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Water treatment processes

Water treatment processes
Lecture 8: Water treatment processes th (Jan 27 2015) by Dr. Arun Kumar (arunkucivil.iitd.ac.in) Objective: Understand functioning of different unit processes for water treatment Courtesy: Dr. Irene Xagoraraki (Michigan State University, East Lansing, USA) January 28, 2015 Arun Kumar 1 (arunkucivil.iitd.ac.in)Previous lecture recap • Introduction of water treatment schematic concepts • Discussion on production of water of different final usages • Discussion on checks – Remove solids before removing bacteria – Remove solids using sedimentation process before using filtration – Remove organic compounds and ammoniabased compounds before disinfection • Discussion on need for calculating solids waste produced from every unit processes and solid waste managementExample 2: River Water Drinking water January 28, 2015 3Example 2 schematic contd. January 28, 2015 4Treatment schematic (SW Potable drinking water) • Sequence of unit processes: presedimentation mixing, flocculation, settlingfiltrationadsorptiondisinfection • Function of unit processes: solids removal  removal of ions, and solids using chemical additionremoval of smaller particlesremoval of organic compounds and ionsoxidation of oxygen demanding wastes and chemical killing of pathogens in water January 28, 2015 5(SW Potable drinking water) contd.. • Chlorine is added in sedimentation and filtration tank to avoid microbial growth • Chorine is used as a oxidizing agent as well as a disinfection solution. January 28, 2015 6Surface Water TreatmentConventional Surface Water Treatment • Screening (remove relatively large floating and suspended debris) • Rapidmix (mixing water with chemicals that encourage suspended solids to coagulate into larger particles that will settle easily) • Flocculation (gently mixing water and coagulant allowing the formation of large particles of floc)Conventional Surface Water Treatment • Sedimentation (flow is slowed enough so that gravity will cause flocs to settle) • Sludge processing (mixture of solids and liquids collected from settling tank are dewatered and disposed of) • Disinfection (ensure that water is free of harmful pathogens) • Distribution system protection (residual disinfection)Solids removal January 28, 2015 10Method 1.SedimentationMethod1. Sedimentation • Solids settle based on their gravitational force (with and without externally added chemicals). • Settling depend on solid physical characteristics (diameter, density) and medium temperature, viscosity, density, etc. • Some solids do not interact with each other during settling (i.e., discrete particles) (no change in their size and shape). The settling is called discrete settling (Type 1 settling). Ex: settling of sand. January 28, 2015 12Method1. Sedimentation • Some solids interact during their settling and change their size and shape (i.e., flocculent particles) (Type 2 settling). Ex: settling of clay; bacteria. January 28, 2015 13Sedimentation • Time for settling = column depth/settling velocity at steady state • Some particle take less time and some particles take longer time to settling. • if tdesigntsettling, particles remove 100. All patcies now constitute to solid waste. • if tdesigntsettling, particles do not remove 100. Remaining particles go to next unit in treatment plant scheme. January 28, 2015 14Method 2. CoagulationFlocculation Sedimentation Some coagulants: Some coagulant aids: aluminum sulfate, activated silica ferric sulfate clay ferric chloride polymersCoagulationFlocculationSedimentation Fullscale Pilotscale BenchscaleSizes of Particles in WaterMethod2. Coagulationflocculation (sedimentation after chemical addition) • Some solids take very long time to settle (size in submicron range or in nanometer range). • Chemicals (ex: alum; ferric chloride) are added in solution to (1) increase size of particles, (2) capture them in hydroxide flocs and then precipitate them. January 28, 2015 18Coagulation Flocculation + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Colloidal particles (0.001 10 μm) floc (1 100 μm)Method2. Coagulationflocculation (sedimentation after chemical addition) • Coagulation methods: (i) ionic layer compressions, (2) charge neutralization and surface complexation, (3) sweep coagulation(iv) polymeric bridging • Ex: ferric chloride gives ferric ions (acidic pH) and ferric hydroxide (basic pH). These species work in 2 different ways to improve particle settling. January 28, 2015 20CoagulationFlocculation • Double Layer Compression • Adsorption of Aluminum to Produce Charge Neutralization • Interparticle Bridging • Enmeshment in Al(OH) Precipitate 3 (sweep floc) Picture Source: Malvern Instruments, ZetaMeter Inc.CoagulationFlocculationSedimentation Fullscale Pilotscale BenchscaleRapid MixingFlocculation th (Source: Water Supply and Pollution Control, 5 ed. W. Viessman, Jr. and M.J. Hammer, Harper Collins College Publ. 1993)SedimentationQuestion 1: GDW to Gardening water • Which parameters do we need to remove • Which unit processes do we need for making gardening water from groundwater Is the order of unit process relevant here • How does the schematic of treatment of GDW to Gardening water differ with that of GDW Drinking water January 28, 2015 26Question 2: Injection of gardening water runoff in soil:Requirements • Can we inject garden water runoff to soil • Which parameters do we measure • Do we need to do treatment before we can inject the water (i.e., pretreatment) • Will it depend on soil type, depth to water table, etc. January 28, 2015 27Question 3: Comparison • Why schematic for GDW DW and SW DW differ • Can one treatment plant treat GDW as well as SW for producing DW January 28, 2015 28Question 4: Domestic WW to Water suitable for discharge to river • Can you draw schematic for treating domestic wastewater to produce water suitable for discharge to river January 28, 2015 29Answer 4: Domestic wastewater  Discharge water to river • Sequence of unit processes: stabilization tanksedimentation biological processsecondry settling  nitrifier unitdenitrifier unitdisinfectiondischarge water suitable for river • Function of unit processes: making incoming flow rate uniformsolid removalbreakdown of organic compound settling of microbial biomass removal of ammonium ionsremoval of nitrate ionskilling of microorganisms January 28, 2015 30Treatment schematic (GW Potable drinking water) • Sequence of unit processes: Aeration chamber  Softening unit  Filtration with chlorination Disinfection • Function of unit processes: Gas removal  cations and solids removalsolids removal microbial removal • Lime, soda ash and chlorine are required daily. • Solid waste generated from softening unit and filtration unit are calculated January 28, 2015 31Next class. January 28, 2015 32