WATER CONDITIONING & ENVIRONMENTAL PROTECTION

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1 WATER CONDITIONING & ENVIRONMENTAL PROTECTION KMU Spring Semester Water conditioning and wastewater treatment have long been essential functions of municipalities. However, the importance of suitably preparing water for the chemical industry is now fully recognized. Industrial wastewaters present a complex and challenging problem to the chemical engineer. Moral and community considerations + Laws prohibiting and limiting the pollution of streams Necessary operating expenses Need for a specific solution to each industry General principles: Increased reuse of wastewater Control of pollution Recovery of products at their source Lagooning of waste Quality and Quantity of available water are very important in choosing the location of a chemical plant. Both the surface water and groundwater can be used in industry, however groundwater is more suitable. Because; they are clean and they have uniform temperature and composition. But such water is generally harder and may cause scale, thus it prevents heat transfer. Therefore, some treatments must be applied to groundwater before using in the industry. Treatment of municipal water supplies is also necessary to produce safe water. Safe water must be free from pathogenic microorganisms and from suspended solids. It is also desirable (but not necessary), that the water be soft and not have an undesired taste or odor. 1

2 Total dissolved solids A few ppm in snow water Several thousands ppm in mineral springs Water with 99.9% purity may contain 1000 ppm dissolved solids!! Other impurities in water Suspended solids (as turbidity) Organic matter Color Dissolved gases Poor quality of water Decision between using alternative water sources and optimum treatment The decision generally depends on the use! Cost minimization Water have been used in several industries for several purposes Solvent Heating or cooling agent Power generation Fabricating/processing Washing Diluting Transporting a product Incorporating water into a product For sanitation needs within the manufacturing facility Water Consumption of Some Industries Some industries that use large amounts of water produce such commodities as food, paper, chemicals, refined petroleum, or primary metals. Product / Industry Water Consumption L H 2 O / kg Product Cement 3 Ammonium Nitrate 10 Beer 100 Sugar 100 Paper 200 Ammonia 400 Explosives Industry is one of the main water users, accounting for about 40 % of total water abstractions. Furthermore the industrial sector is a major water polluter, as only up to 60 % of industrial wastewater receives treatment before being disposed of into the environment. Water abstraction: Volumes of water withdrawn from the different sources Water use: Water that is actually used for industrial processing, excluding returned water Wastewater generation 2

3 WATER AS A RAW MATERIAL Sulfuric Acid Production H 2 O + SO 3 (g) H 2 SO 4 Acetylene Production CaC 2 + H 2 O C 2 H 2 + CaO Calcium Acetylene carbide Phosphoric Acid Production Electrolysis P 2 O 5 + 3H 2 O 2 H 3 PO 4 H 2 O H O 2 HARDNESS Dissolved salts of Ca, Mg, and a little amounts of metals (Fe, Mn,..) cause hardness. They are usually present as bicarbonates, chlorides, and sulphates. These salts give insoluble precipitates with soap. Calcium sulfate, carbonate and silicate form clogging scales with low thermal conductivity in boilers. 3

4 Hardness is usually expressed in terms of the dissolved Ca and Mg salts calculated as CaCO 3 or CaO equivalent. Hardness permanent hardness and temporary hardness (or alkalinity). Temporary Hardness: Bicarbonates of Ca and Mg cause temporary hardness. This form of hardness is also called carbonate hardness or temporary hardness because it can be precipitated and removed by boiling the water. Which is why lime-scale forms in kettles and showerheads! Permanent Hardness: Permanent hardness measures the ions such as, sulphates, and chlorides etc, that are not removed by boiling. Water Hardness Unit International recommended Millimol per liter Physical measures Milli Equivalent per liter America & states ppm English Hardness Clark H French Hardness fh German Hardness dh (gh) The sum of both types of hardness is called the general or total hardness. Definition 100 mg CaCO 3 per 1000 ml water 28 mg CaO or 50 mg CaCO 3 per 1000 ml water 1 part CaCO 3 per million = 1 mg CaCO 3 per 1000 ml water 1 grain CaCO 3 per gallon= 14,3 mg CaCO 3 per 1000 ml water 10 mg CaCO 3 per 1000 ml water 10 mg CaO per 1000 ml water 1 mmol/l 1 meq/l 1 ppm 1 Clark H 1 fh 1 dh

5 Water Supply Classification Property Hardness Degree (gh) Soft Water 9 Slightly Hard Water 9-18 Hard Water Very Hard Water >26 METHODS FOR THE CONTROL AND ELIMINATION OF PRIVATE WATER PROBLEMS Clarification History In water treatment, the removal of turbidity and suspended solids by settling, often aided by centrifugal action and chemically induced coagulation. In 1841, Lime process was patented by Clark for the removal of carbonate hardness. Then, Porter developed the use of soda ash to remove noncarbonate hardness. In 1906, Gens applied zeolite to commercial water softening purposes. Filtration Filtration is the mechanical process that removes particulate matter by separating water from solid material, usually by passing it through sand. 5

6 Disinfection Disinfection technologies kill or screenout biological contaminants present in a water supply. Chlorination, microfiltration, ozone, and ultraviolet light are the four major technologies used to disinfect water. Chlorination adds a concentration of the chemical chlorine or chloramine to the water supply, where the oxidizing ability of this chemical "burns up" the organic contaminants in the water. Chlorination also removes organicallyrelated taste, color, and odor problems. Cl 2 + H 2 O HCl + HClO hypochloride 2HClO 2H + + 2Cl - + O 2 Microfiltration uses a filter media with a pore size smaller than 0.2 microns to physically prevent biological contamination from passing through. While O is converted to O 2, it takes away most of the microorganisms. Ceramic and solid block carbon are commonly used to provide microfiltration. Ozone treatment has typically been used in large-scale commercial and industrial applications; however, there has been a recent growth in the number of ozone units designed for use in a single home or business application. Ozone treatment oxidizes organic contaminants in much the same way that chlorine does. An ozone generator converts the oxygen found in air to ozone. O 3 O 2 + O 6

7 Ultraviolet light has been used to treat water since the beginning of time through natural sunlight as well as UV lamps that are immersed in water. The UV light destroys the genetic material of pathogens, bacteria and all living microorganisms. UV light, which typically has an energy greater than 10 ev, destroys living microorganisms by breaking molecular bonds which are typically 0.1 ev. What factors influence the effectiveness of UV sterilizers? Size and type of organism Power of bulb UV penetration Contact time Adsorption with Activated Carbon (Granular and Solid Block) Activated carbon can adsorb well, meaning that it can take in or collect many organic molecules on its surface. Granular activated carbon filters are typically inexpensive, and maintenance involves replacing six to twelve cartridges a year, depending on the quality of the raw water and the filter media. 7

8 If the water contains large amounts of magnesium and calcium (hard water), softening is still necessary because an activated carbon unit will not remove hardness. It will not remove dissolved metals such as iron, lead, manganese, and copper or chlorides, nitrates, and fluorides. Small activated carbon units can remove only small portions of hydrogen sulfide. These filters are not effective against bacteria. It is most effective for removal of chlorine and potentially dangerous and carcinogenic organic compounds, which may be present in a water system as a result of chlorination or industrial pollution. Reverse Osmosis Osmosis occurs when solutions of different concentrations are separated by a semipermeable membrane. Reverse osmosis is slowly becoming technologically, commercially, and economically feasible for the production of high quality water from alkaline, brackish, or colored water. 8

9 One of the major problems with the reverse osmosis process is the disposal of the reject water, a high salt concentration solution. If this water contains high levels of toxic materials, special provisions for its disposal must be made. Water Softening Softening is the term applied to processes which remove or reduce the hardness of water. Water softening is divided into two classes. These are ion exchange and lime-soda processes. Ion Exchange Ion exchange is a chemical reaction in which mobile hydrated ions of a solid are exchanged, for ions of like charge in solution. Cation is an ion with net positive charge, having more protons than electrons Cation exchange occurs when the fixed charged groups (functional groups) of the exchanger are negative Anion is an ion with more electrons than protons, giving it a net negative charge Anion exchange occurs when the immobilized functional groups are positive. 9

10 In 1852: Removal of ammonia from aqueous liquids passing through certain solids In 1935: Production of purely synthetic organic exchange resins The first products used industrially for ion exchangeènaturally occuring inorganic zeolites (aluminum silicates) with very low exchange capacity Next improvement: increasing the exchange capacity of organic ion exchangers Next improvement: producing organic ion exchangers from sulfonated natural products like coal, lignite High capacity ion exchange resins: Polystyrene-divinylbenzene (SDVB) based Cation-exchange process (Sodium-cation exchange process): The most widely employed method for water softening. During the softening process, Ca and Mg ions are removed from hard water by cation exchange for sodium ions. (HCO 3 ) 2 (Ca,Mg) } SO 4 + 2NaR Cl 2 (HCO 3 ) 2 (Ca,Mg)R 2 + Na 2 } SO 4 Cl 2 The regeneration reaction (Ca,Mg)R 2 + 2NaCl 2NaR + (Ca,Mg)Cl 2 insoluble soluble It is very fast and completed ion-exchanging process. When the exchange resin becomes almost all changed to calcium and magnesium compounds, it is regenerated to restore the Na resin with salt solution and in the ph range between 6 and 8. During the regeneration process, an efficient backwash should be applied. Cation-exchange process with zeolites Zeolites are widely used ion exchangers. They are natural Na sources (NaZ=Sodium zeolite). Zeolites are micro-porous minerals which ares used as catalysts in many industrial purposes such as water purification and air purification. The zeolites are hydrated aluminosilicates and general composition Al x Si y O 2(x+y). 10

11 Cation-exchange process with zeolites (HCO 3 ) 2 (Ca,Mg) } SO 4 + 2NaZ Cl 2 (HCO 3 ) 2 (Ca,Mg)Z 2 + Na 2 } SO 4 Cl 2 The regeneration reaction (Ca,Mg)Z 2 + 2NaCl 2NaZ + (Ca,Mg)Cl 2 insoluble soluble Mechanical degradation of zeolite is nearly 1 % (weight percentage). At high temperatures (T > 40 o C) zeolites are more efficient. Zeolites can be regenerated infinite times with 10% brine (sodium chloride) solution, theoretically. When the CO 2 concentration is too high, NaZ does not work. Limitations of zeolite process High turbidity water cannot be softened efficiently by zeolite process Hot water should not be used as the zeolite tend to dissolve in it Water containing excess acidity or alkalinity should not be used as mineral acids may destroy zeolite bed. The hydrogen-cation-exchange process: The exchange resins contain an exchangeable hydrogen ion and can be employed to remove all cations. (Ca,Mg,Na 2 )(HCO 3 ) 2 + 2HR (Ca,Mg,Na 2 )R 2 + 2H 2 O + 2CO 2 R : Organic radical of the exchanger HR: Hydrogen cation exchanger 11

12 (Ca,Mg,Na 2 )SO 4 + 2HR (Ca,Mg,Na 2 )R 2 + H 2 SO 4 (Ca,Mg,Na 2 )Cl 2 + 2HR (Ca,Mg,Na 2 )R 2 + 2HCl Regeneration with sulfuric acid (the most widely used and economical method) (Ca,Mg,Na 2 )R 2 + H 2 SO 4 2HR + (Ca,Mg,Na 2 )SO 4 Acidic water is not desirable for most purposes, and therefore the effluent from the hydrogen-cation-exchange treatment is neutralized or blended with sodium zeolite-treated water If demineralization is required, an anionexchange process is used. Anion-exchange process: This process is used if demineralization is required. Acidic water is not desirable for most purposes. H 2 SO 4 + 2R 4 NOH (R 4 N) 2 SO 4 + 2H 2 O complex anion exchanger radical There are two types of anion exchanger resins: highly basic resins weakly basic resins Both types: can be used for strongly ionized acids (sulfuric, hydrochloric, nitric) Only highly basic resins will remove weakly ionized acids (silicic, carbonic) Highly basic anion exchangers are regenerated with caustic soda. (R 4 N) 2 SO 4 + 2NaOH 2R 4 NOH + Na 2 SO 4 Weakly basic anion exchangers are regenerated with caustic soda, soda ash (Na 2 CO 3 ) or ammonium hydroxide. Lime-Soda Process Lime-Soda process is a precipitation process. In this process temporary hardness (carbonate hardness) and permanent hardness (non carbonate hardness) eliminated by lime (Ca(OH) 2 ) and soda (Na 2 CO 3 ), respectively. 12

13 For Carbonate Hardness; the cold-lime process the hot-lime process Ca(HCO 3 ) 2 + Ca(OH) 2 2CaCO 3 + 2H 2 O Mg(HCO 3 ) 2 + Ca(OH) 2 2CaCO 3 + MgCO 3 + 2H 2 O For Non carbonate Hardness; Since MgCO 3 is fairly soluble, further addition of lime is necessary. MgCO 3 + Ca(OH) 2 CaCO 3 + Mg(OH) 2 MgCl 2 + Ca(OH) 2 Mg(OH) 2 + CaCl 2 CaCl 2 + Na 2 CO 3 CaCO 3 + 2NaCl CaSO 4 + Na 2 CO 3 CaCO 3 + Na 2 SO 4 MgSO 4 + Na 2 CO 3 + Ca(OH) 2 Mg(OH) 2 + CaCO 3 + Na 2 SO 4 Aeration and Other Methods for Removal of Dissolved Gases The process of aeration is used to improve the physical and chemical characteristics of water for domestic use. The most important functions of this process are the removal of dissolved gases, such as carbon dioxide, methane, and hydrogen sulfide, and the addition of oxygen necessary for the precipitation of iron and manganese. However, oxygen entering the water may increase its corrosiveness. If organic matter is not present, aeration alone is sufficient to cause precipitation of iron and manganese Aeration can also partially remove volatile substances causing problems with odor and taste. 13

14 Other methods of oxidation can be used for removal of dissolved gases like hydrogen sulfide. Oxidation is necessary for conversion of the gas to forms which can precipitate and therefore be filtered. It can be done using oxidizing filters (green sand filters), chlorination, or treatment with hydrogen peroxide, which has been tested lately for this purpose. Deoxygenation Sodium sulfite addition: O 2 + 2Na 2 SO 3 2Na 2 SO 4 Hydrazine addition: O 2 + H 2 N-NH 2 2H 2 O + N 2 Such complete deoxygenation is desirable to minimize corrosion in the modern hightemperature, high-pressure boilers. Coagulation, Flocculation, Sedimentation, and Filtration A large portion of particles suspended in water can be sufficiently small that their removal by sedimentation or filtration is not practicable. Most of these small particles are negatively charged, which is the major cause of the stability of suspended soil particles. The most commonly used coagulant is alum (aluminum sulfate). Coagulation is usually followed by flocculation, which is a slow mixing technique promoting the aggregation of the destabilized (coagulated) particles. Coagulation followed by flocculation as an aid to sedimentation and filtration has been practiced for centuries. It is by far the most widely used process for the removal of substances producing turbidity in water. If water has high turbidity, flocculation followed by sedimentation is often used to reduce the quantity of material prior to entering the filter. Filters for suspended particle removal can be made of graded sand, granular synthetic material, screens of various materials, and fabrics. 14

15 Iron and Manganese Removal If the amount of iron and manganese in water is not very significant, it can be removed by most water softeners along with water hardness. If the iron and manganese concentrations are above 0.1 mg/l (combination of both ions) an iron filter should be used. The manganese greensand iron filter used to be the most common iron removal device. It can be used successfully for iron concentrations up to approximately 6 mg/l. Iron is oxidized by a coating on the surface of manganese greensand bead media. The greensand must be regenerated periodically with potassium permanganate to replenish the oxidant on the surface of the manganese greensand. Nitrate and Nitrite Control Often the best solution for nitrate and nitrite pollution is relocation of the well or drilling the well deeper into an uncontaminated aquifer. The only effective methods of treatment are distillation, reverse osmosis and high quality ion-exchange columns, but these will often not be economically feasible. Volatile Organic Halide Removal The only effective methods for removing volatile organic halides are activated carbon filtration and reverse osmosis. Reverse osmosis would be feasible only if other problems required its use. Studies show that the reduction efficiency for halogenated organics by activated carbon filters ranges from 76 to 99% depending on the size of the equipment. Distillation Distillation produces high quality water by heating the raw water until it turns to steam and this high quality water especially use in pharmaceutic industries. 15

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