Sand filter
Encyclopedia
Sand filters are used for water purification
. There are three main types;
All three methods are used extensively in the water industry
throughout the world. The first two require the use of flocculant chemicals to work effectively whilst slow sand filters can produce very high quality water free from pathogen
s, taste and odour without the need for chemical aids.
. Broadly, there are two types of filter for separating particulate solids from fluids:
In addition, there are passive and active devices for causing solid-liquid separation such as settling tanks, self-cleaning screen filters, hydrocyclones and centrifuge
s.
There are several kinds of depth filter, some employing fibrous material and others employing granular material
s. Sand bed filters are an example of a granular loose media depth filter. They are usually used to separate small amounts (<10 parts per million or <10 g per cubic metre) of fine solids (<100 micrometres) from aqueous solutions. In addition, they are usually used to purify the fluid rather than capture the solids as a valuable material. Therefore they find most of their uses in liquid effluent (wastewater
) treatment.
In addition, particulate solids can be prevented from being captured by surface charge repulsion
if the surface charge of the sand is of the same sign (positive or negative) as that of the particulate solid. Furthermore, it is possible to dislodge captured particulates although they may be re-captured at a greater depth within the bed. Finally, a sand grain that is already contaminated with particulate solids may become more attractive or repel addition particulate solids. This can occur if by adhering to the sand grain the particulate loses surface charge and becomes attractive to additional particulates or the opposite and surface charge is retained repelling further particulates from the sand grain.
In some applications it is necessary to pre-treat the effluent flowing into a sand bed to ensure that the particulate solids can be captured. This can be achieved by one of several methods:
Overall, there are several categories of sand bed filter:
The depth of the sand bed is recommended to be around 0.6-1.8 m (2–6 ft) regardless of the application. This is linked to the maximum throughput discussed below.
Guidance on the design of rapid sand bed filters suggests that they should be operated with a maximum flow rate of 9 m3/m2/hr (220 US gal/ft2/hr). Using the required throughput and the maximum flowrate, the required area of the bed can be calculated.
The final key design point is to be sure that the fluid is properly distributed across the bed and that there are no preferred fluid paths where the sand may be washed away and the filter be compromised.
This filter type will capture particles down to very small sizes, and does not have a true cut off size below which particles will always pass. The shape of the filter particle size-efficiency curve is a U-shape with high rates of particle capture for the smallest and largest particles with a dip in between for mid-sized particles.
The build-up of particulate solids causes an increase in the pressure lost across the bed for a given flow rate. For a gravity fed bed when the pressure available is constant, the flow rate will fall. When the pressure loss or flow is unacceptable the bed is back washed to remove the accumulated particles. For a pressurised rapid sand bed filter this occurs when the pressure drop is around 0.5 bar. The back wash fluid is pumped backwards through the bed until it is fluidised and has expanded by up to about 30% (the sand grains start to mix and as they rub together they drive off the particulate solids). The smaller particulate solids are washed away with the back wash fluid and captured usually in a settling tank. The fluid flow required to fluidise the bed is typically 3 to 10 m3/m2/hr but not run for long (a few minutes). Small amounts of sand can be lost in the back washing process and the bed may need to be topped up periodically.
Passing flocculated
water through a rapid gravity sand filter strains out the floc
and the particles trapped within it reducing numbers of bacteria
and removing most of the solids. The medium of the filter is sand
of varying grades. Where taste and odour may be a problem (organoleptic impacts), the sand filter may include a layer of activated carbon
to remove such taste and odour.
Sand filters become clogged with floc after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed off as a waste-water stream. In some countries the sludge may be used as a soil conditioner
. Inadequate filter maintenance has been the cause of occasional drinking water contamination.
Sand filters are occasionally used in the treatment of sewage
as a final polishing stage (see Sewage treatment
). In these filters the sand traps residual suspended material and bacteria and provides a physical matrix for bacterial decomposition of nitrogenous material, including ammonia
and nitrates, into nitrogen
gas.
Water purification
Water purification is the process of removing undesirable chemicals, materials, and biological contaminants from contaminated water. The goal is to produce water fit for a specific purpose...
. There are three main types;
- rapid (gravity) sand filtersRapid sand filterThe rapid sand filter or rapid gravity filter is a type of filter used in water purification and is commonly used in municipal drinking water facilities as part of a multiple-stage treatment system...
- upflow sand filters
- slow sand filterSlow sand filterSlow sand filters are used in water purification for treating raw water to produce a potable product. They are typically 1 to 2 metres deep, can be rectangular or cylindrical in cross section and are used primarily to treat surface water...
s
All three methods are used extensively in the water industry
Water industry
The water industry provides drinking water and wastewater services to residential, commercial, and industrial sectors of the economy. The water industry includes manufacturers and suppliers of bottled water...
throughout the world. The first two require the use of flocculant chemicals to work effectively whilst slow sand filters can produce very high quality water free from pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s, taste and odour without the need for chemical aids.
Sand bed filtration in context
A sand bed filter is a kind of depth filterDepth filter
Depth filters are the variety of filters that use a porous filtration medium to retain particles throughout the medium, rather that just on the surface of the medium. These filters are commonly used when the fluid to be filtered contains a high load of particles because, relative to other types...
. Broadly, there are two types of filter for separating particulate solids from fluids:
- Surface filters, where particulates are captured on a permeable surface
- Depth filters, where particulates are captured within a porous body of material.
In addition, there are passive and active devices for causing solid-liquid separation such as settling tanks, self-cleaning screen filters, hydrocyclones and centrifuge
Centrifuge
A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...
s.
There are several kinds of depth filter, some employing fibrous material and others employing granular material
Granular material
A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact . The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations...
s. Sand bed filters are an example of a granular loose media depth filter. They are usually used to separate small amounts (<10 parts per million or <10 g per cubic metre) of fine solids (<100 micrometres) from aqueous solutions. In addition, they are usually used to purify the fluid rather than capture the solids as a valuable material. Therefore they find most of their uses in liquid effluent (wastewater
Wastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
) treatment.
Particulate solids capture mechanisms
Sand bed filters work by providing the particulate solids with many opportunities to be captured on the surface of a sand grain. As fluid flows through the porous sand along a tortuous route, the particulates come close to sand grains. They can be captured by one of several mechanisms:- Direct collision
- Van der WaalsVan der Waals forceIn physical chemistry, the van der Waals force , named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral...
or London force attraction - Surface chargeSurface chargeSurface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...
attraction - Diffusion.
In addition, particulate solids can be prevented from being captured by surface charge repulsion
Coulomb's law
Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...
if the surface charge of the sand is of the same sign (positive or negative) as that of the particulate solid. Furthermore, it is possible to dislodge captured particulates although they may be re-captured at a greater depth within the bed. Finally, a sand grain that is already contaminated with particulate solids may become more attractive or repel addition particulate solids. This can occur if by adhering to the sand grain the particulate loses surface charge and becomes attractive to additional particulates or the opposite and surface charge is retained repelling further particulates from the sand grain.
In some applications it is necessary to pre-treat the effluent flowing into a sand bed to ensure that the particulate solids can be captured. This can be achieved by one of several methods:
- Adjusting the surface charge on the particles and the sand by changing the pH
- CoagulationCoagulationCoagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
– adding small, highly charged cations (aluminium 3+ or calcium 2+ are usually used) - FlocculationFlocculationFlocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
– adding small amounts of charge polymer chains which either form a bridge between the particulate solids (making them bigger) or between the particulate solids and the sand.
Operating regimes
They can be operated either with upward flowing fluids or downward flowing fluids the latter being much more usual. For downward flowing devices the fluid can flow under pressure or by gravity alone. Pressure sand bed filters tend to be used in industrial applications and often referred to as rapid sand bed filters. Gravity fed units are used in water purification especially drinking water and these filters have found wide use in developing countries (slow sand filters).Overall, there are several categories of sand bed filter:
- rapid (gravity) sand filters
- rapid (pressure) sand bed filters
- upflow sand filters
- slow sand filters.
Rapid pressure sand bed filter design
Smaller sand grains provide more surface area and therefore a higher decontamination of the inlet water, but it also requires more pumping energy to drive the fluid through the bed. A compromise is that most rapid pressure sand bed filters use grains in the range 0.6 to 1.2 mm although for specialist applications other sizes may be specified. Larger feed particles (>100 micrometres) will tend to block the pores of the bed and turn it into a surface filter that blinds rapidly. Larger sand grains can be used to overcome this problem, but if significant amounts of large solids are in the feed they need to be removed upstream of the sand bed filter by a process such as settling.The depth of the sand bed is recommended to be around 0.6-1.8 m (2–6 ft) regardless of the application. This is linked to the maximum throughput discussed below.
Guidance on the design of rapid sand bed filters suggests that they should be operated with a maximum flow rate of 9 m3/m2/hr (220 US gal/ft2/hr). Using the required throughput and the maximum flowrate, the required area of the bed can be calculated.
The final key design point is to be sure that the fluid is properly distributed across the bed and that there are no preferred fluid paths where the sand may be washed away and the filter be compromised.
Operating parameters for rapid pressure sand bed filters
Rapid pressure sand bed filters are typically operated with a feed pressure of 2 to 5 bar(a) (28 to 70 psi(a)). The pressure drop across a clean sand bed is usually very low. It builds as particulate solids are captured on the bed. Particulate solids are not captured uniformly with depth, more are captured higher up with bed with the concentration gradient decaying exponentially.This filter type will capture particles down to very small sizes, and does not have a true cut off size below which particles will always pass. The shape of the filter particle size-efficiency curve is a U-shape with high rates of particle capture for the smallest and largest particles with a dip in between for mid-sized particles.
The build-up of particulate solids causes an increase in the pressure lost across the bed for a given flow rate. For a gravity fed bed when the pressure available is constant, the flow rate will fall. When the pressure loss or flow is unacceptable the bed is back washed to remove the accumulated particles. For a pressurised rapid sand bed filter this occurs when the pressure drop is around 0.5 bar. The back wash fluid is pumped backwards through the bed until it is fluidised and has expanded by up to about 30% (the sand grains start to mix and as they rub together they drive off the particulate solids). The smaller particulate solids are washed away with the back wash fluid and captured usually in a settling tank. The fluid flow required to fluidise the bed is typically 3 to 10 m3/m2/hr but not run for long (a few minutes). Small amounts of sand can be lost in the back washing process and the bed may need to be topped up periodically.
Uses in water treatment
All three methods are used extensively in the water industry throughout the world. The first two and third in the list above require the use of flocculant chemicals to work effectively whilst slow sand filters can produce very high quality water free from pathogens, taste and odour without the need for chemical aids.Passing flocculated
Flocculation
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
water through a rapid gravity sand filter strains out the floc
Floc
Floc can refer to either:* FLOC is the abbreviation for the Farm Labor Organizing Committee, a labor unionor* Floc is flake of precipitate that comes out of solution during the process of flocculation...
and the particles trapped within it reducing numbers of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and removing most of the solids. The medium of the filter is sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
of varying grades. Where taste and odour may be a problem (organoleptic impacts), the sand filter may include a layer of activated carbon
Activated carbon
Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...
to remove such taste and odour.
Sand filters become clogged with floc after a period in use and they are then backwashed or pressure washed to remove the floc. This backwash water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed off as a waste-water stream. In some countries the sludge may be used as a soil conditioner
Soil conditioner
A soil conditioner, also called a soil amendment, is a material added to soil to improve plant growth and health. A conditioner or a combination of conditioners corrects the soil's deficiencies in structure and-or nutrients.-Purpose:...
. Inadequate filter maintenance has been the cause of occasional drinking water contamination.
Sand filters are occasionally used in the treatment of sewage
Sewage
Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...
as a final polishing stage (see Sewage treatment
Sewage treatment
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...
). In these filters the sand traps residual suspended material and bacteria and provides a physical matrix for bacterial decomposition of nitrogenous material, including ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and nitrates, into nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
gas.