ENVIRONMENTAL SCIENCE AND ENGINEERING CE102 Water Quality Characteristics Physical Characteristics Colour, Turbidity, Taste, Odour, Temperature Chemical Characteristics pH, TDS, Total (Alkalinity, Hardness), Calcium, Magnesium, Fluoride, Chlorides, Sulphates, Nitrates, Total Nitrogen, Chloramines, Residual Chlorine, Iron, Ag, Cu, Zn Biological Characteristics Pathogens Color Color in drinking-water may be due to the presence of organic matter such as Humic Substances, metals such as Iron and Manganese, or highly colored industrial wastes Term ‘color’ is used to mean True Color – Due to Dissolved Solids Term ‘Apparent Color’ includes not only the color due to substances in solution but also that due to Suspended Matter or Colloid Solids Measured by comparing the color of water sample with other standard glass tubes containing solutions of different standard Platinum Cobalt (Pt-Co) color intensities Normally expressed as Hazen Units (Dissolve 1.246 g potassium chloroplatinate (equivalent to 500 mg metallic platinum ) and 1.00 g crystalline cobaltous chloride (equivalent to 250 mg metallic cobalt) in distilled water containing 100 ml of concentrated hydrochloric acid. Dilute to 1000 ml with distilled water. This standard solution is equivalent to 500 Hazen Units(HU) or True Color Unit (TCU) Humic Substances can be defined as “a general category of naturally occurring, biogenic, heterogeneous organic substances that can generally be characterized as being yellow to black in color, of high molecular weight (MW) and refractory” (Aiken et al., 1985b) Indian Standards IS 10500:1983 IS 10500:1991 IS 10500:2012 IS 14543:2004 Desirable Limit (HU/TCU) -5 5 2 Permissible Limit in Absence of Alternate Sources (HU/TCU) -25 15 -- Taste & Odour The Dissolved Organic materials or the inorganic Salts or dissolved gases may impart taste and odor Gasses: H2S,CH4, CO2, O2 + Organic Matter Minerals: NaCl, Iron Compounds, Carbonates & Sulphates Taste expressed by Flavor Threshold Number (TKN) Odour expressed by Threshold Odour Number (TON) The character and intensity of taste and odour discloses the nature of pollution or the presence of microorganisms TON = 𝐴+𝐵 𝐴 = 𝑽𝒐𝒍.𝒐𝒇 𝑹𝒂𝒘 𝑾𝒂𝒕𝒆𝒓 𝑺𝒂𝒎𝒑𝒍𝒆 + 𝑽𝒐𝒍.𝒐𝒇 𝑫𝒊𝒔𝒕𝒊𝒍𝒍𝒆𝒅 𝒘𝒂𝒕𝒆𝒓 𝒖𝒔𝒆𝒅 𝒇𝒐𝒓 𝑫𝒊𝒍𝒖𝒕𝒊𝒐𝒏 𝑽𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝑹𝒂𝒘 𝑾𝒂𝒕𝒆𝒓 𝑺𝒂𝒎𝒑𝒍𝒆 Permissible Limit is 1:3 Nephelometric Turbidity unit Indian Standards Desirable Limit (NTU) Permissible Limit in Absence of Alternate Sources (NTU) IS 10500:1983 Agreeable Agreeable IS 10500:1991 Agreeable Agreeable IS 10500:2012 Agreeable Agreeable IS 14543:2004 Agreeable Agreeable Turbidity Reduction of transparency due to the presence of particulate matter such as clay or silt, finely divided organic matter, plankton or other microscopic organisms - First Attempt was made in the year 1900 to determine turbidity Originally turbidity was measured by Jackson candle turbidity meter A water sample is poured into the tube until the visual image of the candle flame, as viewed from the top of the tube When the intensity of the scattered light equals that of the transmitted light, the image disappears; the depth of the sample in the tube is read against the ppmsilica scale, and turbidity was measured in Jackson Turbidity Units (JTU) The calibration was done based on suspensions of silica - consistency in standard formulation was difficult to achieve Nowadays turbidity is measured by applying Nephelometry - technique to measure level of light scattered by the particles at right angles to the incident light beam Scattered light level is proportional to the particle concentration in the sample Higher the intensity of scattered light, higher the turbidity Formazin polymer is generally used as turbidity standard because it is more reproducible than other types of standards used previously Unit of expression is Nephelometric Turbidity Unit (NTU) Typical series of Formazin turbidity standards shown in NTU/FTU Turbidity - Measurements Turbidity – Limits & its Importance Turbidity < 40 Units - No Issues Turbidity > 40 Units - Dilute the Sample and calculate using the following equation: 𝐴 ∗ (𝐵 + 𝐶) 𝐶 A : Turbidity units found in diluted sample, B : Volume in ml of dilution water used, and C : Volume of sample in ml taken for dilution 𝑇𝑢𝑟𝑏𝑖𝑑𝑖𝑡𝑦 𝑈𝑛𝑖𝑡𝑠 = Indian Standards Desirable Limit (NTU) Permissible Limit in Absence of Alternate Sources (NTU) IS 10500:1983 10 25 IS 10500:1991 5 10 IS 10500:2012 1 5 IS 14543:2004 2 -- Why it is Important ? Turbidity is important because it affects both the acceptability of water to consumers, and the selection and efficiency of treatment processes, particularly the efficiency of disinfection with chlorine since it exerts a chlorine demand and protects microorganisms and may also stimulate the growth of bacteria Solids (Dissolved, Suspended) Total Dissolved Solids: A well-mixed water sample (100 mL) is filtered through a standard glass fiber filter (2.0 µm pore size), and the filtrate is evaporated to dryness in a weighed dish and dried to constant weight at 180°C for 1 hour TDS↑ Hardness ↑ 3 𝑚𝑔 𝑜𝑓 𝑇𝑜𝑡𝑎𝑙 𝐷𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑆𝑜𝑙𝑖𝑑𝑠/𝐿 = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓𝑑𝑟𝑖𝑒𝑑 𝑟𝑒𝑑𝑖𝑠𝑢𝑒 + 𝑑𝑖𝑠ℎ − 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑖𝑠ℎ 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒 ∗ 10 Total Suspended Solids: A well-mixed water sample (100 mL) is filtered through a weighed standard glass-fiber filter (2.0 µm pore size) and the residue retained on the filter is dried to a constant weight at 103 - 105°C for 1 hour If the suspended material clogs the filter and prolongs filtration, it may be necessary to increase the diameter of the filter or decrease the sample volume 𝑚𝑔 𝑇𝑜𝑡𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑑𝑒𝑑 𝑆𝑜𝑙𝑖𝑑𝑠/𝐿 = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓𝑓𝑖𝑙𝑡𝑒𝑟 + 𝑑𝑟𝑖𝑒𝑑 𝑟𝑒𝑑𝑖𝑠𝑢𝑒 − 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑓𝑖𝑙𝑡𝑒𝑟 {𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒} ∗ 103 Solids – Temperature Drying at 103°C ensures the removal of all free water, provided that the drying period is long enough, and minimizes the loss of water of crystallization Water of Crystallization - Water that occurs inside crystals, this water is not bonded to the crystal but embedded during the crystallization of the mineral Ex - CaSO4·2H2O Gypsum contains two water molecules in their crystals Residues dried at 103 - 105°C may retain not only water of crystallization but also some mechanically blocked water Residues dried at 180 ± 2°C will lose almost all mechanically occluded water (water molecules trapped in mineral matrix). Some water of crystallization may remain, especially if sulfates are present 550°C is about the lowest temperature at which organic matter, particularly carbon residues resulting from pyrolysis of carbohydrates and other organic matter, can be oxidized at reasonable speed, while Inorganic salts are stable Solids (Fixed, Volatile) “Fixed Solids” is the term applied to the residue of Total, Suspended, or Dissolved Solids after heating to dryness for a specified time at a specified temperature (550°C for 1 hour). The weight loss on ignition is called “Volatile Solids” Total Fixed Solids - approximation of the Mineral Matter Present (Inorganic) Total Volatile Solids - approximation of the Organic Material Present This test is accomplished by a combustion procedure in which organic matter is converted to gaseous CO2 and H2O, and thus volatilized, while the temperature is controlled to prevent decomposition and volatilization of inorganic substances as much as is consistent with complete oxidation of the organic matter • 𝑚𝑔 𝑉𝑜𝑙𝑎𝑡𝑖𝑙𝑒 𝑆𝑜𝑙𝑖𝑑𝑠/𝐿 = • 𝑚𝑔 𝐹𝑖𝑥𝑒𝑑 𝑆𝑜𝑙𝑖𝑑𝑠/𝐿 = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑟𝑒𝑑𝑖𝑠𝑢𝑒+𝑑𝑖𝑠ℎ 𝑏𝑒𝑓𝑜𝑟𝑒 𝑖𝑔𝑛𝑖𝑡𝑖𝑜𝑛 − 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓𝑟𝑒𝑑𝑖𝑠𝑢𝑒+𝑑𝑖𝑠ℎ/𝑓𝑖𝑙𝑡𝑒𝑟 𝑎𝑓𝑡𝑒𝑟 𝑖𝑔𝑛𝑖𝑡𝑖𝑜𝑛 ∗103 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓𝑟𝑒𝑑𝑖𝑠𝑢𝑒+𝑑𝑖𝑠ℎ/𝑓𝑖𝑙𝑡𝑒𝑟 𝑎𝑓𝑡𝑒𝑟 𝑖𝑔𝑛𝑖𝑡𝑖𝑜𝑛 − 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑖𝑠ℎ/𝑓𝑖𝑙𝑡𝑒𝑟 ∗103 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒 Solids Standards Indian Standards IS 10500:1983 Desirable Limit (mg/L) 1000 Permissible Limit in Absence of Alternate Sources -- IS 10500:1991 IS 10500:2012 IS 14543:2004 500 500 500 2000 2000 -- Environmental Significance Laxative or reverse effects on people whose bodies are not used to the higher levels Imparts taste to water Water stain glassware Experiment - Solids Experiment - Solids Dissolved Solids Suspended Solids Settleable Solids Settleable Solids is applied to solids in suspension that will settle, under gentle (period of inactivity or dormancy) conditions, because of the influence of gravity Fill an Imhoff cone to the 1-L mark with a well-mixed sample. Settle for 45 min, gently agitate sample near the sides of the cone with a rod or by spinning, settle 15 min longer, and record volume of settleable solids in the cone as milliliters per liter (mL/L) Reported as volume (mL/L) basis Solids Analysis Application in Env. Engg. Application TS VS TDS Drinking Water x Natural Waters x Municipal Wastewater Industrial Wastewater x x Sludge x x x TSS VSS x x x x x Example Problem Given the following data: • Weight of a dish = 48.6212 g • 100 mL of sample is placed in a dish and evaporated • Weight of the dish and dry solids = 48.6432 g • The dish is then placed in a 550°C furnace, then cooled. Weight = 48.6300 g • Find the total, fixed, and volatile solids (expressed as mg/L) pH - Universally to express the intensity of the acid or alkaline condition of a solution Expressed on a scale ranging from 0 to 14 Logrithm of Reciprocal of Hydrogen ion activity (moles/L) Recommended pH range for treated drinking waters is 6.5 to 8.5 It is a factor that must be considered in chemical coagulation, disinfection, water softening, and corrosion control In wastewater treatment employing biological processes, pH must be controlled within a range favorable to the particular organisms involved Indian Standards Desirable Limit Permissible Limit in Absence of Alternate Sources IS 10500:1983 6.5 to 8.5 -- IS 10500:1991 6.5 to 8.5 -- IS 10500:2012 6.5 to 8.5 -- IS 14543:2004 6.5 to 8.5 -- pH should preferably be determined at the time of sample collection pH value obtained in the lab may not be the same as that of water at the time of collection of samples due to loss-or absorption of gases, reactions with sediments, hydrolysis and oxidation or reduction taking place within the sample bottle Pure water dissociates to yield a concentration of Hydrogen Ions equal to about 10-7mol/L + − 𝑯𝟐𝑶 ↔ 𝑯 + 𝑶𝑯 − H2O dissociates to− produce one 𝑶𝑯 ion for each H+ ion, it is obvious that about 10-7mol/L of 𝑶𝑯 ion is produced simultaneously By substitution into the equilibrium equation, we obtain, + − 𝑯 {𝑶𝑯 } =𝑲 {𝑯𝟐𝑶} + − 𝑲𝑾 = 𝑯 𝑶𝑯 = {10-7}{10-7} = {10-14} Kw = is known as Ionization Constant for water and changes with change in Temperature The pH of water is a measure of the acid–base equilibrium and, in most natural waters, is controlled by the carbon dioxide–bicarbonate–carbonate equilibrium system An increased carbon dioxide concentration will therefore lower pH, whereas a decrease will cause it to rise The pH of the water entering the distribution system must be controlled to minimize the corrosion of water mains and pipes in household water systems Failure to do so can result in the contamination of drinking-water and in adverse effects on its taste, odour, and appearance For effective disinfection with chlorine, the pH should preferably be less than 8 The pH of water determines the solubility (amount that can be dissolved in the water) and biological availability (amount that can be utilized by aquatic life) of chemical constituents such as nutrients (phosphorus, nitrogen, and carbon) and heavy metals (lead, copper, cadmium, etc.) + 𝑝𝐻 = −𝑙𝑜𝑔 𝐻 𝑜𝑟 1 𝑝𝐻 = log{ +} 𝐻 pH is usually represented from 0 to 14, with pH 7 at 25ᵒC representing absolute neutrality Example: In a water treatment plant, the pH values of upcoming and outgoing water are 7.2 and 8.4 respectively. Assuming a linear variation of pH with time, determine the average pH value of water Iron & Manganese These are present in insoluble forms in significant amounts in nearly all soils Fe exists in soils and minerals mainly as insoluble Ferric Oxides (Fe2O3) and Iron Sulfide (FeS) (pyrite) It occurs in some areas also as Ferrous Carbonate (siderite), which is very slightly soluble Further, groundwaters usually contain significant amounts of CO2, appreciable amounts of Ferrous Carbonate may dissolved by the reaction shown in the equation, FeCO3(s) + CO2 + H2O → Fe2++ 2HCO3Under reducing (anaerobic) conditions, however, the ferric iron (Fe3+) is reduced to ferrous iron (Fe2+) Fe imparts a taste to water which is detectable at very low concentrations Mn exists in the soil principally as Manganese Dioxide (MnO2), which is very insoluble in water containing CO2 Under reducing (anaerobic) conditions, Manganese in the dioxide form is reduced from an oxidation state of III to II Humans suffer no harmful effects from drinking waters containing Fe & Mn Iron & Manganese Groundwaters that contain appreciable amounts of Fe or Mn or both are always empty of dissolved oxygen and are high in CO2 content. The Fe and Mn are present as Fe (II) and Mn (II). The high CO2 content indicates that bacterial oxidation of organic matter has been extensive, and the absence of dissolved oxygen shows that anaerobic conditions were developed It has been shown, on the basis of thermodynamic considerations, that Mn(IV) and Fe(III) are the only stable oxidation states for Fe & Mn in oxygen-containing waters. Thus, these forms can be reduced to the soluble Mn (II) and Fe (II) only under highly anaerobic reducing conditions. Both Fe & Mn interfere with laundering operations, impart objectionable stains to plumbing fixtures, and cause difficulties in distribution systems by supporting growths of iron bacteria Iron bacteria are small living organisms which naturally occur in soil, shallow groundwater, and surface waters. These nuisance bacteria combine Fe (or Mn) and oxygen to form deposits of "rust," bacterial cells, and a slimy material that sticks the bacteria to well pipes, pumps, and plumbing fixtures. The bacteria are not known to cause disease, but can cause undesirable stains, tastes and odors Iron & Manganese Indian Standards IS 10500:1983 IS 10500:1991 IS 10500:2012 IS 14543:2004 Desirable Limit (Fe) (mg/L) 0.3 0.3 0.3 0.1 Permissible Limit in Absence of Alternate Sources 1 No relaxation No relaxation -- Indian Standards Desirable Limit (Mn) (mg/L) Permissible Limit in Absence of Alternate Sources IS 10500:1983 IS 10500:1991 IS 10500:2012 0.1 0.1 0.1 0.5 0.3 0.3 IS 14543:2004 0.1 -- Source of Fluoride: Occurs in Igneous and Sedimentary Rocks Fluoride occurs as Sellaite [MgF2], Fluorite Or Fluorspar [CaF2], Cryolite [Na3AlF6], Fluorapatite [3Ca3(PO4)2 Ca(F,Cl2)], Apatite [CaF2.3Ca3(PO4)], Topaz [Al2SiO4(F,OH)2], Fluormica (phlogopite) [KMg3(Si3Al)O10(F,OH)2], Biotite [K(Mg,Fe)3AlSi3O10(F,OH)2], Epidote [Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH)], Amphibole such as tremolite [Ca2Mg5Si8O22(OH)2], Hornblende [Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2], Mica, Clays, Villuanite and Phosphorite Fluoride minerals such as Fluorite and Cryolite are not readily soluble in water under normal pressure and temperature Under alkaline conditions and range of specific conductivity between 750 and 1750 µS/cm, dissolution rate of fluorite minerals increase (Saxena and Ahmed, 2001) The weathering of Granitic rocks results in increased fluoride content in groundwater Longer residence time in aquifers with fractured fluoride rich rocks enhance fluoride levels in the groundwater Nalgonda, India contain fluoride rich minerals such as Fluorite (0 to 3.3%), Biotite (0.1 to 1.7%) and Hornblende (0.1 to 1.1%) (Ramamohana Rao et al., 1993) Fluoride The discovery of high consumption of Fluoride (two-edge sword) being harmful for humans and animals was made in India in 1937 (Nalgonda-Telangana) When present in concentration of < (0.8-1.0 mg/L) – Cause Dental Caries (Tooth Decay) due to formation of excessive cavities in the teeth of young children during calcination of their permanent teeth Fluoride makes the tooth more resistant to acid attack from bacteria When fluoride compounds are in your mouth, they can actually make teeth stronger and prevent cavities • Higher Fluoride Concentration of (1.5-2.0 mg/L) – Discoloration of teeth (Dental Fluorosis) • Dental Fluorosis: Tooth Enamel (which is stronger than bone) is the outer covering of teeth which is principally made up of Hydroxyapatite (96%) (Ca10(PO4)6(OH)2) and is made from calcium and phosphate • Fluoride displaces the hydroxide ions from hydroxyapatite to form Fluorapatite (Ca10(PO4)6F2) • On prolonged continuation of this process the teeth become hard and brittle • Dental fluorosis : Tooth becoming colored from yellow to brown to black • At still higher concentration, (> 3.0 mg/L) Skeletal Fluorosis which affects the bone and ligaments • Fluorosis is a crippling disease resulted from deposition of fluorides in the hard and soft tissues of body Dental Fluorosis Fluoride - Limits Indian Standards IS 10500:1983 IS 10500:1991 IS 10500:2012 IS 14543:2004 Desirable Limit (F) (mg/L) 0.6 – 1.2 1.0 1.0 1.0 Permissible Limit in Absence of Alternate Sources Desirable 1.5 1.5 1.5 Alkalinity Alkalinity of a water is a measure of its capacity to Neutralize Acids or its ability to maintain a relatively constant pH Alkalinity in natural water is due to: 1. Salts of Weak Acids Carbonate, bicarbonate Borate, Silicate, Phosphate A few organic acids resistant to biological oxidation (humic subs) In polluted or anaerobic waters (Acetic, Propionic acid, H2S) 2. Weak or Strong Bases Ammonia Hydroxides Alkalinity The major portion of the alkalinity in natural waters is caused by three major classes of materials which may be ranked in order of their association with high pH values as follows 1. Hydroxide 2. Carbonate 3. Bicarbonate Alkalinity is an important parameter in evaluating the Optimum Coagulant Dosage Carbon dioxide (CO2) dissolves in water to form carbonic acid (H2CO3), which dissociates and is in equilibrium with bicarbonate (HCO3-) and carbonate (CO3-) ions CO2 (Gas) ↔ CO2 (Dissolved) CO2 (Dissolved)+ H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32- Alkalinity Measured volumetrically by titration with N/50 H2SO4 (N/EW = N/50), and is reported in terms of equivalent CaCO3 It is most convenient to have the standard titrating agent of such strength that 1 mL is equivalent to 1 mg of the material being measured Samples whose initial pH is > 8.3, the titration is made in two steps: In the first step the titration is conducted until the pH is lowered to 8.3, the point at which phenolphthalein indicator turns from pink to colorless (Phenolphthalein Alkalinity) OH- + H+ → H2O CO32- + H+ → HCO3The second phase of the titration is conducted until the pH is lowered to about 4.5, corresponding to the methyl orange end point (Orange to Pink) (Total Alkalinity) HCO3- + H+ → H2CO3 When the pH of a sample is < 8.3, a single titration is made to a pH of 4.5 Alkalinity @ pH 10 - All OH- are neutralized @ pH 8.3 – All CO32- converted to HCO3 Titration till pH 8.3- Phenolphthalein Alkalinity Titration till pH 4.5- Total Alkalinity (till H2CO3) Total Alkalinity is the amount of acid required to lower the pH of the solution to 4.5 Alkalinity - Calculations There are three methods commonly used to make calculations: 1. From Alkalinity Measurements Alone Based on empirical relationships for the calculation of the various forms of alkalinity from Phenolphthalein and Total Alkalinities This is used by technicians and others who do not have a knowledge of the fundamental chemistry involved This method are only approximate for samples with pH > 9 2. From Alkalinity + pH Measurements This procedure gives sufficiently accurate estimates for most practical purposes, and also makes use of Phenolphthalein & Total Alkalinity Measurements In addition, an accurate initial pH measurement is required for the direct calculation of OH- alkalinity 3. From Equilibrium Equations Various equilibrium equations for H2CO3 are used to compute the concentrations of the various alkalinity forms This method gives reasonably accurate results for constituents, even when present in the fractional mg/L range, provided that an accurate pH measurement is made 1. From Alkalinity Measurements Alone From the Measurements of Phenolphthalein and Total Alkalinities - calculation of other three types of alkalinity, Hydroxide, Carbonate, and Bicarbonate, are made Assumption: That both hydroxide and bicarbonate alkalinity cannot exist together in the same sample This permits only five possible situations to be present, which are as follows: 1. Hydroxide Only 2. Carbonate Only 3. Hydroxide + Carbonate 4. Carbonate + Bicarbonate 5. Bicarbonate Only 1. From Alkalinity Measurements Alone When… OH- CO32- HCO3- T=P P 0 0 T= 2P 2P 0 2(T-P) 0 T>2P 0 T-2(T-P) = (2P-T) 0 2P T-2P P=0 0 0 T T<2P 2. From Alkalinity + pH Measurements Measurements are made for pH, Phenolphthalein, and Total Alkalinity This will allow calculation of Hydroxide, Carbonate, and Bicarbonate Alkalinity 𝑲𝒘 1. Hydroxide Alkalinity (from pH Measurement) − 𝑶𝑯 = + [𝑯 ] Since a hydroxide concentration of 1 mol/L is equivalent to 50,000 mg/L of alkalinity as CaCO3 𝑯𝒚𝒅𝒓𝒐𝒙𝒊𝒅𝒆 𝑨𝒍𝒌𝒂𝒍𝒊𝒏𝒊𝒕𝒚 = 𝟓𝟎, 𝟎𝟎𝟎 ∗ 𝟏𝟎(𝒑𝑯 −𝒑𝑲𝒘) At 25ᵒC, pKw = 14.00. However, it varies from 14.94 at 0ᵒC to 13.53 40ᵒC 2. Carbonate Alkalinity Carbonate Alkalinity = 2 (Phenolphthalein Alkalinity – Hydroxide Alkalinity) 3. Bicarbonate Alkalinity Bicarbonate Alkalinity = Total Alkalinity - (Carbonate Alkalinity + Hydroxide Alkalinity) 3. From Equilibrium Equations Total Alkalinity is a measure of the equivalent concentration of all cations associated with the Alkalinity-Producing Anions, Except Hydrogen Ion [H+] + 𝑨𝒍𝒌𝒂𝒍𝒊𝒏𝒊𝒕𝒚 𝟓𝟎,𝟎𝟎𝟎 = [HCO3- ]+ 2[CO32-] + [OH-] The equilibrium equations that must be considered are those for water 𝑶𝑯 + and for the second ionization of carbonic acid is 𝑲𝑨𝟐 = 𝑯 [𝑪𝑶𝟑𝟐 − [𝑯𝑪𝑶𝟑 ] −] − = 𝑲𝒘 + [𝑯 ] Alkalinity – CO2 – pH pH Changes due to Aeration of water Aeration: To remove CO2, NH3, and VOCs CO2 removal from water, tends to decrease [H+] and pH increases Alkalinity of Boiler Waters CO2 is insoluble in boiling water and so is removed with the steam This causes an increase in pH and a shift in alkalinity forms from HCO3- to CO32- and from CO32- to OH-, as indicated below HCO3- ↔ CO32- + H2O+ CO2 CO32- + H2O ↔ 2OH-+ CO2 If Ca2+ levels are high, precipitation of CaCO3(s) may occur. pH – CO2 - Alkalinity Respiration & Photosynthesis affect addition or removal of CO2 Addition of CO2 reduces the pH of a system Removal of CO2 increases the pH of a system Alkalinity – CO2 – pH pH Changes in the presence of Algal Bloom Algae use CO2 for their photosynthetic activity Diurnal variations in pH due to algal photosynthesis and respiration are common in surface waters Many shallow surface waters support extensive algal blooms and they found to grow rapidly when the pH values are as high as 10 Algae can continue to extract CO2 from water until an inhibitory pH is reached, which is usually in the range of pH 10 to 11 As pH increases, alkalinity forms change Total Alkalinity remains constant unless CaCO3 precipitation occurs HCO3- ↔ CO32- + H2O+ CO2 Ca 2+ + CO32- ↔ CaCO3 (s) This precipitation usually happens before pH levels have exceeded 10 The CaCO3 precipitated as a result of removal of CO2 through algal action produces the Marl Deposits in lakes Marl Deposits are the Precursors of Limestone Alkalinity Data - Application Chemical Coagulation – Chemicals used for coagulation of water and wastewater react with water to form insoluble Hydroxide precipitates. The H+ ions released react with the alkalinity of the water. Thus, the alkalinity acts to buffer the water in a pH range where the coagulant can be effective Water Softening – Alkalinity - major point to be considered in calculating lime and soda ash requirements in softening of water Corrosion Control – Used to calculate the Langelier saturation index (degree of saturation of Calcium Carbonate in water) Indian Desirable Limit Permissible Limit in Buffer Capacity Standards (mg/L) Absence of Alternate Used for evaluating the buffering capacity of wastewaters and sludges IS 10500:1983 Also used to assess natural waters IS 10500:1991 ability to resist the effects of acid rain IS 10500:2012 IS 14543:2004 Sources -- -- 200 600 200 600 200 -- Hardness Soft Water - Forms lather easily with soap Hard water - Needs more soap to form lather Because of dissolved chemicals in the hard water react with soap to form a scum Types of Hardness 1. Temporary Hardness (Easily removed by boiling) Due to the presence of HCO3- of Ca2+ & Mg2+ 2. Permanent Hardness (Cannot be removed by boiling) Due to the presence of Cl- & SO42- of Ca2+ & Mg2+ Hardness - Causes Hardness is caused by Multivalent Metallic Cations In general, hard waters originate in areas where the topsoil is thick and limestone formations are present Soft waters originate in areas where the topsoil is thin and Limestone formations are sparse or absent Health Significance Calcium needed for healthy bones and teeth Magnesium needed for effective metabolism Hardness – Determination Complete Cation Analysis – Accurate Atomic Absorption Spectroscopy (AAS) Inductively Coupled Plasma (ICP) Ion Chromatography (IC) 𝑚𝑔 50 2+ (mg/L) * + 𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠 ( 𝑜𝑓 𝐶𝑎𝐶𝑂 ) = MC Ion Specific Electrodes (ISE) 3 𝐿 𝐸𝑊 𝑜𝑓 𝑀𝐶2 Cation Conc. (mg/L) Anion 20 ClNa+ 15 SO42Ca2+ 10 NO3Mg2+ 2 Alkalinity Sr2+ Conc. (mg/L) 40 16 1 50 Cation Ca2+ Mg2+ Sr2+ Conc. (mg/L) 15 10 2 EW 20 12.2 43.8 Hardness (mg/L as CaCO3) =(15*50) /(20) = 37.5 ? ? Hardness – Determination EDTA – Titrimetric Method Initially, water sample is buffered to pH 10.1 using ammonia buffer solution Erichrome Black T (EBT) (indicator dye - Blue Color), is then added to the solution containing Ca2+ & Mg2+ ions. Color of solution turns to wine red MC2+ + EBT → [M.EBT]Complex Ethylenediaminetetraacetic Acid (EDTA), the titrant (0.02 N), complexes with Ca2+ & Mg2+ ions, and the indicator will turn blue which is the end point of the titration MC2+ + EDTA → [M.EDTA]Complex 𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠 ( 𝑚𝑔 𝑜𝑓 𝐿 𝐶𝑎𝐶𝑂3) = 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐸𝐷𝑇𝐴 ∗𝑁∗ 50∗1000 𝑉𝑜𝑙𝑢𝑒𝑚 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒 𝑇𝑎𝑘𝑒𝑛 (𝑚𝐿) Hardness – Types w.r.t the metallic ion (1) Calcium Hardness (2) Magnesium Hardness w.r.t the anions associated with the metallic ions (1) Carbonate Hardness (2) Noncarbonate Hardness Calcium Hardness Calcium usually found in highest conc. in natural water Calcium in water results from deposits of lime stone, gypsum Calcium is one of the principal cations in water hardness These cations form insoluble salts with soap and decrease the cleaning effectiveness of soap They also form hard water deposits in hot water heaters 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠 ( 𝑚𝑔 𝑜𝑓 𝐿 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 𝑖𝑜𝑛 𝐶𝑜𝑛𝑐. = 𝐶𝑎𝐶𝑂3) = 𝐶𝑎 𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝐸𝐷𝑇𝐴 ∗𝑁∗ 50∗1000 𝑉𝑜𝑙𝑢𝑒𝑚 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒 𝑇𝑎𝑘𝑒𝑛 (𝑚𝐿) 𝑚𝑔 𝐶𝑎𝐶𝑂3 𝐿 ∗𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡.𝑜𝑓 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐶𝑎𝑙𝑐𝑖𝑢𝑚 𝐶𝑎𝑟𝑏𝑜𝑛𝑎𝑡𝑒 Total Hardness – Calcium Hardness = Magnesium Hardness Temporary or Carbonate Hardness: Metallic Cations is associated with Bicarbonate and Carbonate ions which tend to precipitate at elevated temperatures such as occur in boilers or during the softening process with lime Ca(HCO3)2 → CaCO3 ↓ + CO2 ↑+ H2O Mg(HCO3)2 → Mg (OH) 2 ↓ + 2CO2 ↑ Permanent or Non-Carbonate Hardness: Contains Chlorides or Sulphates of calcium or magnesium or of both and can’t be removed by boiling CaCl2 → Ca2+ + 2ClMgSO4 → Mg2++ SO42When Alkalinity < Total Hardness: Carbonate Hardness = Alkalinity When Alkalinity ≥ Total Hardness: Carbonate Hardness = Total Hardness Non-Carbonate Hardness = Total Hardness - Carbonate Hardness Pseudo-Hardness: Sea, brackish, and other waters that contain appreciable amounts of Na+. Sodium is not a hardness-causing cation, and so this action which it exhibits when present in high concentration is termed Pseudo-hardness Hardness Reactions for the Precipitation of CaCO3 Reactions for the Dissolution of CaCO3 Hardness - Limits Indian Standards Desirable Limit (mg/L) Permissible Limit in Absence of Alternate Sources IS 10500:1983 300 -- IS 10500:1991 300 600 IS 10500:2012 200 600 IS 14543:2004 Not Available Not Available Hardness – Problem A Sample of Water having pH 7.2, has the following Conc. of ions Ions Ca2+ Mg2+ Na+ K+ HCO3SO42- Conc. (mg/L) 40 10 11.8 7 110 67.2 Cl- 11 Calculate TH, CH, NCH, Alkalinity Hardness – Solution Ions Ca2+ Mg2+ Na+ K+ HCO3SO42Cl- Molecular Conc. (mg/L) Weight (mg/mmol) 40 40.1 10 24.3 11.8 23.0 7 39.1 110 61.0 67.2 96.1 11 35.5 Sum of Cations 175.6 Z 2 2 1 1 1 2 1 Equivalent Conc. (mg/L Conc. (eq/L) Weight as CaCO3) (Col.2/Col.5) (mg/meq.) (Col.6*50) 20.05 1.998 99.8 12.15 0.823 41.2 23.0 0.51 25.7 39.1 0.179 8.95 61.0 1.8 90.2 48.05 1.4 69.9 35.5 0.031 15.5 = Sum of Anions = 175.6 mg/L as CaCO3 99.8 141 166.7 175.65 90.2 160.1 175.6 Hardness – Solution Total Hardness Total Hardness = Sum of (Ca2+ & Mg2+) = 99.8 + 41.2 = 141 mg/L as CaCO3 Carbonate Hardness : Portion of Hardness associated with CO32- or HCO3Total Hardness = 141 mg/L as CaCO3 Carbonate Hardness = 90.2 mg/L as CaCO3 Non-Carbonate Hardness = 141 - 90.2 mg/L as CaCO3 = 50.9 mg/L as CaCO3 • • • • • • • • Chlorides Chloride one of the major inorganic anions in saltwater and freshwater It originates from the dissociation of salts, such as NaCl or CaCl2 in water (taste producing salts) Chloride occurs in all natural waters in varying concentration Low [Cl-] Upland & Mountain Supplies High [Cl-] River & Groundwaters Very High [Cl-] Sea & Ocean Waters As mineral content ↑ chloride content ↑ Human urine, contain Cl- - consumed with food and water - averages about 6 g of Cl- per person per day Indian Standards Desirable Limit Permissible Limit in Absence (mg/L) of Alternate Sources IS 10500:1983 250 -- IS 10500:1991 250 1000 IS 10500:2012 250 1000 IS 14543:2004 200 -- Chlorides - Determination • (1) Argentometric Method (Mohr Method) (2) Potentiometric Method • (3)Mercuric Nitrate Method (4) Ferricyanide Method (5) Ion Chromatography Argentometric Method (Mohr Method) • AgNO3 (0.0141 N) as titrant (1 ml of AgNO3 = 0.5 mg Cl- ion) • Cl- precipitated as white AgCl (Ag+ + Cl- ↔ AgCl(s)) • Indicator: Potassium Chromate • As conc. of Cl- ions approaches extinction, Ag+ ion conc. increases to a level at which the solubility product of Silver Chromate is exceeded and it begins to form a Reddish Brown precipitate (2Ag+ + CrO42↔ Ag2CrO4(s)) • Endpoint: Color change to Reddish-Brown • An excess of Ag+ is needed to produce a visible amount of Ag2CrO4(s), - Blank must be determined & subtracted from all titration Chlorides – Analysis - Precautions 1. 2. 3. A uniform sample size must be used, preferably 100 mL, so that ionic concentrations needed to indicate the end point will he constant. The pH must he in the range of 7 to 8 because Ag+ is precipitated as AgOH(s) at high pH levels and the CrO42- is converted to Cr2O72- at low pH levels A definite amount of indicator must be used to provide a certain concentration of CrO42-; otherwise Ag2CrO4 may form too soon or not soon enough 𝐶𝑙 − ( 𝑚𝑔 ) 𝐿 = 𝑚𝐿 𝑜𝑓 𝐴𝑔𝑁𝑂3 −𝐵𝑙𝑎𝑛𝑘 ∗ 0.0141 𝑁 ∗35.45 ∗1000 𝑉𝑜𝑙𝑢𝑒𝑚 𝑜𝑓 𝑆𝑎𝑚𝑝𝑙𝑒 𝑇𝑎𝑘𝑒𝑛 (𝑚𝐿) Chlorides - Significance • Too much of Cl- in soil, can accumulate in leaf tissue, resulting in leaves with a scorched or burned appearance. • Trees with scorched leaves have brown or dead tissue on the tips, margins, or between the veins of the leaf. • Evapotranspiration tends to increase Cl- and salinity at the root zone of irrigated plants, making it difficult for crops to take up water due to osmotic pressure differences between the water outside the plants and within the plant cell • Interferes in COD Analysis Water Treatment Treatment – Surface Water Treatment – Surface Water Treatment – Ground Water Aeration Air in Water Water in Air Aerator Aeration brings water and air in close contact in order to remove dissolved gases (such as carbon dioxide) and oxidizes dissolved metals such as iron, hydrogen sulfide, and volatile organic chemicals (VOCs) Aeration is often the first major process at the treatment plant Size Range of Particles of Concern in Water Treatment Sedimentation Separation of Unstable & Destabilized SS by force of gravity Applications in Water Treatment: 1) Settling of coagulated and flocculated waters prior to filtration 2) Settling of coagulated and flocculated water in a softening plant 3) Settling of treated waters in Fe and Mn removal plant Applications in Wastewater Treatment: 1) Grit removal 2) Suspended solids removal in primary clarifier 3) Biological floc removal in activated sludge Settling or Sedimentation Settling - A unit operation in which solids are drawn toward a source of attraction. The particular type of settling that will be discussed in this section is gravitational settling. It should be noted that settling is different from sedimentation Sedimentation - The condition whereby the solids are already at the bottom and in the process of sedimenting. Settling is not yet sedimenting, but the particles are falling down the water column in response to gravity. Of course, as soon as the solids reach the bottom, they begin sedimenting. In the physical treatment of water and wastewater, settling is normally carried out in settling or sedimentation basins Settling Depends on.. 1. Characteristics of the Particles I. Discrete particles : Particles whose size, shape and specific gravity do not change with time (Ex: Inert particles such as Sand Grains) II. Flocculating particles : Particles whose size, shape and specific gravity change with time. (Example – Clay) 2. Concentration of Particles in Suspension I. Dilute Suspensions : Suspensions in which the conc. of particles is not sufficient to cause significant displacement of water as they settle or in which the particles will not be close enough for velocity field interference to occur II. Concentrated Suspensions : Suspensions in which the conc. of particles is too great to meet the conditions mentioned for dilute suspensions Sedimentation Tank Basins Rectangular Circular Hopper - Bottom SedimentationTank Design - Circular Basins Settling Basins Advantages 1. 2. 3. 4. 5. Simplest technology Energy input - Less Relatively inexpensive to install and operate No specialized operational skills Easily incorporated into new or existing facilities Disadvantages 1. 2. 3. 4. Low hydraulic loading rates Poor removal of small suspended solids Large floor space requirements Re-suspension of solids and leeching Coagulation Overview Colloidal Particles • Difficult to settle • Pass through small pores of conventional filters How to remove colloidal particles? • By Aggregation (making them bigger sizes) Why aggregation is difficult? • Small size of particles • Physical and Electrical forces How to Aggregate? • Use of Chemical Agents Coagulation & Flocculation Colloidal particles size ranging 10-6 mm (1 nm) - 10-3mm (1 µm) Colloidal impurities in surface water cause water to appear turbid or may impart color It is difficult to separate colloids from water because – Colloids do not settle by gravity – They are so small particles that pass through the pores of most common filtration media Measurement of Colloid Concentration Surface area might be an excellent measure of colloid concentration but it’s a difficult measurement and standard suspended solids measurement won’t work because the colloids will pass through most filters The best method of quantifying colloid concentration is Nephelometry or the measurement of light scattered by the colloids Since colloid size is on the order of the wavelength of visible light they will scatter incident visible light Intensity of incident light is measured at right angles to the light source The percent of “deflected” light is proportional to the colloid concentration A standard colloid concentration is used to calibrate the system The colloid concentration is often expressed as TU (turbidity units) Coagulation & Flocculation Coagulation involves the addition of a chemical coagulants for the purpose of conditioning the suspended, colloidal, and dissolved matter for subsequent processing by flocculation or to create conditions that will allow for the subsequent removal of particulate and dissolved matter. Flocculation is the aggregation of destabilized particles (particles from which the electrical surface charge has been reduced) and precipitation products formed by the addition of coagulants into larger particles known as flocculant particles or, more commonly, ‘‘floc.’ Coagulation The chemistry is extremely complex. Because metal coagulants Hydrolyze to form Acid Products that affect pH that in turn affects the solubility of the coagulant Selection of the type and dose of coagulant depends on, The characteristics of the coagulant Concentration and type of particulates, Concentration and characteristics of Natural Organic Matter (NOM), Water temperature, and water quality Coagulation-Stoichiometry Each mole of trivalent ion will produce 1 mole of the metal hydroxide and 3 moles of hydrogen ions Al3+ + 3H2O ↔ Al (OH)3, am↓ + 3H+ Fe3+ + 3H2O ↔ Fe (OH)3,am↓ + 3H+ The ‘‘am’’ subscript refers to amorphous solids (hours old), which have a much higher solubility product than crystalline precipitates Chlorinated Copperas: Ferric Chloride + Ferric Sulfate Alum Eff. pH : 6.5-8.5 Ferric Chloride Eff. pH : 3.5 - 6.5 & > 8.5 Ferric Sulfate Eff. pH : 4.0 -7.0 & > 9.0 Coagulation-Stoichiometry When alum is added to water and aluminum hydroxide precipitates, the overall reaction is, Al2(SO4)3·14H2O → 2Al(OH)3↓ + 6H+ + 3SO42- + 8H2O Ferric chloride: FeCl3 + 3H2O → Fe(OH)3↓ + 3H+ + 3Cl Ferric sulfate: Fe2(SO4)3 · 9H2O → 2Fe(OH)3↓ + 6H+ + 3SO42- + 3H2O After Al(OH)3 or Fe(OH)3 precipitates, the species remaining in water are the same as if H2SO4 or HCl had been added to the water Thus, adding alum or ferric is like adding a strong acid When alum reacts with bicarbonate alkalinity, Al2(SO4)3·14H2O + 3Ca(HCO3)2 → 2Al(OH)3↓ + 3CaSO4 +14 H2O+ 6CO2 ↑ Alkalinity-Coagulation Relationships Addition of Metallic salts release Hydrogen ions: Al2(SO4)3·14H2O → 2Al(OH)3↓ + 6H+ + 3SO42- + 8H2O Hydrogen ions neutralize alkalinity 1 mg/L alum neutralize 0.5 mg/L alkalinity Low alkalinity must be buffered to maintain coagulation Lime Ca(OH)2 or soda ash (Na2CO3) Coagulation-Stoichiometry Ferrous Sulfate (Copperas): Fe2(SO4)3·7H2O + 3Ca(OH)2 → Fe(OH)2 {Ferrous Hydroxide}+ CaSO4 + 7H2O Fe2(SO4)3·7H2O + 3Ca(HCO3)2 → Fe(HCO3)2 + CaCO4 + H2O Fe(HCO3)2 + 2Ca(OH)2 → Fe(OH)2 + 2CaCO3 + 2H2O 4Fe(OH)2 + O2 + 2 H2O → 4Fe(OH)3↓ Chlroniated Copperas: 6 (Fe2(SO4)3·7H2O) + 3Cl2 → 2Fe2(SO4)3 + 2FeCl3 + 42H2O 2Fe2(SO4)3 + 3Ca(OH)2 → 2 Fe(OH)3 ↓ + 3CaSO4 2FeCl3 + 3Ca(OH)2 → 2 Fe(OH)3 ↓ + 3CaCl2 Alum vs Iron Salts Iron Salts produce heavy floc & remove more suspended matter than Alum Iron Salts being good oxidising agents, can remove H2S and its corresponding taste and odour from water Iron Salts can be used over a wider range of pH Iron Salts cause staining and promote growth of iron and bacteria in distribution system Iron salts imparts corrosiveness to water than Alum Handling of Iron Salts needs more skill and control as they are corrosive and deliquescent No skilled labour for Alum Softening Softening Process of removing hardness from water Carbonate Hardness is defined as the concentration of Ca2+ and Mg2+ and other polyvalent cations in water that are associated with the anions that comprise alkalinity (e.g., HCO3−, CO32−) Noncarbonate Hardness is defined as the concentration of Ca2+ and Mg2+ and other polyvalent cations in water that are associated with no alkalinity anions (e.g., SO42−, Cl−) The presence of hardness causes scale in pipes and hot-water heaters, high soap consumption, and the deterioration of fabrics Removing hardness, termed Softening, may be accomplished either by chemical precipitation as insoluble compounds, ion exchange, or membrane processes Softening by Chemical Precipitation Precipitation softening relies on the relative insolubilities of CaCO3 and MgOH The choice of precipitating chemicals (lime and caustic soda) depends upon the raw-water quality and economics Lime–soda ash softening has been a traditional process for the removal of hardness, consisting of both carbonate and noncarbonate hardness, from water supplies Lime is used to remove chemicals that cause CH Soda ash is used to remove chemicals that cause NCH Softening by Chemical Precipitation Commercial Lime is in the forms of quicklime (CaO) and hydrated lime (Ca(OH)2 ) Quicklime is granular and usually greater than 90 percent CaO with magnesium oxide (MgO) the primary impurity Quicklime is less expensive than hydrated lime, it must be hydrated or slaked to Ca(OH)2 before it is used for Lime Softening can effectively remove softening heavy metals (e.g., Pb, Cr, Hg, As), Fe Powdered, hydrated lime contains about and Mn, turbidity, and some organic 70 percent Ca(OH)2 compounds including a substantial Soda ash is a grayish-white powder and amount of NOM and kill algae, bacteria, is nearly 98 % Na2CO3 and viruses Lime Soda Softening Lime : Soda : Ca(OH)2 or CaO → Ca2+ + OH- Reactions: Na2CO3 → Na+ + CO3= Ca2+ + CO3= ↔ CaCO3 (s) Mg2+ + 2OH- ↔ Mg(OH)2 (s) Lime Softening: Carbonate Hardness CO2 + Ca(OH)2 → CaCO3(s) + H2O Ca2+ + 2HCO3− + Ca(OH)2 → 2CaCO3(s) + 2H2O Detailed Equations: H2CO3 + Ca(OH)2 ↔ CaCO3(s) + 2 H2O Ca(OH)2 ↔ Ca2+ + 2OHH2CO3 + OH- ↔ HCO3- + H2O HCO3- + OH-↔CO3= + H2O Ca2+ + CO3= ↔ CaCO3(s) Lime Softening: Carbonate Hardness Mg2++ 2HCO 3−+ 2Ca(OH)2→ 2CaCO3(s) + Mg(OH)2(s) + 2H2O Detailed Equations: Ca(OH)2 ↔ Ca2+ + 2OH2HCO3- + 2OH- ↔ 2CO3= + 2H2O Mg2+ + 2OH- ↔ Mg (OH)2(s) 2Ca2+ + 2CO3= ↔ 2CaCO3(s) Filtration Filtration Removing particles from water Removal of particles (solids) from a suspension (two-phase system containing particles and liquid) by passage of the suspension through a porous medium The most common granular filtration technology in water treatment is Rapid Filtration The term is used to distinguish it from Slow Sand Filtration, an older filtration technology with a filtration rate 50 to 100 times lower than rapid filtration Principal Features of Rapid Filtration The most important features are 1. A filter bed of granular material that has been processed to a more uniform size than typically found in nature 2. The use of a coagulant to precondition the water 3. Mechanical and hydraulic systems to efficiently remove collected solids from the bed Mechanism Mechanism Mechanical straining Biological Action Electrolytic action Uniformity of Filter Media Media uniformity allows the filters to operate at a higher hydraulic loading rate with lower head loss but results in a filter bed with void spaces significantly larger than the particles being filtered As a result, straining is not the dominant removal mechanism Instead, particles are removed when they adhere to the filter grains or previously deposited particles Particles are removed throughout the entire depth of the filter bed by a process called depth filtration, which gives the filter a high capacity for solids retention without clogging rapidly Coagulation Pre-treatment If particles are not properly destabilized, the natural negative surface charge on the particles and filter media grains cause repulsive electrostatic forces that prevent contact between particles and media Properly designed and operated rapid filters can fail quickly if the coagulant feed breaks down or the raw water quality changes and the coagulant dose is not adjusted accordingly Dual Media Filter Stages in Rapid Filtration Filtration stage, during which particles accumulate, During the filtration stage, water flows downward through the filter bed and particles collect within the bed Backwash stage, during which the accumulated material is flushed from the system During the backwash stage, water flows in the direction opposite to remove the particles that have collected in the filter bed Conventional Filtration Classification of Filters According to type of Granular Medium used 1. Single Media (Sand or Anthracite) 2. Dual Media (Anthracite and Sand) 3. Multi Media (Anthracite, Sand, Garnet) According to Flow 1. Gravity Filter - Open to Atmosphere 2. Pressure Filter - Closed According to Rate of Filtration 1. Rapid Sand 2. Slow Sand Materials Used in Filter Media The common materials are sand, anthracite coal, garnet, and ilmenite Anthracite is harder and contains less volatile material than other types of coal Garnet (group of minerals containing a variety of elements, often appearing reddish or pinkish) and ilmenite (is an oxide of Fe and Ti) are heavier than sand and are used as the bottom layer in tri media filters In addition to these four minerals, Granular Activated Carbon is sometimes used as a filter material when adsorption and filtration are combined in a single unit process Filter- Examples Backwash Water Quality Management Water quality is affected by natural factors: – Historical uses in the watershed – Geometry of the watershed area – Climate of the region Good water quality protects drinking water as well as wildlife Point Sources of Pollutants Point sources include domestic sewage and industrial wastes Point sources - collected by a network of pipes or channels and conveyed to a single point of discharge in receiving water Municipal sewage - domestic sewage and industrial wastes that are discharged into sanitary sewers - hopefully treated Point source pollution can be controlled by waste minimization and proper wastewater treatment Nonpoint Sources Urban and agricultural runoff that are characterized by overland discharge This type of pollution occurs during rainstorms and spring snowmelt Pollution can be reduced by changing land use practices Combined Sewer Flow Nonpoint pollution from urban storm water collects in combined sewers Combined sewers- carry both storm water and municipal sewage - older cities Combined Sewer Overflow Eliminating this involves: – Construction of separate storm and sanitary sewers – Creation of storm water retention basins (Rainwater Harvesting) – Expanded treatment facilities to treat the storm water Combined sewers are not prohibited by the U.S. because removal would disrupt streets, utilities, and commercial activities Oxygen- Demanding MATERIAL Dissolved Oxygen (DO) - fish and other higher forms of aquatic life that must have oxygen to live Oxygen - Demanding Material - anything that can be oxidized in the receiving water resulting in the consumption of dissolved molecular oxygen - BOD, COD Almost all naturally occurring organic matter contributes to the depletion of DO Nutrients Nitrogen and phosphorus are considered pollutants when too much present in high conc. High levels of nutrients cause disturbances in the food web Organisms grow rapidly at the expense of others Major sources of nutrients (N, P): – – – – Phosphorus-based detergent Fertilizer and agricultural runoff Food-processing wastes Animal and human waste Pathogenic Organisms Include bacteria, viruses, and protozoa from diseased persons or animals Water is made unsafe for drinking, swimming, and fishing Antibiotic-resistant bacteria are the most dangerous Bacteria are found in both urban and rural environments with no observable pattern Pathogenic Organisms Serious Outbreaks of these cause great suffering Escherichia(E.) Coli - indicator of fecal coliform bacteria Salmonella (typhoid fever) Shigella (dysentery) Cryptosporidium - protozoa Giardia- protozoa Suspended Solids Suspended solids - organic and inorganic particles that are carried by wastewater into a receiving water A slower flow causes particles to settle and form sediment Colloidal particles - do not settle, cause an increase in the turbidity of surface water Organic suspended solids - exert oxygen demand Inorganic suspended solids - result from soil erosion Suspended Solids Suspended solids - organic and inorganic particles that are carried by wastewater into a receiving water A slower flow causes particles to settle and form sediment Colloidal particles - do not settle, cause an increase in the turbidity of surface water Organic suspended solids - exert oxygen demand Inorganic suspended solids - result from soil erosion Suspended Solids With an increase in the amount of sediment comes – Increase of turbidity – Decrease of light penetration – Increase in amount of bacteria – Increase in solids settled on the bottom which causes animal habitats to be destroyed Salts Total dissolved solids - TDS Water collects salt as it passes over soil during irrigation practice Too much salt can cause crop damage and soil poisoning Heat Impacts An increase in the Temp of water can cause: – Increase in DO which leads to a deterioration in water quality – Large fish kills – Blocked migration of fish – Altered genetic makeup in fish Water Pollution Problems in Streams - Self purification Dilution and decay of degradable, oxygen-demanding wastes and heat in a stream Dissolved Oxygen Depletion (From: Environmental Science: A Global Concern, 3rd ed. By W.P Cunningham and B.W. Saigo, WC Brown Publishers, 1995 Transport characteristics that affect concentration Velocity Dilution (mixing) Dispersion Degradation (mass loss) Adsorption (to soils) Sedimentation (to bottom) Aquatic Life (attached) v Effect of Oxygen-demanding wastes on rivers Depletes the dissolved oxygen in water Threatens aquatic life that require DO Concentration of DO in a river is determined by the rates of photosynthesis of aquatic plants and the rate of oxygen consumed by organisms Biochemical oxygen demand Biochemical oxygen demand (BOD) - oxidation of an organic compound is carried out by microorganisms using the organic matter as a food source Bioassay - to measure by biological means BOD is measured by finding the change in dissolved oxygen concentration before and after bacteria is added to consume organic matter BOD Ultimate BOD - maximum amount of oxygen consumption possible when waste has been completely degraded Numerical value of the rate constant k of BOD depends on: – Nature of waste and T – Ability of organisms in the system to use the waste Temperature Oxygen consumption speeds up as the temperature increases and slows down as the temperature decreases Oxygen consumption is caused by the metabolism of microorganisms BOD rate constants depend on: – Temperature of receiving water throughout the year – Comparing data from various locations at different T values Dissolved Oxygen DO If the discharge of oxygendemanding wastes is within the self-purification capacity, the DO is high If the amount of waste increases, it can result in detrimental changes in plant and animal life Aquatic life cannot survive without DO Objective of water quality management is to assess the capability of a stream to absorb waste Oxygen-Sag Curves Do Sag Curve DO concentration dips as oxygen-demanding materials are oxidized and then rises as oxygen is replenished from atmosphere and photosynthesis Major sources of oxygen: – Reaeration from the atmosphere – Photosynthesis of aquatic plants Factors of oxygen depletion: – – – – BOD of waste discharge DO in waste discharge is less than that in the river Nonpoint source pollution Respiration of organisms and aquatic plants Oxygen-Sag Curves What is an oxygen sag curve? – Is the dip in dissolved oxygen observed when BOD waste water is discharged continuously into a river – The extent of the sag is determined by BOD level in the wastewater stream, by the rate of discharge, and by other factors such as temperature and river characteristics (flow rate, turbulence, etc) – An oxygen sag curve is also observed due to a one-time discharge of BOD waste into a lake – In that case, the DO drop is with time instead of distance downriver – Continuous discharge of BOD waste into a lake results in a decrease in steady-state DO level (not a “sag” followed by a recovery) Dynamics of Oxygen Depletion & Dissolution Figure on left shows a model framework to calculate: DO as a function of distance downstream from a point source discharge; or DO as a function of time after a single “spike” discharge of BOD wastewater DO falls when decomposition rate > dissolution rate and DO rises when decomposition rate < dissolution rate Rate of decomposition (deoxygenation) Linearly proportional to BOD level BOD falls exponentially with time Rate of oxygen dissolution (reaeration) Linearly proportional to the oxygen deficit: DOsat - DOactual Wastewater Treatment RWE Raw water Community WWE Wastewater engineering Quickly drain off the wastewater away from the community To make wastewater fit for dispose or environmental friendly RWE Raw water Community Disposal Treatment of Wastewater Conveyance Collection Wastewater is a term that is used to describe waste material that includes industrial liquid waste and sewage waste that is collected in towns and urban areas and treated at urban wastewater treatment plants Wastewater treatment: A process to convert wastewater - which is water no longer needed or suitable for its most recent use - into an effluent that can be either returned to the water cycle with minimal environmental issues or reused Wastewater Contaminants Suspended solids Biodegradable organics (e.g., BOD, COD) Pathogenic bacteria (e.g., E-coli) Nutrients (N & P) Wastewater Sources Residences human and animal excreta and waters used for washing, bathing, and cooking ― Commercial institution Dairy and industrial establishment ― slaughterhouse waste, dairy waste, tannery wastewater, etc. Where does the water from the washer go? When you flush the toilet where does the contents go? By gravity flow, the waste is on its way to your local wastewater treatment plant! Why to treat wastewater? Causes a demand for dissolved oxygen (lower DO levels of streams) Adds nutrients (nitrate and phosphate) to cause excessive growth Increases suspended solids or sediments in streams (turbidity increase) Objectives of WWT Reduce organic content i.e., BOD Removal/reduction of nutrients i.e., N,P Removal/inactivation of pathogenic microbes Levels of Treatment Primary removal by physical separation of grit and large objects (material to landfill for disposal) Sedimentation and screening of large debris Secondary Biological and chemical treatment aerobic microbiological process (sludge) organic matter + O2 CO2 + NH3 + H2O Primary treatment Typical materials that are removed during primary treatment include fats, oils, and greases sand, gravels and rocks larger settle-able solids including human waste floating materials Methods used in primary treatment Bar screens Grinding Grit Chamber Sedimentation Tank - primary Settling tank Chlorination of effluent Biological treatment activated sludge trickling filter oxidation ponds Secondary treatment Activated sludge process Primary wastewater mixed with bacteria-rich (activated) sludge and air or oxygen is pumped into the mixture Both aerobic and anaerobic bacteria may exist Promotes bacterial growth and decomposition of organic matter BOD removal is approximately 85% Microbial removal by activated sludge 80-99% removal of bacteria 90-99% removal of viruses Trickling filters Trickling filters are beds made of coke (carbonized coal), limestone chips or specially fabricated plastic media Optimize their thickness by insect or worm grazing The primary wastewater is sprayed over the filter and microbes decompose organic material aerobically Low pathogen removal Bacteria, 20-90% Viruses, 50-90% Giardia cysts, 70-90% Stabilization or oxidation ponds Oxidation ponds are a few meters deep, and up to a hectare in size They are low cost with retention times of 1 to 4 weeks. Odor and mosquitoes can be a problem Pathogen removal: Stabilization ponds are the preferred wastewater treatment Bacteria, 90-99% process in developing countries due to low cost, low maintenance - Virus, 90-99% This is balanced by larger land requirement Protozoa, 67-99% Mechanisms include the long detention time, high pH (10- 10.5) generated by photosynthesis, predation, sunlight, temperature Sludge Treatment Processes Thickening (water removal) Digestion (pathogen inactivation and odor control) Conditioning (improved dewatering with alum and high temp, 175-230° C) Dewatering (pathogen inactivation and odor control) Incineration (volume and weight reduction) Final disposal Wastewater Treatment Alternatives Septic Tanks Constructed Wetlands Composting Wastewater treatment 5R Concept Introduction At the center of social and economic development, WATER is vital for energy generation, agriculture, industry, environmental management and cultural development World Bank points out that ~4.5billion people lack sanitation services and ~2.1 billion people lack clean drinking water World Economic Forum’s Global Risks Report, 2017 states that water scarcity is becoming the largest global risk, due in part to climate change, which will lead to more intense droughts, floods, glacial melting, and unpredictable precipitation Other Driving Factors for water scarcity: Improving living standards changing consumption patterns Irrigated agriculture expansion Uneven Water Distribution Uneven distribution of water resources highlights the problems of worldwide water scarcity Worldwide predictions indicated severe water scarcity prevails in locations with either high population density (e.g., in North Africa, Australia, Arabian peninsula, London, San Francisco Bay, and Hainan island) in areas of intensely irrigated agriculture (Great Plains in the United States) or both (India, eastern China, and the Nile delta) Distribution of water availability (bottle size representing annual water stock m3/person). Managing urban water Used water from household activities is called gray water, and it is not heavily polluted Contrasting is black water, which comes from toilet flushing Black water is a relatively small fraction of the flow (parts of toilet flushing, only 30% of total household utilization) but contains most of the BOD, nitrogen, phosphorus, and bacteria of health concern Blue line indicates that storm water can be collected to be part of the supply, and gray water and black water also can be treated and returned as a supply - thus, “wasted waters” become new source “wasted waters” become new sources Projects of “wasted waters” reuse have been applied and were not successful, such as one initiated in Hamburg (German) Public Utilities Board in Singapore processed water reclamation and reuse in1966 but, failed because of costly and unreliable technology Public acceptance of recycled water also withdrawn “wasted waters” reuse project promotion, such as failure cases of Toilet to Tap campaign in California (USA) and Drinking Sewage campaign in Toowoomba (Australia) 5R Generation It approaches to manage urban water It harvests storm water, gray water and black water in several forms It offers promise for moving solutions for urban water scarcity in practice Recover (storm water) Reduce (toilet flushing water) Recycle (gray water) Resource (black water) and Reuse (advanced-treated wastewater) 5R generation integrates newer technologies, available for practice only in recent years, to gradually replace traditional wastewater treatment systems to enhance water utilization Recover Reduce Resource Recycle Reuse Framework of 5R generation Recycle B Applicable in common-use settings, such as community and shopping centers, hotels, and office buildings Recover (storm water) It focuses on urban storm water and includes technologies for collection, treatment, and storage Storm water often requires less treatment than municipal waste-waters, and its collection offers an added benefit of reducing pollution and erosion issues in receiving water bodies After storm water is collected from a drain, creek, roof, or pond, it can be treated to achieve quality requirements for different purposes Storage is a major challenge for Recover, due to space constraints Traditionally, storm water is temporarily held behind dams or in tanks to balance supply and Mobile treatment and storage demand, but alternatives are being sought for storm water Wetlands are also being applied for storm water storage Recover in 5R generation was built to collect storm water from roofs, and mobile treatment and storage of the storm water can be used when rainfall is heavy Reduce (toilet flushing water) Reduce in 5R generation mainly uses high efficiency, water-saving toilets to minimize the flushing volume, which contributes for approximate 30%of the black water flow Reducing the water for flushing is a direct method to address scarcity in urban areas A vacuum toilet uses only 0.5–2 L water (adjustable) per flush, compared to ~10 L/flush for a typical conventional toilet Vacuum flushing already is widely applied for toilets in airplanes and high-speed trains, and it is beginning to be used in homes and hotels Vacuum Toilet Recycle (gray water) Recycle in 5R generation mainly involves processing and recycling gray water in a household or community system Gray water is generated from kitchen and bathroom sinks, showers and bathtubs, and laundry discharges, and it excludes toilet discharge which could be applied for toilet flushing and irrigation after reuse According to lifestyle, living standard, population structure (age, gender), customs, water infrastructure, and the degree of water scarcity, gray water varies typically from 90 to 120 L/person/day, indicating a huge volume recyclable gray water as potential water source Based on civil wastewater calculation, 50%–70% of total domestic wastewater, which could be classified into gray water, only contains 30% of the organic matter and 9%– 20% of the nutrients Resource (black water) Resource in 5R generation mainly involves capturing resources from black water in a household or community water system. Household wastewater normally consists around 30% black water, making black water a non-trivial water source if it is not discharged immediately to the sewer Black water was ignored in the water-recycle system, as it typically was discharged directly into a sewer for transport to a municipal wastewater treatment plant Biochemical oxygen demand (BOD) and nutrients(phosphorus and nitrogen) usually are the most valuable resources in black water Recently, recovery of black water has become a hot topic, and anaerobic digestion is the most effective method for treatment and recovery of black water Reuse (advanced-treated wastewater) Even when Recover, Reduce, Recycle, and Resource are well implemented in a community, some wastewater is going to a conventional wastewater-treatment plant That water also can be reused for a range of purposes, if the effluent quality is sufficient for the purpose Thus, 5R generation includes Reuse to overcome water-quality deficiencies At municipal wastewater treatment plants, advanced treatment, which aims to remove suspended, colloidal, and dissolved constituents remaining after biological treatment, is applied to improve effluent quality Bed filtration, surface filtration, micro- and ultra-filtration, reverse osmosis, electro dialysis, adsorption, air stripping, ion exchange, advanced oxidation, distillation, chemical precipitation, chemical oxidation, and disinfection are the typically advanced technologies used to improve effluent quality