ENG421 (9ab) – Filtration Filtration Types of Filters Filter Media Filtration – design considerations Filtration – design examples

General Water Treatment Technologies (Week 4) Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

Filtration (1 of 11) Widely used unit process in water treatment water passes through a porous medium → suspended solid matter in water removed Filters remove wide size range of impurities in water : microstrainers remove millimetre size range reverse osmosis removes fraction of nanometre size range Filter after water has been pre-treated Removes : clay and silt particles colloidal organic substances algae micro-organisms inorganic precipitates of iron, manganese, aluminium, and calcium Filter bed of packed granular media usually sand can also use : anthracite coal garnet diatomaceous earth aka diatomite, or kieselgur fossilised remains of diatoms (ancient algae) powdered activated carbon Granular media filters operate continuously filter stopped for cleaning and regeneration when filtered water quality drops when filter bed pressure drop is an issue

Filtration (2 of 11)

Filtration (3 of 11) common water treatment practice (conventional treatment) coagulation and flocculation ↓ sedimentation filtration

Filtration (4 of 11) common water treatment practice (direct filtration : water with low turbidity) coagulation and flocculation ↓ filtration

Filtration (5 of 11) common water treatment practice (in-line filtration, aka contact filtration) coagulation ↓ filtration (may be two stage process)

Filtration (6 of 11) advanced water treatment practice

Filtration (7 of 11) advanced water treatment practice (cont)

Filtration (8 of 11) advanced water treatment practice (cont)

Filtration (9 of 11) advanced water treatment practice (cont)

Filtration (10 of 11) advanced water treatment practice (cont)

Filtration (11 of 11) advanced water treatment practice (cont)

Types of Filters (1 of 9) classified by : 1. driving force causes flow through filter - gravity filter - pressure filter - vacuum filter 2. flow direction flow through filter bed - up-flow filter - down-flow filter - bi-flow filter - horizontal flow filter 3. solid removal mechanisms how collected suspended solids are removed from water - cake filtration - depth filtration 4. flow rate how fast water flows to filter - slow sand filter - rapid sand filter - high-rate filter 5. filtration rate how fast water is filtered - declining rate filter - constant rate filter

Types of Filters (2 of 9) - gravity filter open to atmosphere flow through differential head pressure due to gravity very common

Types of Filters (3 of 9) - slow sand filter fine sand bed supported by porous gravel layer biofilm (schumutzdecke) forms on top surface of filter bed physical and biological mechanisms remove suspended particles dissolved matter occurs at top layer of filter (cake filtration) within 0.5 – 3 cm of top layer → scrape off thin top layer filter is cleaned filter restored to original capacity filter run time is interval between cleanings 1 month – 6 months resanding : filter media replenished when depth reduced to 0.4 – 0.5 m

Types of Filters (4 of 9) - slow sand filter (cont) fine sand 0.15 – 0.3 mm fine sand bed depth 1 – 1.5 m supporting gravel 5 – 50 mm gravel layer depth ~ 30 cm perforated underpipes collect filtered water placed 2.5 – 3.5 m centre-to-centre surface area 100 – 200 m2 may use more than one filter flow rates : 0.06 – 0.3 m/h 1.5 – 7 m3/m2.day advantages : easy to build easy to operate (unskilled operators) minimum power requirements produces good quality water disadvantages : higher land requirement clogging caused by algae and fine clay particles lengthy start up procedure build up biofilm

Types of Filters (5 of 9) - rapid sand filter sand 0.6 – 1.2 mm (c.f. slow sand filter : 0.15 – 0.3 mm) particles in water are usually less than pores (spaces) in rapid sand filter media → particles pass through → eventually deposit in a pore in the bed requires chemical coagulation → particles attach to filter media on contact accumulated particles eventually saturate the pores → reduces removal efficiency concentration of impurities may approach water quality limit (breakthrough) → increases headloss headloss may approach maximum headloss provided in plant (available headloss) at predecided headloss (terminal headloss) or filtered water concentrations filter is backwashed water is sent from bottom of filter through filter filter medium expands and accumulated deposits flushed out filter cycle is the time between backwashes

Types of Filters (6 of 9) - rapid sand filter (cont) flow rates : 5 – 15 m/h (c.f. slow sand filter : 0.06 – 0.3 m/h) 120 - 260 m3/m2.day (c.f. slow sand filter : 1.5 – 7 m3/m2.day) filter depth usually 3 – 5 m (but can be 1 – 6 m)

Types of Filters (7 of 9) - rapid sand filter (cont) sand may be graded or ungraded maintained in bed in various configurations (see below) coarse-to-fine water contacts coarse sand grains first → larger suspended matter is retained there as water continues through fine sand grains → more fine particles retained this means better utilisation of bed → better water quality, lower headloss, longer filter run time

Types of Filters (8 of 9) - high-rate filter consists of coarse monomedium or a multimedia bed usually down flow, but can be upflow influent contacts coarse media first → entire bed is effectively used media material : sand anthracite coal ilmenite (iron titanium ore associated with iron oxides – hematite and magnetite, sp.gr. 4.2 – 4.6) garnet (silicates of iron, aluminium, and calcium, sp.gr. 3.6 – 4.2)

Types of Filters (9 of 9) - pressure filter filter media placed inside a pressure vessel tanks are available at standard sizes installed with vertical or horizontal axis concept and operation similar to high-rate filters suitable for smaller communities more economical in smaller plants c.f. sand filters in domestic swimming pools

Filtration – design considerations (1 of 15) factors to consider : influent water characteristics filtrate quality requirements type of filter filter media type size and size range of filter media filtration rate underdrain for filter filter regeneration headloss in filter other units in process train water storage facilities provision for future expansion local conditions weather changes discharge regulations (for filtration unit waste) site topography

Filtration – design considerations (2 of 15) Filter influent conventional treatment system filter influent has small particles (large particles removed in sedimentation) → rapid sand filter (good with water with small suspended particles in low concentrations) direct filtration and contact filtration treatment systems no sedimentation stage → all particles to be removed in filters → particles have broader size range → stratified multi-media down flow filter or graded upflow filter

Filtration – design considerations (3 of 15) Filter media and bed availability and cost of filter media material main aspects in selection filter is chosen on : space requirement flow rate high-rate filter stratified/mixed or monomedium/multimedia decision based on : size distribution of suspended particles in influent concentration of suspended particles in influent backwash arrangements

Filtration – design considerations (4 of 15)

Filtration – design considerations (5 of 15) U.C. = uniformity coefficient

Filtration – design considerations (6 of 15) Filter media and bed (cont) - size classification effective size (d10 or dg) size for which 10% of grains are smaller by weight uniformity coefficient (UC) ratio of d60/d10

Filtration – design considerations (7 of 15) Filter media and bed (cont) - filter efficiency/performance effected by : filter bed and media coagulation and flocculation set-up sedimentation (where installed) effect of coagulants on raw water particles variation in influent characteristics

Filtration – design considerations (8 of 15) Filter media and bed (cont) - filter grain size must select appropriate grain size especially high rate filters grains must not intermix after backwash (must have same settling velocity) to determine size and density of grains :

Filtration – design considerations (9 of 15) Filtration rate size of treatment plant (throughput) determines : number of filters filter size filter flow rate flow control methods when filter taken out of service for regeneration other filters handle increased load at that time throughput (Q) < 100 L/s small plant, at least two filters > 100 L/s at least four filters Rule of Thumb :

Filtration – design considerations (10 of 15) Headloss occurs when water flows through a filter for packed granular medium with uniform size, near spherical grains :

Filtration – design considerations (11 of 15) Headloss (cont) but filter media grains are not spherical characterised by equivalent diameter and shape factor from equation, headloss a function of : flow rate grain size porosity viscosity for stratified beds headloss is determined for each layer then summed for entire bed gravity filters design available headloss for filtration 2.5 – 3.5 m

Filtration – design considerations (12 of 15) Filter underdrain systems support the filter media collects the filtered water distributes backwash water and air for scouring prevents loss of filter media through filtered water outlet orifices (5 -15 mm diameter) in underdrain pipes larger than filter grains → layer of graded gravel placed over underdrain pipes (strains the media grains)

Filtration – design considerations (13 of 15) Filter backwashing accomplished by fluidising filter media pump water or air (or both) at a rate to partially expand the bed individual grains are suspended and abrade by contact with other grains 2 – 3 % of water produced in treatment plant used for backwashing backwashing duration depends on : filter influent characteristics backwashing method avoid operating problems during backwashing : loss of media mudball effect sand boil back wash rate 30 – 45 m/h (6 – 9 times filtration rate) filter bed expansion 15 – 30% (can be up to 80%)

Filtration – design considerations (14 of 15) Filter backwashing (cont)

Filtration – design considerations (15 of 15) Filter backwashing (cont) Rule of Thumb to determine height and spacing of backwash troughs

Filtration – design criteria (1 of 3) Filter media grain size – filter depth relationship

Filtration – design criteria (2 of 3) Slow sand filters

Filtration – design criteria (3 of 3) Rapid sand and high-rate filters

References Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D76 1997), pages 416 – 456 Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H46 2006) , pages 529 - 647, 661 - 714, 715 – 762 Kawamura, S., 2000, Integrated Design and Operation of Water Treatment Facilities, 2nd Ed., John Wiley and Sons, New York (TH4538K38 2000), pages 194 – 291 MWH, 2005, Water Treatment Principles and Design, 2nd ed., John Wiley and Sons, New York (TD430 .W375 2005), pages 867 - 954, 955 – 1034 Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E58 2009), pages 149 - 161 Parsons, S.A. and Jefferson, B., 2006, Introduction to Potable Water Treatment Systems, Blackwell, Oxford (TD430 .P37 2006), pages 72 – 94 Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages 343 - 371