Choose Your Disinfection Weapon
A closer look at the disinfection alternatives available for wastewater treatment
- By Rudy Matousek
- Sep 01, 2004
The 1972 Clean Water Act (CWA) was created to restore and maintain the integrity of the nation's water. One of the specific goals of the CWA is the complete elimination of pollutant discharge into navigable waters. The U.S. Environmental Protection Agency's (EPA) National Pollutant Discharge Elimination System (NPDES) Permitting Program, which supports the CWA, requires permitting for all point source discharges to U.S. waters (i.e., "direct discharges").
One of the efforts undertaken at a wastewater treatment facility to meet the requirements of the CWA and NPDES program is using disinfection technologies to treat wastewater before discharge. In fact, disinfection of discharged wastewater often provides the first line of defense to protect and ensure safe potable water supplies (surface water or groundwater) and promote healthy rivers and streams. Disinfection may be accomplished by chemical or physical methods. Most municipalities choose wastewater disinfection technologies based on factors such as safety, handling, ease of operation, longevity of the equipment, amount of waste generated, size of the actual system, and capital investment. A concise evaluation of all these factors affects how a specific disinfection technology is integrated into a wastewater treatment facility. Although numerous disinfection technologies exist, five preferred methods are highlighted below.
Chlorine Gas Disinfection
Because of its ability to disinfect pathogens, reduce microbiological contamination, and remove other physical and chemical impurities, chlorine is a common method of wastewater disinfection. Chlorine is known to be effective in destroying a variety of bacteria, viruses, and protozoa, including Salmonella, Shigella, and Vibrio cholera.
When added to water, chlorine immediately undergoes hydrolysis to form hypochlorous acid (HOCl), an active disinfectant. Chlorine gas is either produced at merchant chlor-alkali plants and shipped to wastewater treatment facilities as a liquefied gas in pressurized bulk containers or, less commonly, generated on site at the treatment facility. The important benefits of chlorine gas disinfection in wastewater treatment include:
- Disinfection: destroying a broad range of microorganisms including viruses, bacteria, and some protozoa
- Controlling odor and preventing septicity
- Aiding scum and grease removal
- Controlling activated sludge bulking
- Controlling foaming and filter flies
- Stabilizing waste-activated sludge prior to disposal
- Foul air scrubbing
- Destroying cyanides and phenols
- Ammonia removal
The amount of chlorine required for wastewater disinfection is based on inactivation of total coliform. Inactivation capacity is related to chlorine demand (amount) and the available contact time (CT) for the product, which is known as the CT factor. Because the CT factor is constantly changing due to sewage quality and environmental conditions, most plants operate the disinfection portion of the plant with a chlorine residual that would be in the plant effluent. The negative environmental impact of free and available chlorine has been documented; several studies on aquatic life indicate that 0.3-parts-per-million (ppm) chlorine levels have an affect on larger species and levels as low as 0.03 ppm have an affect on smaller species. The results of such studies, along with state regulations, have prompted wastewater plants to add dechlorination systems to the outfall. Control of zero free chlorine is difficult; therefore, most dechlorination systems are designed with a slight excess of sulfur trioxide (sulfuric anhydride) (SO3-ion).
During the dechlorination process, free and combined chlorine residuals are removed to reduce residual toxicity after chlorination and before discharge. Sulfur dioxide, sodium bisulfite, and sodium metabisulfite are the commonly used dechlorinating chemicals. In all cases, the excess sulfites are immediately oxidized to sulfate at the point of discharge. Sulfur is the fourth most common element in natural seawater.
This broad range of capabilities makes chlorine gas disinfection very cost-effective. However, many municipalities have become concerned about the hazards it presents in transportation and storage, the possible creation of harmful disinfection byproducts (DBPs), and its weakness in inactivating Cryptosporidium and other cysts. Despite the safety concerns, gas chlorination still has the best safety record when compared with the alternative methods of chlorine disinfection.
Sodium Hypochlorite Disinfection
Whether generated on site or shipped in bulk form, sodium hypochlorite (NaOCl) is an excellent alternative to gaseous chlorine disinfection. It is widely considered the second most affordable disinfectant after bulk liquefied chlorine gas. It is commercially available at varying solution strengths, ranging from 15 percent to 0.6 percent solutions, and it offers most of the benefits of chlorine gas as a disinfectant and oxidizing agent, but without the risks involved with transporting or storing hazardous chemicals. Hypochlorite efficacy is due to the common chlorine chemistry, which produces hypochlorous acid when either chlorine or hypochlorite is added to the water as the precursor.
Unlike gas chlorine disinfection and on-site generated hypochlorite, bulk sodium hypochlorite tends to decompose exponentially in storage -- with respect to temperature, age, and concentration. However, when employing an on-site sodium hypochlorite generation system, three simple components -- salt, water, and electricity -- are combined to produce sodium hypochlorite, eliminating the common degradation problems of bulk sodium hypochlorite since the disinfectant is produced on demand.
Although studies have shown that chlorine dioxide is an effective wastewater disinfectant, it is limited in its use as a disinfectant in the United States because it is a very unstable gas that cannot be compressed and liquefied. Chlorine dioxide has been applied traditionally to wastewater as a gas that was generated on site using excess chlorine. However, the recent advancement and introduction of chlorine dioxide generating systems that produce the disinfectant on site, through the common combination of sodium chlorite and hydrochloric acid, has helped to eliminate the concerns traditionally attached to working with this chemical disinfectant. In effect, chlorine dioxide is obtained by either oxidizing chlorite or reducing chlorate. Since chlorine dioxide acts primarily as an oxidizing agent and then as a chlorinating agent, it does not produce trihalomethanes (THMs), which are a group of four chemicals that are formed when chlorine reacts with naturally occurring organic and inorganic matter in water. Trihalomethanes are thought to possibly be carcinogenic.
The benefits of using chlorine dioxide in wastewater disinfection include:
- Does not form THMs
- Constant biocide power in a pH range between 6 and 9
- Does not chlorinate organics
- Readily dissolves in water and does not react with ammonia
- Enhances coagulation
- Does not result in toxicity of wastewater discharge
- Does not form organohalogenated compounds
The downside for all chlorine dioxide systems is that they require handling hazardous chemicals of varying degrees in all cases.
Ozone disinfection is similar in most respects to chlorine disinfection. Ozone is a strong oxidizer and is applied as a wastewater disinfection treatment in gas form, generated on site due to its instability. Ozone can be generated from any gas containing oxygen molecules; the most common sources for ozone generation are oxygen gas or atmospheric air. The mechanics of ozonation disinfect wastewater by (a) direct oxidation/destruction of the cell wall with leakage of cellular constituents outside the cell, (b) reactions with radical byproducts of ozone decomposition, and (c) damage to the constituents of the nucleic acids (purines and pyrimidines).
While ozone offers disinfectant properties similar to chlorine, it does not cause the formation of halogentated organics. Its use in wastewater disinfection is relatively new in the United States, mostly because of the high initial capital costs traditionally associated with ozone generation equipment.
Ozonation advantages include:
- More powerful disinfectant than most chlorine compounds
- Inactivates most strains of bacteria and viruses and is noted for destroying chlorine-resistant strains of both, including being highly effective for Cryptosperidium eradication
- Will oxidize phenols with no negative residuals such as trihalomethane production
- Does not produce a disinfection residual that would prevent bacterial growth.
- Degenerates into oxygen, which can elevate oxygen levels in treated water (does not alter pH of water)
- Increases coagulation
- Helps remove iron and manganese
- Has taste and odor control properties
- Requires short contact time
Ozonation disadvantages include:
- More costly than traditional chlorinated disinfection techniques
- Forms nitric oxides and nitric acid, which can lead to corrosion
- Ozone is chemically unstable as a gas, and hazardous to transport -- it must be generated on site and used immediately
- Can form DBPs if bromide is present in the wastewater
Ultraviolet (UV) radiation has become a reliable method for effectively inactivating waterborne pathogens and viruses. In fact, an increased awareness of the many disadvantages of chemical disinfectants, specifically chlorine, has resulted in the selection of UV as an alternative with many attractive features and benefits. The number of installations utilizing UV disinfection systems for the municipal wastewater market began to rise significantly in the United States during the early 1950s.
By the early 1980s, UV disinfection of wastewater had gained popularity due in part to the heightened regulatory requirements affecting the use of chlorine within the wastewater treatment process. The new regulations required dechlorination prior to discharge, the installation of chlorine scrubbers to protect against accidental release of chlorine gas (Uniform Fire Code), and the development of risk management plans in case of accidental release (Occupational Safety and Health Act).
UV light provides a physical process for the disinfection of water and wastewater without the disadvantages associated with chemical disinfection. In total, it is estimated that over 2,000 wastewater UV systems are in operation in the United States and Canada. As an alternative or supplement to chemical disinfectants, such as chlorine, this technology offers a municipality a number of operating advantages. The equipment is quite safe and easy to operate. It has a small footprint and can be readily adapted to fit in existing treatment facilities. Ultraviolet disinfection works by exposing waterborne microorganisms to UV light in the germicidal range of 250 nanometers (nm) to 270 nm, at a specified intensity for a specified period of time. This exposure renders the microorganism "microbiologically dead" by affecting the DNA in such a way that it can no longer reproduce. UV disinfection has effectively treated certain bacteria unaffected by traditional chlorine disinfectio, and UV disinfection does not produce a residual.
When all is said and done, municipalities and wastewater facility managers must weigh the pros and cons of each technology and consider their unique environmental needs before choosing the technology -- or combined technologies -- best suited for their application.
This article originally appeared in the 09/01/2004 issue of Environmental Protection.