Disinfection Protocols
Once water has gone through the major steps of coagulation, flocculation, sedimentation, and filtration, it will appear clean since the particles that cause turbidity have been removed. It’s even safe to say that a large percentage of microorganisms have been removed, as many of those were attached to the particles that were filtered out. However, they are still present - and alive - and present a danger to the health of consumers.
The SDWA demands that standards be met for any public water utility regarding the types and concentrations of potential pathogens in water as it exits the treatment facility, and so disinfection is the final treatment step. In this step, microorganisms such as bacteria, viruses and parasites are killed, or rendered inert.
Still, removing the threat from microorganisms at the water treatment plant is not sufficient, since water can pick up even more as it travels through distribution pipelines. Therefore, the EPA requires that water exiting the treatment plant must retain a certain concentration of disinfectant in the water -- enough to keep it safe all the way to the faucet. There are several ways of accomplishing these ends and, naturally, each method has its own set of advantages and disadvantages.
Chlorine Based Disinfection
In traditional disinfection methods, chemicals like chlorine, chloramine, or chlorine dioxide are added to the water in carefully calculated amounts, which kill germs but are still safe for drinking. Exposure to one of these chlorine compounds oxidizes and destroys the cell membranes of any microorganisms present in the water, killing them. One critical element of this disinfection method is exposure time. In other words, the water must be exposed to the chemical in sufficient concentrations for enough time to completely destroy the pathogens. This is a focus for managers of treatment facilities because it’s not an easy task to accomplish. On the downside, chemical treatment can create taste and odor objections from consumers, and there are potential effects from long-term exposure to the by-products of chlorination. Nonetheless, treatment with chlorine, chloramine, or chlorine dioxide has proven to be effective, widely accepted, relatively simple, and low cost.
Ultraviolet Light Disinfection
Another type of disinfection involves exposing the water to short-wave ultraviolet light. UV is an increasingly popular choice for treatment facilities since UV disinfection equipment requires much less space than other methods and the required exposure time is measured in seconds. Since no known bacteria or virus is completely resistant to UV light, it can actually be more effective than chlorine treatment. However, UV light doesn’t kill the microorganisms, but renders them inactive, or unable to reproduce. It’s important to be aware, then, that a low dosage of UV ultimately may not completely inactivate certain pathogens, and water sterilized by UV doesn’t retain any protection against subsequent exposure in distribution lines. Since disinfectants must be present in potable water before distribution, as required by the SDWA, treatment facilities using UV sterilization are often required to add a chlorine-based disinfectant.
Ozone Disinfection
Ozone is another alternative to chemical disinfection. When ozone gas is injected into the water it produces a broad-spectrum biocide that destroys all bacteria, viruses, and cysts, as well as oxidizing certain metals. Since ozone is a highly unstable molecule, it cannot be stored but must be created onsite in a specialized generator. Like UV light, ozone is more effective against bacteria and viruses compared to chlorine, plus it reduces concentrations of iron, manganese, and sulfur in the water, which reduces taste and odor problems, although post filtration is a necessity. While ozone treatment is highly effective and solves certain additional concerns like odors and objectionable tastes, it’s important to consider that related equipment and operating costs are higher, and ozone does not produce any continued disinfection effect, so the addition of chlorine products after ozone disinfection may seem like a redundant expense. Finally, ozone treatment does create byproducts in the water and, while they’re still being studied, some are known to be carcinogenic. Post filtration can include an activated carbon filter to reduce potential exposure to these byproducts but until more is known about exposure levels, it should still be a concern.
Using Clearwells in Chemical Disinfection
A clearwell is a large tank or other structure where water is disinfected (or stored) in preparation for distribution to consumers. In the disinfection process, the water is dosed with chlorine and passed into the clearwell, where it is mixed thoroughly and travels at a pace that ensures the water has enough contact time for the chlorine to be completely effective. When water is stored in the clearwell, repeat disinfection may be necessary since the amount of residual chemical will be steadily reduced over time.
In any type of disinfection protocol, effective mixing is critical to ensure that all the water, no matter how small the volume, is exposed to the disinfecting mechanism in effective doses. Achieving and maintaining a uniform distribution of chemical in the clearwell tank allows operators to fine tune dosages and minimize issues with inadequate disinfection or high levels of disinfection by-products (DBP). In storage conditions, effective mixing is necessary to eliminate thermal stratification and pockets of aged water.
Simple strategies like adding disinfectant at multiple points along the course of flowing water is inadequate, since the water rarely mixes thoroughly enough to ensure that distribution is completely uniform. In addressing this problem, some water treatment plants employ powered mixers to improve distribution by disrupting thermal stratification, eliminating short-circuits, and evenly distributing the chemicals in question. Some mixers are integrated with dosers that can automatically add and mix disinfectants simultaneously. Mixer technology is constantly improving, and the ideal solution will certainly differ in each situation, but it is one solution to consider.
Many water treatment plants prefer to use a passive method for promoting full distribution of chemicals through the clearwell. Baffles are typically the mechanism of choice and involve the insertion of temporary or permanent walls that direct water along a serpentine path as it travels through the disinfection tank. Somewhat like a maze, it slows the water down and introduces turbulence into the flow, which helps the water mix more effectively. In this way, retention time is increased, and the disinfectant is distributed more uniformly. This is an attractive solution since it is less expensive and requires no external power source. In some cases, operators may employ both methods congruently.
