American Water College 2010

Transcription

1 Vocabulary Disinfection Air Gap An open vertical drop, or vertical empty space, that separates a drinking (potable) water supply to be protected from another water system in a water treatment plant or other location. This open gap prevents the contamination of drinking water by backsiphonage or backflow because there is no way raw water or any other water can reach the drinking water supply. Amperometic Titration A means of measuring concentrations of certain substances in water (such as strong oxidizers) based on the electric current that flows during a chemical reaction. Bacteria Bacteria are living organisms, microscopic in size, which usually consist of a single cell. Most bacteria use organic matter for their food and produce waste products as a result of their life processes. Breakpoint Chlorination Addition of chlorine to water until the chlorine demand has been satisfied. At this point, further additions of chlorine will result in a free chlorine residual that is directly proportional to the amount of chlorine added beyond the breakpoint. Chloramines Compounds formed by the reaction of hypochlorous acid (or aqueous chlorine) with ammonia. Chlorine Demand Chlorine demand is the difference between the amount of chlorine added to water and the amount of residual chlorine remaining after a given contact time. Chlorine demand may change with dosage, time, temperature, ph, and nature and amount of the impurities in the water. Chlorine Residual The concentration of chlorine present in water after the chlorine demand has been satisfied. The concentration is expressed in terms of the total chlorine residual, which includes both the free and combined or chemically bound chlorine residuals. Chlororganic Organic compounds combined with chlorine. These compounds generally originate from, or are associated with, life processes such as those of algae in water

2 Coliform A group of bacteria found in the intestines of warm-blooded animals (including humans) and also in plants, soil, air and water. Fecal coliforms are a specific class of bacteria which only inhabit the intestines of warm-blooded animals. The presence of coliform bacteria is an indication that the water is polluted and may contain pathogenic (diseasecausing) organisms. Combined Chlorine The sum of the chlorine species composed of free chlorine and ammonia, including monochloramine, dichloramine, and trichloramine (nitrogen trichloride). Dichloramine is the strongest disinfectant of these chlorine species, but it has less oxidative capacity than free chlorine. Disinfection The process designed to kill or inactivate most microorganisms in water, including essentially all pathogenic (disease-causing) bacteria. There are several ways to disinfect, with chlorination being the most frequently used in water treatment. Eductor A hydraulic device used to create a negative pressure (suction) by forcing a liquid through a restriction, such as a Venturi. An eductor or aspirator (the hydraulic device) may be used in the laboratory in place of a vacuum pump. As an injector, it is used to produce vacuum for chlorinators. Sometimes used instead of a suction pump. Ejector A device used to disperse a chemical solution into water being treated. Free Available Residual Chlorine That portion of the total available residual chlorine composed of dissolved chlorine gas (CI 2 ), hypochlorous acid (HOCI), and/or hypochlorite ion (OCI - ) remaining in water after chlorination. This does not include chlorine that has combined with ammonia, nitrogen, or other compounds. HTH (pronounce as separate letters) High Test Hypochlorite. Calcium hypochlorite or Ca(OCI) 2. Hypochlorite Chemical compounds containing available chlorine, used for disinfection. They are available as liquids (bleach) or solids (powder granules, and pellets) in barrels, drums and cans. IDLH (pronounce as separate letters) Immediately Dangerous to Life or Health. The atmospheric concentration of any toxic, corrosive, or asphyxiant substance that poses an immediate threat to life or would cause irreversible or delayed adverse health effects or would interfere with an individual s ability to escape from a dangerous atmosphere

3 MPN (pronounce as separate letters) MPN is the Most Probable Number of coliform-group organisms per unit volume of sample water. Expressed as a density or population of organisms per 100 ml of sample water. Oxidation Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons from an element or compound. In the environment, organic matter is oxidized to more stable substances. This is the opposite of Reduction. Pathogenic Organisms Organisms, including bacteria, viruses or cysts, capable of causing diseases (giardiases, cryptosporidiosis, typhoid, cholera, dysentery) in a host (such as a person). There are many types of organisms which do NOT cause disease. These organisms are called nonpathogenic. Reduction Reduction is the addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (no dissolved oxygen present), sulfur compounds are reduced to odor-producing hydrogen sulfide (H 2 S) and other compounds. The opposite of Oxidation. Purpose of Disinfection To destroy pathogenic organisms in wastewater and prevent the spread of disease. Microorganism Removal by Treatment Processes Treatment Process Microorganism Removal Primary Sedimentation 25% - 75% Chemical Precipitation 40% - 80% Trickling Filters 90% - 95% Activated Sludge 90% - 98% Chlorination 98% - 99%

4 Process description Chlorine is added to treated wastewater in the form of gas or liquid. The chlorinated water then is sent through a contact basin to allow the chlorine time to interact with the microorganisms. After appropriate contact time, the water is de-chlorinated and discharged from the plant. Typical Dose Required for Disinfection Treatment Process Chlorine Dose mg/l Primary Clarifier Effluent 5-20 Chemical Precipitation 2-6 Trickling Filters 3-20 Activated Sludge 2-8 Advanced Treatment

5 Pump Station odor/corrosion control BOD Reduction aid settling and Grease removal Screens Chlorine Injection Points Grit Removal odor fly ponding BOD Sedimentation Floatation Trickling Filter Secondary Clarifier Chlorine Contact Basin sludge bulking Activated Sludge Disinfection Chlorine is not only used for disinfection. Chlorine is used throughout the entire system for many different reasons. Some of these include: 1. Odor control (H 2 S) 2. Corrosion control (H 2 S) 3. BOD reduction 4. Aid sedimentation and grease removal 5. Control septic conditions 6. Fly control 7. Ponding control 8. Filamentous bacteria control (sludge bulking)

6 Chlorine Added Breakpoint Chlorination 1. Initial Chlorine Demand 2. Combined Residual Chlorine 3. Oxidation of Combined Chlorine compounds 4. Breakpoint 5. Free Chlorine (residual increase proportionate with dose increase)

7 Factors That Impact Disinfection 1. ph lower is better 2. Temperature higher is better 3. Turbidity/Suspended Solids lower is better 4. Organic Matter lower is better 5. Inorganic Matter lower is better 6. Reducing Agents lower is better 7. Dose higher is better 8. Contact time longer is better Properties of Chlorine Gas 1. Greenish yellow gas 2. Penetrating and distinctive odor times heavier than air 4. Expansive. 450 times when going from liquid to gas 5. Not flammable but supports combustion 6. Strong oxidizer Health Hazards of Chlorine Gas Effect ppm Detectable odor Noxiousness 5 Throat irritation 15 Coughing 30 Dangerous from ½ to 1 hour 40 Death after a few breaths 1,000 Permissible Exposure Limit (PEL) 0.5 ppm Immediate Danger To Life and Health (IDLH) 10 ppm

8 Chlorine Potpourri 1. Fusible plug melts at o F to prevent excessive pressure during fire 2. Max removal rate from 150# cylinder is 40 lb/day 3. Max removal from Ton cylinder is 8 lb/ o F / day (~400 lb/day) 4. Fed to system under vacuum for safety Dechlorination Purpose To remove residual chlorine from treated wastewater prior to discharging to receiving waters. Chlorine can harm the aquatic environment. Methods of Dechlorination 1. Extended detention times 2. Aeration 3. Exposure to sunlight for extended periods 4. Activated Carbon 5. Chemical Reactions (sulfur dioxide, sodium sulfite, sodium bisulfate, sodium thiosulfate) Sulfur Dioxide Facts 1. Reacts instantly with Chlorine 2. Reacts on a one for one basis 3. Colorless gas with pungent odor 4. Non-flammable or explosive 5. Forms H 2 SO 4 in presence of water 6. Health hazards are similar to those of chlorine gas