FiltrationLecture No. 71. Purpose2. Considerations3. Type and Selection GuideA. Available Alternatives.B. Selection Guide4. Hydraulics of Filtration5. Operation and Maintenance6. Design Criteria7. ExampleFiltrationLecture No. 71. Purpose- The fundamental system that removes particulate matter is filtration.- The most common filtration process employs a granular media of a certain size and depth.- The pre-treated water, typically pre-treated by chemical addition, coagulation and sedimentation passes through the filter bed where a majority of the particulates are removed in the top portion of the filter media as well as throughout the entire depth ofthe bed.- Under certain raw water conditions, adequate treatment of the raw water can be carried out in the filter alone, and the need for the ordinary flocculation and sedimentation processes may possibly be eliminated. This process is called direct filtration.2. Considerations- Site Topography. The ideal plant site has a constant slope of 2-3% which allows the filter wash tank, filters and wash-waste holding tank to be easily situated and economically built since there is no need for excessive excavation.Backwashwater tankWaterSourceClear WellLevel during backwashLevel during filtrationFlow control valve- opensgradually as filters becomedirty to maintain constantflow-backwash tank when valve is fully openedTo disinfection andwater distributions systemTo backwash water treatmentFrom backwashwater treatmentFiltration, Page No. 2- Plant Size. Very large plants, > 200MGD, can have their total number of filters exceed 30. The total number of filters my be reduced if the filter size is increased, up to 200ft2,or by using a high filtration rate such as 8 gpm/ft2 or both.- Raw Water Quality and Type of Pretreatment. If the underground water contains high levels of iron or manganese, the best method for removing these compounds is by oxidation with chlorine or potassium permanganate, followed by pressure filtration. In the soil(insoluble)In the water(soluble)In the water(when O2available)Fe+++-Fe++-Fe+++Mn+4anaerobicconditions suchas in a groundwater aquiferMn++O2when waterreaches surfaceair Mn+4colloidalprecipitateExample:Given: Well water. 3.2mg/l of Fe++ and .8mg/l of Mn++ at pH 7.8. 1mg/l of potassium permanganate will oxidize 1.06mg/l of iron and .52mg/l of manganese in accordance with the following equations:3Fe++ + MnO4 - 3Fe+++ + MnO2, pH>7.53Mn++ + 2MnO4 - 5MnO2, pH>9.5Find: the potassium permanganate treatment requirementsKMnO4 (for iron) = 3.2mg/ Fe++ x 1KMnO4 /1.06mg/l of Fe++KMnO4 (for iron) = 3.02mg/lKMnO4 (for Mn) = 0.8mg/ Mn++ x 1KMnO4 /.52mg/l of Mn++KMnO4 (for Mn) = 1.54mg/lTotal KMnO4 requirement = Fe+Mn = 3.02mg/l + 1.54mg/lTotal KMnO4 requirement = 4.56mg/l- Allowing for Future Filter Modifications or Additions. In many cases, water treatment plants “grow” by adding basins and filters in stages. The filter influent channel, effluentchannel and backwash system must be designed to accommodate the new flow rate, based on the original plant specifications. In the future, GAC, granulated activated carbon, may be required to adsorb objectionable organic compounds. GAC beds require a 15-30 minute EBCT, empty bed contact time, necessitating a depth of 6-8’ which can not be provided by ordinary filters.- Filter Washing System. Four basic schemes: elevated wash tank, direct pump, self-backwash (Greenleaf) and continuous backwash. Elevated tank and direct pump are the traditional and therefore proven systems. With respect to the washing system, the basic alternatives are : backwash alone, ordinary air-scour wash, and a combination.- Filter Rate Control: There are 2 modes of filtration: constant rate and declining-rate. Both are capable of producing <1.0NTU effluents. There are 4 basic types of rate control systems: 1.constant rate with a flow meter and flow modulation valve;2. constant level filtration with equal flow splitting inlet weirs, water level sensor and flow modulator valve; 3.declining rate filtration with a submerged inlet; 4.constant ratefiltration with equal flow spitting inlet weirs and a weir to control the inlet level.- Type of Filter Bed. Contingent upon the availability of bed material. Choices include: sand, anthracite coal, GAC and garnet. Third world counties use sand and gravelFiltration, Page No. 3because other types are too expensive. If the main purpose is the filtration of sewage as in a tertiary treatment process, a monosand or mono-anthracite coal is best. For slow sand filtration, the filtration rate is less than .2gpm/ft2. Rapid sand filtration rates range from 2-5gpm/ft2. High rate beds are from 5-10gpm/ft2 and always consist of a reverse graded filter bed or a deep large grain monomedium bed.Example: Given: The village of Salimati in east Africa decides that they need filtration. The population is 1567 people @ 15gpcd.Find: Estimate the size of the filter bedsFilter area = 1567 cap x 15gal/cap.day x 1day/24hours x 1hour/60min / 2gpm/ft2 Filter area = 8.16 ft2- Chemical Application Points. The most common chemicals added are: disinfectants, filter aids, adsorbents (for taste and odor control) and alkali chemicals for pH control. The three major concerns regarding chemical additions are: minimize the application points; certain chemicals have the potential to break through to the treated water; ensure the chemicals are adequately distributed in the water.- Miscellaneous Items. The wash troughs are an essential part of the filter design due to the predominant use of a high backwash rate. Gravity filters generally have an available headloss of 8-10’. The 4 essential factors in selecting the filter underdrain system are : uniform flow distribution of the backwash, durability, reliability and cost effectiveness.3. Type and Selection GuideA. Available Alternatives. - There are three basic types of granular medium filters: slow sand filters, rapid sand filters and high rate filters. In addition there are pressure filters and proprietary filters. - Coagulation and sedimentation pretreatments are common to remove excessive amounts of suspended solids, color and some minerals. - A common design flaw is that the pretreatment does not match the filter requirements. - Bench scale or pilot studies are helpful to evaluate the treatability and seasonal variations involved with the raw water.B. Selection Guide- Type of Filter. If the plant is small,<15MGD, proprietary systems may be considered. Proprietary