Drip irrigation consists of localised irrigation, which provides low flows (up to 16 L h⁻¹ per emitter) [22], and are ideal for securing the soil or providing an adequate amount of water. This technique enables the establishment of a wet bulb to constitute a water reserve sufficient for the development of crop growth without significant losses by deep percolation [23]. Therefore, it is possible to achieve high efficiency in water use. In the holdings of western Almería, consumption of around 6,000 m³ ha⁻¹ yr⁻¹ can be found. By not wetting the entire soil, but only part of it, the plant concentrates its roots in the wet area so that it becomes necessary for nutrients to be taken up from the irrigated water, a practice known as “fertigation” [24]. Fertigation is commonly used in protected horticulture as it improves the benefits of drip irrigation and supports some new in relation to other disinfectants [23], maximises input, uses low-quality resources, and increases the production. This provides the agrosystem with a differential level of competitiveness and justifies the high initial cost of investment.

In places where the soil has been replaced by soilless cultivation techniques, in either the substrates only or by growing directly in a nutritious solution [24], the sustainability of the system relies on the incorporation of technologies that allow for recycling of the substrates. These substrates are present in the non-harvested plant material, which should ideally be recycled as compost [25,26]. However, to achieve this recycling management, the soil must be treated with biodegradable chemical products during fertigation and throughout the productive chain [27].

One drawback in the fertigation system is the blockage of water passage by particles, which compromises correct functioning during the time of installation [24]. The duration of the emitters must be maximised so that uniformity and efficiency of the applied water do not decrease over time; consequently, blockages, which have different origins: physical (suspended soils), chemical (formation of precipitates), biological (bacteria and algae) and their combinations, must be prevented and treated, especially in the case of buried drip irrigation systems, where visual inspection is more difficult. This concern is especially notable in the development of antimicrobial coatings, usually synthetic polymers, for internal irrigation pipes [27].

Paradoxically, clogging produces an intrinsic property, especially in the case of auto-regulation emitters. The irrigation is based in a water volume applied per unit of area and the reagent will be applied to a specific quantity of water for a given cycle. The plants that encounter a blocked emitter will not receive their volume of water, while the ones associated with a non-blocked emitter will receive more than the adequate amount of water (the emitters that function normally will be exposed to increased pressure). This phenomenon negatively influences the coefficient value of variation in the operation of the emitters [27].

The placement of several emitters per plant favours reduction of the clogging effects, because the deviation is lower. However, this greatly affects the uniformity; although two to eight emitters are installed per plant, there is an obstructed percentage of between 1 and 5% [28]. In addition, this measure is not economically viable.

The aspects mentioned above include the biosafety of the agrosystem, including water storage structures. In areas where water resources are scarce, the use of treated wastewater in urban sewage is common, and is increasingly used in protected horticulture. Although wastewater is of great interest, apart from other physical, chemical and physical-chemical problems [29], it may include a microbial level that represents a risk to human health. Under Spanish regulation, in the case of fecal coliforms, these are limited to 1,000 CFU (colony-forming units) per 100 mL for the use of irrigation water in vegetables that are to be consumed raw [30]. It is known that irrigation water is a vector of transmission of intestinal protozoa and nematodes that can cause diseases if ingested through crops irrigated with this water. In south-eastern Spain, there is contamination of irrigation water by several plant pathogens [31,32]. The presence of these pathogens, and in many cases their pathogenicity, has been demonstrated in irrigation waters [33]. Several authors, such as [34] and others, have demonstrated a high level of infectious agents, including some from the Pythium sp, Phytophthora sp, Fusarium sp and Olpidium sp families, in the irrigation waters of this region, even some reaching values above 80%. In the irrigation water, there may also be pathogenic organisms that are potentially harmful for humans and plants. These are introduced from the outside and can survive in irrigation water and adhere to biofilms to act as a reservoir, jeopardising the biosafety of the agrosystem. These biofilms are complex microorganisms and extracellular polymers fixed to a surface; they can be composed of a single or many species. Biofilms are available in any medium, provided that it is an environment with water and nutrients and that it can grow on hydrophobic or hydrophilic, and biotic or abiotic substrates [35]. This might indicate why organic matter is deposited at the interface of the water and surface, forming a layer that changes its physical properties, thereby improving the possibilities of adhesion of the bacteria. Thus, the problem of the clogging of the irrigation infrastructure by causes of biological origin is more obvious if the water used for irrigation is of residual origin. [27] classified the irrigation waters in terms of drip irrigation clogging, as indicated in Table 1.

Within the superficial uptake and use of residual wastewater, the frequent use of groundwater for irrigation is common. Most irrigation water used in arid and semiarid areas is characterised by a significant amount of dissolved bicarbonate. The natural terrains contain variable amounts of soluble salts and organic compounds, which are spread in the natural water supply. As a result, we assume that the global composition of underground water is remarkably constant, regardless of its geographic origin; the only change will be in the concentrations of each component. In regards to the amount of salt and the risk for clogging [27], established the following categories of irrigation water quality:

The dissolved ions normally precipitate by adding a precipitation germ to an initial nucleus (greater than 1 micron) to facilitate and accelerate the formation of saline precipitates that normally respond to microcrystallines. These initial geneses of the process are favoured by solid particles (under suspension) and promoted by the salinity of the fertigation water that contributes to waterlogging. The biofilm acts not only against rainfall due to its inherent biochemistry activity, but also constitutes a physical seat for the subsequent growth of precipitated salt layers [27].

This interaction between the direct chemical precipitation and the synergy with the biofilms plays an essential role for two reasons. First, its direct role on clogging has already been described as an important factor. However, it also provides a “refuge” for pathogens, and thus fails to produce an effect if it is protected [27].

Traditionally, treatment of irrigation water with an acidic liquid solution was thought to be the best option for maintaining the working conditions of the irrigation system [27]. However, other options should be considered in horticulture for sustainable management.

Although the available evidence indicates that the use of chlorinated water (0.5–1.0 mg L⁻¹) is safe for crops, the use of a sodium hypochlorite solution (which releases Cl₂) has been extended in irrigation facilities to avoid clogging problems of emitters by biofilm formation. The biocidal activity of chlorine is dependent on the amount of hypochlorous acid present in the water and in contact with microbial cells. Although the mechanism of action is not entirely clear, it is predicted to inhibit enzymatic reactions and protein denaturation. These have a broad spectrum of activity (bactericidal, virucidal and sporicidal) and free chlorine retention has the most disinfectant activity [36]. The addition of sodium hypochlorite in water has the following chemical reaction: N a O C l + H 2 O ↔ N a O H + H O C l H O C l ↔ H + + O C l −

The balance of this reaction sequence depends on several factors, such as the pH of water, which has an optimum range of 6.5 to 7.5 in order to be highly efficient and stable. In addition, the temperature and presence of organic matter affect its behaviour. The hypochlorous acid reacts with the organic matter present in the washing medium and, as a result of disinfection, sub-products such as trihalomethanes, chlorine fumes [34,37], chloroform (CHCl₃) and bromodichloromethane (CHBrCl₂) are formed. These products may be carcinogenic, mutagenic, teratogenic and/or toxic gases, which have been found to be directly related to the incidence of bladder cancer and birth defects [38–41].