What Is a Slow Sand Filter?

Filters are used for filtering purposes. Some are used to remove suspended matter in water to obtain water with lower turbidity; some are used to remove water from sludge to obtain sludge with lower water content. There are many types of filters, which can be divided into fast filters and slow filters according to the size of the filter speed. At present, most of them are fast filters. The fast filter has a large processing capacity and good effluent quality. ^[1]

The purpose of using a filter in wastewater treatment is to remove fine suspended matter in the wastewater, especially to remove some fine suspended particles and colloidal substances that cannot be removed by biochemical treatment and coagulation sedimentation. Currently. The filter is widely used as a tertiary treatment means to further process the secondary treatment effluent; or as a pretreatment for advanced treatments such as activated carbon adsorption, ion exchange, electrodialysis, reverse osmosis, and membrane separation.

In addition to removing the suspended matter, the filter also has a certain removal effect on turbidity, COD, BOD, phosphorus, heavy metals, bacteria and viruses. In urban sewage treatment, the filter has become an indispensable treatment unit in the water treatment and reuse system.

With the accumulation of filtered suspended matter between the filter layers, the water resistance of the filter layers gradually increases. At this time, although the water turbidity will not change greatly, if the backwash is not performed in time, the structure of the filter material layer will change due to excessive accumulation of sludge; the filter material will see the change in the cross section and shape of the empty island. The layers are compacted and so on. At the same time, due to the increase in water resistance, the filter layer will "rupture", resulting in a short circuit in the filtered water, and the quality of the effluent water will deteriorate. ^[2]

The filtering mechanism of the filter mainly includes:

(1) Mechanical filtering effect. The filter material layer is composed of filter material particles of different sizes. The pores between the filter material particles are like a sieve. When the wastewater flows through the filter material layer, the suspended particles larger than the pores will be trapped in the pores and separated from the water. During the entire filtration process, the pores between the particles of the filter material will become smaller and smaller. Therefore, the filter material also has the function of filtering the fine suspended matter.

(2) Adsorption and contact aggregation. The filter material has a very large surface area and has a strong adsorption capacity. During the passage of waste water through the filter material layer, it must pass through the tortuous water flow channels. There are many opportunities for the suspended particles to contact the filter material. As a result of mutual intermolecular forces, adsorption and contact agglomeration are liable to occur, especially when a flocculant is added before filtration, the contact agglomeration is more prominent. The smaller the filter media particles, the better the effect of adsorption and contact aggregation.

(3) Precipitation. Each small pore in the filter layer acts as a shallow sedimentation tank. When the wastewater flows through, some suspended particles in the wastewater will settle on the surface of the filter material particles.

Filtration process: When the wastewater enters the filter material layer, larger suspended particles are trapped, while the finer suspended particles are trapped by contact with the filter material particles or the suspended particles that have adhered. . Some trapped substances that are not firmly attached will flow with the water to the next layer of filter material under the action of water flow; or due to the large amount of adsorption on the surface of the filter material particles, the pores will become smaller, so the flow velocity will increase, and the water will be washed away The trapped objects can also be carried to the next level. Therefore, with the extension of the filtration time, more substances are trapped in the depth of the filter layer. Even the filter layer is taken out with the water, making the effluent water quality worse.

Because the filter layer is backwashed and hydraulically sorted, the upper filter material has small particles, high contact agglomeration and adsorption efficiency, and some mechanical retention effects. Therefore, most of the suspended matter is trapped in a filter layer with a small thickness on the surface of the filter material, and the amount of suspended matter trapped in the lower layer is small, forming an uneven distribution of the suspended matter in the filter layer.

The ability of the filter material to trap suspended matter can be expressed by the ability to intercept dirt. Pollution interception capacity refers to the mass of pollutants retained by the filter medium per unit volume or unit mass in each working cycle. The unit is kg / m ³ .

There are many types of filters, which can be divided into fast filters and slow filters according to the size of the filter speed. At present, most of them are fast filters. The fast filter has a large processing capacity and good effluent quality. There are also many types of fast filters: according to the form of filter layer, it can be divided into single-layer filter (including homogeneous filter, actual graded filter and ideal filter), double-layer filter and multi-layer filter Material filter (see Figure 1); according to the flow of water, it can be divided into gravity filter and pressure filter; according to the control method, it can be divided into ordinary fast filter, siphon filter, mobile hood filter and Valveless filter, etc.

Filter material is a general term for water treatment filter materials. It is mainly used for the filtration of domestic sewage, industrial sewage, pure water and drinking water.

Filtering media (filtering media) are mainly divided into two categories, one is granular materials used for water filtration in water treatment equipment, usually refers to quartz sand, gravel, anthracite, cobblestone, manganese sand, magnetite filter material, Nutshell filter, foam filter beads, porcelain sand filter, ceramsite, garnet filter, maifanite filter, sponge iron filter, activated alumina ball, zeolite filter, volcanic filter, granular activated carbon, fiber Ball, fiber bundle filter, comet fiber filter, etc. The other is the physically separated filter media, which mainly includes filter cloth, filter screen, filter element, filter paper and the latest membrane.