| to Lab Manual table of contents | ||
| comments? |
What You Can't Tell From the Name
While chlorine dioxide has chlorine in its name, its chemistry is radically different from that of chlorine. The way it works is almost magical. It has to do with the way electrons interact with one another. As we all learned in high school chemistry, we can mix two compounds and create a third that bears little resemblance to its parents.
For instance: Mix two parts of hydrogen gas with one of oxygen and liquid water is the result. Mix equal parts of caustic soda (commonly called lye, a part of everyday soap) and hydrochloric acid (which will dissolve iron) and you get table salt and water. And for chlorine dioxide, mix one part chlorine gas with two parts of oxygen.
We should not be misled by the fact that chlorine and chlorine dioxide share a word in common. Hydrogen is in both water and in hydrogen cyanide. The latter can be a deadly poison. At room temperature, chlorine is a greenish-yellow poisonous gas. When added to water, however, chlorine combines with water to form hypochlorous acid that then ionizes to form hypochlorite ion - 'bleach'
Regarding bleaching, chlorine dioxide and chlorine -- because of their fundamentally different chemistries -- react in distinct ways with organic compounds, and as a result generate very different byproducts. It is this difference that explains the superior environmental performance of chlorine dioxide in paper making and scrubbers. Technically speaking, both chlorine and chlorine dioxide are oxidizing agents -- electron receivers. Chlorine has the capacity to take in two electrons, whereas chlorine dioxide can absorb five (5).
This property, along with the complex, but well known, ways chlorine combines with lignin (the cellular adhesive in wood tissue), explains the basic difference between the two compounds.
In the chlorine-based bleaching process, about 10 percent of the chlorine combines directly with lignin which has "aromatic" components. Aromatic compounds have atoms arranged in rings, and they may have other atoms, such as chlorine, attached to these rings. Within the group of chlorinated aromatics, which can be toxic to some organisms, are the infamous dioxins.
Chlorine dioxide's behavior as a bleaching agent is quite dissimilar. Instead of combining with the aromatic rings, chlorine dioxide breaks these rings apart. In addition, as the use of chlorine dioxide increases, the generation of chlorinated organics falls dramatically. Chlorine dioxide's chemistry explains why it is such an effective oxidant, or bleaching agent. It is 2.5 times more active than chlorine gas, and much more selective. Chlorine dioxide attacks the lignin, but does not react with the desired cellulose in wood tissue. It is cellulose -- the tree's fiber -- that provides the strength in the final paper products.
These advantages make chlorine dioxide the preferred environmental standard for eliminating toxic substances in mill waste water and scrubbers.
Chlorine dioxide is a neutral compound of chlorine in the +IV oxidation state. It has a boiling point of 11 degrees C at atmospheric pressure. The liquid is denser than water, and the gas is denser than air. The molecule is polar with the oxygen atoms separated by 116.5 degrees. Water (H2O) is also polar (105 degrees). The presence of organic matter, nutrients, and microorganisms in the output of sewage treatment plants is measured by three tests: coliform count, algal count, and biochemical oxygen demand (BOD). The coliform count describes the number of E. coli (the characteristic bacteria in animal wastes) present. The algal count is a biological test for microorganisms other than bacteria and viruses which may be present. The BOD measures the volume of oxygen gas taken up by a given amount of water in five days at 20 degrees C, (remember, there is an ultimate test of BOD).
The biochemical oxygen demand analysis is an attempt to simulate the effect a waste will have on the dissolved oxygen of a stream by a laboratory test. The BOD test gives an indication of the amount of oxygen needed to stabilize or biologically oxidize the waste. The advantage of the BOD test is that it measures only the organics which are oxidized by the bacteria. The disadvantage is the 5 day time lag and the difficulty in obtaining consistent repetitive values. The results of the COD (chemical oxygen demand) tests are usually higher that the corresponding BOD test for the following reasons:
Many organic compounds which are dichromate oxidizable are not biochemically oxidizable.
Certain inorganic substances, such as sulfides, sulfites, thiosulfates, nitrites and ferrous iron are oxidized by dichromate, creating an inorganic COD, which is misleading when estimating the organic content of the wastewater.
The BOD results may be affected by lack of seed acclimation, giving erroneously low readings. The COD results are independent of seed acclimation.
In general, chlorine dioxide has been found to produce fewer organic byproducts with naturally occurring dissolved organic material. Chlorine dioxide is an explosive gas, but is stable in water in the absence of light and elevated temperatures ... which is just what we do. ClO2 is capable of oxidizing iron and manganese, removing color, and lowering THM (Trihalomethanes) formation potential. It also oxidizes many organic and sulfurous compounds that cause off-tastes and odors.
Chlorine dioxide is a green-yellow gas that decomposes readily and with explosive force to chlorine and oxygen. It is, therefore, usually manufactured on-site. Chlorine dioxide is a more powerful biocide than free chlorine but does not persist as long as chlorine.
pH: above 11.0
Specific gravity: 2.4
Boiling point: 11 degrees C
Solubility: 2.9 grams per liter
Threshold Odor level: 14-17 ppm/v
Color is a property of the source water caused by the presence of organic and inorganic substances, usually of natural origin, which absorb visible light. The nature of these substances and the molecular basis of the color vary with the source water. In most cases, color is probably caused by natural organics (humic substances) that probably have complexed metals bound into their structures.
CHLORINE DIOXIDE DOSING SYSTEM
Mixing water supply up to pressure (i.e. 60 psig).
Chlorine '1 ton' cylinder valve 1/2 turn open.
Rotameter should read 35 to 40 ppd (pounds per day) when the DAY tank level calls for more solution.
Sodium chlorite supply valves 'cracked open to the chemical metering pump.
Everyday routine: Touch heater on chlorine valve to sense heat on tube making sure that chlorine is gas and not liquid; observe sodium chlorite tank, level, and containment; record ORP values (alarm set for about 700); wait for DAY tank level to call for more generation of chlorine dioxide in order to observe chlorine gas rate, water supply rate and chemical feed pump running pumping sodium chlorite through clear plastic tubing; take sample of generated chlorine dioxide, observe color (yellow-green), light fog of vapor rising, and smell of chlorine; scan piping for leaks, especially any leak of sodium chlorite (when dry it is explosive, a white dry powder); scan scrubber valves, piping, excessive leaks; correct pressure on recirculation piping; observe volume and quantity of overflow in the combined scrubber drain; feel CL310 chemical feed pump for slight thump indicating pumping action; taste the aroma from the sniff tube (sensing the temperature and the pressure), sample effluent air (via 'sniff' tube) slight odor of chlorine is preferred ( like a Laundromat ); look through the Plexiglas access doors on the other side of the scrubber noting flow of water and bubbles or foam, then note the water level in sump section of both stages; check other scrubbers for the same qualities; and scan the fans and motors, listen for unusual noise in belts or bearings. Then, occasionally, during the day, walk around scanning the scrubbers for all the items again.
BE NICE do unto others as you would
have them do unto you.
BE NEAT it looks better
BE SAFE feels better
BE SURE strive to increase your knowledge and skill
Definitions:
Viruses - A submicrosopic disease-causing organism (pathogen) that harbors genetic information that can control a living host organism. Examples include the causative agents for polio and hepatitis
Bacteria - Simple one-celled plants that decompose organic and inorganic wastes and reproduce without sunlight. Examples include Sphaerotillus natans, a filamentous growth in settling chambers; and Escherichia coli, a water quality indicator organism of fecal contamination.
Bacteria are - single-celled microorganisms. They possess no well-defined nucleus, are devoid of chlorophyll, and reproduce by binary fission. Legionella: 26 species of Legionella have been identified. Seven are etiologic agents of Legionnaires disease, they are relatively resistant to chlorine, and small numbers are expected in the finished water of systems employing full treatment. Cooling towers have also been suggested as sites of Legionella proliferation.
Fungi - Microscopic plants that do not require sunlight to grow. Their filamentous nature can lead to poor settling floc. Growths usually occur under low pH conditions.
Algae - Microscopic plants that require sunlight to grow. They digest the primary decomposers (bacteria and fungi) in the aquatic food chain.
Algae are green plants requiring sunlight and the proper temperature, and chemicals for growth. While they are microscopic is size, they can cover an entire surface of a body of water if the conditions are right. This phenomena is called a 'bloom'. Algae affect water quality in many ways. Through the process of living, they can change the pH and hardness of water. This process reverses during the night. Algae are unicellular, generally nonmotile plants. All use photosynthesis as their primary mode of nutrition. Algae are typically not of health concern; however, certain species may produce endo- or exotoxins, which, if ingested at high enough concentrations, may be harmful. Dissolved nutrient materials, such as nitrogen and phosphorus, are used by green Algae which are actually microscopic plants floating and living in the water. The algae use carbon dioxide and bicarbonate to build body protoplasm. In so growing, they need nitrogen and phosphorus in their metabolism much as land plants do. Like land plants, they release oxygen and some carbon dioxide as waste products. In aerobic ponds or in the aerobic layer of facultative ponds, organic matter contained in the wastewater is first converted to carbon dioxide and ammonia, and finally, to algae in the presence of sunlight. Algae are simple one- or many-celled microscopic plants which are essential o the successful operation of both aerobic and facultative ponds.
Protozoa - Microscopic single-celled aquatic animals. They digest bacteria. fungi, and algae and are commonly found in natural surface bodies of water. Each of these categories of microorganisms can be classified according to their source of food, where they obtain their oxygen, and their optimum temperature for best reproduction and growth. Classification according to food source is based on whether the food is organic or inorganic in composition. Heterotrophs use organic matter, whereas autotrophs use inorganic matter. Classification with respect to oxygen source is broken down into three divisions. Aerobes require free DO found in the water. Anaerobes use oxygen bound in the foods they decompose. Facultative microorganisms prefer free DO but can also live in its absence like an anaerobe. The last classification is according to the temperature of the environment that organisms are most suited for growing in. Mesophilic bugs grow between 10 to 40 degrees C ; thermophilic bugs grow between 45 to 75 degrees C.
ORP Oxidation Reduction Potential. Measurement of a minute (small) electrical voltage. Refer to a chemistry textbook concerning oxidation-reduction reactions. The chemical reactions are, in effect, a battery. Probes are designed for reactions calibrated for pH, DO, and ORP among many possible configurations. We use probes (in stream) calibrated for chlorine dioxide and for pH. Manufacturers have many models available.
There are 6 factors which will routinely govern all the growth of aquatic microorganisms:
1. Temperature - When the temp is above 37 or below 13 degrees C, the organisms will start slowing their activities and begin to hibernate (cysts).
2. Availability of Food - Organisms will starve without the "right kind" of food in sufficient quantities. On the other hand, they will continue to reproduce until they run out of the "right kind" of food.
3. Oxygen Supply - The source of oxygen and the amount available determines the rate of biological activity. Aerobes reproduce very rapidly whereas, anaerobes are very slow.
4. pH - Most organisms grow best in a pH range of 6.5 to 8.5 (except fungi). Outside of this range, their biological activity decreases rapidly and many of them will die.
5. Toxins - Materials which act as poisons to the organisms will slow down their biological activity and may kill them. An example of a toxic material is chlorine which is used to disinfect water.
6. Availability of Nutrients - In addition to food, Microorganisms must have certain quantities of nitrogen, phosphorus and other trace elements to stimulate growth. These elements are their 'vitamins' and must be present for growth to proceed.Chlorine dioxide is a powerful oxidant that is always prepared on site most economically by the reaction of chlorine and sodium chlorite. One mole of chlorine gas plus 2 moles of sodium chlorite (2NaClO2) produces 2 moles of chlorine dioxide and 2 moles of NaCl (salt). Our operation typically is: 40 pounds/day (rate) of chlorine gas in 6 gallons per minute of well water and sodium chlorite metering pump set for 70/80 (strokes/rate). The end result is sampled, observed for color and pH (around 3 plus or minus .3 SU ).
Oxidation processes involve the exchange of electrons between chemical species so as to change the oxidation state (valence) of the species involved. Strictly speaking, oxidation processes should be referred to as reduction - oxidation (redox) processes because one species loses electrons or is oxidized while another gains electrons or is reduced.
Chlorine dioxide was first produced from the reaction of potassium chlorate and hydrochloric acid by Davy in 1811. Not until the industrial-scale preparation of sodium chlorite, from which chlorine dioxide may more readily be generated however, did its widespread use occur.