Role of micro irrigation filters and farm wastewater treatment to improve the performance of emitters

Irrigated agriculture is the biggest consumer of water in the world. Competition for fresh-water resources with the domestic and industrial sectors and those of the agricultural sector is very strong, mainly in populated arid and semi-arid regions of the world. At the same time, increasing urbanisation is producing large volumes of wastewater that have become a serious environmental problem in most countries. Sustainable water use in these conditions involves wastewater reuse in irrigation. Municipal wastewater (WW) is the most suitable for irrigation reuse but requires irrigation methods which minimise health and environmental risks. Micro irrigation, and, in particular, surface (DI) and subsurface (SDI) drip irrigation, is suitable for WW use, mainly for plants with edible parts growing above the soil (tomatoes, oranges, etc.) as highlighted by different researchers. Physical, chemical and biological particles contained or developed in WW could lead to a high risk of system failure due to clogging of the emitters and filtering difficulties. The clogging of emitters is the largest maintenance problem with micro irrigation systems. It is difficult to detect and expensive to clean, or replace, clogged emitters. Partial or complete clogging reduces emission uniformity and, as a consequence, decreases irrigation efficiency. Clogging risk depends on the level of treatment the WW has undergone. In theory, there exist treatment technologies to achieve any desired water quality, but extremely stringent quality standards would lead to unsustainable costs especially in developing countries and in small communities, where the development of sewage treatment plants does not follow that of water use. In these conditions, a good trade-off between treatment costs and micro irrigation system performance has to be found. Filtering is the main defence against emitter clogging, but does not prevent it completely and, in many cases of low-quality water the filters clog, causing system management problems. The main filter types used in irrigation are screen, disk and gravel-sand media filters. The most commonly are screen and disk filters; they are simple, economical and easy to manage. Gravel-sand media filters are particularly suitable for water with a high suspended solids content, but they are relatively more complex and expensive. Chemical injection (with chlorine or acid compounds) may be necessary to dissolve mineral precipitates or prevent the growth of slime. Gravel-sand media filters and chemical injection are more suitable for farms with high technological and professional standards. In many areas, farm water treatment systems are generally very elementary and irrigators prefer large-sized emitters, such as sprayers and sprinklers, which are less appropriate to minimise health and environmental risks. Field tests using WW showed that the main problems of micro irrigation systems using this type of water are: complete protection of the emitters from clogging may require expensive filtration and chemical treatment equipments; the usual filters may need frequent cleaning, which involves both failure of the irrigation system and the continuous presence in the field of the irrigator (for manual filters), or the use of automatic cleaning filters which, in turn, contribute towards an increase in equipment costs; frequent cleaning of the filters involves contact between the irrigator and WW and the production of large amounts of dirty water to eliminate. Instead, to encourage the use of micro irrigation systems with WW it is of fundamental importance the costs of equipment and management to be comparable with those of a system using clean water. Farmers need emitters which assure good performance (uniformity) and filters which guarantee a good trade-off between cost and operating times between cleaning operations. To reduce the cost of water analysis it is also important to identify a limited number of water quality parameters to foresee the emitter clogging risk. Micro irrigation system performance using WW is a result of a complex interaction between effluent characteristics, emitter type and location along the lateral, filter type and system management. It is for this reason that several researchers have studied filters and emitters using different effluents and irrigation equipment. In the chapter, the main results of experimental trials on the performance of several kinds of filter are detailing dealt with.