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What comes out of the end of the pipe of your onsite treatment facility may seem to be clean. But even with the best technology, sometimes pathogens—bacteria, viruses and protozoans—will still be present. Selecting the right system to disinfect your discharge can be a challenge, especially when selecting between chlorination, ozone, ultraviolet, or chemical. But understanding the mechanics of disinfection and the various benefits and costs of each system can help you make a choice that affects both the green in your wallet and the green in your community.
Like a good video game, the mission for your disinfection death ray is the inactivation or destruction of pathogenic organisms that have the potential of wreaking havoc on unsuspecting residents by transmitting life-threatening diseases. Like miniature aliens from another world, these pathogens have the ability to multiply rapidly, given the right environmental conditions, and can enter the human body through ingestion, inhalation, or skin contact. While there is no electronic score for each kill and you won't have the opportunity to move to the next level, if you're successful in your mission, you will have saved humanity from the ravages and miseries on a scale of galactic proportions.
Pathogens in the Pipe
Understanding how to disinfect the water coming from your treatment system requires knowing your enemy. Pathogens are any microorganisms that can cause disease to downstream users and the environment. There are three general categories of pathogens that are most commonly identified and associated with waterborne diseases. These include bacteria, protozoans, and viruses.
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Bacteria are unicellular organisms that are found throughout the environment and are responsible for a variety of processes and functions, from the manufacture of various food products to nitrogen uptake by plants. Only a small percentage of all bacteria present in the world cause diseases. Wastes from warm-blooded animals are a primary source for bacteria that are found in wastewater. Some of the pathogenic bacteria of concern to water quality include Esherichia coli, which can cause gastroenteritis, whose symptoms include vomiting and diarrhea; Salmonella typhi, which causes typhoid fever; and Vibrio cholerae , which causes cholera.
Anyone who had high school biology probably observed protozoans under a microscope sampled from a pool of stagnant water. There are approximately 35,000 known species of protozoans, occurring primarily in aquatic environments, with nearly 10,000 being known to cause disease. They exist in the environment as cysts that hatch, grow, and multiply after ingestion. The cysts provide a protective shell to help the protozoans to survive harsh conditions such as high temperatures or salinity. According to the EPA, two waterborne protozoans of concern are Giardia lamblia , which causes giardiasis, with symptoms that include mild to severe diarrhea, nausea, or indigestion; and Cryptosporidium parvum, which causes cryptosporidiosis, whose symptoms include diarrhea, and can cause death in susceptible populations such as those with weakened immune systems.
Viruses are technically not living things; they cannot survive without a host organism. These infectious agents are particularly difficult to manage due to their ability to mutate readily, which is of major concern to world health managers each year at flu season. The most significant virus group affecting water quality and human health originates in the gastrointestinal tract of infected individuals and is excreted in feces. Viruses include Hepatitis A that can cause jaundice and fever, and Enteroviruses including those that cause polio, encephalitis, and conjunctivitis.
Selecting Your Weapon
Selecting the right disinfection weapon requires understanding the factors governing the particular site and the wastewater to be treated. In general, the selection of an appropriate disinfection system should be evaluated against the following six criteria:
- Safety . How does the disinfectant work and what types of precautions are needed to transport, store, use, and operate the disinfectant system and associated chemicals? If a system will require significant safety protection—such as use of breathing apparatus and protective clothing—as well as high levels of operator training, it may be advisable to explore other, less intensive systems. In addition, while the disinfectant may be relatively safe to use, consideration also has to be made for the effects of both intentional and unintentional releases to the environment.
- Effectiveness . How effective is the disinfectant against the pathogens present in the wastewater? Since the intent is to reduce the levels of pathogens to acceptable standards, understanding how effective the proposed disinfectant system is in achieving those target levels, as well as the system's ability to reliably achieve the result, will be important to selecting the right system.
- Cost . What are the costs associated with the disinfection system, both in terms of capital outlay and ongoing operations and maintenance? Operating costs can vary in terms of the time it takes to service the disinfectant system regularly, and the costs of supplies and components.
- Complexity of use How does the system operate and does it take specialized training to keep the system within tolerances? Since the outflow from the treatment facility may be subject to various standards and regulations, if the system is too complex it may require additional staff time to ensure that it operates within the desired parameters.
- Environmental/Adverse Effects. What are some of the potential downsides to the operation of the system as it relates to the watershed in which the treated effluent is discharged? While some systems may provide a net-positive environmental benefit through increased oxygenation of the receiving waters, other systems may need to have additional treatment of the disinfected effluent in order to render it benign when released.
- Flow and Wastewater Characteristics . Can the system handle fluctuations within the flow or with changing characteristics of the wastewater being processed? If a system has a narrow tolerance for the amount of wastewater flow, this could impact the effectiveness of the overall system. In addition, if the system cannot adjust for off-site concerns such as dry or wet weather flow rates of the receiving water body, this may also effect the system's appropriateness for your application.
With those criteria in mind, there are primarily four basic disinfection systems currently available—chlorination, ozone gas, ultraviolet radiation, and chemical treatment other than chlorine.
Chlorine Systems
According to the EPA, chlorination systems for disinfection are the most widely used for treating wastewater because they destroy the organisms through oxidation of their cellular material. In other words, the chemicals eat through the skin of the pathogens, killing them. Chlorine is available in several forms, including chlorine gas, hypochlorite solutions, and other compounds in either solid or liquid forms. Anyone who has a swimming pool is familiar with the use of chlorine as a disinfectant, but the formulation for use in a swimming pool is significantly different from that used in onsite treatment operations.
"It's kind of an interesting reaction as you get in the wastewater," says Donald A. Bach, vice president of the Chemical Division of NORWECO in Norwalk, OH. "It obviously is a lot different than swimming pools or drinking water treatment. Wastewater is a lot dirtier. When you put chlorine in the water, the first thing that it does is react with the ammonia, which is very common in wastewater. So that's an instantaneous chemical reaction of converting the ammonia into the chloramines. That reaction is usually called chemical chlorine demand. Once that reaction is complete, whatever leftover chlorine you've got is going to do the biological chlorine demand; in other words, oxidize the membranes of the bacteria that's in the water, the E. coli or coliform, anything that comes from human [waste]. After that reaction is complete—after about 40 minutes—if you have any chlorine left, then you start to attack the viruses, such as smallpox. So after the bacteria, you then destroy the viruses if it's [the chlorine] in there long enough. Following the viruses, if you have enough chlorine in detention time left, then you can get into the real strange ones, like cryptosporidium . They're the last on the chain that are destroyed by chlorine. Typically in onsite wastewater treatment, we don't chlorinate to the point that we get into the cryptosporidium because you run out of chlorine beforehand or it's discharged. But that is your process."
For most onsite treatment facilities, the most common type of chlorination system is the tablet chlorinator. This system is relatively low-cost, doesn't require power and control systems and can be sized for flows from less than 100 gallons per day to a million-gallon-flow per day. "It used to be that tablet disinfection basically was used from less than 100 gallons a day up to about 100,000 gallons a day," states Bach. "Then we went into liquid bleach, which started at about 50,000 gallons—so there's an overlap—and went up to a half million. Then after that half million, everybody used gas—and that was all an economic decision. However, nowadays it's becoming a safety decision. Gas chlorine is considered a weapon of mass destruction. It's not unusual nowadays for us to do a tablet chlorination system for 1-million-gallon-a-day flow, because it's easy, it doesn't cost too much, and the product is safe."
Ozone Disinfection
The next system that is used commonly to disinfect wastewater is ozone. Ozone is a strong oxidant and virucide and is produced when oxygen molecules are dissociated by an energy source into atoms and subsequently collide with another oxygen molecule. Ozone is a highly unstable gas with a half-life of 20 minutes prior to decomposing back to elemental oxygen. The significant advantage of ozone is that it disinfects by directly oxidizing or destroying the cell wall, causing leakage of the cell's contents, as well as causing damage to the constituents of nucleic acids. Ozone is literally the "Terminator" of bacteria.
Some of the advantages of ozone: it is more effective than chlorine in destroying viruses and bacteria; it only requires a short contact time with the wastewater to be effective; and there are no harmful residuals that need to be removed after ozonation because the ozone decomposes rapidly. Plus, ozone has the ability to raise the dissolved oxygen content of the effluent and can eliminate the need for re-aeration. The downside is that ozone is a tremendous oxidizer and any piping or tanks used for ozone should be made of a corrosion-resistant material such as stainless steel. Since ozone is usually generated onsite by the application of electrical current, the cost and demand for electricity will be higher by the use of ozone.
Most ozone systems are custom-designed around a standard model. "It's usually stock equipment that you custom-design for the system," says Barbara Schilling with Ozonia North America (Elmwood Park, NJ). "It is a system, not just a piece of equipment. So the system is put together in a way that the customer or the engineering company sets forth. In ozone systems, the way that you can tell, for instance, if things are working the way you want is you monitor ozone dosage to the liquid. You can monitor and measure either Oxidation Reduction Potential, TOC, or COD. You can measure ozone off gases from a tank. You just have to put proper instrumentation that tells you that your practices are where you wanted to be. There are real time and laboratory tests that you can do."
One of the advantages of the use of ozone is that it can be produced onsite, thus eliminating the need to transport a hazardous material. "Ozone is generated from oxygen," Schilling says. "That means either from the oxygen component of dry air or from concentrated oxygen that is fed to your ozone generator. It comes discharged from the ozone generator as a percentage of ozone and a percentage of feed gas. So it's not 100% ozone; it's a percentage of gas from an ozone generator. That gas is then applied to either a liquid or they'll use it as gas to do what they want to do. For instance, often in odor control it's used as a gas, but in the wastewater stream it's injected typically into the water via a conductor. Depending on what the system is, it's either used by itself with a certain retention time or it's used after a certain amount of time that it goes into a UV system."
Ultraviolet Disinfection
UV disinfection transfers electromagnetic energy from a mercury arc lamp to a pathogen's DNA material, thus affecting its ability to replicate itself. UV's effectiveness depends on the characteristics of the wastewater, the intensity of the UV radiation being emitted, the length of time that the wastewater comes in contact with the UV radiation, and the arrangement of the UV reactor.
UV has the advantage of being effective at inactivating viruses and because it's a physical process rather than a chemical process, there are no residual constituents remaining in the treated wastewater after exposure to UV. Also, the contact time for the wastewater with the UV source is the shortest of any of the disinfectant strategies, lasting no longer than 20 to 30 seconds. Disadvantages include the effects of turbidity in the water reducing the infiltration and therefore the effectiveness of UV and the need to provide an effective cleaning and replacement program for the UV components.
"If you just need pure disinfection you probably would tend towards UV rather than ozone," says Schilling. "The cost of the UV just for disinfection is usually less than ozone, and the amount of equipment needed is less. But every waste treatment has to be looked at individually in order to get what you want. Sometimes you cannot use UV because the waste treatment is too turbid; if you can filter it to get the turbidity levels down, maybe you'll use UV."
Primarily, there are two designs for UV systems. One system involves a non-contact design in which the UV-light system is suspended away from contact with the wastewater. The second system is the contact reactor-type design in which the lamps are encased in a quartz sleeve that is submerged in the waste waster. This contact-type system is further separated by whether it is an open- or closed-channel system. The open-channel system submerges the lamps in either a horizontal or vertical arrangement. A closed-channel system is within a sealed chamber that can be used in a pressurized system. The open-channel system is mostly used in wastewater treatment.
Ensuring that the UV maintains good contact with the water requires control of the water level within the channel to ensure that the UV is making total contact at the designed depths. Also, because of the heat generated by the electric components of the system, adequate ventilation and cooling must be applied to the UV arrays to reduce heat build-up, otherwise the ballasts could fail. UV lamps have a rated life of up to 14,000 hours, and should be routinely replaced at 12,000 hours or roughly every 1.5 years of continuous operation. The electrical consumption of this system, combined with the cost of routine replacement of ballasts and shields, should be considered against other systems.
Chemical Treatments
Peracetic acid (PAA) is the latest product in the disinfectant arsenal. It is a very strong oxidant and has the added advantage of controlling odors in sludges. PAA is formed through an equilibrium mixture of hydrogen peroxide and acetic acid. This mixture is highly unstable and explosive when transported, so it must be prepared onsite to be used. When exposed to wastewater, PAA decomposes into acetic acid, hydrogen peroxide, and oxygen—all easily biodegradable substances.
Maintenance for PAA should include inspection of all supply and storage lines for leakage and spills; all spills should be cleaned up immediately upon discovery. Due to the explosive nature of the chemical agents, fire codes require the installation of suppression systems. Other restrictions include special protective clothing and positive-pressure breathing apparatus to be worn by all workers handling PAA.
Deciding to Disinfect
A variety of factors come into play in deciding which type of disinfectant system is right for your operation. The decision to install a system could be the result of local concerns and potential to mitigate health risks, as well as improved community relations. In any event, the operator of an onsite wastewater treatment plant needs to consider some of the safeguards that need to be in place as well. "Typical safeguards include operator training and instrumentation monitoring that will perform a shutdown function if something goes above a certain level," says Schilling. "If they detect [for example] an ozone leak, you can do an interconnect and do a plant shutdown. UV has safeguards where you have monitors that tell you what your dosage is, and if you're over or under your dosage it will perform some kind of warning of whatever you want to do."
State and local regulations vary considerably in their requirements to disinfect wastewater, so the decision of what type of system to use can be affected by the chemical and physical composition of the wastewater stream, the environment to which it will be discharged, and the concerns of the local health department. "It's all over the place," says Bach. "The chemical itself is a pesticide and is regulated by the US EPA. The states will specify what sort of E. coli or coliform counts you're going to have on discharge, and the regulations vary all over the map in the states. You've got a lot of things like that throughout the country and it goes down to ultimately the views of local health departments and reflects the local topography, their local population density, and also their experience of whether or not people have gotten sick."
LYNN MERRILL is a consultant based in southern California.
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- September/October 2005
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