Presentation on theme: "Environmental Engineering 343"— Presentation transcript:
1 Environmental Engineering 343 Philadelphia UniversityFaculty of EngineeringDepartment of Civil EngineeringFirst Semester, 2013/2014Environmental Engineering 343Lecture 8:Water Treatment (3)Filtration & Disinfection
2 FiltrationFiltration is a process for separating suspended or colloidal impurities from water by passing through a porous medium, usually a bed of sand or other medium.Settled water (Sedimentation effluent) turbidity range 1-10 TU – due to residue of flocs particles. So turbidity need to be reduce to less than 0.3Common materials for granular bed filters:Sand (slow, rapid or high)Anthracite coalDual media (Coal plus sand)Mixed media (coal, sand & garnet)
3 Filtration- Filter media Loading rateWhere,Vo = water velocity (m/s)Q = water flow (m3/s)Ac= cross surface area (m2)ExampleInstalling a Sand filter after the sedimentation tank . The design loading rate to the filter is 200m3/d.m2 How much filter surface area should be provided for the design flow rate of 0.5 m3/s
4 FiltrationPurpose to reduce the turbidity from TU to less than 0.3 TUSlow sand filtersLow filtration rate with the use of smaller sandFilter sand is less uniformParticles are removed on the surface of the filter (forming a mat of materials , called schmultzdecke)Schmultzdecke forms a complex of biological community that degrade some organic compounds.Pretreatment is not importantRequire large area of land and are operator intensiveLoading rate m3/d.m2
5 Filtration Rapid sand filters ( most common) Graded (layered) within the bed to optimize the passage of water while minimizing the passage of particulate materCleaned in place by backwashing processPretreatment to destabilize particles is essentialLoading rate 120 up to 235 m3/d.m2For larger loading rate, a minimum of 4 filters is suggested
8 Filtration Dual-media Filters Constructed of silica sand and anthracite coal.Depth of sand is about 0.3m and coal 0.45 m. Size and uniformity is selected to produce a distinct separation after backwashingDisadvantage is that filtered materials are held loosely in the anthracite layer and can dislodge with sudden changes in hydraulic loading. The material can then bind to the sand layerLoading rate up to 250 m3/d.m2 or more
9 Filtration Mixed Media Filters The perfect filter is composed of a grading of large media at the top to small at the base. This is best achieved by the use of 3 or more media with ranging size, density and uniformity coefficientTypical installation – Overall bed depth 0.75m; 60% anthracite; 30% silica sand; 10% garnet sandSize range from 1.0mm anthracite to 0.15 garnet sandFiltration rates range from 10 to 20m/hr
10 FiltrationTerminal head loss: As the filter clogs, it become very hard to force water through the bed and the filter bed must be cleaned for backwashing processHead loss: (pressure drops) that occurs when clean water flows through a bed of clean filter media. It is important in design value to determine the overall head require to operate the filterBackwashing process; allowing large flow of water (clean water) to be pumped to enter the filter bed. This forces the sand bed to release the colloidal particles that trapped in the pores and released and escaped with the washwater.
11 Filtration Design Key Elements HydraulicsParameters to be measure during operationThe head loss across the filterThe turbidity of the effluent
14 DisinfectionDisinfection is used in water treatment to reduce pathogens (diseases-producing microorganisms) to an acceptable level
15 Disinfection- Introduction Treatment of water with chemicals to kill bacteria”Two objectives:Primary disinfection : Kill any pathogen in waterSecondary (residual) disinfection : Prevent pathogen re growth in the water.Method use :Should be harmless and unobjectionable to the consumerShould be able to retain a residual disinfecting effect for a long period
16 Disinfection- properties Destroy bacteria / pathogens within a practicable period of time, over an expected range of water temperatureEffective at variable compositions, concentration and conditions of water treated.Neither toxic to humans and domestic animals nor unpalatableNot change water propertiesHave residual in a sufficient concentration to provide protection against recontamination
17 Disinfection- properties-con’d 6) Can be determined easily, quickly, and preferably automatically.7) Dispensable at reasonable cost8) Safe and easy to store, transport, handle and supply10) Not form toxic by-products due to their reactions with any naturally occurring materials in water.
19 DISINFECTION METHODS-CHLORINATION Free chlorine disinfectionEffective and the most common applicationAvailable in granular, powdered, liquid or gases formDeveloped by using chlorine gas (Cl2), sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(OCl)2).Reacts in water to produce dissolved chlorine gas(Cl2(aq)), hypochlorous acid (HOCl) and hypochlorite (OCl-).
20 Disinfection –water reaction H2O+Cl2 (g) H+ + HOCl + Cl-HOCl H++ OCl- (pH > 8)H++ OCl HOCl (pH < 7)HOCl= hypochlorous acidOCl- = hypochlorite ionHOCl is more effective than OCl-Chlorine gas will be injected for water pH less than 3
22 Ammonia Reaction Con’d The proportion of chloramines depends onpH.TemperatureTimeInitial chlorine to ammonia ratio*Combined chlorine: Combination of chlorine with ammonia nitrogen or organic nitrogen compounds. Combined chlorine is less reactive.
23 Chlorination- Advantage Provide chlorine residual for secondary disinfection.Chlorine residue must be maintained in the treated water to the end user. This secondary disinfection functioned to control pathogen distribution during water distribution.Increase lifetime of chlorine residual by adding ammonia to treated water.Ammonia reacts with free chlorine to form chloramines (NH2Cl, NHCl2 and NCl3) which are termed combined chlorine.-Chloramines less effective as oxidants than HOCl, seldom used as primary disinfectant. However, more persistent than free chlorine and maintain secondary disinfectant.
24 Chlorination- Disadvantage Effectiveness is less with protozoan cysts, Giardia lamblia and Crytosporidium and virus.Formation of halogenated disinfectant by- products (DBPs). Includes trihalomethanes (THMs) such as chloroform (CHCl3) and haloacetic acids (HAAs).- THMs and HAAs created when free chlorine combines with natural organic substances that may still be present in the water.
25 Chlorine Effectiveness -Factors 1.Dosage – Sufficient high concentration to inactive pathogen 2.Contact time – Physical contact with pathogen for sufficient time to achieve inactivation. 3.Turbidity – Present particles (turbidity) hides pathogen from the disinfectant. 4.Other reactive species – Consume disinfectant, reduce concentration available for inactivation. 5. pH – Most effective at pH values less than Water temperature – Disinfection increases, however chlorine become less stable.
26 Disinfection - OzoneSweet smelling, unstable gas-Form of oxygen in which three atoms of oxygen combined to form the molecule O O+O2 O3Air in generating equipment contain up to 13% ozone. Most powerful disinfectant and widely used in waterMore effective against cysts and viruses than free chlorine.Leaving no taste or odor problems.Faster contact timeNo residual remainNot forming THMUnaffected by the pH or the ammonia content of the water High cost – Construction, Operation and Maintenance
27 Disinfection - UVEmployed by submerged UV lamps into the water to be treatedEnergy is absorbed by genetic material in the microorganism, interfering with their ability to reproduce and survive.Multiple lamps used to provide greater coverageAbility of UV lights to pass through the water to get the target organism.Much higher energy level than visible lightPotential to inactivate pathogens.Performs well against both bacteria and viruses.Leaves no residual protection for distribution system.High cost