Filtration, Page No. 3

Filtration

Lecture No. 7

1. 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 of the 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.

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 O2 available)

Fe+++(Fe++(Fe+++

Mn+4anaerobic conditions such as in a ground water aquiferMn++O2

when water reaches surface air Mn+4colloidal precipitate

Example:

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>7.5

3Mn++ + 2MnO4 ( 5MnO2, pH>9.5

Find: the potassium permanganate treatment requirements

KMnO4 (for iron) = 3.2mg/ Fe++ x 1KMnO4 /1.06mg/l of Fe++KMnO4 (for iron) = 3.02mg/l

KMnO4 (for Mn) = 0.8mg/ Mn++ x 1KMnO4 /.52mg/l of Mn++KMnO4 (for Mn) = 1.54mg/l

Total KMnO4 requirement = Fe+Mn = 3.02mg/l + 1.54mg/l

Total 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, effluent channel 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 50mg/l, ferric iron salts should be evaluated as the primary coagulant.

Example:

Given: The above plant at alum of 3mg/l

Find: How much alum do they use per day.

eq \F(lb,day) = X mg/l x 8.34 eq \F(lb/MG,mg/l) x Q(MGD)

eq \F(lb,day) = 3 mg/l x 8.34 eq \F(lb/MG,mg/l) x 600(MGD)

eq \F(lb,day) = 15,012 lb/day = 7.51 tons/day

Note: QADF was used, report the answer in units that mean something

Evaluation and Adjustment of the Filter Washing Procedure. Three methods to evaluate the effectiveness of the washing procedure: visually inspect the bed before and after washing; measuring the turbidity of the backwash water at 1 minute intervals: core sample the bed before and after the wash. A filter bed is clean but also ripened i.e. the media grains of a ripe filter are coated with the proper amount of floc or polymer. The operators look for mud balls, cracks and worms and debris on the surface. Overwashing may be detrimental; the wash may be terminated when the turbidity of the wash waste ranges from 10-15NTU which usually occurs after 5-6 minutes of non air-scour washing.

Mud Deposition Profile. A mud profile is bed depth vs. floc deposition p.208 measured as turbidity. A filter bed is properly conditioned if it is in a ripened stage: each grain is coated with a thin film of coagulant hydroxide or polymer. A bed that is too clean usually exhibits a distinct turbidity breakthrough at the beginning of each filter cycle lasting anywhere for 30-60 minutes. A turbidity of > 300NTU is indicative of a mud ball problem. When mud balls are found in the filter bed, they are measured by % volume. If the percentage is < .1%, the filter bed is clean; 1-5% indicates a bad condition. If the volume is > 5%, the bed must be replaced with new media.

Air-Binding. Air-binding is when large amounts of air bubbles accumulate in the filter bed. Air-binding may be alleviated by initiating a filter wash whenever the filter headloss reaches 4-5ft a practice which prevents the creation of negative pressure and the concomitant negative pressure in any part of the filter bed.

6. Design Criteria

See information starting on p.225 including T3.2.7-1, p.234

7. Example

Given: A 75MGD water treatment plant using a conventional process.

Find:

1.) number of filters

2.) filter bed, filtration rate, filter washing scheme

3.) size of filters

4.) filter arrangement

5.) filter media

6.)backwash rate, underdrain system, headloss through each orifice(7.5

3Mn++ + 2MnO4 ( 5MnO2, pH>9.5

Find: the potassium permanganate treatment requirements

7B. Given: Des Moines, Iowa feel that they need filtration. The population is 230,000 @ 150gpcd. Assume they will use a high rate system.

Find: Estimate the area of the filter beds.

7C. Given: The above problem.

Find: Estimate the number of filters

7D. Given: A bed run at 5 gpm/ft2 has a normal filter run of 30 hours.

Find: What would be the filter run if the rate were 20 gpm/ft2

7E.Given: The size of the sand is .50mm and its density is 2.60. The density of the anthracite is 1.60. The fluid is water at density 1. UC=1.7

Find: What size coal particle is required to ensure the same settling velocity, use F3.2.4-7, p 213, NOT the equation.

7F. Given: A .5m wide wash trough that is carrying .25m3/s.

Find: How high is it?

7G. Given: The same problem as the lecture notes. Rapid sand filter. D=20. (=.42, T=55(F. filtration rate = 2gpm/ft2. Spherical particles.

Rose equation:

hl = eq \F(1.067,k)

eq \F(D,g)

eq \F(vA2, (4)

eq \F((CDx,() , for stratified beds, where

CD = 24/NrSieve Analysis% of sand retained

given as a %,

apply as a decimalGeometric mean size, ((ftx10-3)

14-201.053.28

20-286.652.29

28-3215.701.77

32-3518.841.51

35-4218.981.25

42-4817.721.05

48-6014.24.88

60-655.15.75

65-1001.66.59

(=100%

Find: headloss using the Rose equation.

7H. Given: A 130 MGD water treatment plant using a conventional process.

Find:

1.) number of filters

2.) filter bed, filtration rate, filter washing scheme

3.) size of filters

4.) filter arrangement

5.) filter media

6.)backwash rate, underdrain system, headloss through each orifice(