Generally, a micro-irrigation system consists of three parts: the head hub composed of pumps, instruments, control valves and filters, the pipe network system composed of various main pipes, branch pipes and sub-pipes responsible for water transmission and distribution, and various types of emitters that directly supply water to the roots of crops. Among them, the emitters are the key equipment at the final stage of completing the irrigation task and are known as the heart of the micro-irrigation system. To ensure the normal operation of the emitters, the selection and working performance of the filtration equipment are of vital importance. If the wrong selection of the filtration equipment leads to the clogging of the emitters, it will cause uneven water distribution and a decline in system performance, and even result in the paralysis of the entire micro-irrigation system. In this case, a large amount of manpower and financial resources will have to be spent to remove the clogging or rebuild the system.

There are the following types of filtration equipment:

(1) Trash racks (screens) are mainly used in irrigation water sources such as rivers and ponds that contain large volumes of debris to intercept dead branches, leaves, weeds and other large floating objects, preventing debris from entering sedimentation tanks or reservoirs. The trash rack has a simple structure and can be designed and manufactured according to the actual situation of the water source. The primary trash screen (net) is a kind of grid-type trash screen installed at the inlet of the water pump in the water source, that is, the primary purification treatment facility. It is mainly used to block mud at water sources containing a large amount of aquatic plants, debris and algae, etc.

(2) Sedimentation tank: The sedimentation tank is one of the primary treatment facilities for irrigation water quality purification. Although it is a simple and ancient water treatment method, it is an effective and economical way to solve the water quality purification problems of various water sources. The function of the sedimentation tank is manifested in two aspects; ① Remove solid substances present in the water. When there is too much sediment in the water, the screen filter and medium filter used will lose their function due to frequent flushing. In such cases, the sedimentation tank can play a primary filtration role. ② Remove iron substances. The carbon dioxide dissolved in groundwater escapes in the sedimentation tank due to the decrease in pressure and the increase in water temperature, causing the pH value of the water to rise and leading to the oxidation and precipitation of iron substances.

(3) Centrifugal filters (also known as sand separators) are mainly used to remove sediment from well water. The working principle of a centrifugal filter is that the centrifugal force generated by high-speed rotating water flow separates sand particles and other heavier impurities from the water body. It has no filter screen inside and no detachable parts, making maintenance and upkeep very convenient. The sand accumulation chamber at the bottom of the centrifugal filter must be flushed frequently to prevent the accumulated sand and mud from being brought back into the system again. For organic matter or impurities with a specific gravity less than that of water, the separation effect of centrifugal filters is very poor. Only within a certain flow range can centrifugal filters play their due role in purifying water quality.

(4) Sand and gravel filters are mainly used for the pre-filtration of organic matter in reservoirs, ponds, canals, rivers and other open water surface water sources. It uses sand and stones as the filtering medium. Sewage enters the filter tank through the inlet and then is intercepted and captured through the pores between the sand and stones to achieve the filtering purpose. The sand and gravel filter is reliable in filtration and has a high degree of cleanliness. Among all filters, the sand and gravel filter is the most effective in treating organic and inorganic impurities in water and can provide uninterrupted water supply. Whenever the organic matter content in water exceeds 10mg/L, regardless of the amount of inorganic matter, a sand and gravel filter should be selected.

(5) Screen filter (also known as screen mesh filter) is a simple yet effective filtration device with a relatively low cost. It is the most widely used in irrigation systems both at home and abroad. Its filtering medium is nylon screen or stainless steel screen. It is mainly used to filter out powder particles, sand and scale and other contaminants in irrigation water. It can also be used to filter irrigation water containing a small amount of organic contaminants. However, when the organic matter content is slightly high, the filtration effect is very poor, especially when the pressure is large, a large amount of organic contaminants will squeeze through the screen and enter the pipeline, causing blockage of the system and the irrigation device. There are many types of screen filters. It can be classified into vertical and horizontal types according to the installation method. It can be classified into plastic and metal according to the manufacturing materials. According to the cleaning method, it can be divided into manual cleaning and automatic cleaning. It can be classified into closed type and open type according to whether it is closed or not. The screen filter is mainly composed of a screen, a housing, a top cover and other parts. All parts of the filter should be made of pressure-resistant and corrosion-resistant metals or plastics. The screen of the main filter of the system is generally made of stainless steel wire. The screen of the filter used on the branch pipe and the main pipe can also be made of nylon or copper wire.

(6) Laminated filter: This filter is composed of a large number of very thin circular laminated plates stacked together and locked to form a cylindrical filter element. Each circular laminated plate has two surfaces: one side is distributed with many S-shaped filter grooves, and the other side has a large number of concentric annular filter grooves. If the filtering capacity of the disc filter is different, the dimensions of the S-shaped and ring-shaped filter grooves on its discs will also be different. Just like the screen filter, the filtering capacity of the disc filter is also expressed in mesh size. Generally, the filtering capacity of this type of filter is between 40 and 400 mesh.

Before choosing a filtration device, the following factors should be determined first:

① Find out what impurities are contained in the irrigation water.

② Determine the flow channel diameter of the selected irrigation device, and use this diameter to select the filtering capacity of the filter.

③ Determine the peak flow rate of the irrigation system and use this flow rate to select the filtration capacity of the filter.

The mesh size of the selected filter, that is, the filtering capacity of the filter, depends on the diameter of the flow channel of the water injector. For the filter of the micro-sprinkler system, it must be able to filter out all impurities larger than 1/7 of the nozzle diameter. For drip irrigation systems using long flow channel emitters, the filtering capacity of the filter must be sufficient to filter out all impurities larger than 1/10 of the drip hole diameter.