Water Treatment in
Clean the water filter
Three factors can be considered to assess when a water treatment filter needs back washing.
- Water Treatment Filter run reaches a given hour of operation.
- The head loss is so high that the water filter no longer produces water at the desired filtration rate and
- Floc starts to break through the filter and the turbidity in the water filter effluent increases.
Some plants use the length of the filter run, arbitrarily scheduling backwashing after 24 to 48 hours of water treaments filter operation. Other plants monitor turbidity of the effluent water and head loss within the filter to determine when the filter is clogged enough to need cleaning. Head loss is a loss of pressure by water flowing through the water treatments filter. When water flows through a clogged filter, friction causes the water to lose energy, so that the water leaving the filter is under less pressure than the water entering the filter. Head loss is displayed on a head loss gauge. Once the head loss within the water filter has reached between 250 and 350 em, a filter should be backwashed.
Procedure of water treatments filter backwash
The following steps have found to give the best results:
- Close the influent valve but continue the water treatments filter in operation until the water level drops to a point about 15 cm above the sand. This conserves the settled water and also permits inspection of the filter sand.
- Close the effluent valve.
- Inspect the water filter for mud balls, cracks, mounds, or evidence that the sand is drawing away from the sidewalls.
- Open the drain valve. This may be done any time after the water on filter is below the wash water gutter.
- Open the wash-water valve gradually during a period of 45 to 60 see to the point where expansion is about 15%, or flow is about 400 Lpm/m2. This prevents wash water from entering so fast that the compacted surface sand is lifted as a body until a portion cracks and causes high horizontal flow towards crack, this flow carrying the cleaner bottom sand and pea gravel with it.
- The backwash water must have enough velocity and volume to agitate the sand and carry away the foreign matter which has collected there.
- Backwashing should begin slowly. If begun too quickly, backwash water can damage the underdrain system, gravel bed, and media due to the speed of the water. Beginning backwashing too quickly will also force air bound in the filter out, further damaging the filter.
- Wash at the low rate for about 3 min, to provide ample time for the sand grains to impinge on one another and dislodge the floc.
- Open the wash-water valve that gives the maximum sand expansion desired, such as 40%, and continue for about 1 minute or until the wash water appears to be relatively clear and the agitated sand can be observed.
- The dirty backwash water is collected by the wash troughs and can be recycled after sedimentation to.the beginning of the plant or can be allowed to settle in a tank, pond, or basin.
- Close the wash-water valve after the cleaning.
- Open the influent valve slowly, so as to avoid undue turbulence and disturbance of the sand.
- Open the influent valve wide when the water level reaches normal Open the filtered water outlet valve.
Design criteria of under-drainage system consisting of central manifold and laterals:
- Size of perforations (staggered at slight angle from the vertical axis); 5 to 12mm
- Spacing of perforations: Vary from 80mm for 5mm perforations to 200mm for 12mm perforations.
- Ratio of total area of perforations to total area of cross section of laterals: 0.25 to 0.5 (for 5 mm to 12mm perforations respectively).
- Percentage of total area of perforations to entire water treatment filteration area:0.3%
- Ratio of length to diameter of the lateral: < 60
- The spacing of laterals approximately equal to the spacing of orifices: 300mm.
- Nozzle density depends upon the type of nozzles and is about 40 nozzles/ m2.
- Ratio of cross sectional area of manifold to the total cross sectiona) area of laterals: 1.5 to 2:
Backwashing preceded by water- wash:
- Maximum clear distance between wash-water troughs; 180cm
- Horizontal travel of wash – water to trough : not more than 90 cm
- Backwash rate: 400 to 600 Lpm/sq.m of filter surface area
- Desired pressure of wash-water: 5.0 m at under drains
- Duration of backwashing: 10 minutes
- Sand bed expansion: 130 to 150% of undisturbed volume
Backwashing preceded by air- wash:
- Duration of air-wash: 5 minutes
- Rate of air-wash: 600 to 900 Lpm (0.6 to 0.9 m3)/ sq.m of filter surface area
- Duration of water-wash following air-wash: 5 minutes
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