Transcript Slide 1
Water Treatment Dr. Martin T. Auer MTU Department of Civil & Environmental Engineering Drinking Water Treatment Objectives Potable: safe to drink - may be consumed with low risk of immediate or long term harm. Palatable: pleasant to taste London 1854 Philadelphia Milwaukee Cryptosporidium 1993 - 403,000 – 104 Dead U.S. EPA Standards National Primary Drinking Water Regulations Microorganisms Disinfectants & Disinfection Byproducts Inorganic Chemicals Organic Chemicals Radionuclides Enforceable U.S. EPA Standards National Primary Drinking Water Regulations Microorganisms Viruses: hepatitis A, gastroenteritis Bacteria: cholera, dysentery, legionellosis, typhoid Protozoa: Giardia and Cryptosporidium Turbidity: standard is 1 NTU; microorganism contamination is associated with turbidity; particles also shield microorganisms from agents of disinfection. Indicator organisms: standard for microbial contamination is based on E. coli , a species of bacteria originating from animal or human fecal material. Most strains of E. coli are not pathogenic, but their presence indicates the presence of fecal material and thus, potentially, pathogenic microbes. No E. coli may be present in finished drinking water. U.S. EPA Standards National Primary Drinking Water Regulations Chemicals Inorganic: arsenic, cadmium, copper, lead, mercury, nitrate Organic: herbicides (e.g. atrazine), insecticides (methoxychlor), industrial residues (e.g. polychlorinated biphenyls, dioxin) Radionuclides: and particles, radium, uranium MCLGs: Maximum Contaminant Level Goals – level below which there is no know or expected risk to health. Allow a margin of safety and are not enforceable. MCLs: Maximum Contaminant Levels – highest level allowed in drinking water. MCLs are enforceable and are set as close to MCLGs as feasible using BAT, taking into account economic considerations. U.S. EPA Standards National Primary Drinking Water Regulations Disinfectants Chlorine Chlorine dioxide Chloramines Disinfectants & Disinfection Byproducts Trihalomethanes U.S. EPA Standards Secondary Drinking Water Regulations Cosmetic Effects (tooth color, excess fluoride) Total Dissolved Solids (chloride, sulfate) Taste, Odor, Color Non-Enforceable (federally) U.S. EPA Standards National Primary Drinking Water Regulations Microorganisms Viruses: hepatitis A, gastroenteritis Bacteria: cholera, dysentery, legionellosis, typhoid Protozoa: Giardia and Cryptosporidium Turbidity: standard is 1 NTU; microorganism contamination is associated with turbidity; particles also shield microorganisms from agents of disinfection. Indicator organisms: standard for microbial contamination is based on E. coli , a species of bacteria originating from animal or human fecal material. Most strains of E. coli are not pathogenic, but their presence indicates the presence of fecal material and thus, potentially, pathogenic microbes. No E. coli may be present in finished drinking water. Drinking Water Process Train Basic treatment for turbidity and pathogens Particle Settling Velocities Particle Diameter (mm) Velocity (m/s) Sand 1.0 2x10-1 Fine sand 0.1 1x10-2 Silt 0.01 1x10-4 Clay 0.001 1x10-6 Source: Vesilind & Morgan Stokes Law v d 2 g ( p l ) 18 Thus, the small, clay particles settle extremely slowly. Coagulation + + + Chemicals such as alum + + Al2 (SO4 )3 + + + + + + + + + + + + + + Addition of the chemical occurs in a flash mix of 1-3 minutes where the chemical dissolves and mixes with the raw water. + + + + + + are added to neutralize the negative charge and destabilize the particle populations and allow them to come together, i.e. coagulate. + + + + Particle populations are stable because their net negative charge repels one another. + + Flocculation The alum reacts with calcium bicarbonate naturally present in most waters to form a precipitate or floc, aluminum hydroxide. Al2 (SO4 )3 3Ca( HCO3 )2 Al (OH )3 3CaSO4 6CO2 Destabilzied smaller particles can be attracted to the floc or simply swept up by the larger particles (sweep floc) as they settle and removed from the system. Flocculation proceeds through a slow mix of 10-30 minutes. 7 1 107 Alum Sweep Floc 6 8 106 8x10 Alum Sweep Floc 6 0 min 6 106 6x10 14 min p Volume distribution Distribution Volume Particle 3 (m (V/ log d )/mL) in m3 cm-3 1x10 6 4 106 4x10 6 2 106 2x10 0 0.0 0.0 0.4 0.4 0.80.8 1.2 1.2 log ofof particle diameter measured in m log particle diameter (d in m) p Courtesy Dr. Desmond Lawler, University of Texas 1.6 1.6 Sedimentation Sedimentation takes place over a period of 1-4 hours. Filtration Water is filtered until significant head loss is experienced, then the filter is backwashed. Disinfection Chlorination sorption Cl2( g ) H2O HOCl HCl HOCl OCl H Ka 107.5 , pKa 7.5 HOCl dominates below pH 7.5 and OCL dominates above pH 7.5 HOCl is the stronger disinfectant and thus it is best to chlorinate at pH<7.5 Disinfection – The CT Relationship Design Chick’s Law dN sorption kN dt The Surface Water Treatment Rule Requires a 4-log or 99.99% removal Nt ln N 0 t k Nt N0 ek t Nt e k t N0 % Red Nt/N0 log Nt/N0 Log Red 90 0.1 -1 1 99 0.01 -2 2 99.9 0.001 -3 3 99.99 0.0001 -4 4 t ln(0.0001) k Disinfection Design Ct = concentration, time The Surface Water Treatment Rule Requires a 4-log or 99.99% removal It = irradiance, time Disinfection Residual Disinfection Disinfection By-products (DBPs) Formed through reaction of chlorine and natural organic matter (NOM) e.g. trihalomethanes such as chloroform, Cl2 Design – reduce NOM through pre-oxidation with ozone. Iron and Manganese Hardness Primarily Ca2+, Mg2+ Limestone: calcium carbonate, CaCO3 Dolomite: calcium magnesium carbonate, CaMg(CO3)2 The limestone formation underlying much of Miami, Florida Lime – Soda Ash Process Adding lime, Ca(OH)2 Add Ca(OH)2 to raise pH and convert HCO3 CO32 Ca(HCO3 )2 + Ca(OH)2 2CaCO3 + 2H2 0 Mg HCO3 2 2Ca OH2 MgCO3 + 2CaCO3 2H2O But MgCO3 is soluble. Add more Ca(OH)2 to further raise pH. MgCO3 Ca OH2 Mg OH2 CaCO3 Adding soda ash, Na2CO3 Sometimes there is insufficient HCO3 to convert to CO32 ; adding soda ash Na 2CO3 2Na 2 CO32 Granular Activated Carbon The most commonly used adsorbent is granular activated carbon (GAC). These irregular particles, 0.2-5 mm in diameter, are a char of carbon material (wood or coal). They are ‘activated’ or made more porous by exposure to steam at high temperature. Activated carbon has 1000 m2 of adsorbing surface area per gram (~ 1 teaspoon) or equivalent to that of a 40 acre farm in one handful! Source: Chemviron Carbon Source: Millenium Inorganic Chemicals Source: Sontheimer et al. 1988 The Adsorption Process Organic chemicals are typically removed from a water supply prior to distribution through the process of adsorption: the physical-chemical attraction of a solid material for a chemical in solution. In adsorption, the chemical being adsorbed is termed the adsorbate and the solid to which it sorbs is the adsorbent. Influent stream Effluent stream … tendency to sorb Concentration (mg/L) For adsorption to be effective, the chemical must sorb strongly. Poorly soluble (hydrophobic) compounds (e.g. the components of gasoline) adsorb more strongly than highly soluble (hydrophilic) compounds (e.g. table salt). 6 Poorly sorbed 4 2 Strongly sorbed 0 0 2 4 Time (d) 6 8 … application in water treatment In drinking water treatment, adsorption with GAC is accomplished using a packed bed column. The untreated water is introduced at the top of the column and trickles down through the GAC. Contaminants are removed en route and clean water emerges at the bottom of the column. In application, columns 6 feet in diameter and 30 feet in height are not uncommon. Carbon Bed GAC columns … column operation Water flows thru the column and contaminants are adsorbed. With time, the GAC becomes saturated (sorption capacity is reached) and contaminants exit the bed (breakthrough). The exhausted carbon must then be replaced. Cin Cout Ceq exhaustion breakthrough Asbestos/Arsenic/Metals Removal Sorption with Ferric sulfate: asbestos, arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, selenium, silver Coagulation with Alum: asbestos, nickel, uranium Membrane Processes Ultrafiltration polypropylene fiber 300 µm ID 500 µm OD Ultrafiltration 0.2 µm nominal pore size raw water in epoxy seal permeate flow Ultrafiltration Ultrafiltration contaminants banks of fiber bundles backwashing Ultrafiltration contaminants backwashing Home Water Treatment Reverse osmosis unit (salt) Softening by ion exchange (hardness) Home Water Treatment Three step process: • sieve and bottom filter – rust, sand, turbidity • activated carbon filter – chlorine, color and SOCs • ion exchange resin – metals Achieves 99.99% removal of Giardia and Cryptosporidium cysts, but does not remove all pathogenic organisms. Bottled Water: $8 /gallon Tap Mount: $0.25 / gallon Municipal: $0.0015 / gallon