Sand filter

filtration

A sand filter is a device that physically removes water particles such as flowers, paints, bacteria and particles from corrosion by water filtration

Sand filters

Sand filters, which pass water through the sand are responsible for the separation of suspended particles (usually up to 50 micron) from the circulating water as a precursor to the transparency and clarity of water, but they don’t remove solutes and other organic matter in the water

Structure of sand filter

Sand filters are designed as cylindrical or pressure vessels. Inside the sand filter tank is filled with silica sand or anthracite, which is usually a layer of coarse particles on the nozzles to prevent clogging and then a layer of the particles is poured on them.

As water passes through the spaces between the sand particles

As water passes through the spaces between the sand particles, the suspended material is trapped between the sand and the water is almost free of suspended material at the sand filter outlet. However, the accumulation of suspended particulate matter in the filter, filtration increases the pressure drop of the filter bypassing the filter

The most common sand filter in homes

The most common sand filter in homes is the refrigerator water filter, which has a sand filter can absorb large particles only and provide clear water for consumption.

Sand filter usages

Sand filter for shallow water (rivers or wells) pool water removal

paint removal

cooling tower re circulation

industrial and human wastewater filtration

beverage and industrial use filtration (city and industrial wastewater treatment) placed

Notable points

Problems with not using sand filters in pools for example, in case of the surface of the pool is open, water stagnates and becomes contaminated if pool water is not replaced or filtered intermittently. External substances such as leaves, soil, walnut dust and the growth and proliferation of germs, all kinds of microbial fungi and parasitic eggs in the water cause diseases such as jaundice, intestinal fever, bloody diarrhea, skin diseases and more. This should be taken into consideration especially in public pools

Carbon filters

Carbon filters are used to remove free chlorine, color, taste, odor and waterborne organic matter

Pressure sand filters

Pressure sand filters are the most common type of filters for pools that provide the ideal filtration at the right speed. During filtration, dirty water from the pool enters the filter through the inlet. Pump pressure and gravity force the water from the bottom of the gravel downward and find sand collects any debits and waste from the pool water

point

The use of sand filters in pools, in addition to separating the above suspended particles, greatly absorbs the floating fats in the water

At the bottom of the sand filters

There are nozzles with very small holes that will not allow sand to pass through out the water passes through these nozzles and drains out of the outlet pipe under the main sand filter tank

Sand filters are two type

Gravity filters

Pressure filters

It can be manually and fully automated with two water path selections from up flow to down flow

:Pars dama sand filter

Pressure sand filter

Pressure sand filters are the most common type of filter used for pools, providing an ideal filtration rate with the right filtration speed. During filtration, dirty water from the pool enters the filter through the inlet. The pressure of the pump and the force of gravity drives the water down through the sand and the fine sand collects any trash and waste from the pool water. At the bottom of the sand filters are very fine-holes nozzles that will not allow sand to pass through, but water will pass through the nozzles. Beneath the reservoir of filtered water comes out of the outlet pipe

The shapes of sand filters

Pressure sand filters are manufactured in different shapes, sizes, and types, the most common of which are general-purpose dual-lens sand filters with a specified diameter of ST37 (or galvanized) steel sheet thicknesses specified by ASME for wall and body as well as thick sheeting in ASME for double and lens upper and lower lenses made by double dish, which after rusting on the inside with an epoxy coating layer and on the outside with a stainless steel and two special color coating layers. Also to improve the filtration quality and service life of pressure filters, pre-rust filters can be blasted from inside

notable point

These types of filters are filtered with galvanized diffusers in the upper part that distribute water to the substrate for better filtration

Also at the bottom of these filters and underneath the last layer of silica inside those brass strainers (brass nozzles) are to collect and pass water. These are also filters in the market that use plastic nozzles, which are of lower quality than the brass type and cost up to 100 times lower than the brass nozzles. One of the pillars of filter selection is to have brass nozzles

Inside the sand filters there should be four layers of silica coated with a uniformity coefficient of 1/35 with fine-grained rock type with a purity of more than 95% 

(Optimal flow rate in pressure sand filters (speed

To maximize the effect of the filter it must be assumed that the water is at its optimum discharge.  The most important findings from the test show that the high flow rate effects the filtration and the time needed to collect the contaminants

High velocity, shorten filtration time and make filtration more difficult. High velocity means excessive velocity in a tube that is measured in ft/.sec of gom/ft ². For example, fps 10 is a high speed. This is approximately 70 gpm in the 1inch pvc tube and 100 gpm in the 2inch pvc tube

Some manufacturers claim higher speed = better filtration but this is contrary to what happens in practice. If you pass the water through a fast flowing filter, the filter will virtually no chance of taking germs and bacteria. The problem with velocity speed is that you only drive the current through the filter without any special operation on the water

Suggestion speeds are between 5 to 9 gpm/ft ²

Pars dama sand filters

Automatic sand filter and semi-automatic steel

In two types of mn4 17 steel and galvanized sheet

Automatic sand filter and FRP sand filter 

Pars dama steel sand filter

Pars dama sand filter made of mn4 17 sheet with two layers of internal epoxy paint and one layer of stainless steel and outer layer of industrial paint for upper and lower sockets double-dish method according to ASME standard torispherical made. The machine also has diffusers in the upper part for water distribution and brass strainers in the lower part for water collection

Pay attention

The device is equipped with two viewing valves, one at the top and the other at the bottom. It should be noted that the silica is granulated and is of the Hamedan type of rocks with a purity of 98%

Silica

All silica spilled on sand filters will reach a total height of about one meter, which is not related to the filtration capacity since after testing on sand filters it has been proven that materials from one height to the next are used. The filters are not capable of further purification of water and have a maximum height of 1.5 meters and are considered to be 1.5 meters in height with the filter and water dispenser

Notable point

The filter grains provide more surface area and consequently, more water pollution, but require more pumping energy for fluid transfer. Most compression sand filters use a range of 0.2 to 1.2 mm

Particles larger than 100 micrometers tend to close the filtration pores, which larger sand grains can overcome. For this reason, the depth of the sand headstock is about 0.6 to 1.5 meters depending on the usage

pay attention

The design of the sand filters indicates that they must operate at a maximum velocity of gpm/ft ² .  using the required power and maximum discharge, the required area can be calculated. The final point of the design is to make sure that the fluid is flowing properly in the floor and that no amount of fluid is diverted by sand from the flow path

When rinsed after allowing dirt to accumulate in the sand for a while, it allows less water to pass. However, the waste material in the filter will stay but should be removed. Now is the filter backwash time

When the filter accumulates a large amount of soil, the pressure drops and the water can’t freely exit the filter. Backwash sends water back out to trapped through dust filter.  After the backwash, the increase in the return pressure to the pool is clearly evident, and if the pressure gauge is mounted on the filter it is observed that the pressure more than 0.5 bar

How to design and calculate sand filters

Calculating the exact dimensions of a sand filter may require only a little information about water and fluid hydraulics

The basis of the sand filter design is based on the linear velocity of the water. In other words, the clearer the fluid and the lower the turbidity, it is expected to pass through the sand filter floor at a high rate

Linear speed, the basis of sand filter design work

Fluid linear velocity is determined by analysis and laboratory data sheet based on the available turbidity. Linear velocity is defined by the dimension or dimension of cubic meters per square meter per hour. Its more expert definition means the volume of water the sand filter can pass per square meter per hour

:In sand filter design, engineers usually consider four types of linear velocity

Linear velocity 20-25 cube meter on square meter per hour

If the raw water turbidity of the inlet to the sand filter is less than 5 and there is no specific process on the water and the water is of a good quality, the sand filter design is based on this linear velocity.

Such as filtration of indoor swimming pools or water filters for make-up coolers and etc.

Linear velocity 15-20 cube meter on square meter per hour

The TSS factor along with the water turbidity is one of the important factors in selecting the fluid linear velocity. If the water turbidity is below 10, velocity 15 can be considered in the sand filter design

Such as water of some wells (wells drilled in high water areas), as well as outdoor pools and some waterfronts throughout the city

Linear velocity is 10-15 cubic meters per square meter per hour

If the amount of suspended solids in water is higher than the 250 mg of the liter, and the opacity is reported about 10, it is better to compare the caution for the design and calculation of the sandy cultivation of the caution and all calculations based on linear velocity

Well water, for example, falls into this category in low-water areas

Linear velocity below 4 cubic meters per square foot per hour

Fluid quality in this category is considered low. The filtration effluent is usually used for wastewater filtration. The amount of colloidal material and suspended material in the fluid is high

Back wash

Back wash in the sand filter simply bypassing the water flow through the pool. The water flows from the bottom to the top of the sand filter by the waste material that gets trapped in the sand filter

To perform the backwash, you must first turn off the pump and backwash the filter valve and then turn on the pump again. Usually the backwash time is between 2-3 minutes (this time depends on the type of silica used). After the backwash, the pump had to be switched off and the valve was discharged, then switched on the pump for 30 to 60 seconds to make sure all the dirty water didn’t go into the sewer and didn’t enter the pool. At this point stop pumping for the last time and put the valve in filter mode. Now turn on the pump and filter again to purity the pool water

The pressure gauge installed at the inlet and outlet tells the pool operator that the water passage in the filter is being closed. If the gauge shows too much pressure at the inlet than the outlet, the operator will notice that a lot of waste has accumulated in the sand. This means that it’s the time for filter backwash. For backwash

operator, adjusts valves to divert water, closes pipe back to pool, and opens outlet pipe as water enters sewer

Sand filter wash

After some time, the particulate matter in the sand filter interfaces increase, causing the pressure drop to increase, and if the pressure drop exceeds a certain level, the desired sand filter must be restored. Different sand filters have different ways of resuscitation. Slow sand filter resuscitation is accomplished by removing a layer 10 cm above it. But resuscitation of the sharp sand filter (gravity and pressure) is done by backwash

Filtration mechanisms

:Removal of floating particles and water by a sand filter is carried out under various mechanisms, including

Sieve mechanism

The sieve mechanism prevents particles larger than the diameter of the pore smaller than the diameter of the pore, and thus floats particles in the sand pore lid and is removed from the sand filter when rinsed with water

Adsorption mechanism

Waterborne particulates are influenced by van der walls, electrostatic and adsorption, adsorption of the sand surface and its pores. These forces stabilize suspended particles on the sand bed and prevent them from floating again

Cooling tower sand filter

The sand filter used for cooling towers must have two characteristics: one is to use top-grade silica in the filter and to extract high quality water, and the other is to linearly filter the water passing through the filter25m/h. The sand filter used for water filtration of the cooling tower can be steel and carbon steel or composite and it is also determined by its sand filter capacity 

Sand filter as a third wastewater treatment

One of the main uses of sand filters is its use for filtration of refined wastewater treatment plants. Wastewater effluent usually has high turbidity that uses metal sand filters to reduce turbidity. The linear velocity of the water into the sand filter is assumed to be between 7 to 10 m/h and the sand filter design is based on this. In many cases, carbon filters are used after the sand filter as supplementary equipment. The silica sand used in this sand filter is selected in four sizes, which in some cases also use anthracite charcoal

Difference of metal sand filter with FRP filter

Differences in how the FRP-like sand and carbon filters work are always one of the questions that come up for the buyer. Each of these filters has both positive and negative parameters on their process and lifetime.  These include the level of filtration in the filter. In a metal sand filter, because of the nozzle plate on which the nozzles are mounted, there is always a higher level of filtration due to the nozzle spread than the FRP sand filter and filter water is always clearer in this type of filter. But one of the reasons for choosing a FRP sand filter that is strongly emphasized by both the sand filter buyer and the seller is either corrosion resistance due to the internal PE layer. If the manufacturing companies are careful in their coloring and painted after the blast document, at least 15 years and no repainting will occur 

Conditions for water passage through the sand filter

Water passes through coagulants directly without the use of chemicals and detergents

During the process of water clarification by the sand filter the clotting process is also performed

The clotting and setting process is carried out before the water enters the sand filter 

Important tips in sand filter maintenance

The minimum temperature of the sand filter installation should be 5 ⁰ c

Note the timing of secondary sand filter washing

Use caution when carrying the machine

Take note of the manufacture’s safety tips

For better performance and shock to the sand filter, the inlet pump is turned off when switching on

Select pump with head and discharge suitable for sand filter

If the inlet pressure is too high, be sure to use a pressure breaker

Drain water into the filter for a long time without using a sand filter

Check the sand filter valves regularly

Don’t use acid to wash silica stone

Kernel filtration

It is a long-lasting physical method remove suspended particulate matter in water. The sand filter is a granular filtration device and in the form of a pressurized cylindrical tank containing a bed of granulated sand and sand (silica sand for anthracite) that the physical filtration of water removes suspended particles such as mud, paint, organic matter, plankton, bacteria and particles from corrosion. The body of these filters is made of standard steel with double-check valves to prevent corrosion of all surfaces adjacent to the water inside the two-layer epoxy paint tank and all exterior surfaces of the device with air paint oil coated. The standard pressure in these filters is between 3-7 atmospheres, and if the pressure is too high, it causes a groove phenomenon

These filters are made both vertically and horizontally. Standard vertical filters have a diameter of about 0.5 to 3 meters and a height of between 0.7 to 1/5 meters. Vertical filters are preferred to horizontal filters as they are physically treated in all their height. In vertical and horizontal filters, the incoming water and the water from the wash are split or collected through the nozzle grid above and below the device

good point

Water entered the upper part of the column and filter passing through the sand filter bed, gravel was sanded at 98% purity and a lattice plate with 50 to 70 pacifiers per square meter was designed from leaving the machine is out of the lower part

The size of sand particles that are aggregated and classed on the grid plate is determined by the size of the waterborne particles. The order of magnitude of these particles from the top of the bed to the bottom of the bed is coarse to fine

Generally, the higher the thickness of the filter bed, the higher the removal efficiency of the suspended material, but in practice the thickness of the filters is within the range of 70-150 cm. Depending on the particle size of the granular bed and the pressure on the bed per square meter of surface area of 5-20 cubic meters of water can be treated per hour

notable point

In the filtration cycle, the suspended particles are deposited in the filtration bed and gradually fill the pores. Thus, the pressure drop gradually increases over a certain period of time depending on the amount of turbidity and inlet water content

By the time the sand filter pressure drop has been recorded about 0/6 to 0/8 bar, it is time for the filter to go out of service and the substrates to be washed by water flow back wash

When properly designed, these filters can remove particles up to 10 microns, and removal of particles up to 1-2 micron will be possible if water is injected into the coagulants. Typical efficiencies of these systems are removal of 50 to 60 % of colloids with a size of 10-15 um. The turbidity of inlet water to the RO pretreatment system should be less than NTU. In the high NTU10 capacity of the backwash period these systems will be too low to obtain acceptable quality water