Sand Filters

Sand filtration is a frequently used very robust method to remove suspended solids from water. The filtration medium consists of multiple layers of sand with a variety in size and specific gravity. Sand filters make use of sand to filter water, usually using one grade of sand alone as the filtration media. The unfiltered water is distributed at the sand filter's top and will slowly sink through the filter bed. The water to be purified slowly sinks through the sand of the sand filter, leaving the dirt particles from the water behind in the fine pores of the sand.

Industrial sand filters can have graded support media layers or be directly installed over or on a distribution plate. This water treatment technique can also be combined with other filtration techniques such as activated carbon.

sand filter working pressure is between 4-10 bar 

Sand filters are employed as primary treatment in the water/wastewater industry. Raw water with various particle sizes of colloidal needs to be coagulated/flocculated with coagulants and flocculants. The filter is filled with media such as garnet, sand of varying sizes, and anthracite. These filters could be dual-media or multimedia based on the raw water characteristics. A very high degree of clarity is achieved in the filtered water on account of finer particles of garnet at the bottom which trap finer turbidity particles. Turbidity is trapped and held throughout the entire bed depth, rather than the top 1 or 2 in.

Mechanical filtration will remove almost all forms of turbidity. In many cases, filters containing specially graded and sized gravel and sand are effective in screening out turbid particles. With such units, periodic backwashing to remove the filtered material is all the maintenance necessary.

 

Sand Filter Media Characteristics


The media characteristics of sand-based treatment systems are among the most important design criteria. The primary sand media characteristics affecting filtration performance are the effective grain size and uniformity coefficient. These characteristics tend to affect the retention time of liquid passing through the media and the potential for clogging.

Filtration media :

One layer of Anthracite 1.4-1.5 mm

Two-layer of Sand 0.4-0.8 mm and 1.0-2.0 mm

Two-layer  of Gravel 2.0-3.15 mm and 3.15-5.6 mm

Anthracite/sand filter beds normally provide all of the advantages of single-media filtration but require less backwash water than sand or anthracite alone. Similar claims have been made for anthracite/sand/garnet mixed units. The major advantages of dual-media filtration are higher rates and longer runs. Anthracite/sand/garnet beds have operated at normal rates of approximately 5 gpm/ft² and peak rates as high as 8 gpm/ft² without loss of effluent quality.

Sand Filters Coating:

In preparation for the coating to be applied, filters are provided with a special layer of zinc phosphate. This treatment ensures proper adhesion of the coating and protects against rusting-through from the inside and after that steel is coated with an epoxy layer. Subsequently,

bed depth in sand filter is usually 1 meter in minimom.

 

Applications for sand filtration:

  • Treatment of wastewater

  • Filtration of grey or surface water

  • Removal of iron 

  • Preparation of cooling water

  •  Filtration in swimming pools

  • Pre-filtration of disinfection equipment

  • Preparation for micro and ultrafiltration

  • Pre filtration for membrane systems

  • Recirculation systems

  • Cooling water processes
  • Irrigation systems
  • Swimming-pool filtration
  • Basin water filtration
  • Groundwater purification

Sand Filter Backwash :

The time for the cleaning is determined by one of the following criteria:

  1. Pressure drop across the filter
  2. operating time

When the filters are loaded with particles the filter will show a high-pressure drop across the bed. Thus, it requires a backwash to clean the bed. the backwash has to be performed once the pressure drop across the bed drops below a target threshold. A standard pressure drop ranges between 0.2 to 0.5 BAR (3 to 7 PSI), so it has to be considered before initiating a backwash., the flow direction is reversed and the flow is increased to clean the filter again. This step is called a backwash. A backwash should be performed each time the media reaches capacity which should be about once a week for a properly designed system.A sand filter has a dirt holding capacity of 3 to 6 kg TSS / m2 of the sand surface.