ENVIRONMENTAL ENGINEERING. Chemical Engineering department

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1 ENVIRONMENTAL ENGINEERING Chemical Engineering department

WATER TREATMENT Many aquifers and isolated surface waters are of high water quality and may be pumped from the supply and transmission network directly to any number of end

However, such clean water sources are the exception to the rule, particularly in regions with dense populations or regions that are heavily agricultural.

2 WATER TREATMENT Many aquifers and isolated surface waters are of high water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes, and fire control. However, such clean water sources are the exception to the rule, particularly in regions with dense populations or regions that are heavily agricultural. Here, the water supply must receive varying degrees of treatment prior to distribution. A typical water treatment plant. A typical water treatment plant is diagramed in Figure. Such plants are made up of a series of reactors or unit operations, with the water flowing from one to the next to

3 achieve a desired end product. Each operation is designed to perform a specific function, and the order of these operations is important. Described below are a number of the most important of these processes. 1-Softening Some waters (both surface waters and ground waters) need hardness removed to use them as a potable water source. Hardness is caused by multivalent cations (or minerals such as calcium, magnesium, and iron that dissolve from soil and rocks (particularly limestone). While hardness does not cause health problems, it does reduce the effectiveness of soaps and cause scale formation. The reaction of hardness ions with soaps causes bathtub ring and reduces lather. Soaps are typically made up of long, chain-like molecules with two distinct ends. The hydrophilic end interacts with water while the hydrophobic end interacts with oil and grease. When the hydrophobic end interacts with hardness ions instead, the soap bunches together, forming a soap scum, or film. Besides leaving a bathtub ring, soap scum can cause skin irritation by disrupting the skin s ph and can make hair dull. The same effect happens to laundry detergents, and the scum formed can make clothes look drab and feel stiff and can cause fabric to wear out faster. Scale, which forms when calcium carbonate precipitates from heated water, is a more serious problem because it reduces heat transfer efficiency by coating water heaters, boilers, heat exchangers, tea pots anything in which water is Heated and can eventually clog pipes. Hardness also sometimes causes objectionable tastes. Total hardness (TH) is defined as the sum of the multivalent cations in the water. Calcium (Ca 2+ ) and magnesium (Mg 2+ ) tend to be the largest components of hardness, so TH is typically approximated as the sum of these two components. However, iron (Fe 2+ and Fe 3+ ), manganese (Mn 2+ ), strontium (Sr 2+ ), and aluminum (Al 3+ ) may also be present in water supplies.

4 2- Coagulation and Flocculation Coagulants and flocculants are chemicals used to precipitate insoluble substances. The purpose of coagulation and flocculation is to cause small pollutant particles such as metals to aggregate and form large enough floc so that they can be separated from the wastewater through sedimentation. There are three main types of coagulants that are used to overcome the repulsive forces of particles, thus causing them to aggregate. Electrolytes, organic polymers, and synthetic polyelectrolyte are added to wastewater and then flocculation tanks mix the water to promote floc and subsequent physical separation Rate of flocculation is dependent upon many factors including concentration of particles, particle contact, and range of particle sizes. Coagulation targets dissolved ions such as metal and radionuclide. Some difficulties with this technology include the frequent need to adjust ph levels, the creation of toxic sludge that must be eventually mitigated, and the difficulty that results in trying to address the chemical nature of multiple compounds. This technology has been used consistently in the electronics and electroplating industry as well as for applications in groundwater treatment. 3-Membrane Filtration The three main types of membrane-based filtration technologies include reverse osmosis Nano filtration Ultra filtration Although categorized as different technologies, the three types of membrane filtration have a great deal in common. All three act as membranes created by coating a thin layer of a very porous polymer, or plastic, onto a backing material. The end result is the finest form of filtration presently known, with reverse osmosis being the

5 smallest, nano filtration being a slight step larger and ultra filtration being a bit larger again. The pore sizes are typically measured in angstroms (one billionth of a meter) and thus are extremely tiny. These membrane technologies offer a host of advantages over traditional filtration. Due to the fine pore space and indiscrimination of influents of these membrane filtration systems, a very high quality effluent emerges. Additionally, membrane technologies take up only a fraction of the space needed for other tertiary treatment systems. The disadvantage of having extremely fine pores means that clogging is a frequent and costly problem with membrane filtration technologies. Water Purification Water purification forms a critical link in promoting public health and safety. It involves variety of steps which depend upon the type of impurities in the raw water source. The major operations done are sedimentation, flocculation, filtration and disinfection. Raw water becomes potable after this treatment (Figure below). Impurities in raw water include suspended, dissolved, colloidal solids; bacteria; toxic substances; color; odor and mineral or organic matter. These can be categorized as

6 chemical, physical and microbiological. Table1 indicates the drinking water standards in India. Different unit processes and operations are performed to remove different impurities (Table below). Table: The Bureau of Indian Standards defined levels of substances in water and their permissible levels S.NO Substance / Test Unit Desirable limit Maximum permissible limit* 1. Physical turbidity NTU Chemical ph Number No relaxation 3. Hardness as (CaCO 3 ) mg/l Chloride as Cl mg/l Iron as Fe mg/l Nitrate as N mg/; 45 No relaxation 7. Fluoride as F mg/l Residual chlorine mg/l No relaxation 9. Arsenic as As mg/l 0.05 No relaxation 10. Coliforms MPN/100 ml 10** No relaxation 11. E. coli MPN/100 ml 0 No relaxation * Where there is no alternative source for drinking ** Coliform should not be detected in 100 ml of any two consecutive samples Table: Unit processes and operations and specific impurities removed S. No. Unit Processes / operations Effect 1. Aeration, chemical oxidation, ion exchange, sedimentation 2. Chemical precipitation, (dosing, mixing, flocculation, settling) ion exchange 3. Chemical coagulation, (dosing, mixing, flocculation, settling) filtration Color and precipitate removal Softening (Ca, Mg removal) Turbidity removal 4. Aeration, chemical oxidation, adsorption Taste and odour removal 5. Irradiation, ozonation, chlorination Disinfection

7 1-Sedimentation Is separation of suspended particles by natural aggregation and gravitational settling carried out in sedimentation tank or settling basin? Some degree of sedimentation occurs during storage. Quiescent sedimentation for a period of days may result in purification equivalent to filtration. Bacteria and viruses still persist. Sedimentation is combined with coagulation. Coagulation brings about destabilization and agglomeration of the particles. Metal salts like aluminium sulfate, ferric chloride, calcium oxide or hydroxide are used as coagulants, metal hydroxide form precipitates in which the colloidal particles get enmeshed and settle along with them. In flocculation the agglomeration of the destabilized particles is induced by mechanical means into compact fast settlable particles called flocs. The aluminium sulfate added reacts with the natural alkalinity present in water to form flocs. Betonite, clay, activated silica etc. aid the coagulants. These agents bear negative charge when present with positively charged metal hydroxide gives a tough dense flocs thereby hastening flocculation. Polyelectrolytes which have ionizable carboxyl, amino, sulfonic groups are synthetic coagulants. The flocs are allowed to settle and the supernatant is taken for filtration.

Delvin DeBoer, Ph.D., PE. MN/ND/SD SWTW April 29, 2014 OUTLINE

Delvin DeBoer, Ph.D., PE. MN/ND/SD SWTW April 29, 2014 OUTLINE Physical/Chemical Process FUNDAMENTALS Delvin DeBoer, Ph.D., PE MN/ND/SD SWTW April 29, 2014 OUTLINE Properties of turbidity and organic matter Mechanisms of coagulation, coagulant chemicals and jar testing