Sand Filter :

Filtration process, especially grain filtration is of the oldest and most common methods of water treatment. Sand filter that is a kind of grainy filtration has the most use in different situations. Suspended solids in the water while passing through the grain bed are trapped and separated from the water flow. Sand filter generally consists of a pressure tank within which is filled with grains of sand.

In general, as the thickness of the filter bed is more, the efficiency of removal of suspended solids in it will increase but in practice, the thickness of these filters is in the range of 70-150 cm.  Depending on the diameter of the seedbed particles and the bed pressure, per square meter of substrate, 5-20 cubic meters of water per hour can be refined.

Sand filter structure

Sand filter under pressure are made in all shapes, sizes and of different materials, the most common of vertical sand filters with lens at the ends. The body of sand filter is formed of steel body or layers of silica rock with definite graining do shell, which is as a cylinder closed at both ends and water treatment process. This is made with specified diameter out of steel ST-37 (or galvanized) with the announced thickness in ASME for wall and body as well as sheet with thickness announced at ASME for the upper and lower lenses made by DOUBLE-DISH method. The inner surface of the column covered by a thick layer of epoxy paint and the outer surface of the column-lined. The inner surface of the column is covered by a thick layer of epoxy paint and outer surface of column covered by a layer of stainless and two layers of oil color.

Silica inside the filter

Inside sand filters, there should be four different layers of silica sand with uniformity coefficient of 35.1, fine-grained of special rocks and purity of over 95%.

  • Grade one silica from 5.0 mm to 5.1 mm in diameter

  • Grade two silica of 2 mm to 5.3 mm in diameter

  • Grade three silica of 4 mm to 8 mm in diameter

  • Grade four silica of 8 mm to 12 mm in diameter

All silica spilled in sand filter in total will reach a height of one meter and this number does not depend on the capacity of the filter because after experiments done with sand filters, this has been proven. From a special height on, the material used in the filters are not capable of treating more water and a maximum height of 1.5 meters is considered, and by considering the filter bed filter and water distributor, the height of sand filters reaches a height of 1.5 meters. Sand with finer granularity provides more surface area and thus takes more water pollution. However, it needs more pumping energy for transmission of fluid. In any case, in the majority of pressure sand filters, grains of range 0.2 to 1.22 mm are used. Particles larger than 100 micrometers tend to close filtration pores. The larger grains of sand can overcome this problem. That is why the sand bed depths of about 0.6 to 1.5 meters are different depending on the application. Designing sand and grain filters shows that, they should work at a maximum flow rate of 10gpm/ft2. Using the power required and maximum discharge, the needed area can be calculated. The final point of designing is to make sure the liquid is properly flowing in the bed and no amount of fluid flow is deviated from the flow path.

When pressure of the two heads of sand filters is fixed at 0.8 – 0.6, it is the time to remove filter from service and to wash the beds with the entry of water in Back Wash form. Washing time depends on the amount of turbidity of input water. When filter piles up large amounts of soil, pressure drops and water cannot come out freely through the filter. Back wash sends water back so that by filtering trapped dust is led out. After back wash, pressure back increase to the pool is obvious and if the pressure gauge is installed on the filter, it can be seen that pressure is improved more than 0.5 bar.

Back wash in the sand filters under pressure:

Backwash is done simply in the sand filter through the back flow of water o the pool. Thus, by passing the reverse flow from the bottom to the top of the sand filter, debris stuck to the sand filter is passed to the sewage.

Pump should be turned off for backwashing and valve of the filter should be set to state of “back wash” and then turn on the pump again. Backwashing in the pools usually lasts 2 to 3 minutes. After the back wash, pump bypass is turned off and valve should be set to position “close” then turn on the pump for 30 to 60 seconds to make sure that all the dirty water in the backwashing has been sent to sewage and will not go to the pool. Pump is stopped for the last time, and valve is set to “Filter” now, we turn on the pump and filter starts to purify pool water again.

Pressure gauge at the inlet and outlet makes the pool operator understand that water flow path in filter is closing. If gauge shows much higher pressure in the inlet compared to the outlet, the operator realizes that a lot of junk has accumulated among sand. This means that now is the time for “back wash” of the filter. For “back wash,” the operator adjusts valves to redirect the water, closes return pipe to the pool, and opens the outlet pipe so that water enters the sewer system.

The profile of sand filter made by Arvin Bokhar:

  • Design and fabrication based on ASME standard

  • Body of the device is made of metal frame of sheet steel ST44 or mild steel RST37

  • The capacity from 2m2 / hr to 300m2 / hr

  • With certification and Iran standard no

  • Tank inner surface is covered with two layers of epoxy paint

  • Hydraulic, pneumatic, or electric valves

  • Manual or automatic operation