to remove such as: nitrogen, phosphorous, COD and BOD5 compounds makes ozonation a good choice for pretreatment leading to improved process stability, predictability and overall process performance.