Understanding the ins & outs of disinfection technologies for water treatment
Water is a natural carrier of disease-causing pathogens, including many that impact human health, such as E.coli and Giardia. Over the course of human history, various cultures found that treating water could improve human health. The ancient Egyptians, for example, are believed to be the earliest adopters of advanced treatment techniques, using alum to coagulate water and aid in improving clarity and filtration. However, widespread water treatment did not start until the early twentieth century.
In 1906, the number of annual typhoid fever cases in the U.S. resulted in it being declared a nationwide epidemic. Increasing cases of waterborne illnesses prompted many large U.S. cities to filter water on a large scale. With the addition of filtration to drinking water, the number of annual typhoid fever cases dropped from 10,000 to approximately 1,000 in less than 10 years.
However, filtration alone was not adequate to reduce the rate of illnesses. In the mid-to-late 1800s, chlorine was sporadically used to control infection in hospitals and drinking water. In 1914, the U.S. Department of the Treasury enacted a standard requiring a maximum of two coliforms per 100 mL in drinking water, effectively ushering in the modern age of disinfection in the U.S. Chlorination is credited with virtually eliminating waterborne epidemics and with increasing life expectancy by 50%. Disinfection, with its ability to kill up to 99.99% of certain pathogens, is arguably the single largest advancement in the improvement of human health in the history of humankind.
Evaluating the Technology
There are a number of effective technologies available for the disinfection of water and wastewater. Figure 2 on page 37 indicates some of the most common disinfection technologies and their relative disinfection power. The key to choosing the right disinfection option is understanding the treatment process goals.
Chlorine remains the universal and most widely used disinfectant, used by more than 75% of the water and wastewater treatment facilities in the U.S. It generally is administered as chlorine gas, bulk sodium hypochlorite, onsite generated sodium hypochlorite or calcium hypochlorite. Generally, all forms of chlorine eventually dissociate in water into two compounds: hypochlorite (OCl) and hypochlorous acid (HOCl). Although the chemical reaction of each form of chlorine essentially is the same from a disinfection standpoint, the effectiveness, safety and ease of maintenance are different for each delivery system.
In general, chlorine has the advantage of being a strong oxidant that maintains a long-lasting residual, which makes it the most common disinfectant for U.S. water treatment plants. This same quality, however, is a disadvantage in wastewater treatment plants—or water reclamation facilities—where states limit the discharge of chlorine to certain grades of inland surface waters. In these cases, a dechlorination chemical, such as sulfur dioxide, sodium bisulfite or meta-bisulfite, must be used to remove excess chlorine after disinfection prior to the discharge of wastewater effluent to the environment.
Chlorine gas has been used as a disinfectant in water treatment plants since the early 1900s. It is pure chlorine, typically delivered directly to the site in pressurized 150-lb cylinders, 1-ton containers or rail cars. It is the most cost effective, most efficient and easiest method for disinfecting with chlorine. Vacuum-operated feed systems enhance the safety of feeding chlorine gas. Control modes used are flow proportioning, residual control or compound loop (flow plus residual).
Bulk sodium hypochlorite is manufactured at approximately 12.5% to 15% chlorine by weight with a pH greater than 11. Bulk sodium hypochlorite delivery systems often consist of a storage tank, a chemical dosing pumping system with associated valves and piping, and a control method such as flow control, residual control or compound loop (flow and residual). Bulk sodium hypochlorite is classified as a hazardous chemical and requires secondary containment and hazardous chemical manifests. It can cost more per pound of chlorine than either chlorine gas or on-site hypochlorite generation (OSHG) but is perceived as simpler to maintain and operate. Bulk sodium hypochlorite concentration decays over time and higher volumes must be fed to achieve the same result. Chlorates are a byproduct and a concern for the expected new maximum contaminant level (MCL) of 210 ppb.
OSHG is a method of administrating chlorine without the dangers of gas under pressure or the high cost of bulk sodium hypochlorite. An OSGH system uses electrolysis to generate a 0.4% to 0.8% solution of hypochlorite on-site as needed, passing a dilute brine solution through an electrolytic cell in which a current passes through the brine solution and converts the chloride ion from the salt to hypochlorite. The process typically uses
3 lb of salt, 2 kW hours of electricity and 15 gal of water to produce 1 lb of chlorine in 15 gal of solution. This is the equivalent of the active chlorine present in 1 gal of 12.5% bulk hypochlorite or 1 lb of chlorine gas. OSHG systems have moderate maintenance requirements, producing chlorine at about 25% to 60% of the cost of bulk sodium hypochlorite per pound of chlorine produced.
Calcium hypochlorite is a solid tablet form of chlorine, typically 60% available. It is most commonly used in swimming pools and occasionally in water and wastewater treatment plants. Calcium hypochlorite utilizes either a simple tablet feed system or a dilution tank in which the calcium hypochlorite is dissolved into solution then dosed with a metering pump. Because the cost of calcium hypochlorite is expensive per pound of chlorine and is difficult to accurately dose, it often is used in smaller remote water and waste treatment plants where other methods of chlorine feed are not as feasible.
When in the presence of ammonia, chlorine combines with the ammonia to form either monochloramine, dichloramine or trichloramine depending on the ammonia-to-chlorine ratio. Monochloramine is a relatively weak disinfectant, but it maintains a stable, long-lasting residual in water and is therefore used as a secondary disinfection method in systems with a long water age. Chloramine systems can be challenging to control. For this reason, operators must take care to monitor and maintain their dosing systems for maximum accuracy.
Chlorine dioxide is a gas that typically is made on site by mixing acid or chlorine gas and sodium chlorite in a controlled way. The gas then is mixed with the ejector water to form a solution that is applied to the process. A selective and effective oxidant for both water and wastewater, chlorine dioxide is a strong oxidant and disinfectant across a wide pH range. Chlorine dioxide also does not react with ammonia to become a weaker disinfectant like chlorine. This especially is important in systems where the plant has ammonia already present in the water and the operator does not want to remove it or use it to create monochloramines. This use can yield lower operating costs.
Chlorine dioxide is effective as a disinfectant against many types of organisms, including Giardia, Cryptosporidium and Legionella. Chlorine dioxide often is used in water treatment plants as a primary disinfectant early in the treatment process to prevent the formation of trihalomethanes (THM). Since the residual does not last as long as chlorine, it is not used in the U.S. as a secondary disinfectant in potable water distribution systems.
Chlorine dioxide is a highly toxic and unstable gas, and cannot be compressed and liquefied so it is generated and mixed into the water on site close to its intended use. Production is considered simple, safe and reliable.
Ozone comprises three oxygen atoms and is a strong oxidant. It is more effective than chlorine or chlorine dioxide in quickly destroying bacteria and other difficult-to-kill pathogens such as Cryptosporidium. Ozone deteriorates rapidly to oxygen and usually is generated on-site using either air or pure oxygen.
Ozone does not produce disinfection by products (DBPs) associated with chlorine and can be used as a primary disinfectant for water treatment to reduce THMs and DBPs. Ozone also is used for taste, odor and color control in potable water treatment, as well as iron/manganese removal when THMs are a concern. Ozone also can be used to remove micropollutants including pesticides at disinfection dosages.
Ozonation typically is not economical for primary disinfection of wastewater effluent with high levels of suspended solids (SS), biochemical oxygen demand (BOD), chemical oxygen demand, or total organic carbon since the cost of treatment can be relatively high in capital and in power intensiveness. However, ozone can be used for color and odor in the reuse of wastewater effluent and has the advantage that there are no harmful residuals that need to be removed after ozonation because ozone rapidly decomposes and it also elevates the dissolved oxygen concentration of the effluent.
Ultraviolet (UV) light is not chemically dosed, but rather uses the energy form UV light to disinfect water. UV light energy at 254 nm wavelength is absorbed by the DNA of a microorganism, which alters its genetic structure and stops the reproductive process. As a result, the microorganism is non-infective and considered microbiologically dead. UV systems operate at varying pressure and output, depending on application. The required UV dosage can vary significantly depending on the target pathogen and water quality involved.
UV has gained popularity as a disinfectant of choice for wastewater effluent due to regulatory requirements involving more stringent chlorine discharge limits for receiving streams. UV effectively removes pathogens from drinking water including Cryptosporidium, Giardia and various viruses that have proven to be resistant to traditional disinfection methods such as chlorine and filtration.
PAA (also known as peroxyacetic acid), is an organic compound with the formula CH3CO3H. Peracetic acid is a liquid that functions as a strong oxidizing agent, has an acrid odor and also can be used as a disinfectant. PAA is generally commercially available as an equilibrium mixture of 12% to 15% peracetic acid and 18% to 23% hydrogen peroxide. PAA is available in 330-gal totes and in bulk, requires stainless steel piping and is administered using a metering pump.
It has been used for years as a component of antimicrobial washes for poultry carcasses and fruit, and it recently has been tested for the disinfection of wastewater effluent. Since PAA is a highly effective bactericide that does not form DBPs, has minimal dependency on pH and does not leave a residual, it has received consideration for this wastewater effluent disinfection purpose.
A View of the Future
Human-made contamination and resulting regulations continue to play an important role in disinfectant selection, and it is becoming increasingly difficult to achieve all of the requirements for safe disinfection with one treatment alone.
For this reason, a more layered approach for disinfection with multiple technologies being used together has emerged. As an example, a surface water potable water treatment facility may use chlorine dioxide at the head of the plant to gain disinfection credit without creating chlorine byproducts, but also may apply chlorine to the finished water to maintain a residual through the distribution system.
With a clear understanding of the treatment process goals of the project, and knowledgeable partners working together, developing the most appropriate, simple and budget-conscious disinfection plan can be a smooth process.