Filtration

Question 1

Define Filtration

Answer 1

Filtration is a process in which water is passed through a filter where the particulate materials are trapped through the effect of different mechanisms (phsical,mechanical,electromechanical) resulting in a highly clarified effluent.

Question 2

Exam question

Explain the mechanism involved in the removal of particles in the filtration process.

Answer 2

1-Strainig: particle size is larger than the openings in sand.
a)Straining by chance: particle get stick at the point of contact.
2-Sedimentation(Inertial impaction): water hits the sand particle and the particle following the water trajectory will go out of the trajectory and settle on sand grain
3-Interception: distance between flowline and sand grain is larger than the radius of the particle .
4-Adhesion: particles that already sticked on the sand will attract more particles to stick on them
5-Flocculation: particle moving at higher velocity take with it a particle moving at lower velocity implies it can get bigger than the opening thus removal occurs.

Question 3

How are granular filters classified

Answer 3

Granular filters are classified according to :
1-)Type of medium (Sand,anthracite,…)
2-)Rate of flow (slow,rapid,high)
3-)Direction of flow (upflow-downflow,biflow,fine to coarse,coarse to fine)
4-)Pressure of flow (Gravity flow-pressure flow)

Question 4

Exam question

What are the factors that affect the efficiency of filtration; Explain briefly

Answer 4

1-Suspended particles size: larger particle size are easy to get stuck in the filter unlike the small particles.
2-Filtration velocity: If the rate of filtration is slow it will be more efficient unlike rapid filtration.
3-Grain type and size: if we use anthracite for example, we will have the bed of the filter graded from coarse to fine which increase the efficiency of filtration.
4-Filter liquid temperature: At higher temperatures the viscousity of water will decrrease.
5-Chemical properties of the influent water: if it contains any organics or metals.

Question 5

Tabulate the different characteristics of a slow sand filter and a rapid sand filter

Answer 5

Slow sand filters:
1-Low filtration rates (10 m/d) to 36m/d.
2-The raw turbidity of water can go up to 100mg/l.
3-The medium used is mostly sand with effective size of 0.1 to 0.3 mm and a uniformity coefficient of 2 to 3.
4- Cleaning is done by scraping the top 12 to 50 mm of sand.The removed sand is cleaned and stored to be used back at latter stage.
5-Duration of slow sand filter run is 40 to 60 days.
6-Standard units up to 6000m2 may be used , the size depend on the need to provide the required flow when on is out.
7-Requires development of gelatinous biological surface composed of bacterai and funji for optimal use, the growth of such coat takes between 6 hrs and 30days.
Rapid sand filters
1-High filtration rates beetween 100 and 475 m/d, normally between 120-240 m/d.
2-Maximum Raw turbidity of water is 10mg/l.
3-Medium used usually contains both sand and anthracite where:
effective size of sand 0.45-0.55mm / uniformity coefficient between 1.2 and 1.7
4-cleaning done by backwashing using filtered water.
5-Duration of rapid sand filter run is between 12 to 24 hrs but must not exceed 72 hrs.
6-penetration ofsuspended matter is deep thus we will have more mechanisms of filtration.
7-Depth of bed is 0.5 m.
8-prior treatment is required.

Question 6

Exam question
How would u get about selecting the size of anthracite particle in a dual media filter if the size of sand particle is known.

Answer 6

The density of anthracite is less than that of sand (1.5<2..65),For equal settling velocity for both grains we will get a bigger size(diameter) for anthracite, the required anthracite particle size is calculated by the following formula:
d1(anthracite)/d2(sand)= ((raw2-rawwater)/(raw1-rawwater))power2/3
If we want a coarse to fine bed gradation we choose a diameter