CEE 371 Water and Wastewater Systems

Transcription

1 Updated: 21 November 2009 CEE 371 Water and Wastewater Systems Print version Lecture #18 Drinking Water Treatment: Granular Media Filtration Reading: Chapter 7, pp David Reckhow CEE 371 L#18 1 Pg 25, from Fair & Geyer, 1954 David Reckhow CEE 371 L#14 2 Lecture #18 Dave Reckhow 1

2 Filtration A polishing solid/liquid separation step Intended to remove particles some particles are pathogens Other impacts biodegradation of organic compounds organics adsorption (especially to GAC) Mn and Fe adsorption Placement after sedimentation or flotation (clarification) after RM or flocculation (direct, or in-line filtration) David Reckhow CEE 371 L#18 3 TYPES OF FILTRATION Granular media filters (depth filtration, most common): Particles smaller than pores, removal primarily il by transport t to collector (media) surface and attachment Filters: slow sand, rapid sand, high rate, roughing Membrane filters Particles larger than pores, removal mostly by mechanical sieving Continuous or periodic removal of deposited particles microfiltration, ultrafiltration bag and cartridge filters microscreen filters (pre-treatment usually) Cake Filtration initial deposit on a membrane, deposited particles become the filter media, i.e., a "cake" or layer of particles diatomaceous earth (pre-coat) filters in DW treatment vacuum filters, belt presses for sludge dewatering David Reckhow CEE 371 L#18 4 Lecture #18 Dave Reckhow 2

3 Size Spectrum of Some Filter Media Types Particle Diameter m 1 Å 1 nm 10 nm 100 nm 1 μm 10 μm 100 μm 1 mm Filter Papers Micro Sieves Sieves Membranes Activated Carbon Pores Diatomaceaous Earth Sand, Anthracite GAC grains 1 Å 1 nm 10 nm 100 nm 1 μm 10 μm 100 μm 1 mm (After Stumm, ES&T, Vol. 11, p. 1066, 1977) David Reckhow CEE 371 L#18 5 Types of Filtration Granular media filters slow sand filters rapid sand filters high-rate granular media filters Membrane filters microfiltration, ultrafiltration, nanofiltration Cake filtration diatomaceous earth David Reckhow CEE 371 L#15 6 Lecture #18 Dave Reckhow 3

4 Typical rapid sand filter Concrete box about 9ft deep With 2 ft of sand, or more if the media is larger in size Typical rates 2-5 gpm/ft 2 H&H figure 7-11, pg 220 David Reckhow CEE 371 L#18 7 General Gravity Flow, Granular Media Filter Schematic Filter Influent Waste Filter Backwash (FBW) Water Above Filter Filter Mdi Media Underdrain System A = filter surface area V 0 = Q/A = hydraulic loading rate Units of gpm/ft 2 or m/hr Filter Effluent Filtration ti Rate Control - Constant Rate - Declining Rate David Reckhow CEE 371 L#18 8 Backwash Water Supply (Backwash Air Supply) Filter to Waste Lecture #18 Dave Reckhow 4

5 General characteristics of granular media filters FILTR. RATE MEDIA SIZE BED DEPTH TYPE (m/hr) (mm) (m) Slow Sand < Rapid-Rate High-Rate Precoat (DE) Note: 4.9 m/hr = 2 gallons per minute per square foot (gpm/ft 2 ) Filtration rate = (Flow) / (Cross-sectional Area) = Q/A = V 0. This sometimes called the superficial velocity Pore velocity = V 0 /ε, which is the superficial velocity divided by the porosity Selection of filter type depends on factors such as: total flow rate raw water quality desired effluent quality experience David Reckhow CEE 371 L#18 9 GRANULAR FILTER MEDIA Natural Silica (quartz) Sand Crushed Anthracite Coal often used with sand in a dual media, downflow filter Garnet Sand (higher density than silica) often used in mixed (or tri) media, downflow filter Granular Activated Carbon (GAC) may use in place of anthracite as dual media with sand used alone as coarse, deep bed mono-media also Typical Dual Media Typical effective sizes Porosity anthracite ( mm diam) ε=0.40 sand ( mm diam) ε=0.36 Tri Media anthracite (largest size, lowest density) silica sand (intermediate size and density) garnet sand (smallest size, highest density) David Reckhow CEE 371 L#18 10 Lecture #18 Dave Reckhow 5

6 GRANULAR FILTER MEDIA (cont.) Various other coarse media: gravel, stones plastic (often upflow as a roughing filter) Media Sizing: Effective Size = d 10 = 10% by weight smaller than this size Determine from a sieve analysis: % Finer (weight) Uniformity Coefficient (UC) = [d 60 /d 10 ] 10 More uniform as UC gets lower Criteria: UC should be < 1.7 for rapid rate filters Sieve sizes (i.e., 8 x 20 GAC) Media passing #8 sieve but retained by #20 sieve d 10 d 60 Diameter (mm) David Reckhow CEE 371 L#19 11 H&H figure 7-14, pg 223 Underdrain I Many types High density polyethylene block Central feeder lateral and outside compensating laterals David Reckhow CEE 371 L#18 12 Lecture #18 Dave Reckhow 6

7 Rapid Media Filter showing underdrain Source: AWWA and ASCE, David Reckhow CEE 371 L#18 13 Underdrain II Nozzle type Air & water backwash H&H figure 7-15, pg 224 David Reckhow CEE 371 L#18 14 Lecture #18 Dave Reckhow 7

8 Pressure Filters Common for small systems treating groundwater Focus is on Fe/Mn removal e.g., Wellesley, MA David Reckhow CEE 371 L#18 15 Filter Operation David Reckhow CEE 371 L#15 16 Lecture #18 Dave Reckhow 8

9 Flow control H&H figure 7-12, pg 220 Normal operation Backwash Initial filter to waste: valves 1 & 3 are open, all others closed Production: Valves 1 & 4 are open, 2, 3, &5 are closed Filter flow typically regulated by valve #4 Valves 1 & 4 are closed, 2 is opened Filter drains (air added?) Valve 5 is opened and Backwash pump started Discharge from underdrain David Reckhow CEE 371 L#18 17 Filtration: Mechanisms Interception lines of flow strike media sedimentation diffusion straining too large to fit between spaces flocculation promoted by increased turbulence David Reckhow CEE 371 L#15 18 Lecture #18 Dave Reckhow 9

10 Deposition in a Filter David Reckhow CEE 371 L# micron 10 microns Relative size of particles and filter media Filter Media Grain 0.5 mm diam Given that most particles removed in water are in the range of microns, straining is not really an important mechanism 100 microns David Reckhow CEE 371 L#18 20 Lecture #18 Dave Reckhow 10

11 Filtration transport mechanisms In addition, particles must be able to stick. This requires chemical destabilization (i.e. coagulation). Sedimentation Interception Media Diffusion or Brownian Motion David Reckhow CEE 371 L#15 21 To next lecture David Reckhow CEE 371 L#18 22 Lecture #18 Dave Reckhow 11