Wastewater Treatment Basic Terminology: What Is Sand Filtration
The contact media commonly used in China mainly include FRP or plastic honeycomb media, three-dimensional corrugated media, soft fibre media, semi-soft media and irregular granular media (sand, gravel, slag, coke, anthracite), etc. [2]. FRP or plastic media have smooth surfaces, poor biofilm adhesion and a tendency to age, and in practice they often suffer clogging of varying degrees; the flow regime in soft fibre media is not ideal, and the media are easily bound together by biofilm to form balls, which greatly reduces their surface area and creates anaerobic conditions inside the balls, impairing treatment performance. Irregular granular media create high flow resistance and easily clog the oxidation tank. China has also carried out research on the use of flaky ceramsite to treat slightly polluted source water. Flaky ceramsite is an irregular granular medium; although it has good biofilm formation performance, it offers high flow resistance, clogs easily, has poor strength, breaks easily and does not resist water scouring, which limits its use to the treatment of slightly polluted source water and prevents its application in wastewater treatment. It is precisely these shortcomings of traditional contact media that have limited the application of biological aerated filters in wastewater treatment in China. Therefore, researching and developing a good contact medium is the key to the successful application of biological aerated filters in wastewater treatment in China [3].
Quartz sand filter media are polygonal or spherical in appearance and pure white, and feature high mechanical strength, a strong solids-capture capacity and good acid resistance. Quartz sand is the most widely used filter medium and performs the filtration function: much like water percolating through sand and gravel into the ground, it intercepts the suspended solids in the water, mainly the fine suspended particles [4].
Sand filtration is a rapid method for separating insoluble substances from water in water supply and wastewater treatment. Since the 1960s, severe water shortages have put wastewater reuse on the agenda. To obtain high-quality effluent, filtration technology has been widely used for advanced treatment of sewage and wastewater. Filtration is usually applied after chemical coagulation and biological treatment; it is a water treatment operation in which water passes through a bed of porous media such as sand, coal particles or diatomite to separate suspended solids. Its main function is to remove dispersed suspended inorganic and organic particles, including various plankton, bacteria, filterable viruses, floating oil and emulsified oil.
This is a water filtration process that uses activated carbon as the filter medium. The abundant pores of activated carbon give it a very large specific surface area (500-1700 m2 per gram of material, with a specific gravity of 1.9-2.1), so it can adsorb fine substances in various liquids. Activated carbon filter beds are divided into fixed beds, moving beds and fluidised beds. For a fixed carbon layer, the thickness is 1.5-2.0 m, the filtration rate is 8-20 m/h, the backwash water rate is 28-32 m/h and the backwash time is 4-10 min. Activated carbon is regenerated by cleaning with high-temperature steam or by microwave heating, with a recovery rate of 80%-94%. In advanced treatment of water supply and wastewater, activated carbon filtration is mainly used for decolourisation, deodorisation, dechlorination and removal of organics, heavy metals, synthetic detergents and viruses [1].
This is a water filtration process that uses canvas, felt or nylon cloth as the filter medium. It is used in water treatment to remove fine suspended solids (particles of 10-100 um, such as lint in textile wastewater treatment), or to dewater industrial wastewater sediment (such as gypsum) and sludge produced in water treatment. Common sludge dewatering machines include vacuum rotary drum filters and plate-and-frame filter presses, used to dewater sludge from textile, printing and dyeing, chemical, metallurgical and papermaking wastewater treatment [1].
