City improves water quality with ozone system
Water impurities in communities around the country are raising concerns with city officials and consumers. A public notice recently issued in Washington, D. C., warned that a high level of bacteria in the water made it unsafe for dialysis patients, AIDS patients, organ transplant patients, the elderly and infants. One far-reaching proposal, already passed by the U.S. House of Representatives, would require municipal water systems to tell the public what contaminants have been found in local drinking water.
In the midst of this heightened concern over water quality, Andover, Mass., is using an ozone water system to help remove odors, tastes and color from the town’s supply of drinking water at two-thirds the cost of the old treatment process.
“We doubled the capacity of our plant and assured ourselves of meeting increased demand as we move into the next century,” says Water Superintendent John Pollano. More impressively, according to Pollano, the system is currently saving the town approximately $64,000 annually in chemical costs.
Town officials had several water-quality problems to address, most of which could be traced to unchecked algae growth. Algae in the area can be severe and unpredictable, especially in the summer and fall months when nutrients and sunlight are at their peaks.
Andover needed a system that could solve the taste and odor problems, reduce concentrations of total organic matter and color and limit the formation of disinfection by-products such as trihalomethanes (THMs), according to Pollano.
The original water treatment process provided:
* chemical coagulation with alum;
* sedimentation;
* taste and odor control with powdered activated carbon (PAC);
* filtration with rapid sand filters for further turbidity reduction;
* pH adjustment with potassium hydroxide; and
* disinfection with sodium hypochlorite.
The existing process could not consistently handle the algae or the taste and odor problems and was limited in its ability to remove natural organic matter and minimize disinfection by-products.
Andover worked with the Electric Power Research Institute (EPRI), Palo Alto, Calif., and Massachusetts Electric, Westborough, Mass., to find a technology for the town’s needs. EPRI’s Northeast Regional Community Environmental Center supports collaborative research, development and demonstration projects and provides technical support to its member utilities, as well as their customers.
“The [ozone] system uses electric generators to produce ozone gas by passing dry air or oxygen through a high-voltage electrical field,” says Ann Hatcher, senior analyst with the NEES Companies, Westborough, Mass. “Ozone water treatment has successfully controlled the effects of algae blooms and eliminated water quality problems. THM formation potential has been reduced by an average of 75 percent.”
The new process adds ozone, clarification and granular activated carbon (GAC) filters. Treatment involves a multi-step process, which begins with adding ozone to the town’s water supply. The water is treated with alum, clarified and passed through GAC filters that sift out fine particles and residual matter. Lastly, sodium hypochlorite and potassium hydroxide are used to disinfect the water.
“The ozone system itself is not complex,” says Wilson. “It consists of the three stages of air preparation, ozone generation and adding the ozone to the water supply.”
The air, purified to remove fine particles and moisture, passes through the ozone generator, where an electric current transforms some of the oxygen in the air into ozone gas. The gas is then added to water in a contractor to oxidize organic matter and destroy or inactivate disease-causing organisms. Off-gas from the contractor is sent to an off-gas destruction unit and discharged.
Hatcher noted that ozone treatment compares favorably with standard treatment facilities in electricity use. Ozone uses 11 kilowatt-hours of electricity to produce a pound (kWh/lb) of ozone, compared to the 10 kWh/lb to 12 kWh/lb rate with traditional air-feed systems. The total cost of water production is about $82 per million gallons.
In addition to upgrading drinking water, ozone water treatment addresses public health issues as well. Ozone preconditions the water for more effective removal of contaminants with fewer chemicals. The system provides protection against microorganisms such as Cryptosporidium and Giardia, which can cause gastrointestinal illnesses.
The new system has also been advantageous in that plant operators can now handle nearly all plant maintenance without assistance. Additionally, because ozone water treatment does not require employees to handle as many chemicals such as powdered activated carbon and sodium hypochlorite, there is a decreased risk of injury.
