Filters aim to protect and preserve a drip irrigation system. They have to prevent the clogging of drippers and of the drip tape. It is essential to choose them basing on the characteristics of the water and the desired flow rate.
When irrigation water have suspended solids (sand in quantities < 2gr/l – for larger quantities, it is better to equip yourself with a hydrocylone filter – residues of rust, silt and clay, insoluble residues of fertilizers used for fertigation, etc.), filters to be used are net filters or disc filters.
The filters should be chosen both by type and by size, based on the characteristics of the water and the flow rate of the system.
It is important to know the filtering capacity of a filter, that is its capacity to block particles larger of a certain value. A filter with a filtering capacity of 100 microns (0.100mm) is a filter capable of blocking particles larger than 100 microns. The filtering capacity is generally expressed in mesh, an Anglo-Saxon measure that indicates the number of links per square inch. This system may be easily misinterpreted, as it does not give you a clear value of passage capacity of the holes, which can vary a lot for the same mesh value depending on the diameter and dimensions of the entire structure. It is also important to know the filtering surface, which must be sized to the passage water: an incorrect evaluation of the two parameters often increases the speed water passing through the filtering mass. As a result, some particles that would be stopped at a lower speed are “forced” by the flow and therefore, they end up in the distribution system.
There are different types of filters for drip irrigation: among the most used are disc filters and net filters.
Disc filters are devices consisting of a metal or plastic casing with a filtering mass inside. The latter consists of a series of discs with a knurled surface, piled up tightly around a support structure. The filtering capacity is determined by the knurling on disc surface.
Small discs are grooved on both sides with a specific micron size. A series of these discs are then piled up and compressed on a specially designed spine. When the discs are stacked, the groove on the upper part slides opposite to the one on the lower part, creating a filtering element. A statistically significant number of intersecting disc grooves manages to block the solids. During filtration all discs are tightly compressed by a combination of elastic power and differential pressure, thus providing high filtration efficiency.
Net filters are devices made up of a metal or plastic casing with a filtering mass inside (cartridge), composed of a net (stainless steel or polyester) supported by a metal or plastic structure. The net determines the filtering capacity: if filtered water is heavily loaded, net filters are more likely subject to lacerations of the filtering mass, compared to disc filters.
Both net and disc filters should be selected in such a way to respect the following rule: under the maximum flow rate of the system, the pressure drop determined by them is (ideally) BETWEEN INLET AND OUTLET OF THE FILTER OF 0.2 bar and in any case not above 0.3 bar with clean water. When dimensioning, the load of suspended solids present in the water must be also considered.
The larger the active filtering surface of the filter is, the longer is the time interval between one cleaning operation and another. It is given by pressure differential increase between filter inlet and outlet: in this regards, the presence of pressure gauges is important, in order to monitor the cleanliness of the filter.
Which filter to choose for drip irrigation?
The difference between two examined filters lies in the composition of their filtering mass. The choice must be made on the basis of the analysis of the water used in the irrigation system.
• On the one hand, net filters are suitable for removing inorganic particles (well water) and for water with not high suspended solids value. They are used as field safety filters in case of long mainline pipes and downstream of sand filters.
• On the other hand, disc filters are suitable for the removal of inorganic particles and reduced quantities of organic substances. However, they are increasingly used as safety filters, too.