The First Line of Defense
Disinfection and monitoring can be used as measures to protect drinking water plants against terrorism
- By Yong Kim, PhD
- May 01, 2003
Public drinking water plants seeking to guard against the threat of terrorist attacks might do well to review and enhance some of the technology they already have on site.
Namely, disinfection facilities and monitoring systems should be considered as potential tools that -- properly applied -- can be effective in reducing the impact of criminal chemical or biological contamination. In adequate measures, chlorination or other disinfection technologies, such as enhanced ultraviolet light (UV), can destroy biological contaminants. Additionally, comprehensive monitoring of water quality can provide early and accurate detection of a breach in any treatment process and is essential to engage countermeasures.
Over the years, the drinking water industry (approximately 168,000 public water systems nationwide) has spent considerable resources learning and applying techniques to make plants safer. However, with the possibility of random terrorist attacks affecting thousands of people in public places, our industry must adjust its tactics accordingly. The traditional "dilution is the solution" approach is not adequate in the face of this new threat.
The first step in developing security and control schemes at a water plant is to identify the points of high vulnerability. In this assessment, managers can collect and review all the information required for implementing and modifying the existing security system applicable to the facility. These physical and electronic measures include controlled access to the facility, adequate lighting, visual or audible alarm systems, closed circuit television monitoring and more. Plus, many vulnerable points within the plant that are open to personnel -- such as clarifiers, chemical storage tanks and clear-wells -- can be made more secure.
Furthermore, there are conventional processes in the water treatment plant that can protect public health from chemical and biological contamination. Physical removal of contaminants by clarification and filtration are prime examples. Well-operated, they can remove several orders of magnitude of viruses, bacteria and other protozoa.
Disinfection: Even More Critical
The major barrier to biological agents, however, is disinfection. Chlorine has been used to disinfect drinking water for more than 100 years, and is responsible for the dramatic reduction of waterborne disease, such as cholera and typhoid, that killed tens of thousands people in the early 1900s. It is currently used to disinfect over 95 percent of the treated drinking water in the United States, but may be even more important in this new era of water security. Chlorination also gives the flexibility for achieving higher levels of treatment, depending on the degree of contamination of the source water.
In an important study of potential waterborne threats to the drinking water industry, Burrows and Renner assessed 18 infectious agents and 9 biotoxins, including Anthrax, Variola (Smallpox) and Tularemia, which are classified as category A biological agents of high concern by the Center for Disease Control.1 They reported that the vegetative forms of B. anthrax and P. tularensis are easily inactivated by chlorine disinfection under field conditions; the effect on Variola was not discussed. Although they recommended microfiltration or ultrafiltration for more complete removal of infectious agents, the ultimate protection against infectious agents is chlorination at the conventional water treatment plant.
For the last 80 years, the chlorine industry has worked towards making the feeding of gas chlorine the safest possible. However, in this era of heightened concern about terrorism and criminal sabotage, managers at public utilities may have even more reason to pay serious attention to the heightened security around the chlorination process. All personnel at the utilities should be well informed of the characteristics and health effects of chlorine gas, as well as proper procedures to follow in case of emergency. There should be intensive training programs for proper transportation, storage and handling. Physical protection of the site must be secured in the form of fences, video cameras and alarm systems.
Alternative Disinfection Methods
Although gas chlorination continues to be a major source of guarding public health from biological contaminants, the water plant may seek other alternative disinfection processes to eliminate the risk of chlorine tanks being a target in the wake of unprecedented terrorism.
On-site generation of chlorine from brine or seawater is one of many ways of reducing the inventory of hazardous chemicals at water utilities, as well as maintaining the availability of disinfecting chemicals. The first working systems were introduced as early as the 1930s, but the cost of equipment and the troubles associated with electrodes were major problems. For the last two decades, however, interest in this system has been growing with the advancement of technology. It is becoming popular as it limits the exposure of operating personnel to chlorine gas or other hypochlorite compounds. It has a definite advantage over the use of sodium hypochlorite solution because commercial hypochlorite solution will degrade in the storage tank. Degradation is significant: eight percent decay in two weeks and nearly 50 percent degradation in three months, in which the temperature and pH become accelerating factors.
The principle of an on-site hypochlorite generation system is straightforward. Brine solution goes through electrolysis to produce sodium hypochlorite with electrical energy:
NaCl + H2O + Electricity = NaOCl + H2
Supply water is softened and combined with the saturated brine solution to form a three percent brine solution that enters the electrolyzer. The brine solution, being a good conductor of electricity, supports an electric current applied between the electrodes, thus electrolyzing the sodium chloride solution. Chlorine gas (Cl2) is produced at the anode, while sodium hydroxide (HNaO) and hydrogen gas (H2) are produced at the cathode. Chlorine gas further reacts with the hydroxide to form 0.7 percent to 0.9 percent sodium hypochlorite. The hypochlorite solution, together with the hydrogen gas produced during electrolysis, discharges into a solution storage tank. To ensure that the hydrogen gas is diluted and removed, an air blower is used to force vent the storage tank.
In the United States, UV technology has been widely used for the disinfection of municipal wastewater (over 2,000 systems), as opposed to drinking water. But interest in UV for drinking water disinfection in North America is increasing because of the recent discovery that UV is extremely effective for the inactivation of protozoan such as Cryptosporidium, Giardia and Microsporidia.
These studies demonstrated that 4-log inactivation of these microorganisms can be achieved at relatively low UV doses. This is similar to the dose necessary for the inactivation of bacteria and lower than that for an equivalent inactivation of viruses and bacterial spores. At these low doses, the power cost, which is the main component of the operation and maintenance cost for UV systems, is a fraction of a penny per thousand gallons of water treated. Its efficiency for a wide variety of microorganisms, low operating cost and the fact that it does not form harmful disinfection by-products, makes UV a popular choice for water treatment plants facing the present and future microbial/disinfection-by-products rules.
Commercially available sources of UV light include low and medium pressure mercury arcs. Selection of the proper UV lamp is critical, as lamps significantly differ in UV output (and consequently number of lamps required), energy efficiency number of UV photons emitted in the 230-300 nanometers (nm) range per kilowatts (kW) of input power and expected lifetime. In contrast to chemical disinfectants, UV radiation does not have a destruction effect on microorganisms: the inactivation is due to a specific dimerization of purine bases within the double helixes of the DNA or RNA that carry the genetic information. This dimerization prevents the replication of the genome and consequently prevents the microorganism from multiplying in and infecting hosts.
As mentioned previously, UV has shown to be effective for a wide variety of microorganisms, including bacteria, viruses, protozoans, fungi, yeasts and algae. It can therefore be anticipated that UV will be equally effective for microorganisms that could be used for biological warfare. However, only one study has been published to date that has demonstrated the relative efficiency of UV for bacillus anthracis. 2 UV dose-responses, performed under carefully controlled bench-scale conditions, need to be developed for all microorganisms of concern.
Should chemical or biological contamination occur, there is no better way of getting early warning than the use of online monitoring instrumentation. Effective monitoring can be achieved by many factors, such as types of instruments, monitoring frequency and proper locations.
It is practically impossible to detect all contaminants in a reasonable time period with any single monitoring system alone. The more parameters to be monitored, the better chance there is to detect abnormalities in water quality. Monitoring parameters include both physical and chemical measurements of various constituents in water, and many of them can be monitored on an online basis that will provide early warning of potential contamination. These parameters are pH, conductivity, chlorine residual, ORP (oxidation reduction potential), turbidity, particle count and TOC or ammonia.
- pH/Conductivity: These two parameters detect the change in water due to dissolved substances and serve as an early indicator of changes from baseline. Conductivity is a measure of total dissolved ions in the water that is often called the universal solvent. It is a very simple parameter to measure, and a sudden change in the conductivity may indicate the introduction of hazardous chemical contaminants.
- Chlorine residual: Continuous monitoring of chlorine residual at key locations both at the water plant and in the distribution system is critical to maintaining the proper disinfection of water, especially at this time of possible terrorism. It includes the point of chlorination process, distant branches of the distribution system and storage reservoirs. An increase in demand could result from the introduction of significant amounts of contaminants into the water supply or distribution system.
- ORP: ORP yields very valuable information that is often overlooked in the water industry. Because most chemical reactions in aqueous phase can be explained by the transfer of electrons from one substance to another, change in chemical constituents in the water will result in an immediate change of ORP as megavolt (mV) readings. ORP can be easily measured continuously by using an oxidation-reduction type electrode in conjunction with a reference electrode.
ORP has the unique ability to detect the introduction of a very small amount of chemicals because ORP responses are logarithmic, making them most sensitive at extremely low levels of oxidant and reductant. This logarithmic response is not limited to chlorine or sulfite, but works for many other chemical substances, which places ORP as a critical parameter in detecting chemical/biological contamination of water.
- Turbidity/Particle count: Turbidity is one of the most widely used parameters in the water industry. It does not have any direct relationship to contamination, but can serve as a primitive indicator of water quality. Because particle counters are much more sensitive than turbidimeters to the change in particulate distribution in water, it would provide early detection of filter breakthrough to prevent contaminated water from entering distribution systems.
- Dissolved oxygen (DO): The dissolved oxygen in the water is a major parameter for the survival of aquatic lives and organisms. Contamination by organic compounds usually results in the decrease of DO levels.
- Ammonia: An ion-selective electrode provides continuous monitoring of ammonia content in water. An increase in ammonia level may be the result of an introduction of agricultural or sewage discharge, triggering an increase in the demand of chlorine at the treatment plant. Various ion-selective electrodes can monitor other chemicals of concern such as cyanide, nitrate, nitrite, chloride, copper, lead and so on.
- Total organic carbon (TOC): A measure of the total organic carbon gives an indication of the total organic load of the water. The analyzer measures the carbon dioxide concentration after oxidizing all carbon containing compounds by UV irradiation in the water and can be measured continuously as online instrumentation. Once reference baseline data is established, the change in the TOC can be indicative of contamination from organic compounds.
There is a critical need for rapid online and other field monitoring technologies for detecting and quantifying both infectious and toxic agents for both industrial and municipal industries. The continuous monitoring of water quality has a couple of purposes: one is to ensure that the water treatment process is functioning properly; the other one is to make sure that treated water meets the requirements set by regulations.3 However, since water security issues have risen to the surface, online monitoring is believed to be an excellent tool to detect the sudden change in water quality that may result from accidental or intentional contamination. For this purpose, online monitoring instruments have definite advantages over bench-top or portable instruments.
Since early detection of the change of water quality is critical in responding effectively to potential contamination by biological or chemical agents, it is important to increase the frequency and sites of monitoring throughout the treatment plant. Selection of monitoring locations should be made based on the result of vulnerability assessment. The U.S. Environmental Protection Agency (EPA) recommends raw water intakes, finished water storage reservoirs, distribution system entry points and key monitoring locations within the distribution system. Based on well-established ordinary historical data and normal fluctuations, a sudden increase or decrease of any parameter could be indicative of malicious contamination.
Several emerging technologies are commercially available, and others are still in the exploratory stage. For example, it appears to be feasible to replace the use of the human nose with the electronic nose. That would be useful in an early warning system since it operates continuously with less bias and greater precision. The other area is a rapid detection of bacterial existence in the water. Such a rapid test in the raw water would lead to proper disinfectant dosing. Recent advancement in wireless telecommunication technology also enables water utilities to continuously monitor for sudden changes in water quality anywhere within the distribution network. Data signals from many remotely located monitoring sites can be collected and monitored at a central location through supervisory control and data acquisition (SCADA) systems.
1. Burrows, W.D. and S.E. Renner (1999), "Biological Warfare Agents as Threats to Potable Water," Environmental Health Perspectives, Vol. 107 (12), 975.
2. Knudson, G.B. (1986), "Photoreactivation of Ultraviolet-irradiated, Plasmid-bearing, and Plasmid-free strains of Baciluus Anthracis." Applied and Environmental Microbiology, Vol. 52(3), 444.
3. Henley, M. (2002), "Water Quality Monitoring Remains Important water Treatment Segment," Ultrapure Water, Vol. 19(2), 15.
By permission, adapted from AWWA 2002 Annual Conference Proceedings, copyright 2002, American Water Works Association.
This article originally appeared in the 05/01/2003 issue of Environmental Protection.