3.1. Types of emitter
A range of emitter types is available to suit particular situations.
Drippers. Drippers are designed to give a slow application at frequent intervals, targeting the crop root zone. The drippers are located either on the outside of the lateral (on-line) or installed inside the lateral at manufacture (in-line). Laterals are installed permanently in die case of vineyards and orchards, either laid out on die ground or suspended from a wire (where they are more visible and clear of vegetation). In orchards and plantations of larger trees at wider spacings, multiple drippers are used around each tree, attached to an off take from the lateral. In die case of row crops, the laterals are laid out for the duration of the irrigation season, then wound up on a large reel for later re-use. For plants grown in containers, drippers can be attached to the lateral by a short length of microtube. It is also possible to install in-line dripper laterals below the ground, which may have application in landscape irrigation, lucerne and other crops, but the risks of root penetration and die lack of visibility of die dripper may deter some users.
Figure 7.1 Drip irrigation of winegrapes. The dripline is carried on a wire above the ground. The wetted surface area is visible.
For standard drippers, there is a particular relationship between water pressure in die lateral and dripper output. To achieve a uniform output for each dripper in die field (a flow variation of 10 per cent or less is usually acceptable), it is necessary to provide a reasonably uniform water pressure along and between each lateral. Consequently, a drip irrigation system requires careful hydraulic analysis of die pipe network to ensure uniformity. This is difficult to achieve for long laterals and steeper sites, but pressure compensated drippers are available. These are manufactured with a small diaphragm mat automatically adjusts the dripper orifice size to provide a constant output over a range of water pressures. A high degree of manufacturing accuracy is necessary to ensure this occurs.
Figure 7.2 In this nursery, each container receives water from the lateral through a small piece of microtube.
Figure 7.3 Drip irrigation of corn. The driplines, connected to hand-shift aluminium sub-mains, are wound up each season and re-used.
Dripper spacing needs to be matched to the soil's hydraulic conductivity and dripper output, and die peak irrigation requirement of die crop must be met width die required irrigation interval. In-line drippers can be selected to suit a wide range of standard spacings. On-line drippers can be installed at any position in die lateral, provided die water pressure along die lateral is kept reasonably uniform.
Blockage of drippers is a major risk, because of die small orifice size used. Manufacturers go to great lengths to minimise die risk of blockage. Rather man a single pin-hole type of orifice, the flow path through die dripper should be a tortuous one, because this enables a short but larger diameter flow path. Also, the flow through it is more likely to be turbulent, (rather man laminar) which will help keep suspended material moving through die dripper. The inlet to die flow path should have its own screen to prevent particles entering. Some online drippers can be disassembled and cleaned manually.
Other factors to consider in dripper selection include:
The ability of me dripper (and diaphragm) materials to resist chemicals in the irrigation water.
The uniformity of output and range of operating pressures for pressure compensated drippers.
Temperature changes on dripper performance (as well as expansion and contraction of laterals).
Perforated or permeable tube. There are various versions of perforated tube available where water is delivered through a pin hole through the wall of the tube, with perforations made at fixed intervals along the length of the tube. The holes are relatively small and easily blocked, so some types utilise secondary chambers built into the tube to allow a larger hole to be used.
Permeable tubes are also available, where water oozes continuously along the full surface of the tube. One type is manufactured from a plasticised paper material, and another from a special re-cycled rubber which gives to release particles which might cause a blockage.
These may have application in vegetable, flower or greenhouse crops and can be buried below the soil or growing medium.
Microtubes. These are pieces of small diameter plastic spaghetti tubing inserted into the wall of a lateral. Discharge rate is governed by the length of microtube and occasionally a spreading device is inserted on the end. Their relatively narrow diameter can result in easy blockage, and although coiled types are available, the microtube can get in the way and be damaged or pulled out. They have been mostly superseded by other emitters.
Microjets. This term describes a family of emitters which spread water over a much larger surface area than a single dripper. This may be necessary in larger tree crops or ornamental applications, or where a higher discharge rate is required. Microjets are in common use in domestic garden watering.
Various types of fan type spray patterns are available (full circle, part circle, directed jets, mist). Most use small perforations in the top of the jet to produce die wetting pattern, or alternatively, the water stream strikes a stationary anvil to cause spreading.
Microjets have no moving parts (which distinguishes them from minisprinklers). Operating pressure is nominated by the manufacturer and discharge rate is selected by choice of orifice size in the base of the jet. Discharge rates up to 200 L/hr are typical, with wetted diameters to around 2 m. The wetting pattern at the surface is often uneven, but this may not be a problem with low application rates, considering lateral infiltration in the root zone. Microjets need to be clear of vegetation often encountered at ground level, and need to be oriented vertically. Although they can be inserted directly into the wall of a lateral, they are best installed on a short riser or stake, with a connecting tube from the lateral to the jet.
Figure 7.4 Cross section through one type of in-line dripper, indicating the tortuous flow path designed to help prevent blockage.
Blockage of microjets is still a potential problem, since water must pass through a single, small-diameter orifice.
Minisprinklers. This describes another family of emitters, which use a rotating component to spread water over a larger area (2-5 m). This type of emitter also delivers a higher rate of discharge. Orifice size is often larger, so blockages may be less likely.
Many different types are available. Some use a spinning disc or cap, others a rotating arm or arms, and others resemble small versions of conventional sprinklers. Selection is based on crop requirements for frequency and amount of watering, operating pressure and discharge rate, wetting pattern, and emitter spacing.
Figure 7.5 Microjet irrigation
Figure 7.6 Minisprinklers have moving parts to spread water over a larger wetted area whilst maintaining reasonably low operating pressure
Water quality and filtration.
Filtration is essential with microirrigation systems, to remove particles suspended in the water. Unfortunately, filters alone cannot cope with all possible water quality problems, so additional treatments may be necessary. The choice of filter with depend on:
The nature of the suspended particles and their concentration.
The flow rate and pressure that must be accommodated.
The diameter of the emitter orifice or flow path.
The ease with which die filter must be cleaned, and the amount of water required to do this.
Types of filters available.
Because of the importance of filtration, the different specifications available, and die range of possible problems with water quality, expert advice should be sought.
Wire mesh or gauze. These need to be of a large surface area and relatively fine mesh size to suit microirrigation applications. They are generally used as back-up filters, although self-cleaning versions are available. The mesh provides only a single surface to remove particles, and particles of irregular shape or those that are compressible might still pass through. Cleaning can be difficult, because the particles may be lodged hard into die mesh.
Centrifugal or spiral filters. These do not have a screen of any type, but cause the water to spin in passing from inlet to outlet. Particles are removed from the water by centrifugal force, collecting at a disposal outlet at die base of die filter. These can be used as initial or primary filters to reduce die load on the main filters, particularly when sandy material is present in die water.
Figure 7.7 Multiple disc-type filters, mounted in a tower. This version is self-cleaning
Disc type. These consist of many plastic discs pressed together in a cylinder. The surface of die discs is manufactured with angled grooves at opposing angles, so mat water passes through a tortuous pathway of intersecting grooves between adjacent discs, removing particles as it travels. This provides a longer pathway for filtration to occur, compared to a screen, with different disc and groove patterns corresponding to a range of equivalent mesh sizes.
Multiple cylinders are connected together to give die required flow rate. Cleaning is accomplished by loosening die discs and flushing with clean water. Self-cleaning types are available, which use only a moderate amount of water for backflushing.
Figure 7.8 Media filtration unit. Multiple tanks are necessary to accommodate the required flow rate, and to provide for automatic backflushing.
Mediafilter. Water is passed vertically through a large tank containing graded gravel or sand. Multiple tanks are used to provide die required flow rate. Although die most expensive, these are generally considered the most effective type, particularly where the water supply is stored in dams. Although routinely specified for microirrigation systems in Australia, some irrigators have experienced problems with media filters being clogged very quickly, requiring frequent backflushing, which could be a result of improper selection of filter size and capacity. Some systems could benefit from some form of pre-cleaning equipment. In some cases, the filter could be removing material which will pass through die dripper anyhow, which could overwork the filter.
Cleaning is achieved by temporarily reversing the flow through the tank (backflushing). During this operation, the media is suspended, releasing contaminants. A reasonably large amount (and velocity) of backflush water is required, normally going to waste. Automatic backflushing is available; as the filter commences to collect suspended material, there is an increase in die pressure required to push die water through. This results in a larger difference in pressure between upstream and downstream sides of me filter, which increases over time. When die pressure difference reaches a pre-set limit, die filter is automatically switched to backflush mode. Whilst being flushed, the filter cannot be in normal working mode, so installation requires at least two modules.
A back-up filter is required immediately after die media tanks to capture any media mat might escape die tank.
Aspects of water quality. In me event of a pipe repair in die field, there is a high risk of material entering die pipe network after die filter. Flushing will be essential immediately after repair, and some projects have manual clean back-up filters installed at die inlet to each block.
Unfortunately, filters are only best at removing suspended particles, and may not remove other problem material mat may be present:
• Algae present in die water supply may accumulate and grow in or downstream from die filter. Periodic chlorine treatment may be necessary, with the concentration of chlorine selected to suit the particular situation. Sometimes, continuous chlorine injection may be required, for example where wastewater is used.
Figure 7.9 In this system, a manual clean disc flier is installed with the valve at the entrance to each irrigation block as a back-up to the main filters (system under construction)
Clay colloids are small enough to pass through the emitter, but certain chemical reactions can occur with other elements dissolved in die water, which causes the clay particles to flocculate. Some chemicals can be flocculated in storage reservoirs prior to use. Salts of iron, common from bore water, will react with oxygen, hence it may be necessary to oxygenate me water prior to use.
Bacteria can form a slime inside pipes, which can detach from the pipe wall and cause blockages. Chlorine treatment may be necessary.
Acid may need to be injected to lower pH and to control build up of calcium based deposits.
Temperature fluctuations can initiate chemical reactions, although burying the pipeline will minimise hot temperatures. Fertiliser injection should be done before the end of die irrigation to enable clean water to flush the chemical through.
Insects crawling into the emitter can be a problem. Keeping the emitter above ground level helps, as does periodic flushing with lateral ends open. Some emitters are designed with "anti-ant" closing devices.
Flushing of die laterals should be a regular practice; at the commencement of each season, and during die season as required. For manual flushing, die lateral end is folded onto itself and held by a clip. The clip is removed for flushing, men replaced. Laterals can be fitted with automatic flushing-valves mat release some water on every start-up. For larger projects, where flushing of individual laterals is inconvenient or too expensive, blocks of laterals can be connected into a common flushing sub-main, die whole block flushed by opening one or two valves. A reasonable water velocity is required during flushing, to dislodge and move die contaminating particles. Sub-main ends should also be fitted with a flushing valve.
Material may also accumulate in mainlines, either a gradual build-up of fine material, or through a pipe repair. Each irrigation block can be fitted with a back-up filter, but die low points of die mainline should be fitted with a scour valve to enable flushing of die mainline.