How to Deal with Taste and Odor Problems in Potable Water Production Trial Lecture: doctoral theses at NTNU, 2011 (69) Igor Ivanovic How to Deal with Taste and Odor Problems in Potable Water Production Supervisor: Prof. TorOve Leiknes

Outline Background Sources of taste and odor in potable water Examination of taste and odor Measurements for taste and odor control New approaches Conclusions Reference

Potable water quality: General considerations Background What is Potable Water? Potable water is water of sufficiently high quality that it can be consumed or used without risk of immediate or long term harm. Such water is commonly called drinking water. Potable water quality: General considerations Chemical aspects Microbial aspects Disinfection Radiological aspects Acceptability aspects http://en.wikipedia.org/wiki/Drinking_water

Introduction Taste and odor are aesthetic qualities of drinking water and their measures are dependent on human perception. The sense of taste is a combination of responses from olfactory receptors located in the nasal cavity. The mechanism that controls odor detection, however, is much more sensitive and complex. Surface water supplies are more likely to be affected by substances causing undesirable tastes and odors. Nearly all water treatment plants utilize chemicals or adsorbents to control these problems. Treating water so that it meets taste and odor standards set by authorities makes up a significant portion of total water treatment costs.

What Causes Taste and Odor Problems in Potable Water Production?

Odor The types of odors in drinking water are directly related to the compounds present in the source water. Groundwater that is rich in salts and minerals often possess undesirable odors. Hydrogen sulfide (H2S), a dissolved gas which is characterized by a rotten egg odor, is a common odor causing compound. Chemical dumping, landfill disposal, and agricultural and industrial activities have contaminated groundwater supplies, increasing odor problems in drinking water. Phenolic or medicinal odors can often be detected in drinking water that originates from industrial rivers. The most significant source of odor in water supplies is the growth and decay of microorganisms in surface water. Blue-green algae, green algae, diatoms, and flagellates are the four algae groups responsible for the most common odor complaints. Geosmin is an odor causing metabolite produced by blue-green algae, which possess an earthy, musty smell. Moldlike bacteria called actinomycetes are also present in surface water and produce MIB, another common odor compound.

Odor (2) Four algae groups responsible for the most common odor complaints Blue-green algae, green algae, diatoms, and flagellates http://www.cee.vt.edu/

Taste Inorganic compounds are the main source of unwanted tastes in drinking water. Ferrous and manganese ions (Fe2+ and Mn2+) are often present in surface waters due to the mixing and pH changes that thermal stratification and oxygen depletion cause. If these ions are present in sufficient concentrations, produce metallic tastes in drinking water. Chlorine added at the treatment plant for disinfection, however, is by far the most common source of complaint, as it has an extremely distintictive taste.

Testing for Taste and Odor Problems Testing procedures for measuring taste and odors in potable water are described in Standard Methods, (by the American Public Health Association). Flavor profile analysis, involves a panel of selected persons who attempt to detect taste and odor causing contaminants at various dilutions. The panelists define the nature of the taste or odor, either in their own words or through the use of standard descriptions. The point at which the compound is just detected determines the threshold number, which is given by the equation: Threshold number = (A + B) / A where A is the volume of the sample tested and B is the volume of the distilled dilution water used. The method is a very subjective due to the human error involved. Tests are mainly used to determine overall trends and taste and odor origins so that corrective measures may be applied. Data is presented in a Weber-Fechner plot. http://www.cee.vt.edu/

Testing for Taste and Odor Problems (2) Development of so called "mechanical sniffers", is more accurate method (unlike humans, can smell an unlimited number of samples). Example of such device is the Aromascan, which is capable of detecting odor vapors at concentrations much lower than the human nose. This is accomplished through sensors which are located on a ceramic cartridge. The combination of the responses of these sensors allows the Aromascan to interpret what compounds are present and if they are at concentrations higher than that of distilled water. Other methods include use of GS-MS, …. http://www.cee.vt.edu/

Dealing With Taste and Odor Problems Basically there are four groups of processes that can handle taste and odor problems within the treatment plant: Oxidation Adsorption Aeration Membrane filtration

Oxidation (1) Most taste and odor causing compounds exist in reduced form, making oxidation an effective means of removal. The most commonly used compounds for oxidation include: chlorine, permanganate, ozone, and chlorine dioxide. Chlorine has been found to be very effective in removing low-level inorganic odors such as hydrogen sulfide (H2S). Chlorine is also effective with organic sulfides, disulfides, and mercaptans, but can increase the odors of algal bi-products, such as geosmin and MIB. Drawback of this technique, is its complexity due to the need for air stripping or dechlorination.

Oxidation (2) - potassium permanganate The use of potassium permanganate (KMnO4) is especially useful in treating industrial and algal odors, but if too much is added, manganese dioxide (MnO2) will form and turn the water black.

Oxidation (3) - Ozone Ozone is excellent oxidizing agent for taste and odor problems, although high dosages are required to remove industrial odors. It is mainly used to reduce natural aquatic odors. Drawback of the ozone is the fact that ozone quickly dissipates, it must be generated on-site at a greater expense to the treatment plant.

Oxidation (4) - Chlorine dioxide Chlorine dioxide is successfully used to control algae and phenolic odors. Chlorine dioxide has a high cost it is as not as widely used as other oxidizing agents. Drawback is that chlorine dioxide gas must be generated on-site and aqueous chlorine dioxide is too costly for regular use.

Adsorption – activated carbon Activated carbon adsorption is considered to be the most reliable method of taste and odor removal. It can be used in two forms: powdered activated carbon (PAC) or granular activated carbon (GAC). PAC is primarily relied on to remove common taste and odor problems, but increased industrial compounds have required the use of GAC. Powdered activated carbon doses commonly range between 3 and 15 mg/L. When relatively low doses are required, PAC is most effective when added immediately before filtration. For high doses, it is best to add PAC before coagulation so that removal can take place during sedimentation.

Adsorption -

Adsorption -

Aeration - Aeration is only useful for the most easily removed substances such as sulfides or volatile organics. Groundwater supplies can be aerated to remove hydrogen sulfide Ferrous ion at low concentrations may be oxidized through aeration. Manganese ion cannot be effectively removed by aeration.

Aeration -

Membrane fitration

Conclusions Potable reuse is a viable application of reclaimed water There should be a careful, thorough, project-specific assessment Potable reuse projects should include multiple, independent barriers. Conduct continuous toxicological monitoring. Potable reuse is an option of last resort. 22

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