Ozonation System Tested in Anticipation of Disinfection By-Product Rule
Contract operation, maintenance and management (OM&M) of a municipal water treatment plant are fairly straightforward concepts. For a fixed fee, the private OM&M firm operates the plant according to regulatory requirements and supplies a prescribed volume of drinking water to the municipality. OM&M contracts may be detailed and lengthy, but the essentials remain the same-service and compliance for a reasonable price.
While it might be a simple concept, unforeseen issues can influence cost and compliance. These problems can be related to correcting a pre-existing problem (often the motivation for a municipality to contract with a private operator) or anticipating new and increasingly strict environmental regulations that could have a significant impact on a plant's compliance record. This was the case at Newark, N.J.'s Pequannock Water Treatment Plant located in West Milford, N.J. A pilot study of an alternative water treatment process was conducted earlier this year in response to proposed disinfection by-product rules.
Reducing Disinfection By-Products
The Pequannock Water Treatment Plant serves the City of Newark and a number of nearby communities. The 80 million-gallon-per-day (MGD) direct filtration facility draws raw water from a nearby reservoir and uses chlorination for disinfection. The plant, completed in 1990, has been operated by the city's private OM&M firm since March 1993. Since then, the plant has experienced no major operational or regulatory problems.
The potential for compliance difficulties exists because of additional rules for disinfection and disinfection by-products (D/DBP) now being proposed by the Environmental Protection Agency (EPA). The new rules would require the level of trihalomethanes (THMs), a by-product of chlorination, be reduced in the city's drinking water. The City of Newark is currently meeting the disinfection by-product maximum contaminant level (MCL) of 100 parts per billion (ppb) for THM, but the EPA is proposing to reduce the MCLs for THMs to 80 ppb. Additionally, haloacetic acids (HAA) are expected to be included in the new D/DBP rules. The MCLs for HAAs are anticipated to be 60 ppb.
The city and its private OM&M firm have been working together to ensure that the plant complies with the proposed EPA regulations. The Newark treatment plant cannot simply lower the amount of chlorine used in disinfection to reduce the level of THMs. This would increase the risk of inadequate disinfection and could increase the presence of cryptosporidium and giardia in the city's drinking water.
An EPA Information Collection Rule (ICR) requires cities larger than 100,000 people to monitor for cryptosporidium. Of the 300 cities affected by the ICR, the National Association of People with AIDS has singled out six metropolitan areas, including Newark, as being locations of special concern. People with weakened immune systems are particularly at risk for the presence of cryptosporidium.
The Ozone Alternative
Studies have found that ozone, used as an alternative to chlorine for disinfection, not only reduces the formation of THMs and HAA, but is also effective as a disinfectant for the control of cryptosporidium and giardia. Ozonation has also been found to improve the taste, odor and color of water.
Ozone is an extremely strong oxidizing agent. In fact, ozone is so reactive to various water contaminants that it generally does not survive very long in the treatment process. Ozone reacts with natural organics in the water supply and forms lower molecular weight organic by-products that tend to be biodegradable.
Ozone has been used in water treatment systems for more than 90 years, mostly in Europe and especially in France. An increasing number of water treatment systems in the United States are utilizing ozone for disinfection due to the increase in regulatory requirements. Ozone installations in the United States have increased from less than ten in 1980 to more than 100 in 1994, with the majority of new plants using the technology having capacities more than ten MGD.
Newark Ozone Study
Newark officials and the city's private OM&M firm, Professional Services Group, Inc. (PSG), Houston, Texas, explored the possibility of installing an ozonation system at the Pequannock Water Treatment Plant. In order to determine the feasibility of such a system, an on-site pilot test was conducted.
The private firm was responsible for overseeing the design, installation and operation of the pilot ozonation system at no cost to the city for personnel or equipment. Through its corporate affiliates company, Compagnie Generale des Eaux, Praxair-Trailigaz and Metcalf & Eddy, Inc., PSG had access to advanced technologies and the engineering expertise necessary to install and test the customized pilot system. INUSA Instruments supplied the necessary instrumentation to measure the ozone and the ozone residual during the pilot study. PSG conducted the microbiological analyses and the City of Newark provided the organic analyses through a private laboratory. The study was initiated to determine the most effective dosage, contact time, D/DBP effects and general microbiological effectiveness associated with ozonation. The study was conducted over a three-month period in early 1997.
The ozonation system was assembled in the plant's pretreatment screen building, where a raw water supply was readily available. This system was designed to be simple and functional, while also being easy to dismantle at the end of testing. The pilot plant consisted of a 20-foot, six-inch clear PVC column with a diffuser located at the bottom of the column. Raw water flowed into the column from the top to the bottom, with sample taps located at various places along the column. The instrumentation consisted of a raw water flow meter, ozone generator, ozone feed gas analyzer, ozone analyzer, oxygen generator, high-quality air compressor and related piping. The support structure for the flow meter, contact column and instrumentation was mounted on two, 4¢ x 8¢ sheets of 3¦4-inch thick plywood. The other related generating units were floor mounted. Ozone destruction units were installed on all discharge points on the column and the instrumentation.
The ozone generator used in the study utilized a dry oxygen feed for the production of ozone. The oxygen feed unit is a separate system that consists of an air compressor and the oxygen generator that supplies the oxygen to the ozone generator. Oxygen is introduced at this juncture to the ozonator and distributed in a set of stainless steel cells containing the dielectric tubes where the oxygen-to-ozone conversion is conducted.
Dosages and Contact Times
Contact times of 5, 10, and 15 minutes were selected for the study because they could best represent real-time treatment plant contact times during low, average and high-flow conditions. Three ozone dosage levels were investigated (1 parts per million [ppm], 1.5 ppm and 2 ppm). The dosages selected were based on the microbiological studies done by plant personnel during the pilot unit's startup. Initial biological analyses were completed to determine the most effective dosage and contact times.
The microbiological testing protocol was the Heterotrophic Plate Counts (HPC) that were run on tryptone glucose yeast agar and incubated at 35° C for 48 hours. The coliform analyses were done on the membrane filter method using m-Endo broth and incubated for 24 hours. These analyses were conducted at each dosage level. Testing for HAAs, Total Trihalomethanes (TTHM), Total Trihalomethane Potential (TTHMP) and Total Organic Carbon (TOC) was also conducted at each dosage level. The same analyses were also conducted on chlorinated raw and on the distribution water.
Each run was completed in a day and the samples were submitted for analyses the same day. Three runs were performed each month during the three-month study. Analyses were run on raw water, chlorinated raw water and the plant's distribution water. A raw ozonated/chlorinated sample also was run to determine the effect of chlorine on an ozonated sample. The raw ozonated sample received approximately the same dosage of chlorine (1.0 ppm) that was applied to the distribution water with a contact time of 60 minutes. This gave a free-chlorine residual level of 0.8 to 0.9 ppm to the test samples. No filtration was conducted on the ozonated samples prior to the chlorine addition.
The purpose of the pilot study was to determine if ozonation could reduce the disinfection by-products under consideration for regulation, as well as those currently regulated. The study also addressed the effectiveness of microbiological disinfection.
Analytical data developed from the three runs clearly indicated THMs and the HAAs could be reduced and could meet the proposed and current disinfection by-product rules. The ozonated sample that was chlorinated also met proposed MCL levels for THMs and HAAs. No significant levels of bromates (BrO4) were found. The study also found TTHMP levels were not great in the ozonated and ozonated/chlorinated samples.
The microbiological data indicated that ozone is as effective as chlorine for disinfection. Both the HPCs and the coliform analyses indicated acceptable disinfection. In all cases, the ozonated samples at all dosages and contact times were negative for coliform.
As indicated by the data, ozonation is an alternative disinfectant for the Pequannock Water Treatment Plant. The MCLs for microbiological and disinfection by-products were met at 1 ppm, 1.5 ppm and 2 ppm dosages of ozone and at all contact times investigated. The one drawback of utilizing an ozonation system is its increased power consumption.
This study conducted by the private firm and the city indicates ozonation to be a superior microbiological disinfection system, resulting in the reduction of other DBPs. Other studies have indicated no major health risk by-products are produced by using ozonation as a disinfectant.
A Solution for Others
A pilot study similar to that conducted in Newark also may be useful to other municipalities across the United States facing the same proposed disinfection by-product rules. Since the ozonation system is a portable unit, it can be specifically designed for other water treatment facilities across the country. Many publicly operated water treatment systems could not conduct a study of this depth due to cost limitations and lack of ozone experience. Newark's private OM&M firm was able to save the city approximately $200,000 in costs related to the study.
In the best sense, a municipality and its private OM&M operator are partners with a true mutuality of interest. If a water plant is out of compliance, neither party's interests are served. Working together, the City of Newark and the private OM&M firm anticipated potential compliance problems rather than responding to them after the fact.