Basics of wastewater disinfection

• Feb 01, 2001

Requirements for a disinfectant used in wastewater treatment include producing an effluent that meets U.S. Environmental Protection Agency and state regulations and having low or no disinfectant residual. In addition, the disinfectant should produce little or no disinfectant byproducts, have minimal impact on the receiving stream, be readily available, easy to handle, easy to feed or use and cost effective. The options of disinfectants available today present the designers and operators of the facilities with a series of choices. Each offers some - but not all - of these features in one package.

Disinfection technologies used in wastewater treatment include: UV light, calcium hypochlorite, sodium hypochlorite (bulk supply), sodium hypochlorite (on-site generated), chlorine gas, chlorine dioxide and ozone. Chlorine compounds have historically been the most widely used disinfectants in the United States and the world. Under the lead of the Water Environment Federation's (WEF) Disinfection Committee, the wastewater treatment industry conducted studies on disinfectant usage at wastewater treatment plants in 1979, 1989 and 1994. Their results have been reported by the committee in the WEF journals and summarized here in Table 1. These data provide information that shows the usage of these various disinfectant choices and suggests some trends that are developing.

Based upon Table 1, one can surmise that interest in the use of chlorine gas is decreasing and is being replaced by other chlorine compounds (sodium hypochlorite, calcium hypochlorite or chlorine dioxide), ozone or ultraviolet light (UV). Chlorine sources, however, still maintain a formidable position as the primary disinfectant. In fact, in wastewater treatment, use of chlorine based disinfectants remain the first choice by far. The use of UV light has been increasing (Table 1), which has been helped by regulatory concerns of excess chlorine residuals, presence of disinfection byproducts in the effluent, and safety concerns caused when some forms of chlorine are used.

The type of chlorine compound used depends upon product availability, system needs and operator capability. In general, the chlorine compounds used in today's regulatory environment must be automatically controlled in a reliable fashion and, more importantly, do not complicate downstream water treatment. In Table 2, data from the surveys suggest changes are continuing in control of feeding methods for chlorine compounds. Although the results indicate that manual feed is still being employed, there is a distinct increase in use of automatic control methods.

Most states now require a chlorine residual in the effluent less than 0.05 milligrams per liter (mg/L). This has resulted in the increased practice of dechlorination of wastewater plant effluents. This practice also increases the cost and complexity of the plant. Reducing compounds (e.g., sulfur dioxide or any sulfite solution) provides improved control of chlorine residual. By 1996, almost 75 percent of the wastewater plants using chlorine compounds for disinfection used dechlorination for some form of dechlorination feed control. Compared to 45 percent in 1989, this shows a major turnaround by the wastewater industry in a short period of time.

Of the available technologies, UV Light is a physical process, while the others are chemical technologies. Any of these technologies should be used only after some form of primary/secondary pretreatment. With UV, tertiary treatment/filtration is preferable.

Pretreatment may involve the addition of flocculating agents, settling and removal of sludge, aeration or a biological process; it is always performed prior to the disinfection step at discharge. The following provides some details on each of the more common disinfection methods.

Ultraviolet light disinfection systems use an ultraviolet lamp as the light source. The lamps used are most often mercury vapor lamps that emit a wavelength of 253.7 nanometers (nm). This concentration of energy from mercury vapor lamps is within the known biocidal range of 240 to 280 nm. The UV lamp is surrounded by a quartz sleeve, which allows the energy, generated to penetrate the wastewater surrounding the lamp while protecting the lamp from direct contact with the wastewater. Other UV lamp types are also being used.

UV wastewater systems use an open channel with the lamps submerged below the water level. The amount of UV exposure required is a function of lamp output, and exposure time is usually measured in microwatt-seconds per centimeter squared.

With UV lamps, there is no concern of exposure to the handling of chemicals. Only a source of electric power is necessary. Care must be maintained so that the operator is not exposed to the UV rays, which could cause blindness. The quality of the water being treated can impact the performance of UV units. Wastewater systems that are cloudy or have turbid discharges from the process will prevent penetration of the water by the UV irradiation. An example is water containing calcium or magnesium compounds that would form oxides on the surface of the lamp sleeve due to heat generated by the lamps.

High wet weather flows and biological upsets can particularly be a problem for UV. Cleaning in place with mechanical wipers can be practiced or the racks of lamps can be removed and washed in a light acid system for cleaning. At this time there appears to be no byproduct formation. Use of UV does not create a disinfectant residual for wastewater systems, which is considered a plus.

Sodium hypochlorite, sometimes called liquid bleach, is available in bulk or is generated on-site. Bulk solutions can range from 5 to 12.5 percent strength. On-site generated solutions are about 0.8 percent strength. The bulk solution is classified as corrosive, however the dilute (0.8 percent), site-generated hypochlorite is not. All materials used for piping must have sufficient chemical resistance and mechanical strength. Typically, polyethylene (PE) or polyvinyl chloride (PVC) is used for piping, tankage and instrumentation protection. When used, iron or steel materials must be protected or lined with a corrosion resistant material, such as rubber.

All hypochlorite tanks or shipping containers must be provided with containment, such as a dike or suitable basin large enough to hold any accidental releases, except in the case of on-site generated solution. Since the bulk solutions of sodium hypochlorite are 88 to 95 percent water, and on-site generated solutions are 99+ percent water, possible freezing conditions must be reviewed. Outdoor installations in areas that can experience temperatures below 0 degrees Celsius (32 degrees Fahrenheit) should provide for heated tanks and traced and insulated piping. Heat is required not only to protect from freezing in the tank and piping, but also to aid pumping, since low temperatures will affect viscosity. Twelve percent solutions of hypochlorite will freeze at about -15 degrees Celsius (5 degrees Fahrenheit).

Caution should be observed with the storage of large amounts of bulk commercial sodium hypochlorite solutions since these solutions decompose over time and can produce undesirable chlorates. Decomposition is not an issue with site-generated hypochlorite due to its lower concentration. The amount of bromate in sodium hypochlorite solutions contributes to the formation of trihalomethanes (THMs). Presence of chlorate and bromate needs to be avoided when disinfecting drinking water, although there is less concern in wastewater. Workers that handle sodium hypochlorite should wear rubber gloves and goggles to protect them from exposure to liquid when servicing, filling, unloading or otherwise using the chemical.

Calcium hypochlorite, sometimes called powder bleach, is 65 to 70 percent strength and is available either as a powder, tablets or briquettes. If the powder form is used, calcium hypochlorite must be made into a solution for use. Mixing tanks and feed tanks are required to form the solution. The solution formed from the powder contains solids and can form calcium carbonate that can impact the ability to pump. As a result, these solutions are normally decanted from the mixing tank to a feed tank so that the byproduct solids are minimized in the feed tank. Calcium hypochlorite solutions act similarly to sodium hypochlorite solutions in that they can decompose to chlorates, but at a slower rate.

Compressed tablets or calcium hypochlorite use erosion type feeders or a newly developed spray feeder. Storage and handling of the tablets is easier and the operation of erosion type feeders has no moving parts. The compressed tablets/briquettes are less prone to decomposition than the powder type.

The quantities of chlorine needed for small wastewater systems are supplied in cylinders of 45 or 68 kilograms (Kg) (100 or 150 pounds) containing pressurized liquid chlorine, also known as elemental chlorine. Larger containers of 908 Kg (2,000 pounds), known as ton containers, are also available.

Most systems feed gas from the source under a vacuum through container-mounted gas feeders. These systems use plastic piping, either PE or PVC for this purpose as long as the gas pressure is less than 41 kPa (six (6) psig). Chlorinated polyvinyl chloride (CPVC) has been used in some cases. If gas is to be fed through a pressure manifold to the gas feeder, the manifold piping must be carbon steel, schedule 80. Flexible connectors, used to connect the cylinder to the pressure manifold, are usually copper tubing, although other suitable materials, recommended by the Chlorine Institute (CI), have been used. Chlorine/water solutions formed in the feeder system for addition to the wastewater use rigid PVC piping, or rubber hose, designed to handle the system water pressures.

Any system using chlorine gas, sodium hypochlorite or calcium hypochlorite should expect to have a disinfectant residual in the distribution system. The residual can be either a free chlorine or combined chlorine. Chlorine gas degradation is not a problem. Safety concerns over gas handling are the primary drawback to its use versus alternative chlorine sources.

Chlorine dioxide

Chlorine dioxide has been used in water treatment for 50 years. It is a strong selective oxidant, an excellent disinfectant (bacteria, viruses, cysts), does not react with ammonia, is basically unaffected by pH and will not form chlorinated organics. Although these characteristics would seem ideal for wastewater disinfection, few municipal wastewater systems currently use chlorine dioxide. The drawbacks to chlorine dioxide are high chemical cost, complexity and volatility.

Chlorine dioxide gas or solutions are not sufficiently stable to be transported and therefore must be generated on-site, typically by combining sodium chlorite with chlorine gas. Chlorite/hypochlorite/acid and chlorite/acid systems are also available.

The application of chlorine dioxide must also be supported by analysis of the generator's performance and residuals. In general, the analytical procedures are adequate but time-consuming. The analysis is complicated by the fact that as many as five different residual oxidants (ClO_2, ClO₂-, ClO₃-, HOC1, and NH2Cl) may be present at the same time.

In an EPA evaluation, chlorine dioxide and chlorine were found to disinfect several activated sludge and nitrified effluents at roughly an equivalent rate and dose. In a filtered nitrified effluent, chlorine dioxide performed better than chlorine. Chlorine dioxide is also at least five times more effective against the pathogens giardia and cryptosporidium. The cost of chlorine dioxide however, is about ten times that of chlorine.

Control and addition of chlorine dioxide may also be problematic. Unlike most water treatment plants, flow and quality variations in wastewater effluents will require a wide ClO₂ production range. Turn down on conventional generators is limited, and the efficiency can drop off dramatically at lower production rate. In solution, chlorine dioxide is a dissolved gas. The combination of mixing requirements and high dose may result in ClO₂ off gassing. A covered contact tank may be required.

Ozone is a disinfection technology of interest particularly since there are no formations

of the potentially hazardous residuals, THMs. It has been used for years, mostly in Europe, for the disinfection of water supplies, but its use for disinfection of wastewater has been infrequent. Ozone (O₃) is a highly reactive gas. Like chlorine dioxide, it must be generated on-site as it is needed. All that is required for the generation process is a source of clean, dry air or oxygen, which is passed between electrodes across which an alternating high-voltage potential is maintained.

Ozone can be used for a host of purposes including odor control, chemical oxidation and disinfection. Some advantages to using ozone include: fewer safety problems that typically result from transportation and storage of disinfectants; excellent virucidal and bactericidal properties; up to two thirds shorter treatment times than with traditional disinfectants; and wastewater quality improvements (including effluent color, odor and turbidity reductions).

Authorization of the Safe Drinking Water Act in 1986 led to an increase of ozone use in water disinfection, because of its efficacy against viruses, bacteria, and protozoa and the reduction of disinfection byproducts. However, to be effective and economical ozone requires high quality effluent, which makes it difficult to use in wastewater applications. Also, ozonation is energy and capital intensive and much more expensive than traditional disinfection methods.

The choice of a disinfectant for wastewater systems must meet a set of objectives. The disinfected effluent must leave the water safe for the receiving stream to absorb. The disinfectant must minimally impact the flora and fauna of the receiving stream and must act as a barrier to the transmission of disease to any downstream drinking water intake or recreation area. It must also create a minimal amount of byproducts, meet local regulations, be cost-effective, simple to operate as well as safe to use, transport and store.

To determine the disinfectant system that is best for a given installation, a matrix can be established so that the relative merits of each disinfectant are determined by a scaling system, such as that shown in Table 3. Disinfectants are set in the columns and the features or benefits set in the rows. Each section of the matrix can be filled in with one of three values: (-1, 0 or +1). These numerical values represent the contribution that the disinfectant would produce for the system. The values represent whether the result is undesirable (-1), has no impact/not relevant (0) or whether the result is desirable (+1).

The values are totaled and the result provides a numerical value that would provide the designer or owner with a method to use for choosing the disinfectant. The fact that the total contribution from each disinfectant is not weighted on a scale from one to 10 removes the emotional impact on the choice. Table 3 provides a grid for a typical system for wastewater with a flow rated at two million gallons per day (MGD) (3,150 cubic meters per hour (M³ / h)).

All of the technologies can be used in the system but some are more suitable than others depending upon local conditions. The availability and capability of the operator is most important for small systems, and some technologies are more readily accepted under this condition - UV and calcium hypochlorite tablets, for example. If no power is available, calcium hypochlorite may be the choice. Sodium hypochlorite and chlorine gas are also reasonable options.

The example may not include all of the choices; users of this technique may add different choices or requirements. After the numerical value is established, the user should add the most important needs of his or her system. Additional requirements that could be included in the matrix might include:

• Operating cost

• Capital cost

• Maintenance cost

• Servicing requirements

• Chemical delivery time

• Training needs

• Controls required

Other requirements or similar factors can be added to the grid as the user/operator/designer wants. At the end of the day, more points only help make a choice of the desirable system simpler.

1. Johnson, B, Wastewater Disinfection - Current Practices, JWPCF, 52 (7), pp1844-1868, 1980.
2. Ross, M., et al, 1997 Assessment of the 1996 Disinfection Practices Survey, Report of the Disinfection Committee of the Water Environment Federation, Presented at WEFTEC, Chicago, 1997.

• Reference 2.

This article originally appeared in the 02/01/2001 issue of Environmental Protection.