FILTRATION. Water Treatment Course

Transcription

1 FILTRATION Course, Zerihun Alemayehu FILTRATION Filtration involves the removal of suspended and colloidal particles from the water by passing it through a layer or bed of a porous granular material, such as sand. 1

2 CLASSIFICATION OF FILTERS Based on the filter media Sand filters, e.g. natural silica sand Anthracite filters, e.g. crushed anthracitic coal Diatomaceous earth filters, e.g. diatomaceous earth Metal fabric filters (microstrainers), e.g. stainless steel fabric filter. CLASSIFICATION OF FILTERS Based on the depth of filter media Deep granular filters, e.g. sand, dual media and multi media (combination of two or more media), granular activated carbon Precoat filters, e.g. diatomaceous earth, and powdered activated carbon, filters 2

3 CLASSIFICATION OF FILTERS Based on the rate of filtration, sand filters can be further classified as Gravity filters Slow sand filters rapid sand filters high rate sand filters Pressure filters RATE OF FILTRATION Rate of filtration (loading rate) is the flow rate of water applied per unit area of the filter. It is the velocity of the water approaching the face of the filter: where v a = face velocity, m/d = loading rate, m 3 /d.m 2 Q = flow rate onto filter surface, m 3 /d A s = surface are of filter, m 2 v a Q A s 3

4 EXAMPLE A city is to install rapid sand filters downstream of the clarifiers. The design loading rate is selected to be 160 m 3 /(m 2 d). The design capacity of the water works is 0.35 m 3 /s. The maximum surface per filter is limited to 50 m 2. Design the number and size of filters and calculate the normal filtration rate. EXAMPLE SOLUTION 4

5 MECHANISM OF FILTRATION The theory of filtration basically involves, transport mechanisms, and attachment mechanisms. The transport mechanism brings small particles from the bulk solution to the surface of the media. a) gravitational settling, b) diffusion, c) interception and d) hydrodynamics. MECHANISM OF FILTRATION They are affected by physical characteristics such as size of the filter medium, filtration rate, fluid temperature, size and density of suspended solids. As the particles reach the surface of the filter media, an attachment mechanism is required to retain it. This occurs due to (i) electrostatic interactions (ii) chemical bridging or specific adsorption. 5

6 SLOW SAND FILTERS In SSF water is allowed at a slow rate through a bed of sand, so that coarse suspended solids are retained on or near the surface of the bed. Loading rate of 2.9 to 7.6 m 3 /d.m 2 The raw water turbidity has to be < 50 NTU. The filtering action is a combination of straining, adsorption, and biological flocculation. 6

7 SLOW SAND FILTERS Gelatinous slimes of bacterial growth called schmutzdecke form on the surface and in the upper sand layer, consists of bacteria, fungi, protozoa, rotifera and a range of aquatic insect larvae. The underlying sand provides the support medium for this biological treatment layer. Slow sand filters slowly lose their performance as the Schmutzdecke grows and thereby reduces the rate of flow through the filter. requires refurbishing CLEANING SLOW SAND FILTERS Scrapping: the top few mm of sand is carefully scraped off using mechanical plant and this exposes a new layer of clean sand. Water is then decanted back into the filter and re circulated for a few hours to allow a new Schmutzedecke to develop. The filter is then filled to full depth and brought back into service. wet harrowing: lower the water level to just above the Schmutzdecke, stirring the sand and thereby suspending any solids held in that layer and then running the water to waste. The filter is then filled to full depth and brought back into service. 7

8 TYPICAL SLOW SAND FILTER Raw water Schmutzecke Supernatant water Sand filter bed Weir Grave l System of underdrains Finished water TYPICAL SLOW SAND FILTER 8

9 TYPICAL SSF CONSTRUCTION DETAILS ADVANTAGES AND DISADVANTAGES Advantages Simple to construct and supervise Suitable where sand is readily available Effective in bacterial removal Preferable for uniform quality of treated water Disadvantages Large area is required Unsuitable for treating highly turbid waters Less flexibility in operation due to seasonal variations in raw water quality 9

10 DESIGN CRITERIA FOR SSF Parameter Recommended level (UK experience) Design life year Period of operation 24 h/day Filtration rate m/h Filter bed area m 2 /filter (minimum of two filters) Height of filter bed Initial m Minimum m Effective size mm Uniformity coefficient < 3 Height of underdrains + gravel layer m Height of supernatant water 1 m EXAMPLE. SSF DESIGN Design a slow sand filter to treat a flow of 800 m 3 /day. Solution: assuming a filtration rate of 0.15 m/h, Required tank area = (800/24) x (1/0.15) = 222 m 2 Use a tank 23 m long x 10 m wide. From Table 6.1, the height of the tank require is: System underdrain + gravel 0.5 m Filter bed 0.9 m Supernatant water 1 m Therefore, total tank height = 2.4 m and tank dimension becomes 23 m long x 10 m wide x 2.4 m high 10

11 RAPID SAND FILTERS The most common type of filter for treating municipal water supplies. During filtration, the water flows downward through the bed under the force of gravity. When the filter is washed, clean water is forced upward, expanding the filter bed slightly and carrying away the accumulated impurities. This process is called backwashing. ADVANTAGES AND DISADVANTAGES Advantages Turbid water may be treated Land required is less compared to slow sand filter Operation is continuous. Disadvantages Requires skilled personnel for operation and maintenance Less effective in bacteria removal Operational troubles 11

12 TYPICAL GRADATION OF RSF after backwashing, the larger sand grains settle to the bottom first, leaving the smaller sand grains at the filter surface. Allows in-depth filtration: provides more storage space for the solids, offer less resistance to flow, and allows longer filter runs. TYPES OF RSF RSF based on filter material, three types: Single media filters: these have one type of media, usually sand or crushed anthracite coal Dual media filters: these have two types of media, usually crushed anthracite coal and sand. Multi media filters: these have three types of media, usually crushed anthracite coal, sand, and garnet. 12

13 RAPID SAND FILTER OPERATION OF A RSF Terminal head loss. Constant rate filtration 13

14 GRAIN SIZE CHARACTERISTICS Sieve analysis a plot on semi log paper of the cumulative frequency distribution, Geometric mean (X g ) and Geometric standard deviation (S g ) Effective size, E, or 10 percentile, P 10, E = P 10 = (X g /S g ) Uniformity coefficient, U, or ratio of the 60 percentile to the 10 percentile, P 60 /P 10. U = P 60 /P 10 = (S g ) RSF FILTER MEDIA TYPICAL PROPERTIES PROPERTY UNIT GARNET LMENITE SAND ANTHRACITE GAC Effective Size, ES mm Uniformity Coefficient, UC UC Density, ρ ρ g/ml Porosity, ε % Not available Not available Hardness Moh Low 14

15 FILTER HYDRAULICS The loss of pressure (head loss) through a clean stratified sand filter with uniform porosity was described by Rose: where h L = frictional head loss through the filter, m v a = approach velocity, m/s D = depth of filter sand, m C D = drag force coefficient f = mass fraction of sand particles of diameter d d = diameter of sand grains, m ϕ = shape factor and = porosity FILTER HYDRAULICS 15

16 FILTER HYDRAULICS The hydraulic head loss that occurs during backwashing is calculated to determine the placement of the backwash troughs above the filter bed. where De = depth of the expanded bed, m = porosity of the bed and s = porosity of the expanded bed f = mass fraction of sand with expanded porosity Laminar Turbulent SETTLING VELOCITY 16

17 REYNOLDS NUMBER EXAMPLE 3 A dual medium filter is composed of 0.3 m anthracite (mean size of 2.0 mm) that is placed over a 0.6 m layer of sand (mean size of 0.7 mm) with filtration rate of 9.78 m/h. Assume the grain sphericity is = 0.75 and a porosity for both is Estimate the head loss of the filter at 15 o C. 17

18 SOLUTION Calculate head loss for anthracite Calculate head loss for sand EXAMPLE 4 Estimate the clean filter headloss for a proposed new sand filter using the sand. Use the following assumptions: loading rate is 216 m 3 /d.m 2, specific gravity of sand is 2.65, the shape factor is 0.82, the bed porosity is 0.45, the water temperature is 10 o C, and the depth of sand is 0.5 m. Sieve No % retain d(mm)

19 SOLUTION SOLUTION 19

20 SOLUTION EXAMPLE 5 Determine the depth of the expanded sand filter bed being designed for Example 4. 20

21 SOLUTION Any Questions? 21