Disinfection Byproducts: Treatment Options And Challenges For Public Water Suppliers
By Shahid Parvez, assistant professor, Indiana University Fairbanks School of Public Health
Regulated and emerging disinfection byproducts (DBPs) pose several challenges for water suppliers, but there is a variety of cost-effective cures.
Most public water suppliers are interested in findings ways to comply with existing regulation on disinfection byproducts (DBPs) at minimal treatment cost. The Stage-2 DBP Rule is pushing water suppliers to find alternatives for maintaining the level of trihalomethanes (THMs) and haloacetic acids (HAAs) below regulatory standards. Several water suppliers preferred to switch to chloramine for secondary disinfection. Although chloramine is a less powerful disinfectant than chlorine, it is adequate to inhibit microbial growth in finished drinking water. Water suppliers love the chloramine approach because it helps them keep the level of THMs and HAAs below regulatory limits and creates low temporal-spatial variability in the distribution system (Parvez et al., 2011). This makes more economical sense to water suppliers than treating water with high levels of THMs and HAAs.
However, chloramine is blamed for producing toxic nitrosamines and extracting lead from old distribution pipes. This is particularly true for water systems with higher pH and lower redox potentials. Some of these problems can be controlled by frequent flushing of the system, minimizing the use of free chlorine, and reducing the water retention time, but they cannot be eradicated. Also, chloramine does not prevent DBP formation in the preceding disinfection stage (primary disinfection) where chlorine reacts with organics (natural organic matters) to form high concentrations of THMs and HAAs.
To eliminate or minimize DBP formation, water engineers need to focus on two fundamental things: First, find ways to incorporate technology for the removal of organics; and second, identify a good alternative disinfectant for chlorine. Fortunately, there are technology and alternative disinfectants available to address both issues. However, it is important to note that no single, stand-alone technology or disinfectant can prevent DBP formation. The important key is to use them in the best possible combination to meet the individual water supplier’s treatment needs. Let me discuss some of the important treatment options and disinfectants that can be useful to minimize DBP formation.
Options For Organics Removal
The removal of precursor organics before disinfection prevents DBP formation. Enhanced coagulation (EC), powdered activated carbon (PAC), and granulated activated carbon (GAC) are effective for removing organic precursors. EC is quite affordable and commonly used by water suppliers. However, this has low organic removal efficiency (45 to 50 percent) and works best when removing negatively-charged, large-molecular-weight organics. Therefore, most water suppliers prefer PAC to boost removal in wet seasons when high organic load is expected in raw water. PAC in general has better removal efficiency than EC, but it works best for low-molecularweight and uncharged organics. A combined EC and PAC approach works best for maximizing removal efficiency (75 to 80 percent) for both high- and low-molecularweight organics (Kristiana et al., 2011).
To achieve superior organic removal efficiency, GAC filters work best and are preferred by water suppliers with high organic load in raw water. Iodine and molasses numbers are typically used to characterize GAC filters. These numbers describe the quantity of small- and large-pore volumes in a sample of GAC. For GAC filters, a minimum iodine number of 500 is recommended by the American Water Works Association (AWWA). The granular volume of the filter allows higher flow rates and provides good molecular adsorption for a range of contaminants. It also improves the taste and odor of drinking water by removing chlorine.
However, high installation, operation, and maintenance costs make GAC unaffordable to many water suppliers. It is also susceptible to biological growth, which helps to degrade organics and other biodegradable compounds, but reduces the performance of the filter. To control biological growth, frequent backwashing (once a week) is required. Disinfection is recommended after GAC adsorption of organics to prevent biological growth in the distribution system. In addition to the technologies discussed above, other treatment technologies include ion exchange and nanofiltration. However, engineers and researchers need to do further work to make these technologies more efficient and affordable for commercial use.
As the problem of DBPs has risen, so has the popularity of UV.
Photo credit: WEDECO – a Xylem brand
Alternative Primary Disinfectants
DBP formation is primarily influenced by disinfectant type, dose, reaction time, temperature, and pH. Chlorine is the most commonly used oxidizing agent for primary disinfection. But its use has been criticized because it produces high amounts of THMs and HAAs. For this reason, many suppliers want to switch to alternative means, such as ozone and ultraviolet (UV) disinfection, which are superior choices for primary disinfection and produce little to no THMs and HAAs. Although the raw drinking water quality, the nature of organic precursors, and the amount of bacterial contamination will rule the decision on disinfectant choice, ozone and UV are gaining popularity among water suppliers.
Ozone has been a popular choice for bottled water suppliers, but most water suppliers hesitate to try this technology for a variety of reasons. The technology is widely accepted in the UK and Netherlands, but the U.S. has some catching up to do. Ozone is a powerful disinfectant and produces very low concentrations of DBPs. However, the issue with ozone is that it does not remove organics from water, which triggers DBP formation after secondary disinfection. Also, it is not a good disinfectant to treat water with high pesticide and bromate contents because it oxidizes them into more toxic epoxides and bromates. It also produces unknown byproducts that trigger microbial growth in GAC. It is recommended to use GAC filters before ozone treatment to minimize byproducts formation and subsequent regrowth of microorganisms in finished water. Consequently, ozone-based disinfection requires periodic replacement of GAC filters, thus raising the overall cost of water treatment.
UV is another attractive alternative for primary disinfection because it produces no byproducts. It is more effective than chlorine in killing cryptosporidium and other pathogens. New York City recently opened the world’s largest UV drinking water treatment facility. This $1.5 billion facility serves 9 million residents and provides treatment specifically for cryptosporidium and giardia. UV works best when combined with high-grade GAC filters to minimize DBP formation. Although the high cost of UV-bulbs has been a discouraging factor for several water suppliers, low maintenance and operational expenses can lower the overall treatment cost. The business of UV-based treatment technology has tripled in the U.S. in last six years and is rapidly gaining acceptance in the water industry. Also, several countries in Europe, the Middle East, and Asia are offering subsidies to promote UV technology in the water treatment industry.
Paradigm Shift
The DBP paradigm is shifting from the nine regulated THMs and HAAs to emerging DBPs such as nitrosamines, halonitriles, haloamides, halonitromethanes, and iodinated aldehydes. With the growing evidence of nitrosamines’ toxicity in humans, they are anticipated to be included in the national drinking water regulations as they are at the state level in California and Massachusetts. Recent studies have found that nitrosamines are more genotoxic, cytotoxic, and carcinogenic than the regulated THMs and HAAs (Richardson et al., 2007). The toxicological review of such studies takes years and will not impact water suppliers in the near future. However, it is possible that some of the emerging DBPs will eventually end up being on the list of regulated DBPs. The treatment difficulties due to the variability in raw water quality, the complex nature of DBPs, and growing health evidence will continue to pose challenges for water suppliers to comply with the regulations. They need to remain in dynamic mode and continue to upgrade their technology to keep up with ever-changing regulations on DBPs in order to protect community health.
References
Kristiana, I., Joll, C., Heitz, A. Powdered activated carbon coupled with enhanced coagulation for natural organic matter removal and disinfection byproduct control: application in a Western Australian water treatment plant. Chemosphere 2011, 83 (5), 661-667.
Parvez, S., Rivera-Núñez, Z., Meyer, A., Wright, J.M. Temporal variability in Trihalomethane and Haloacetic Acid concentrations in Massachusetts Public Drinking Water Systems. Environmental Research 2011, 111, 499-509, 2011
Richardson, SD, Plewa, MJ, Wagner, ED, Schoeny, R, and DeMarini, DM. Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection byproducts in drinking water: A review and roadmap for research. Mutation Research 2007, 636, 178-242.
Shahid Parvez is an assistant professor at the Indiana University Fairbanks School of Public Health. He studies disinfection byproducts for their formation, exposure, toxicity, and health risk. He completed his postdoctoral training at the U.S. Environmental Protection Agency and earned a Ph.D. from the Indian Institute of Technology in Bombay.