Environmental Systems Engineering_Notes.pdf

Environmental Systems Engineering Dr. Pritha Chatterjee Department of Civil Engineering IIT Hyderabad APPLICATIONS Water Supply and Treatment Develop systems to store, treat, and convey water for various u...

Environmental Systems Engineering Dr. Pritha Chatterjee Department of Civil Engineering IIT Hyderabad APPLICATIONS Water Supply and Treatment Develop systems to store, treat, and convey water for various uses Wastewater Treatment From domestic wastewater to industrial wastewater Air Pollution Management Vehicle exhausts and industrial flue gas stack emissions. To some extent, this field overlaps the desire to decrease carbon dioxide and other greenhouse gas emissions from combustion processes Environmental Impact Assessment Environmental engineers apply scientific and engineering principles to evaluate if there are likely to be any adverse impacts to water quality, air quality, habitat quality, flora and fauna, agricultural capacity, traffic, ecology, and noise. If impacts are expected, they then develop mitigation measures to limit or prevent such impacts. Pritha Chatterjee, 2 29-08-2024 Department of Civil Engineering  Technology: To make life comfortable Civil Engineering: Deals with the design, construction and maintenance of public utility works roads, railways, bridges, canals, dams, airports, sewerage systems, water supply, pipelines, buildings Environmental Engineering – Branch of engineering that protects the environment from the adverse impacts of human activity and protects the human population from the adverse effects of the environment Pritha Chatterjee, 3 29-08-2024 Department of Civil Engineering ENVIRONMENTAL SYSTEM COMPONENTS 29-08-2024 Department of Civil Engineering, IIT Hyderabad 4 SYLLABUS Part I: Background Knowledge Part III: Physical Unit Processes Characteristics of Water and Wastewater Softening Part II: Unit Operations Stabilization Screening, Settling and Floatation Coagulation Mixing and Flocculation Chemical Precipitation Filtration Ion-exchange Aeration and Stripping Disinfection Part IV: Biological Unit Processes Activated Sludge Process Anaerobic treatment processes 29-08-2024 Department of Civil Engineering, IIT Hyderabad 5 GRADING (1.5 credits) Assignments: 20 Quiz 1: 30 Quizzes can be surprise, so I suggest to prepare everyday before class Assignments and quizzes can be in any form, viva/presentation/written exam To pass 40% in all assignments and quizzes 29-08-2024 Pritha Chatterjee 6 BOOKS Physical-Chemical Treatment of Water and Wastewater: Arcadio P. Sincero Sr. and Gregoria A. Sincero, IWA Publishing Wastewater Engineering: Treatment, Disposal, and Reuse: Metcalf & Eddy, Inc. (1991). McGraw-Hill, New York, 375. Environmental Engineering. Peavy, H. S., D. R. Rowe, and G. Tchobanoglous (1985). McGraw-Hill, New York. 29-08-2024 Pritha Chatterjee 7 ASSIGNMENT (5/08) Move around the campus and bring photos and description of any of these Sustainable mobility Water conservation Renewable energy Resource recovery Waste minimization Reduce/Reuse/Recycle/Recover 29-08-2024 Department of Civil Engineering, IIT Hyderabad 8 IMPACT OF HUMAN UPON ENVIRONMENT To satisfy natural needs – Air, Water, Food and Shelter To satisfy acquired needs – Automobiles, Appliances, Comfort, Processed Food and Beverages https://www.sciencedirect.com/science/article/pii/S0959652604000289 Pritha Chatterjee, 9 29-08-2024 Department of Civil Engineering IMPACT OF ENVIRONMENT UPON HUMAN Rivers became stagnant Sky became smoke-shrouded Dumping grounds became odoriferous Communicable diseases due to polluted air and water Loss of cultural and aesthetic heritage Economic threats Irrespective of these populations ignore the impact of environment until they become aware of the ill-effects Some examples are use of DDT, dioxin, CFCs Pritha Chatterjee, 10 29-08-2024 Department of Civil Engineering WATER DISTRIBUTION Water covers approximately ¾ of the surface of earth 97% of the total water is saline, present in oceans and seas Out of the remaining 3%, a little over 2% is tied up in ice caps and glaciers and as atmospheric and soil moisture The remaining 0.62% is accessible 29-08-2024 Pritha Chatterjee 11 WASTEWATER TREATMENT STATUS IN INDIA As per the CPCB reports the total wastewater generated by 23 metro cities is 9,275 MLD [CPCB, 1997] The ratio of industrial to municipal wastewater varies from 0.06% to 2% Around 100 million m3 of untreated wastewater is discharged into the river Ganga per day (Birol and Das, 2010) 73% of the sewage generated in our country – is discharged untreated Out of 115 centralised sewage treatment plants located in our country – 45 failed to achieve the prescribed effluent discharge standards Pritha Chatterjee, 12 29-08-2024 Department of Civil Engineering ROLE OF AN ENVIRONMENTAL ENGINEER Natural Processes Dilutions Biochemical conversions Convert waste to more acceptable forms No longer is enough Chemical Reactions Environmental Engineers design facilities based on the principles in nature in an amplified and optimized way, to handle larger volumes and treat in shorter time frame Occasionally environmental engineers design to reverse or counteract natural processes Pritha Chatterjee, 13 29-08-2024 Department of Civil Engineering PRINCIPLES Sustainable Development - Use of resources to satisfy current needs without compromising future availability of resources Eg: Develop solutions that protect both our quality of life and the environment, Organic Agriculture, Technology that reduces pollution, Recycling rather than disposal, Alternative fuels Circular Economy - Innovative waste treatment and management solutions aiming not only at treating the waste, but also providing other benefits such as facilitating reuse of treated waste, resources recovery in the form of energy or other value added products or nutrient reuse 4R – Reduce, Reuse, Recycle and Recover Pritha Chatterjee, 14 29-08-2024 Department of Civil Engineering 29-08-2024 Pritha Chatterjee, Department of Civil Engineering 15 WATER SCARCITY 21 percent of communicable diseases in India are linked to unsafe water and the lack of hygiene practices. More than 500 children under the age of five die each day from diarrhea in India alone. 163 M people lack access to safe water 29-08-2024 Pritha Chatterjee 16 SOURCES OF WATER 1. Ground water Shallow well (take water from aquifers at depth < 30 m) – prone to bacterial contamination, free from turbidity Deep tube well (around 300 m) – no bacterial contamination but may contain inorganic pollutants like fluorides or arsenic Generally have Fe and Mn (not much health effect other than colour to the water) 2. Surface water Maybe receiving untreated effluents, making it most polluted Can have pathogens, inorganic pollutants, organic pollutants and turbidity Pritha Chatterjee 17 29-08-2024 Department of Civil Engineering SOURCES OF WATER 3. Sea water Saline and has high TDS 4. Treated effluent Gardening, flushing, air conditioning Drinking (??) Pritha Chatterjee, 18 29-08-2024 Department of Civil Engineering SOURCES OF WATER Raw water quality depends on the source of water and the intake point From Rudraprayag Ganga travels 2525 km before it meets the Bay of Bengal at Gangasagar Haridwar – Religious gatherings Kanpur – Industrial waste (tanneries) Allahabad – religious center (Puja samagri) Varanasi – Puja samagri and dead bodies Patna – Industrial and domestic waste 29-08-2024 Pritha Chatterjee 19 TREATMENT EVOLUTION WW 1: Public health Protection (Organic matter, Solids, Pathogen removal) WW 2: Ecological Protection (Removal of Nitrogen and Phosphorus to resist eutrophication) WW 3: Water Reclamation (Removal of Micropollutants) WW 4: Resource Recovery (Water, Nutrients, Energy) WW 5: Sustainability (Carbon footprint, Energy neutral) Pritha Chatterjee 29-08-2024 20 Department of Civil Engineering DEFINITIONS Pollution – Presence of impurities in such quantity and of such nature as to impair the use of the water for a stated purpose, usually caused by the presence of a foreign body which is organic or inorganic in nature Pathogen – disease causing microorganisms Wastewater- All forms of liquid waste. Industrial wastewater - Wastewater generated from the industrial and commercial areas. This wastewater contains objectionable organic and inorganic compounds that may not be amenable to conventional treatment processes. 29-08-2024 Pritha Chatterjee 21 DEFINITIONS Sewage - Wastewater generated from the lavatory basins, urinals and water closets of residential buildings, office building, theatre and other institutions. Also called domestic wastewater or sanitary sewage It contains numerous pathogenic or disease producing bacteria, along with high concentration of organic matter and suspended solids. Sewage Treatment Plant - is a facility designed to receive the waste from domestic, commercial and industrial sources and to remove materials that damage water quality and compromise public health and safety when discharged into water receiving systems or land. Sewer - It is an underground conduit or drain through which sewage is carried to a point of discharge or disposal. Sewerage - The infrastructure which includes device, equipment and appurtenances for the collection, transportation and pumping of sewage, but excluding works for the treatment of sewage. 29-08-2024 Pritha Chatterjee 22 DEFINITIONS Stormwater - Rain water of the locality. Subsoil water - Groundwater that enters into the sewers through leakages is called subsoil water. Sullage - This refers to the wastewater generated from bathrooms, kitchens, washing place and wash basins, etc. Composition of this waste does not involve higher concentration of organic matter and it is less polluted water as compared to sewage Sludge - Wastewater treatment residual 29-08-2024 Pritha Chatterjee 23 DEFINITIONS Unit Operation – Contaminant removal by physical forces Unit processes – Contaminant removal by biological and/or chemical processes Reactor – Refers to the vessel or containment structure along with all of its appurtenances, in which the unit operation or unit process take place Wastewater Treatment System – Combination of unit operations and unit processes designed to reduce certain constituents of wastewater to an acceptable level Primary Treatment – removes solid material from the incoming wastewater Secondary Treatment – biological conversion of the dissolved and the colloidal organics into biomass that can be removed at later stages Tertiary Treatment – involves further removal of solids and nutrients 29-08-2024 Pritha Chatterjee 24 DEFINITIONS Chemical treatment - A process applied to water and wastewater in which chemical changes occur. Physical treatment - A process applied to water and wastewater in which no chemical changes occur. Physical–chemical treatment - A process applied to water and wastewater in which chemical changes may or may not occur. 29-08-2024 Pritha Chatterjee 25  CHARACTERISTICS OF WATER AND WASTEWATER ❖ Water Quality ❖ Water Quantity 29-08-2024 Pritha Chatterjee, Department of Civil Engineering 26 STANDARDS Water Quality Requirement vary according to the proposed use Water Quality Standards represent a statutory requirement Eg: A farmer may know that saline water will damage crops, hence require water with less salinity, however, there may not be any official water quality standard that says that irrigation water cannot be saline. Standards Instream or surface water standards Potable Water Standards Wastewater Effluent Standards Pritha Chatterjee, 27 29-08-2024 Department of Civil Engineering DRINKING WATER STANDARDS, INDIA Parameters Desirable Permissible Risks Source Treatment pH 6.5-8.5 No relaxation Low pH - corrosion, metallic taste High pH – Natural Neutralization bitter/soda taste, deposits TDS 500 mg/L 2000 mg/L Hardness, scaly deposits, sediment, cloudy colored Livestock waste, septic system Reverse water, staining, salty or bitter taste, corrosion of pipes Landfills, nature of soil Hazardous Osmosis and fittings waste landfills Dissolved minerals, iron and manganese Hardness 300 mg/L 600 mg/L Scale in utensils and hot water system, soap scums Dissolved calcium and magnesium Softening from soil and aquifer minerals containing limestone or dolomite Alkalinity 200 mg/L 600 mg/L Low Alkalinity (i.e. high acidity) causes deterioration of Pipes, landfills Neutralization plumbing and increases the chance for many heavy metals in water are present in pipes, solder or plumbing fixtures Manganese 0.1 mg/L 0.3 mg/L Brownish color, black stains on laundry and fixtures at Landfills Deposits in rock and soil Chlorination.2 mg/l, bitter taste, altered taste of water-mixed beverages Sulphate 200 mg/L 400 mg/L Bitter, medicinal taste, scaly deposits, corrosion, Animal waste, By-product of coal RO laxative effects, "rotten-egg" odor from hydrogen mining, industrial waste Natural sulfide gas formation deposits or salt Nitrate 45 mg/L 100 mg/L Methemoglobinemia or blue baby disease in infants Sewage, Fertilizers RO 29-08-2024 Pritha Chatterjee 28 DRINKING WATER STANDARDS, INDIA Parameters Desirable Permissible Risks Source Treatment Fluoride 1 mg/L 1.5 mg/L Brownish discoloration of teeth, bone damage Natural, Industrial Waste RO Chloride 250 mg/L 1000 mg/L High blood pressure, salty taste, corroded pipes, fixtures Fertilizers, Industrial Waste, Sea RO and appliances, blackening and pitting of stainless steel Water Arsenic 0.05 mg/L No Weight loss; Depression; Lack of energy; Skin and nervous Pesticides, Natural RO, Softening relaxation system toxicity Chromium 0.05 mg/L No Skin irritation, skin and nasal ulcers, lung tumors, Sewage, Industrial Waste, RO relaxation gastrointestinal effects, damage to the nervous system and Natural circulatory system, accumulates in the spleen, bones, kidney and liver Copper 0.05 mg/L 1.5 mg/L Anaemia, digestive disturbances, liver and kidney damage, Leaching from copper water RO gastrointestinal irritations, bitter or metallic taste; Blue- pipes and tubing, algae green stains on plumbing fixtures treatment Industrial and mining waste, wood preservatives Natural deposits Cyanide 0.05 mg/L No Thyroid, nervous system damage Ferticlizer, E-waste RO, Chlorination relaxation Lead 0.05 mg/L No Reduces mental capacity (mental retardation), interference Paint, discarded batteries, Activated Carbon, relaxation with kidney and neurological functions, hearing loss, blood paint, leaded gasoline Natural Reverse Osmosis disorders, hypertension, death at high levels deposits 29-08-2024 Pritha Chatterjee 29 DRINKING WATER STANDARDS, INDIA Parameters Desirable Permissible Risks Source Treatment Mercury 0.001 mg/L No relaxation Brownish discoloration of teeth, bone Natural, Industrial Waste RO damage Zinc 5 mg/L 15 mg/L Metallic Taste Leaching of galvanized pipes and fittings, Softening, RO paints, dyes Natural deposits Iron 0.3 mg/L __ Metallic Taste, brown color to clothes Plumbing RO Manganese 0.05 mg/L __ Medicinal Taste Plumbing RO Sodium Increased blood pressure Pathogen 95% of samples No relaxation Gastrointestinal illness Wastewater Chlorination should not contain coliform in 100 ml 29-08-2024 Pritha Chatterjee 30 WATER QUALITY PARAMETERS Physical Parameters Suspended solids (mg/L), Turbidy (NTU), Colour (TCU), Taste and Odour (TON), Temperature (℃) Chemical Parameters Total Dissolved Solids (mg/L), Alkalinity (mg/L of CaCO3), Hardness (mg/L of CaCO3), Fluorides, Metals (mg/L), Organics (BOD, COD, TOC), Nutrients (Nitrogen and Phosphorus) Biological Parameters Pathogens 29-08-2024 Department of Civil Engineering, IIT Hyderabad 31 IMPACTS Physical Parameters – aesthetics and normally a marker of other pollutants in water Dissolved Solids – imparts colour and taste to water Alkalinity – imparts a bitter taste to water, precipitations of cations in pipelines and other appurtenances Hardness – Scaling or fouling of water heaters or hot-water pipes, laxative effect from magnesium Fluorides – mottling of teeth Non-toxic metals – Hardness ions, sodium, iron, manganese, aluminium, copper and zinc. Sodium being the most common. Bitter taste Toxic metals – Arsenic, barium, cadmium, chromium, lead, mercury, silver. Hazardous and can cause diseases. These are concentrated by the food chain, thereby causing maximum damage to the top of the food chain Organics and Nutrients – Increases oxygen demand Nitrate – Methamoglobinamea in infants Pritha Chatterjee, 32 29-08-2024 Department of Civil Engineering TURBIDITY Measure of the extent to which suspended matter in water either absorbs or scatters radiant light energy impinging upon the suspension Measurement A beam of light from a source produced by a standardized electric bulb is passed through a sample vial The sample “scatters” the light that impinges upon it The scattered light is then measured by putting the photometer at right angle from the original direction of the light generated by the light source This measurement of light scattered at a 90-degree angle is called Nephelometry Unit - Nephelometric turbidity unit (NTU) 29-08-2024 Pritha Chatterjee 33 COLOR A perception registered as radiation of various wavelengths strikes the retina of the eye Color may be objectionable not for health reasons but for aesthetics Color due to suspended matter is apparent color and that due to dissolved solids is true colour Source - Materials decayed from vegetation and inorganic matter impart color to water Unit – One milligram per liter of Pt in potassium chloroplatinate is one unit of color TASTE A perception registered by the taste buds Measurement – A volume of sample A (in mL) is diluted with distilled water volume B (in mL), so that the taste of the resulting mixture, is just barely detectable 𝐴+𝐵 𝑇ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑 𝑇𝑎𝑠𝑡𝑒 𝑁𝑢𝑚𝑏𝑒𝑟 𝑇𝑇𝑁 = 𝐴 29-08-2024 Pritha Chatterjee 34 ODOR Perception registered by the olfactory nerves Measurement – A volume of sample A (in mL) is diluted with distilled water volume B (in mL), so that the odor of the resulting mixture, is just barely detectable Source – Ammonia, Mercaptans, Amines, Diamines, Sulphides 𝐴+𝐵 𝑇ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑 𝑂𝑑𝑜𝑟 𝑁𝑢𝑚𝑏𝑒𝑟 𝑇𝑂𝑁 = 𝐴 TEMPERATURE Temperature affects the efficiency of treatment units. For example, in cold temperatures, the viscosity increases. This, in turn, diminishes the efficiency of settling of the solids because of the resistance that high viscosity offers to the downward motion of the particles. Pressure drops also increase in the operation of filtration units, again, because of the resistance that the higher viscosity offers. 29-08-2024 Pritha Chatterjee 35 ACIDITY pH is defined as the negative logarithm to the base 10 of the hydrogen ion activity expressed in gmols per liter: 𝑝𝐻 = −𝑙𝑜𝑔10 {𝐻+ } 𝑝𝑂𝐻 = −𝑙𝑜𝑔10 {𝑂𝐻− } Ion Product of water 𝑘𝑤 = 𝐻 + 𝑂𝐻− = 1 × 10−14 at 25 ℃ pH is an important parameter both in natural water systems and in water and wastewater engineering For example, nitrification plants are found to function at only a narrow pH range of 7.2 to 9.0. In water distribution systems, the pH must be maintained at above neutrality of close to 8 to prevent corrosion Above pH 8, the water could also cause scaling, which is equally detrimental when compared with corrosion 29-08-2024 Pritha Chatterjee 36 ALKALINITY Alkalinity is the measure of the ability of water to neutralize acids Alkalinity imparts a bitter taste to water Measured by titrating the water sample with acid (1 mL of 0.02 N H2SO4 can neutralize 1 mg of alkalinity as CaCO3) Constituents - Bicarbonate, Carbonate and Hydroxide ions – originates from the dissolution of minerals and also from carbon dioxide - Phosphates originate from detergents, fertilizers and insecticides - Hydrogen sulphide and ammonia are products of microbial decomposition of organic material 𝐶𝑂2 + 𝐻2 𝑂 𝐻2 𝐶𝑂3 𝐻2 𝐶𝑂3 𝐻 + + 𝐻𝐶𝑂3− 𝐻𝐶𝑂3− 𝐻 + + 𝐶𝑂32− 𝐶𝑂32− + 𝐻2 𝑂 𝐻𝐶𝑂3− + 𝑂𝐻− 29-08-2024 Pritha Chatterjee 37 ALKALINITY Conversion of carbonate to bicarbonate is essentially complete at pH 8.3 However, because bicarbonate is also an alkalinity species, an equal amount of acid must be added to complete the neutralization. Thus, the neutralization of carbonate is only one~half complete at pH 8.3. Conversion of hydroxide to water is virtually complete at pH 8.3, all of the hydroxide and one~half of the carbonate have been measured at pH 8.3. At pH 4.5 all of the bicarbonate has been converted to 𝐻 + + 𝑂𝐻 − 𝐻2 𝑂 carbonic acid, including the bicarbonate resulting 𝐶𝑂32− + 𝐻+ 𝐻𝐶𝑂3− from the reaction of the acid and carbonate 𝐻𝐶𝑂3− + 𝐻 + 𝐻2 𝐶𝑂3 Thus, the amount of acid required to titrate a sample to pH 4.5 is equivalent to the total alkalinity of the water 29-08-2024 Pritha Chatterjee 38 HARDNESS Concentration of multivalent cations in water Hardness associated with alkalinity causing anions are called carbonate hardness and the rest is non carbonate hardness Hardness is measured by titrating with EDTA using EBT as an indicator 29-08-2024 Pritha Chatterjee 39 SOLIDS Total solids - materials left after water has been evaporated from the sample. The evaporation is normally done at 103–105 °C. The filterable fraction contains the colloidal particles and the dissolved solids that pass through a filter The nonfilterable fraction (total suspended solids) contain the settleable and the nonsettleable fractions that did not pass through the filter The settleable fraction is the volume of the solids after settling for 30 minutes in a cone-shaped vessel called an Imhoff cone Fixed portions of the solids are those that remain as a residue when the sample is decomposed at 600 °C. Those that disappear are called volatile solids Volatile solids and fixed solids are normally used as measures of the amount of organic matter and inorganic matter in a sample, respectively. Magnesium carbonate, however, decomposes to magnesium oxide and carbon dioxide at 350°C. Thus, the amount of organic matter may be overpredicted and the amount of inorganic may be underpredicted if the carbonate is present in an appreciable amount. Unit – mg/L 29-08-2024 Pritha Chatterjee 40 QUESTIONS 1. What causes apparent colour and true colour? Ans: Colour due to suspended matter is apparent colour and that due to dissolved solids is true colour. 2. What are the sources of taste and odour in water? Ans: Dissolved minerals and organic matter 3. What is alkalinity? Ans: Alkalinity is the measure of the ability of water to neutralize acids. It is measured by titrating the water sample with acid (1 mL of 0.02 N H2SO4 can neutralize 1 mg of alkalinity as CaCO3). 4. What are the constituents of alkalinity in water? Ans: Bicarbonate, Carbonate and Hydroxide ions 5. What is hardness? Ans: Concentration of multivalent cations (Ca, Mg, Fe, Mg, Sr, Al). Measured by titrating with ethylene diamine tetra-acetic acid (EDTA) using eriochrome black T (EBT) as indicator. 6. What is carbonate hardness and non-carbonate hardness? Ans: Hardness equivalent to alkalinity is carbonate hardness and the rest is non-carbonate hardness 7. What are the sources of organics and nutrients? Ans: Wastewater Discharge 29-08-2024 Department of Civil Engineering, IIT Hyderabad 41 EXAMPLE 1. A suspended solids analysis is run on a sample. The tared mass of the crucible and filter is 55.3520 g. A sample of 260 mL is then filtered and the residue dried to constant mass at 103 °C. If the constant mass of the crucible, filter, and the residue is 55.3890 g, what is the suspended solids (SS) content of the sample? 2. How many grams of calcium will be required to combine with 90 g of carbonate to form calcium carbonate? 3. What is the equivalent calcium carbonate concentration of (a) 117 mg/L of NaCl, (b) 2*10^-3 mol of NaCl? 4. Tests for common ions are run on a sample of water and the results are shown below: Ca2+ 55 mg/L, Mg2+ 18 mg/L, Na+ 98 mg/L, HCO3- 250 mg/L, SO42- 60 mg/L, Cl- 89 mg/L. Draw a bar diagram representing the ions. 5. A 200 mL sample of water has an initial pH of 10. To titrate the sample to pH 8.3, 11 mL of 0.02 N H2SO4 is required. 30 mL of 0.02 N H2SO4 is required to titrate the sample to pH 4.5. What is the total alkalinity of the water in mg/L s CaCO3? Determine the quantity of each contributing species. 29-08-2024 Pritha Chatterjee 42 BIOCHEMICAL OXYGEN DEMAND Amount of oxygen consumed by organisms in the process of stabilizing waste - quantifies the amount of oxygen-consuming substances that a wastewater may contain. Measurement - Incubating a sample in a refrigerator for five days at a temperature of 20 °C and measuring the amount of oxygen consumed during that time. The difference in dissolved oxygen (DO) concentration between the final and the initial time after a period of incubation at some controlled temperature is measured - The saturation value of DO for water at 20 ℃ is only 9.1 mg/L, it is usually necessary to dilute the samples to keep final DO level, at the end of incubation period, above 1.5 mg/L - The total volume of the BOD bottle used for test is usually 300 mL - The dilution water (distilled water) is aerated for sufficient time to correct DO close to the saturation value - Nutrients and buffer solutions are added to the dilution water to provide nutrient for bacterial growth and maintain pH near neutral - Sufficient amount of seed is added to the BOD bottle to ensure adequate concentration of bacterial population to carry out the biodegradation - It is necessary to subtract the oxygen demand of the seed from the mixed sample, because organic matter present in this 1 to 2 mL of seed will also exert oxygen demand - It is important that in BOD work, the concentration of DO in the incubation bottle should not fall 2.0 mg/L, to avoid errors in measurement 29-08-2024 Pritha Chatterjee 43 BOD (𝐷𝑂𝑖 −𝐷𝑂𝑓 )−(𝐵𝑖 −𝐵𝑓 )(1−𝑝) 𝐵𝑂𝐷 = 𝑝 𝐷𝑂𝑖 and 𝐷𝑂𝑓 = DO of mixture, initial and final values, respectively, 𝐵𝑖 and 𝐵𝑓 = DO of blank, initial and final values, respectively, p = Vw/Vm = Volume of wastewater in mixture / Total volume of mixture Unless it is specified, BOD is measured at the standard temperature of incubation of 20 °C, for 5 days Biochemical oxidation is slow process and theoretically takes an infinite time to go to completion i.e. complete oxidation of organic matter. In the first five days, the period used for BOD determination, 60 to 70% oxidation is complete, 𝐵𝑂𝐷5 = 0.6 × 𝐵𝑂𝐷𝑢 Under Indian conditions, the BOD values are acceptable for 3 days incubation at 27 ℃ temperature 29-08-2024 Department of Civil Engineering, IIT Hyderabad 44 ULTIMATE BOD If incubation is done for a long period of time such as 20 to 30 days, it is assumed that all the BOD has been exerted. The BOD under this situation is the ultimate; therefore, it is called ultimate BOD or BODu Composed of carbonaceous (carbon content) and nitrogenous (ammonia content) portions If the BOD reaction is allowed to go to completion with the ammonia reaction inhibited, the resulting ultimate BOD is called ultimate carbonaceous BOD or CBOD Nitrosomonas and Nitrobacter, the organisms for the ammonia reaction, cannot compete very well with carbonaceous bacteria (the organisms for the carbon reaction), the reaction during the first few days of incubation up to approximately five or six days (BOD5) is mainly carbonaceous 29-08-2024 Pritha Chatterjee 45 NITRIFICATION IN BOD Non-carbonaceous matter, such as ammonia is produced during the hydrolysis of proteins. In addition, when the living things die, excreta waste, and nitrogen organic compounds, the nitrogen tied to organic molecule is converted to ammonia by bacterial and fungal action. Under aerobic conditions, this ammonia will be converted to nitrate, called as nitrification as per the reactions given below by two types of autotrophic bacteria - Nitrosomonas and Nitrobacter 2𝑁𝐻3 + 3𝑂2 → 2𝑁𝑂2− + 2𝐻 + + 2𝐻2 𝑂 (Nitrosomonas) 2𝑁𝑂2− + 𝑂2 → 2𝑁𝑂3− (Nitrobacter) 29-08-2024 Pritha Chatterjee 46 MATHEMATICAL ANALYSIS OF BOD Assumption - rate at which the oxygen is consumed is directly proportional to the concentration of degradable organic matter remaining at any time The kinetics of BOD reaction can be formulated in accordance with first order reaction kinetics as: ⅆ𝐿𝑡 = −𝐾𝐿𝑡 ⅆ𝑡 𝐿𝑡 = 𝐿0 𝑒 −𝑘𝑡 The amount of BOD that has been exerted Lt = amount of first order BOD remaining in wastewater at time t 𝐵𝑂𝐷𝑡 = 𝐿0 1 − 𝑒 −𝑘𝑡 K = First order BOD reaction rate constant, time-1 Lo or BODu at time t = 0, is the ultimate first stage BOD initially present in the sample 29-08-2024 Department of Civil Engineering, IIT Hyderabad 47 BOD REACTION RATE CONSTANT The BOD reaction rate constant depends on: 1. Nature of waste – Higher rate constant for easily degradable waste like sugar and lower k for starch Q – Which one has higher K, treated sewage or raw sewage? 2. The ability of the organisms in the system to utilize the waste 3. Temperature - biochemical reactions are temperature dependent and the activity of the microorganism increases with the increase in temperature up to certain value, and drop with decrease in temperature BOD rate constant is adjusted to the temperature of receiving water: 𝐾𝑇 = 𝑘20 𝜃 (𝑇−20) T = temperature of interest, ℃ KT = BOD rate constant at the temperature of interest, day-1 K20 = BOD rate constant determined at 20oC, day-1 θ = temperature coefficient (1.056 in general and 1.047 for temp.> 20 ℃ 29-08-2024 Department of Civil Engineering, IIT Hyderabad 48 EXERCISE 1. A wastewater sample is expected to have BOD5 of about 200 mg/L. The initial DO of dilution water is 8.0 mg/L. Calculate the dilution requirement for BOD determination. 2. A test bottle containing only seeded dilution water has its DO level drop by 1.0 mg/L in a 5-day incubation. A 300 mL BOD bottle filled with 10 mL of wastewater and the rest seeded dilution water experiences a DO drop of 6.2 mg/L in the same time period. What would be five day BOD of the wastewater? 3. Treated wastewater is being discharged into a river that has a temperature of 15 ℃. The BOD rate constant determined in the laboratory for this mixed water is 0.12 per day. What fraction of maximum oxygen consumption will occur in first four days? 4. The dissolved oxygen in an unseeded sample of diluted wastewater having an initial DO of 9.0 mg/L is measured to be 3.0 mg/L after 5 days. The dilution fraction is 0.03 and reaction rate constant k = 0.22 d-1. Calculate a) 5 day BOD of the waste, b) ultimate carbonaceous BOD, and c) What would be remaining oxygen demand after 5 days? 29-08-2024 Pritha Chatterjee 49 EXERCISE 5. Determine ultimate BOD for a wastewater having 5 day BOD at 20 ℃ as 160 mg/L. Assume reaction rate constant as 0.2 per day (base 10). 6. The BOD of a sewage incubated for one day at 30 ℃ has been found to be 100 mg/L. What will be the five day 20 ℃ BOD? Assume K = 0.12 (base 10) at 20 ℃, and θ = 1.056 7. Determine the 1 day BOD and ultimate first stage BOD for a wastewater whose 5 day 20 ℃ BOD is 200 mg/L. The reaction rate constant k (base e) = 0.23 per day. 29-08-2024 Pritha Chatterjee 50 SOLUTION 1. BOD = 200 mg/L; DOi = 8.0 mg/L, Minimum DO that should be left after five days of incubation is 2.0 mg/L, Say final DO = 2.0 mg/L Hence, dilution required = 200 / (8.0 – 2.0) = 33.33 say 35 to 40 times. 2. Dilution factor p = 10/300 Therefore, BOD5 = [6.2 – 1.0 (1 – (10/300))] / (10/300) = 157 mg/L 29-08-2024 Pritha Chatterjee 51 SOLUTIONS 3. 𝐾15 = 𝑘20 1.056(𝑇−20) = 0.091 𝑑 −1 𝐵𝑂𝐷4 = 𝐿0 1 − 𝑒 −4𝑘 𝐵𝑂𝐷4 Therefore, fraction of oxygen consumption = = 1 − 𝑒 −4𝑘 = 0.305 𝐿0 (𝐷𝑂𝑖 −𝐷𝑂𝑓 )−(𝐵𝑖 −𝐵𝑓 )(1−𝑝) 9−3 4. a) Oxygen demand for first 5 days, 𝐵𝑂𝐷 = = = 200 𝑚𝑔/𝐿 𝑝 0.03 𝐵𝑂𝐷5 200 b) Ultimate BOD,𝐿0 = = = 300 𝑚𝑔/𝐿 1−ⅇ −5𝑘 1−ⅇ −5×0.22 c) After 5 days, 200 mg/L of oxygen demand out of total 300 mg/L would be satisfied. Hence, the remaining oxygen demand would be 300 – 200 = 100 mg/L 29-08-2024 Pritha Chatterjee 52 SOLUTIONS 5. 𝐵𝑂𝐷𝑡 = 𝐿0 1 − 𝑒 −𝑘𝑡 , 160 = 𝐿0 1 − 10−0.2×5 Therefore, 𝐿0 = 177.8 mg/L ~ 178 mg/L 6. BOD at 30 ℃= 100 mg/L, K20 = 0.12 𝐾30 = 𝑘20 1.056(30−20) = 0.207 𝑑 −1 𝐵𝑂𝐷𝑡 = 𝐿0 1 − 𝑒 −𝑘𝑡 100 = 𝐿0 1 − 10−0.207×1 𝐿0 = 263.8mg/L (This is ultimate BOD, the value of which is independent of incubation temperature) 𝐵𝑂𝐷5 = 𝐿0 1 − 𝑒 −5𝑘 = 263.8 1 − 𝑒 −0.12×5 = 197.5 mg/L = 200 mg/L 29-08-2024 Pritha Chatterjee 53 SOLUTIONS 𝐵𝑂𝐷5 200 7. Ultimate BOD, 𝐿0 = = = 293 𝑚𝑔/𝐿 1−ⅇ −5𝑘 1−ⅇ −5×0.23 Therefore, one day BOD, 𝐵𝑂𝐷1 = 𝐿0 1 − 𝑒 −𝑘 = 293 1 − 𝑒 −0.23 = 60 𝑚𝑔/𝐿 29-08-2024 Pritha Chatterjee 54 CHEMICAL OXYGEN DEMAND (COD) Measure the oxygen equivalent content of a given waste by using a chemical to oxidize the organic content of the waste Q. What is higher, BOD or COD? In some types of wastes, a high degree of correlation may be established between COD and BOD5 If such is the case, a correlation curve may be prepared such that instead of analyzing for BOD5, COD may be analyzed, instead. This is practically advantageous, since it takes five days to complete the BOD test but only three hours for the COD test 29-08-2024 Pritha Chatterjee 55 NITROGEN Nitrogen in wastewater comes from protein in our food – this is organic nitrogen Sum of the organic, free ammonia, nitrite, and nitrate nitrogen is called total nitrogen Sum of ammonia and organic nitrogen is called total Kjeldahl nitrogen (TKN) Nitrite nitrogen is very unstable and is easily oxidized to the nitrate form. Because its presence is transitory, it can be used as an indicator of past pollution that is in the process of recovery Nitrate nitrogen is the most oxidized form of nitrogen. Excess nitrate can cause methemoglobinemia (blue baby syndrome) in babies 29-08-2024 Pritha Chatterjee 56  Quantity estimation ❖ Water Demand ❖ Variation in Demand ❖ Population Forecasting 29-08-2024 Pritha Chatterjee, Department of Civil Engineering 57 WATER DEMAND 1. Residential or domestic - 135 lpcd for proper living of human beings and if water scarcity is there a minimum of 70 to 100 lpcd 2. Institutional - water consumption in hospitals, hotels, boarding schools, restaurants Hospital, bed < 100 – 450 L/bed, bed > 100 – 340 L/bed Hotel – 180 lpcd Hostels – 135 lpcd Restaurants – 70 lpcd Ports – 70 lpcd Schools, colleges, offices – 45 lpcd Cinema Halls – 15 lpcd 3. Public or civic use - road washing, public park, sanitation, fire fighting 4. Industrial use – 20-25% per capita 5. Water system losses – 20% per capita Pritha Chatterjee 58 29-08-2024 Department of Civil Engineering RESIDENTIAL USE Bathing – 55 Washing of clothes – 20 Flushing – 30 Washing house – 10 Utensils – 10 Cooking – 5 Drinking - 5 Other than human use, domestic animals should also be considered Cow and buffalo – 40 to 60 Horse – 40 – 50 Dog – 8 – 12 Sheep or Goat – 5 to 10 These demands we have to include depending upon the area whether they have the domestic animals and live stocks with human beings. Pritha Chatterjee 59 29-08-2024 Department of Civil Engineering PUBLIC USE 1. Road washing – 5 lpcd 2. Sanitation – 3 – 5 lpcd (public facilities) 3. Public parks – 2 to 3 L/m2.d 4. Fire fighting When there is fire enough water should be available for a short period under very high pressure Fire hydrants of 15 to 20 centimeter diameter is normally provided and these fire hydrants are usually connected to water supply mains and not the branch pipes. Pressure of water nozzle required for extinguishing the fire is around 1 to 2 kg/cm2. For a fire of moderate nature, three streams of 1100 liters per minute of water is essential Pritha Chatterjee 60 29-08-2024 Department of Civil Engineering FIRE DEMAND Kuchling’s formula 𝑄 = 3182 𝑃 , Q is quantity of water in L/min, P – Population in thousands Buston’s formula 𝑄 = 5663 𝑃 Freeman’s formula 𝐹 𝑄 = 1136 + 10 ,𝐹 = 2.8 𝑃 , F is the number of simultaneous fire streams required. 5 National board of fire underwriters 𝑄 = 4637 𝑃 1 − 0.01 𝑃 Pritha Chatterjee 61 29-08-2024 Department of Civil Engineering WATER LOSSES 1. Leakage and overflow from service reservoirs 2. Leakage from main and service pipelines 3. Leakage and losses on consumer’s premises 4. Under registration of supply meter 5. Leakage from public taps In a well maintained water distribution system the losses hardly exceeds 20%. But if a system is partly metered and partly unmetered it can go up to 50% because if the water is not metered people will be having the tendency to use more and more water so definitely that will be increasing the water loss. Any water which is going unmeasured is also coming under water losses so the water losses will be increasing. Pritha Chatterjee 62 29-08-2024 Department of Civil Engineering FACTORS AFFECTING WATER DEMAND Size and type of community – big city less fluctuation, Standard of living – car washing, lawns etc., type of community Climate Quality of water Pressure in the supply – high pressure, more losses System of supply – intermittent-continuous, piped or not Sewerage system Metering Pritha Chatterjee 63 29-08-2024 Department of Civil Engineering VARIATIONS IN DEMAND Maximum seasonal consumption – 130% of annual average daily rate of demand Maximum monthly consumption – 140% of the annual average daily rate of demand Maximum daily consumption – 180% of annual average daily rate of demand Maximum hourly consumption – 150% average for the day Filters and pumps are design for 1.5 times average daily demand. Distribution mains are designed for the maximum hourly demand for a A city has a projected population of 60,000 spread over area of maximum consumption day. That means 50 hectare. Find the design discharge for the separate sewer 1.8 to 1.5 that means 2.7 times the average daily demand. line by assuming rate of water supply of 250 LPCD and out of this total supply only 75 % reaches in sewer as wastewater. Sedimentation tanks and water reservoirs are designed for average Make necessary assumption whenever necessary. daily demand. Pritha Chatterjee 29-08-2024 Department of Civil Engineering 64 DESIGN PERIOD Number of years in future for which the given facility is available to meet the demand Dams – 50 years Treatment units – 30 years Pumping station – 30 years Pumps – 15 years Distribution system – 30 years Overhead tanks – 15 years Factors 1. Service time 2. Cost involved in expansion 3. Interest rate Pritha Chatterjee, 65 29-08-2024 Department of Civil Engineering POPULATION PROJECTION 1. Arithmetic Increase Method 𝑷 = 𝑷𝟎 + 𝒏𝑰 P0 is the population at the last known decade, and n is the number of decades and I is the average increase rate per decade, Assumption: Increase of population is constant per decade 2. Incremental Increase Method 𝒏(𝒏+𝟏) 𝑷 = 𝑷𝟎 + 𝒏𝑰 + 𝒓 r is the average incremental increase rate per decade 𝟐 3. Geometric Increase Method 𝑰𝒈 𝒏 𝑷𝒏 = 𝟏 + 𝟏𝟎𝟎 Ig is the average percentage increase 4. Graphical Method – extrapolation of growth curve 5. Logistic Method - mathematically modeled with a logistic or S shaped growth curve 6. Method of Density – the city is divided into zones Pritha Chatterjee, 66 29-08-2024 Department of Civil Engineering PROBLEMS What will be the population in the year 2021 for the following two cities. What methods will you use? City A: 1971 – 10000, 1981 – 16000, 1991 – 22000, 2001 – 28000, 2011 – 33000 City B: 1971 – 72000, 1981 – 85000, 1991 – 110500, 2001 – 144000, 2011 – 184000 Pritha Chatterjee 67 29-08-2024 Department of Civil Engineering  WATER PURIFICATION IN NATURAL SYSTEMS 29-08-2024 Pritha Chatterjee, Department of Civil Engineering 68 PHYSICAL PROCESSES Dilution Success depends upon discharging small quantities of waste into large water bodies. If the volumetric flow rate and the concentration of a given material are known in both the stream and waste discharge, the concentration after mixing can be calculated as follows: 𝐶𝑠 𝑄𝑠 + 𝐶𝑤 𝑄𝑤 = 𝑄𝑚 𝐶𝑚 where C represents the concentration (mass/ volume) of the selected material, Q is the volumeric flow rate (volume/time) and the subscripts s, w and m designate stream, waste, and mixture conditions. 29-08-2024 Department of Civil Engineering, IIT Hyderabad 69 PHYSICAL PROCESSES Example 1: A treated wastewater enters a stream. The concentration of sodium in the stream at point A is 10 mg/ L, and the flow rate is 20 m3/s. The concentration of sodium in the waste stream is 250 mg/L, and the flow rate is 1.5 m3/s. Determine the concentration of sodium at point B assuming that complete mixing has occurred. A B Example 2: A municipal wastewater treatment plant receives a seasonal discharge from a fruit processing plant. Influent flow and strengths of the wastewater when the industry is both on and offline are shown below. Determine the contribution of each constituent by the industry. Industry online Industry offline Flow (m3/d) 18750 13275 BOD5 (mg/L) 300 215 SS (mg/L) 420 240 Ammonia (mg/L) 64 15 Chloride (mg/L) 29 41 Alkalinity (mg/L) 57 125 29-08-2024 Department of Civil Engineering, IIT Hyderabad 70 PHYSICAL PROCESSES Sedimentation and Resuspension Most large solids will settle out readily. Particles in the colloidal size range stay in suspension for long periods of time though eventually most of these will also settle out. Anaerobic conditions are likely to develop in sediment deposits and any organics trapped in them will decompose, releasing soluble compounds into the stream above. Sediment deposits can also alter the streambed by filling up the pore space and creating unsuitable conditions for the reproduction of many aquatic organisms The development of banks of silt and mud along the bottom or streams can alter its course or hamper navigation activities. Sediment accumulations also reduce reservoir storage capacities and increase flooding due to channel fill-in. Resuspension of solids is common in times of flooding or heavy runoff 29-08-2024 Department of Civil Engineering, IIT Hyderabad 71 PHYSICAL PROCESSES Filtration As water percolates from the surface downward into groundwater aquifers, filtration occurs and if the soil layers are deep and fine enough, removal of suspended material is essentially complete by the time the water enters into the aquifer Gas Transfer - Replenishment of oxygen lost due to bacterial degradation of organic matter - Transfer of gases evolved in water due to chemical and biological processes - Solubility of a gas in equilibrium with a liquid is quantified by Henry’s Law: 𝑥 = 𝑃/𝐻 In which x is the equilibrium mole fraction of the dissolved gas at 1 atm, H is the coefficient of absorption (Henry’s coefficient, which is unique for each gas-liquid system at a particular temperature), P is the pressure of the gas above the liquid. Example 3: Calculate the solubility of air in water at 0 ℃ and 1 atm pressure. H = 4.32 *10^4 atm/mol fraction, for air 28.9 g/gmol 29-08-2024 Department of Civil Engineering, IIT Hyderabad 72 PHYSICAL PROCESSES Gas Transfer Rate ⅆ𝐶 = (𝐶𝑠 − 𝐶)𝑘𝑎 ⅆ𝑡 Where dC/dt is the instantaneous rate of change of the concentration of gas in the liquid, 𝐶𝑠 and C are the saturation and actual concentration, respectively 𝑘𝑎 is a constant and depends on temperature, interfacial area available for gas transfer and resistance of movement from one phase to another 29-08-2024 Department of Civil Engineering, IIT Hyderabad 73 PHYSICAL PROCESSES Heat Transfer Bodies of water lose and gain heat slower than land or air masses. This makes aquatic plants and animals less adaptable to sudden changes in temperature Thermal Stratification – Fresh water has maximum density at 4 ºC with density declining as water moves to freezing point or gets warmer Water divides into a layer of warm circulating water called epilimnion and a lower layer of cool relatively undisturbed layer called hypolimnion. The two layers are separated by thermocline which has a steep temperature gradient 29-08-2024 Department of Civil Engineering, IIT Hyderabad 74 CHEMICAL PROCESSES The most important chemical process taking place in water bodies help stabilize the pH 𝐻2 𝑂 + 𝐶𝑂2 𝐻2 𝐶𝑂3 𝐻2 𝐶𝑂3 𝐻 + + 𝐻𝐶𝑂3− 𝐶𝑎𝐶𝑂3 + 𝐻+ 𝐶𝑎2+ + 𝐻𝐶𝑂3− This bicarbonate acts as a buffer that protects the stream from pH fluctuations 29-08-2024 Department of Civil Engineering, IIT Hyderabad 75 BIOCHEMICAL PROCESSES Metabolism is the sum of the processes by which living organism assimilate and use food for subsistence, growth and reproduction Catabolism provides the energy for the maintenance of other cell functions Anabolism provides the material necessary for cell growth. When an external food source is interrupted, the organisms will use stored food for maintenance energy in a process called Endogenous catabolism Enzymes may be considered as organic catalysts that influence reactions without becoming a reactant themselves. In biochemical processes, enzymes lower the activation energy necessary to initiate reactions. The enzyme then reverts to its original form for reuse 29-08-2024 Department of Civil Engineering, IIT Hyderabad 76 BIOCHEMICAL PROCESSES Aerobic microorganisms: When molecular oxygen is used as terminal electron acceptor in respiratory metabolism it is referred as aerobic respiration. The organisms that exist only when there is molecular oxygen supply are called as obligately aerobic. Anoxic microorganisms: For some respiratory microorganisms oxidized inorganic compounds such as sulphate, nitrate and nitrite can function as electron acceptors in absence of molecular oxygen; these are called as anoxic microorganisms. Obligate anaerobic: These are the microorganisms which generate energy by fermentation and can exist in absence of oxygen. They are only active in total absence of oxygen (free oxygen or bound oxygen) Facultative anaerobes: These microorganisms have ability to grow in absence or presence of oxygen. These can be divided in two types: (a) True facultative anaerobes: those can shift from fermentative to aerobic respiratory metabolism, depending on oxygen available or not; (b) Aerotolerant anaerobes: these follow strictly fermentative metabolism and are insensitive if oxygen is present in the system 29-08-2024 Department of Civil Engineering, IIT Hyderabad 77 BIOCHEMICAL PROCESSES Organisms that use inorganic materials as carbon source are called autotrophs Organisms that use organic materials as carbon source are called heterotrophs 29-08-2024 Department of Civil Engineering, IIT Hyderabad 78 BIOCHEMICAL PROCESSES Aerobic Anaerobic 29-08-2024 Department of Civil Engineering, IIT Hyderabad 79 RESPONSE TO BIODEGRADABLE ORGANICS Dissolved Oxygen Balance When biodegradable organics are discharged into a stream, microorganisms begin the metabolic processes that convert these organic matter into new cells and oxidized end products. The quantity of oxygen required for this conversion is called the biochemical oxygen demand (BOD) This oxygen consumed must be replenished: 1. Dissolution of oxygen from the atmosphere (Reaeration) 2. Production of oxygen by algal photosynthesis 29-08-2024 Department of Civil Engineering, IIT Hyderabad 80 DISSOLVED OXYGEN MODEL Most dissolved oxygen model relates to a model proposed by Streeter and Phelps in 1925 This model predicts DO deficit as a function of BOD exertion and stream reaeration Rate of Oxygen Removal The rate at which DO disappears from the stream coincides with the rate of BOD exertion, where D is the DO deficit 𝑑𝑦 𝑑𝐷 = 𝑑𝑡 𝑑𝑡 Increase in the rate of BOD exertion result in an increase in the rate of change of oxygen deficit 𝑦 = 𝐿0 − 𝐿𝑡 Here, 𝐿0 is the ultimate BOD and has a fixed value, therefore, 𝑑𝐷 𝑑𝑦 𝑑𝐿𝑡 = =− = 𝑘𝐿𝑡 𝑑𝑡 𝑑𝑡 𝑑𝑡 This rate of change of oxygen deficit can be depicted as 𝑟𝐷 = 𝑘1 𝐿𝑡 (𝑘1 and k are same) 29-08-2024 Department of Civil Engineering, IIT Hyderabad 81 DISSOLVED OXYGEN MODEL Rate of Oxygen Addition Rate of reaeration is a first-order reaction with respect to the magnitude of the oxygen deficit 𝑟𝑅 = −𝑘2 𝐷 Where 𝑟𝑅 is the rate at which oxygen becomes dissolved and 𝑘2 is reaeration rate constant. The negative sign reflects the fact that an increase in the oxygen concentration due to reaeration reduced the oxygen deficit The reaeration rate constant is also temperature dependent 𝐾𝑇 = 𝑘20 𝜃 (𝑇−20) T = temperature of interest, ℃ KT = reaeration rate constant at the temperature of interest, day-1 K20 = reaeration rate constant determined at 20 ℃, day-1 θ = temperature coefficient (1.016 in general) 29-08-2024 Department of Civil Engineering, IIT Hyderabad 82 OXYGEN SAG CURVE The oxygen deficit in a stream is a function of both utilization and reaeration The rate of change in the deficit is the sum of the two reactions 𝑑𝐷 = 𝑟𝐷 + 𝑟𝑅 = 𝑘1 𝐿𝑡 − 𝑘2 𝐷 𝑑𝑡 The actual oxygen concentration has a characteristic dip, resulting in the term Oxygen Sag Curve 29-08-2024 Department of Civil Engineering, IIT Hyderabad 83 OXYGEN SAG CURVE Critical Deficit (Dc) - The point of lowest DO concentration. The time of travel to this point is called the critical time (tc) 𝑘1 𝐷𝑐 = 𝐿0 𝑒 −𝑘1𝑡𝑐 𝑘2 1 𝑘2 𝑘2 − 𝑘1 𝑡𝑐 = ln[ 1 − 𝐷0 ] 𝑘2 − 𝑘1 𝑘1 𝑘1 𝐿0 𝑘1 𝐿0 𝐷= 𝑒 −𝑘1𝑡 − 𝑒 −𝑘2𝑡 + 𝐷0 𝑒 −𝑘2𝑡 𝑘2 − 𝑘1 𝑥 𝑡= 𝑢 29-08-2024 Department of Civil Engineering, IIT Hyderabad 84 ORGANIC DISCHARGE AND STREAM ECOLOGY Degradation zone – turbid water with sludge deposits and floating debris Active Decomposition Zone – grayish water with scum and septic conditions Recovery Zone – Clear water Clean Water Zone – Natural stream conditions are restored 29-08-2024 Department of Civil Engineering, IIT Hyderabad 85 OXYGEN SAG CURVE A municipal wastewater treatment plant discharges secondary effluent to a surface stream. The worst conditions are known to occur in summer months when stream flow is low and water temperature is high. Under these conditions, measurements are made in the laboratory and in the field to determine the characteristics of the wastewater and stream flows. The wastewater is found to have a maximum flow rate of 15000 m3/d, a BOD5 of 40 mg/L, a DO of 2 mg/L and a temperature od 25 ºC. The stream (upstream from the point of wastewater discharge) is found to have a minimum flow rate of 0.5 m3/s, a BOD5 of 3 mg/L, a DO of 8 mg/L and a temperature od 22 ºC. Complete mixing of the wastewater and stream is almost instantaneous, and the velocity of the mixture is 0.2 m/s. From the flow regime the reaeration constant is estimated to be 0.4 day-1 at 20 ºC. Sketch the DO profile for a 100 km stretch of the stream below the discharge. 29-08-2024 Department of Civil Engineering, IIT Hyderabad 86  ENGINEERED SYSTEMS FOR WATER PURIFICATION 29-08-2024 Pritha Chatterjee, Department of Civil Engineering 87 TREATMENT OF GROUND WATER Most GW are clear and pathogen free and do not contain significant amount of organic materials Otherwise GW may contain dissolved gases or solids – Physicochemical treatment might be necessary Process for hard ground water Chemicals added Waste Stream Aeration: Removes undesirable gases and/or oxidation Gases to atmosphere of iron and manganese Lime CaCO3 Sludge to be removed and Softening: Removes calcium and/or magnesium disposed off, possible hardness Soda ash Mg(OH)2 recovery and reuse of lime Filtration: Removes residual CaCO3 crystals and Mg(OH)2 Backwash water decanted, sludge flocs left over from softening combined with previous sludge Disinfection: Destroys pathogens, enough added to Chlorine provide a residual Storage: Provides contact time for disinfection and stores water for peak demands Pritha Chatterjee, 88 29-08-2024 Department of Civil Engineering TREATMENT OF SURFACE WATER Often contain suspended and colloidal solids, organic matter and pathogen. Hardness is usually not present in surface water. Process for turbid surface water Chemicals added Waste Stream Plain sedimentation: Maybe necessary if water comes Sludge removed periodically and from fast-flowing streams. Removes larger suspended disposed off by spreading land solids Coagulation and flocculation: to remove colloidal solids Alum Sludge removed continuously, disposal by Polymers landfilling after dewatering Filtration: Polishes to remove remaining turbidity Backwash water decanted, sludge combined with previous sludge Adsorption: Maybe necessary if water contains dissolved organics; may consist of activated carbon columns Disinfection: Destroys pathogens, enough added to Chlorine provide a residual Storage: Provides contact time for disinfection and stores water for peak demands Pritha Chatterjee, 89 29-08-2024 Department of Civil Engineering AERATION 1. Removal of iron and manganese from ground water. 2. In the removal of hardness, the presence of high concentrations of carbon dioxide may result in high cost for lime, as CO2 reacts with lime. Thus, excess concentrations of this gas may be removed from the water by stripping or spraying the water into the air. 3. Dissolved gases like H2S, ammonia 4. Volatile solvents like benzene, carbon tetrachloride, p-dicholorobenzene, vinyl chloride, and trichloroethylene 5. Major purpose of dissolving air is to provide oxygen to be used by microorganism in the process of wastewater treatment – aeration employed in the activated sludge process. 29-08-2024 Pritha Chatterjee 90 SOFTENING Hardness in water used for domestic purposes is not completely removed. Hardness is normally removed to the level of 75 to 120 mg/L as CaCO3 Calcium hardness [Ca(HCO3)2, CaCl2] is removed through precipitation of CaCO3 Carbonate hardness is removed by providing the hydroxide radical 𝐶𝑎 𝐻𝐶𝑂3− 2 + 2𝑂𝐻 − → 𝐶𝑎𝐶𝑂3− ↓ +𝐶𝑂32− + 2𝐻2 𝑂 Noncarbonate hardness is removed by providing a carbonate ion. Usual source is soda ash 𝐶𝑎2+ + 𝐶𝑂32− → 𝐶𝑎𝐶𝑂3 ↓ Magnesium hardness is removed in the form of Mg(OH)2. Carbonate hardness - a source of the OH− ion is added to precipitate the Mg(OH)2 𝑀𝑔 𝐻𝐶𝑂3 2 + 4𝑂𝐻 − → 𝑀𝑔(𝑂𝐻)2 ↓ +2𝐶𝑂32− + 2𝐻2 𝑂 Noncarbonate hardness - associated with the sulfate anion; although, occasionally, large quantities of the chloride and nitrate anions may also be found. Also removed as hydroxide precipitate 𝑀𝑔𝑆𝑂4 + 2𝑂𝐻 − → 𝑀𝑔(𝑂𝐻)2 ↓ +𝑆𝑂42− 𝑀𝑔𝐶𝑙4 + 2𝑂𝐻 − → 𝑀𝑔(𝑂𝐻)2 ↓ +2𝐶𝑙 𝑀𝑔(𝑁𝑂3 )2 + 2𝑂𝐻 − → 𝑀𝑔(𝑂𝐻)2 ↓ +2𝑁𝑂3 29-08-2024 Pritha Chatterjee 91 LIME SODA PROCESS – LIME REACTIONS Soda ash (Na2CO3) and lime (CaO) may be used for the removal of hardness caused by calcium and magnesium 1. Slaking - CaO first reacts with water to form slaked lime 𝐶𝑎𝑂 + 𝐻2 𝑂 → 𝐶𝑎(𝑂𝐻)2 2. Bicarbonates are neutralized 𝐶𝑎(𝐻𝐶𝑂3 )2 +𝐶𝑎(𝑂𝐻)2 → 2𝐶𝑎𝐶𝑂3 ↓ +𝐻2 𝑂 𝑀𝑔(𝐻𝐶𝑂3 )2 +2𝐶𝑎(𝑂𝐻)2 → 𝑀𝑔(𝑂𝐻)2 ↓ +2𝐶𝑎𝐶𝑂3 ↓ +𝐻2 𝑂 3. Rest of the hardness is neutralized 𝑀𝑔𝑆𝑂4 + 𝐶𝑎(𝑂𝐻)2 → 𝑀𝑔(𝑂𝐻)2 ↓ +𝐶𝑎𝑆𝑂4 For every mole of magnesium hardness removed, there is 𝑀𝑔𝐶𝑙4 + 𝐶𝑎(𝑂𝐻)2 → 𝑀𝑔(𝑂𝐻)2 ↓ +𝐶𝑎𝐶𝑙2 a corresponding mole of by-product noncarbonate 𝑀𝑔(𝑁𝑂3 )2 + 𝐶𝑎(𝑂𝐻)2 → 𝑀𝑔(𝑂𝐻)2 ↓ +𝐶𝑎(𝑁𝑂3 )2 calcium hardness produced 29-08-2024 Pritha Chatterjee 92 LIME SODA PROCESS – SODA REACTIONS Removal of only calcium is initially targeted to achieve standards If the desired hardness level is not met by the removal of only the calcium ions, then removal of the magnesium may be initiated by addition of soda ash to remove the resulting noncarbonate hardness of calcium 𝐶𝑎𝑆𝑂4 + 𝑁𝑎2 𝐶𝑂3 → 𝐶𝑎𝐶𝑂3 ↓ +𝑁𝑎2 𝑆𝑂4 𝐶𝑎𝐶𝑙2 + 𝑁𝑎2 𝐶𝑂3 → 𝐶𝑎𝐶𝑂3 ↓ +2𝑁𝑎𝐶𝑙 𝐶𝑎(𝑁𝑂3 )2 +𝑁𝑎2 𝐶𝑂3 → 𝐶𝑎𝐶𝑂3 ↓ +2𝑁𝑎𝑁𝑂3 The precipitation of CaCO3 (9 – 9.5) and Mg(OH)2 (11) is pH dependent. This rise in pH is accomplished by addition of excess lime. If dissolved carbon dioxide is present in the water, it will also react with lime 𝐶𝑂2 + 𝐶𝑎𝑂 → 𝐶𝑎𝐶𝑂3 ↓ 29-08-2024 Pritha Chatterjee 93 STABILIZATION Complete removal of hardness cannot be accomplished by chemical precipitation, with up to 40 mg/L of CaCO3 and 10 mg/L of Mg(OH)2 remaining in the softened water, which eventually will precipitate either in the pipelines or in the storage facilities. These are converted back to their soluble bicarbonate forms by adding carbon dioxide. This also helps in lowering the pH - Recarbonation 𝐶𝑎𝐶𝑂3 + 𝐶𝑂2 + 𝐻2 𝑂 → 𝐶𝑎(𝐻𝐶𝑂3 )2 𝑀𝑔 𝑂𝐻 2 + 𝐶𝑂2 → 𝑀𝑔(𝐻𝐶𝑂3 )2 SOFTENING OPERATIONS 1. Mixer – mixing of chemicals with water 2. Flocculator – to aid in precipitate growth 3. Settling Basin – settling of precipitates 4. Stabilization – Carbon dioxide is added under pressure 29-08-2024 Department of Civil Engineering, IIT Hyderabad 94 PROBLEM A water with the ionic characteristics shown in the bar diagram below is to be softened to the minimum calcium hardness by the lime-soda ash process. Magnesium removal is not deemed necessary. Calculate the chemical requirements and solids produced in milliequivalents per liter. For a flow of 25,000 m3/d, calculate the daily chemical requirement and the mass of solids produced: Assume that the lime used is 90 percent pure and the soda ash is 85 percent pure 1 5 6 8 meq/L Ca2+ Mg2+ Na+ CO2 HCO3- SO42- meq/L 3.5 8 29-08-2024 Pritha Chatterjee 95 SOLUTION 𝐶𝑂2 + 𝐶𝑎 𝑂𝐻 2 → 𝐶𝑎𝐶𝑂3 ↓ +𝐻2 𝑂 𝐶𝑎(𝐻𝐶𝑂3 )2 +𝐶𝑎(𝑂𝐻)2 → 2𝐶𝑎𝐶𝑂3 ↓ +𝐻2 𝑂 𝐶𝑎𝑆𝑂4 + 𝑁𝑎2 𝐶𝑂3 → 𝐶𝑎𝐶𝑂3 ↓ +𝑁𝑎2 𝑆𝑂4 Lime required = for CO2 + Ca(HCO3)2 = 1+ 2.5 = 3.5 meq/L Soda Ash required = for CaSO4 = 1.5 meq/L Solids produced = 1 + 2.5*2 + 1.5 = 7.5 meq/L Equivalent mass of lime = 28 mg/meq Equivalent mass of soda ash = 53 mg/meq Therefore lime required = 1/0.9*28*3.5*25*106 = 2722 kg/d Soda ash required = 1/0.85*53*1.5*25*106 = 2338 kg/d Mass of solids produced = 50*7.5*25*106 = 9375 kg/d 29-08-2024 Pritha Chatterjee 96 ION EXCHANGE Replacing calcium and magnesium in the water with another non-hardness cation usually sodium A synthetic resin coated with the desirable exchange material is used In equal quantities. calcium and magnesium are adsorbed more strongly to the medium than is sodium 𝐶𝑎 𝐶𝑎 + 𝑎𝑛𝑖𝑜𝑛 + 2𝑁𝑎 𝑅 → 𝑅 + 2𝑁𝑎 + 𝑎𝑛𝑖𝑜𝑛 𝑀𝑔 𝑀𝑔 The reaction is virtually instantaneous and complete as long as exchange sites are available Advantages: Produces softer water than chemical precipitation Avoids the large quantity of sludges encountered in the lime-soda process The physical and mechanical apparatus is much smaller and simpler to operate 29-08-2024 Pritha Chatterjee 97 ION EXCHANGE Precautions: The water must be essentially free of turbidity and particulate matter or the resin will function as a filter and become plugged - Surfaces of the medium may act as an adsorbent for organic molecules and become coated Iron and manganese precipitates can foul the surfaces if oxidation – Softening of clear groundwater should be done immediately (before aeration occurs) Surface water should receive all necessary treatment, including filtration, prior to softening by ion exchange Design Problem: An ion exchange softener is to be used to treat the water in the previous example. The medium selected has an adsorption capacity of 90 kg/m3 at a flow rate of 0.4 m3/min.m2. Regeneration is accomplished using 150 kg of NaCl/m3 of resin in 10% solution. Determine the volume of medium required, chemical requirement and regeneration time. Assume regeneration flow rate as 1/10th of adsorption. 29-08-2024 Pritha Chatterjee 98 FILTRATION A polishing treatment to remove small flocs or precipitant particles It involves passing the water through a stationary bed of granular medium – solids in the water are retained by the filter medium. Filter Components 1. Filter box – constructed of reinforced concrete, structurally must be strong enough to support the weight of the underdrainage system, filter medium and water column, additionally must be watertight 2. Underdrainage system – collect and remove the filtered water and to disperse the backwash water. 3. Filtering media - Traditionally, silica sand has been the medium most commonly used in granular- medium filters. Modern filter applications often make use of anthracite coal and garnet sand in place of, or in combination with, silica sand Additionally piping systems, pumps, valves, backwash troughs, and other appurtenances for controlling the flow of water to and from the filter are necessary 29-08-2024 Department of Civil Engineering, IIT Hyderabad 99 TYPES OF FILTERS A. Based on Force applied 1. Gravity Filter - filters that rely on the pull of gravity to create a pressure differential to force the water through the filter 2. Pressure Filter - pressure differential used to drive the filtration is induced by impressing a high pressure on the inlet side of the filter medium 3. Vacuum Filter – pressure differential is created by ensuring a vacuum at the outlet end of the filter B. Based on Media used 1. Perforated plate 2. Wooden septum 3. Granular filter 29-08-2024 Pritha Chatterjee 100 TYPES OF FILTERS C. Based on rate of filtration 1. Slow Sand Filter (SSF) operate at a rate of 1.0 to 10 m3/d.m2 2. Rapid Sand Filter (RSF) operate at a rate of 100 to 200 m3/d.m2 Both of these are granular gravity filters Slow sand filter Rapid sand filter Coagulation Not required Required. Rate of filtration 100-150 LPH/Sqm. 100-150 LPM/Sqm. Distribution of grain size Uniform, effective size 0.2 mm Non uniform, fine at top and course at bottom, effective size 0.55 mm Cleaning period 1 to 3 months 2 to 3 days Method of cleaning Scrapping the top layers Agitation and back washing Efficiency More efficient for bacterial removal and less for Less efficient for bacterial removal and more turbidity and colour removal efficient for turbidity and colour removal. Back washing quantity No back washing required. Top sand layer is 2 to 4 % of filtered water. required replaced. Under drainage system Laid to receive filtered water Laid to receive filtered water and also to pass backwash water at a higher rate. 29-08-2024 Pritha Chatterjee 101 DISINFECTION Disinfection: unit process involving reactions that render pathogenic organisms harmless Sterilization: killing of all organisms Other water-treatment processes assist in removing pathogens: 90% of the bacteria and viruses is usually removed by coagulation, settling, and filtration. Excess lime softening is an effective disinfectant due to the high pH involved. However, to meet the standard of one coliform organism per 100 mL and to provide protection against regrowth, additional disinfection must be practiced Turbidity-producing colloids shields organisms from disinfectants, therefore before disinfection the water must be treated to remove turbidity. 29-08-2024 Pritha Chatterjee 102 DISINFECTANTS Chemical Disinfection: Historically, the most widely used chemical agent is chlorine. Other chemical agents that have been used include ozone, ClO2, the halogens bromine and iodine and bromine chloride, the metals copper and silver, KMnO4, phenol and phenolic compounds, alcohols, soaps and detergents, quaternary ammonium salts, hydrogen peroxide, and various alkalis and acids Physical Disinfection: ultraviolet light (UV), electron beam, gamma-ray irradiation, sonification, and heat Properties of a good disinfectant toxic to microorganisms at concentrations well below the toxic thresholds to humans and higher animals should have a fast rate of kill should be persistent enough to prevent regrowth of organisms in the distribution system 29-08-2024 Pritha Chatterjee 103 CHLORINE DISINFECTANTS Molecular chlorine [Cl2] A pale-green gas, which turns into a yellow-green liquid when pressurized Supplied from liquid chlorine that is shipped in pressurized cylinders Very poisonous and corrosive - the capturing hood vents should be placed at floor level, because the gas is heavier than air Calcium hypochlorite [𝑪𝒂 𝑶𝑪𝒍 𝟐] Available in powder or granular forms and compressed tablets or pellets Should be stored in a cool dry place and in corrosion-resistant containers Sodium hypochlorite [NaOCl] Available in solution form in strengths of 1.5 to 15% with 3% the usual maximum strength. The solution decomposes readily at high concentrations Stored in a cool dry place and in corrosion-resistant containers 29-08-2024 Pritha Chatterjee 104 CHLORINATION 𝐶𝑙2 𝑎𝑞 + 𝐻2 𝑂 𝐻𝑂𝐶𝑙 + 𝐻+ + 𝐶𝑙 − 𝑁𝑎𝑂𝐶𝑙 𝑁𝑎+ + 𝑂𝐶𝑙 − 𝐶𝑎 𝑂𝐶𝑙 2 𝐶𝑎2+ + 2𝑂𝐶𝑙 − By convention, the concentrations of the three species are expressed in terms of the molecular chlorine, Cl2 𝑚𝑔 𝐶𝑙 2×35.5 𝑚𝑔 1 𝐻𝑂𝐶𝑙 = 2 = = 1.35 𝐶𝑙2 𝐿 𝐻𝑂𝐶𝑙 1+16+35.5 𝐿 𝑚𝑔 𝐶𝑙2 2×35.5 𝑚𝑔 1 𝑁𝑎𝑂𝐶𝑙 = = = 0.95 𝐶𝑙2 𝐿 𝑁𝑎𝑂𝐶𝑙 23+16+35.5 𝐿 𝑚𝑔 2𝐶𝑙2 4×35.5 𝑚𝑔 1 𝐶𝑎 𝑂𝐶𝑙 2 = = = 0.99 𝐶𝑙2 𝐿 𝐶𝑎 𝑂𝐶𝑙 2 40+2×(16+35.5) 𝐿 29-08-2024 Pritha Chatterjee 105 REACTIONS IN CHLORINATION Disproportionation One of the chlorine atoms is oxidized to +1 and the other reduced to −1 𝐶𝑙2 𝑎𝑞 + 𝐻2 𝑂 𝐻𝑂𝐶𝑙 + 𝐻+ + 𝐶𝑙 − Dissociation 𝐻𝑂𝐶𝑙 𝐻 + + 𝑂𝐶𝑙 − The sum of HOCl and OCl- is called the free chlorine residual and is the primary disinfectant employed 29-08-2024 Pritha Chatterjee 106 REACTIONS IN CHLORINATION Reaction with inorganics Ferrous, manganous, nitrites and hydrogen sulphides react with chlorine and interferes it effectiveness as a disinfectant 2𝐹𝑒 2+ + 𝐻𝑂𝐶𝑙 + 𝐻 + 2𝐹𝑒 3+ + 𝐶𝑙 − + 𝐻2 𝑂 𝑀𝑛2+ + 𝐻𝑂𝐶𝑙 + 𝐻+ 𝑀𝑛4+ + 𝐶𝑙 − + 𝐻2 𝑂 𝑁𝑂2− + 𝐻𝑂𝐶𝑙 𝑁𝑂3 − + 𝐶𝑙 − + 𝐻 + 𝐻2 𝑆 + 4𝐻𝑂𝐶𝑙 𝑆𝑂4 2− + 4𝐶𝑙 − + 6𝐻 + 29-08-2024 Pritha Chatterjee 107 REACTIONS IN CHLORINATION Reaction with Ammonia Ammonia upon reaction to chlorines form Chloramines Chloramines are disinfectants like chlorine, but they are slow reacting Ammonia is often added in water treatment plants, to provide residual disinfectant in the distribution system. Formation of chloramines assures that when the water arrives at the tap of the consumer, a certain amount of disinfectant still exists 𝑁𝐻3 + 𝐻𝑂𝐶𝑙 → 𝑁𝐻2 𝐶𝑙(𝑚𝑜𝑛𝑜𝑐ℎ𝑙𝑜𝑟𝑎𝑚𝑖𝑛𝑒) + 𝐻2 𝑂 𝑁𝐻2 𝐶𝑙 + 𝐻𝑂𝐶𝑙 → 𝑁𝐻𝐶𝑙2 (𝑑𝑖𝑐ℎ𝑙𝑜𝑟𝑎𝑚𝑖𝑛𝑒) + 𝐻2 𝑂 Monochloramine and dichloramine are called combined chlorine 𝑁𝐻𝐶𝑙2 + 𝐻𝑂𝐶𝑙 → 𝑁𝐶𝑙3 (𝑡𝑟𝑖𝑐ℎ𝑙𝑜𝑟𝑎𝑚𝑖𝑛𝑒) + 𝐻2 𝑂 29-08-2024 Pritha Chatterjee 108 REACTIONS IN CHLORINATION Reaction with organic Nitrogen Chlorine reacts with organic amines (-NH2, -NH-, -N=) to form organic chloramines 𝐶𝐻3 𝐶𝑙 + 𝐻𝑂𝐶𝑙 → 𝐶𝐻3 𝑁𝐻𝐶𝑙 + 𝐻2 𝑂 𝐶𝐻3 𝑁𝐻𝐶𝑙 + 𝐻𝑂𝐶𝑙 → 𝐶𝐻3 𝑁𝐶𝑙2 + 𝐻2 𝑂 Breakpoint Reactions Decomposition reactions before the breakpoint are called breakpoint reactions. Breakpoint is that concentration, beyond which any chlorine applied appears as residual chlorine. 29-08-2024 Pritha Chatterjee 109 REACTIONS IN CHLORINATION Up to point A: From zero chlorine applied at the beginning to point A, the applied chlorine is immediately consumed. This consumption is caused by reducing species such as Fe2+, Mn2+, H2S, and NO2- Point A to B: Chlorine forms chloro-organic compounds and organic chloramines. Ammonia will be converted to monochloramine at this range of chlorine dosage Beyond point B: Chloro-organic compounds and organic chloramines break down. Also, at this range of chlorine dosage, the monochloramine starts to convert to dichloramine. As the curve continues to go “downhill” from point B, the dichloramine converts to the trichloramine and other decomposition products, the conversion being complete at the lowest point indicated by “breakpoint.” Ammonia chloramines are all destroyed Organic chloramines are not good disinfectants – so at breakpoint and only organic chloramines stay. chlorination should be done to point B 29-08-2024 Pritha Chatterjee 110 REACTIONS IN CHLORINATION Superchlorination - The practice of chlorinating up to and beyond the breakpoint is called superchlorination – though it causes complete disinfection but it leaves only free chlorine residual that disappear quickly If superchlorination is to be practiced, then ammonia must be added after disinfection, to bring back monochloramines for chlorine residual Reactions with Phenol Chlorine reacts readily with phenol and organic compounds containing the phenol group by substituting the hydrogen atom in the phenol ring with the chlorine atom. These chloride substitution products are extremely odorous. Phenol breaks down to odourless low molecular weight decomposition products using excess HOCl. Formation of trihalomethane Reactions of chlorine with organic compounds such as fulvic and humic acids and humin produce undesirable by-products. These by-products are known as disinfection by-products, DBPs. Examples of DBPs are chloroform and bromochloromethane; these DBPs are suspected carcinogens. 29-08-2024 Department of Civil Engineering, IIT Hyderabad 111 DISINFECTION USING OZONE Ozone is a very strong oxidizer and has been found to be superior to chlorine in inactivating resistant strains of bacteria and viruses. It is very unstable, however, having a half-life of only 20 to 30 min in distilled water at 20 °C. It is therefore generated on site before use. Typical ozone dosage is 1.0 to 5.3 kg/1000 m3 of treated water at a power consumption of 10 to 20 kW/kg of ozone. The immediate ozone demand parallels that of the immediate chlorine demand that is for ferrous, manganous, nitrites, and hydrogen sulfide 2𝐹𝑒 2+ + 𝑂3 + 2𝐻 + → 2𝐹𝑒 3+ + 𝑂2 + 𝐻2 𝑂 𝑀𝑛2+ + 𝑂3 + 2𝐻 + → 𝑀𝑛4+ + 𝑂2 + 𝐻2 𝑂 𝑁𝑂2− + 𝑂3 → 𝑁𝑂3− 𝐻2 𝑆 + 4𝑂3 → 𝑆𝑂42− + 4𝑂2 + 2𝐻+ 29-08-2024 Pritha Chatterjee 112 DISINFECTION USING UV LIGHT UV light destroys bacteria, bacterial spores, molds, mold spores, viruses, and other microorganisms. Low-pressure mercury arc lamp is the principal means of producing ultraviolet light. This is used because about 85% of the light output is monochromatic at a wavelength of 253.7 nm which is within the optimum range for germicidal effect 29-08-2024 Pritha Chatterjee 113 SEPARATION OF SOLIDS Separation may occur by floatation if the water is denser that the solid matter. For potable water, virtually all solids requiring removal are heavier than water, therefore sedimentation with gravity as the driving force is the most common separation technique. Sedimentation is classified into various types depending upon the characteristics and concentration of suspended material Particles whose size, shape and specific gravity do not change with time are called Discrete Particles Particles whose surface properties are such that they aggregate, or coalesce, with other particles upon contact, thus changing size, shape, and perhaps specific gravity with each contact, are called flocculating particles. Suspensions in which the concentration of particles is not sufficient to cause significant displacement of water as they settle or in which particles will not be close enough for velocity field interference to occur are termed dilute suspensions. Suspensions in which the concentration of particles is too great to meet these conditions are termed concentrated suspensions. 29-08-2024 Department of Civil Engineering, IIT Hyderabad 114 TYPES OF SETTLING Four types of settling occur: types 1 to 4. Type 1 settling - removal of discrete particles, Type 2 settling - removal of flocculent particles, Type 3 settling - removal of particles that settle in a zone Type 4 settling - compression or compaction of the particle mass is occurs during settling 29-08-2024 Pritha Chatterjee 115 DISCRETE SETTLING When particles in suspension are dilute, they tend to act independently; thus, their behaviors are therefore said to be discrete with respect to each other As a particle settles in a fluid, three forces work on it, Weight, 𝑓𝑔 = 𝜌𝑝 𝑔𝑉𝑝 𝜌𝑝 - mass density of particle, 𝜌𝑤 - mass density of water, 𝑉𝑝 - volume of the particle Buoyant force, 𝑓𝑔 = 𝜌𝑤 𝑔𝑉𝑝 g - acceleration due to gravity, 𝑣𝑠 - terminal settling velocity 𝐶𝐷 𝐴𝑝 𝜌𝑤 𝑣𝑠 2 𝐶𝐷 - drag coefficient, and Drag force, 𝑓𝑔 = 𝐴𝑝 - projected area of the particle normal to the direction of motion. 2 Total force, 𝐹 = 𝑚𝑎, (m is the mass of the particle and a its acceleration) The particle will ultimately settle at its terminal settling velocity, the acceleration a is equal to zero 29-08-2024 Pritha Chatterjee 116 DISCRETE SETTLING Terminal Settling Velocity, 4 𝜌𝑝 −𝜌𝑤 ⅆ 𝑣𝑠 = 𝑔 3 𝐶𝐷 𝜌𝑤 24 where,𝐶𝐷 = , for 𝑅ⅇ < 0.3 (the equation becomes Stoke’s Law)