1. Introduction
When referring to water reuse, we mean taking treated wastewater (TW) and then reusing it for other purposes rather than dumping it into the sea or a river. To be able to reuse it, treated water has to undergo an extra water treatment process because conventional processes do not suffice. Having drinking water is one of the sustainable development goals (SDG no. 6) that is not always easy to ensure, especially in countries like Spain or arid regions marked by water deficit. In Spain and other Mediterranean Region countries, reusing treated water should be considered when the main stakeholders and policy makers plan water resources because it is an area with strong water stress. In other words, water is short, and “wasting” water instead of reusing it is a luxury we cannot afford.
According to a new study by UN University’s Canadian-based Institute for Water, Environment and Health [
1], today, some 380 billion cubic meters of wastewater are produced annually worldwide. Furthermore, the paper says, wastewater volumes are increasing quickly, with a projected rise of roughly 24% by 2030, 51% by 2050. In Spain, the 2016 reuse data published at the end of 2018 by the INE (National Statistics Institute) [
2] indicate that the TW volume is 12,949,076 m
3 per day, with 1,350,536 m
3 per day for reused treated water. What this implies is that only 10.5% of the treated water is actually used, and the rest is wasted. Notwithstanding, Spain is the EU country that reuses the biggest wastewater volume and is one of the top ten world countries because the average figure in Europe only comes to 2.4%. The most widespread reused water use is agricultural as 70% of the total volume is reused, which is estimated to be slightly over 400 hm
3.
Technically speaking, it is feasible to close the cycle, i.e., we can treat water sufficiently to be able to drink it again. Yet for the time being, legislation does not allow this, at least not directly. Legal wastewater treatment is regulated by European legislation, namely Council Directive 91/271/EEC [
3], which has been adopted in each Member Country’s legislation and recently, in 2018, a document entitled “Proposal for a regulation of the European Parliament and of the Council on minimum requirements for water reuse” was published by European Comission [
4]. The proposal would contribute to the implementation of several other EU policies, in particular the EU climate change adaptation and disaster prevention policies and the resource-efficient Europe flagship initiative under the Europe 2020 Strategy. Council Directive 91/271/EEC is about urban wastewater treatment and protecting the environment from adverse effects of wastewater discharges. Among many other matters, this directive expects treatment data to be monitored and acquired, which will provide an overall idea of the water treatment situation in Europe. Similarly, Spanish Royal Decree 1620/2007 [
5] sets out a series of definitions to clarify the reclaimed water concept. It also deals with aspects of the legal regime. Finally, it sets out the quality conditions that regenerated water must meet to be used by indicating the permitted and prohibited uses, and responsibilities related to quality maintenance, by setting out five major use types: urban, agricultural, industrial, recreational, environmental.
Reusing wastewater for irrigation remains a controversial issue. Although reused water has remarkably increased, reuse is still far from being a representative and important water source worldwide. It is stated that only 55 countries have reliable information about wastewater production, treatment and reuse [
6]. Water reuse is essential for water supply diversification strategies in order to ensure water security in a context that is adapting to climate change in countries or basins with structural scarcity-type problems, and to also cushion pressure on aquatic ecosystems, which comes in the form of pollution from numerous chemical substances. Most of the studies where residual water is applied to irrigate agree that this type of water contains beneficial nutrients for the crops but nevertheless, it must be considered that they are generally waters that can increase soil salinity [
7,
8,
9]. Another potential problem that can be found to the use wastewater for irrigation is increasing heavy metals in soil, especially if water comes from industrial areas [
10,
11]. Finally, to achieve a safe irrigation, it is essential to have strict microbiological irrigation control. Microbiological contamination is not exclusive to wastewater [
12] and must be controlled depending on its use. In any case, the limit values to consider water suitable for irrigation in this regard depend on the legal regulations of each country and each region.
Traditionally, rain-fed conditions have been used to grow vineyards and olive groves. Nonetheless, irrigation practices have improved yields [
13]. Hence optimizing water resources is essential to properly manage both these crops. With grapevines, the life cycle assessment of wine, particularly the water footprint related to production, needs more efficient water use to either maintain or extend today’s economic markets [
14]. When freshwater is scarce in traditional olive and vineyard growing areas, alternative water sources have been employed, including reclaimed irrigation wastewater, which normally contains large amounts of minerals and organic matter [
15]. Moreover, soil salinity has increased, but does not necessarily affect crops [
16] as each rain event could reduce excess salinity by allowing soil’s ability to self-cleanse [
17]. Hence, good recycled water management as an alternative source to irrigating with conventional well water is fundamental to first obtain optimum harvest values so that farmers can maintain their adequate economic level, and second, to improve the sustainability of scarce water resources in countries like Spain or others in the Mediterranean Region within a European circular economic framework.
Agronomic TW suitability has been proven in many studies, designed and conducted under controlled conditions [
18,
19,
20]. However, experiments need to be directly carried out in the field to check whether this effectiveness can be applied to real situations. Researchers know that agronomic follow-up in these situations is a particularly complex practice because many factors must be considered when assessing outcomes. All in all, it is necessary to perform such experiments to be able to actually apply former knowledge.
Our study aims to determine differences among soils of the same vegetation type (vineyard, olive grove), but that have been irrigated with water of distinct qualities (freshwater, reclaimed water). We hypothesized that continuing irrigation on the studied agricultural land with treated wastewater would not harm soil or woody crops. The principle objective of this study was to assess soil response after being irrigated with treated wastewater for years. The aims of the present study were to: (i) initially compare the compositions of soils irrigated for years with reclaimed wastewater to others that have always been irrigated with freshwater in the same zone; (ii) monitor both soil and the applied irrigation water in two typical woody crops of the Mediterranean Region for 3 years running.
5. Conclusions
Utilization of wastewater for irrigation may be included in a fertilization strategy based on the sustainability aspect. Moreover, this type of water could be an alternative water resource.
The wastewater samples herein studied do not pose problems for being used in agricultural irrigation in relation to heavy metal contents (Al, As, Cd, Cu, Cr, Fe, Mn, Hg, Ni, Pb, Se, Zn), ecotoxicity, organic compounds like PAHs or PCBs or microbiological composition (helminth eggs: Taenia Saginata; Taenia Solium; Legionella sp and Salmonella sp).
Soils irrigated with TW versus those irrigated with conventional well water had higher values for electrical conductivity, N, K, Na, Mg, Mn and CEC. This represents an advantage for increasing the nutritional value of agricultural soil (bigger quantity of N, K, Mg and Mn) and better CEC compared to control soils. Nevertheless, the increase in Na in soil and high EC values could prove to be a long-term difficulty. Hence, the main parameter to be considered, for its potential harmful effect on soil, is a potential increase in salinity, although neither soil pH nor texture changed. All in all, the results obtained by the continuous monitoring of soil and water on agricultural plots with two different woody crops (olive grove and vineyard) are revealing as regards the agronomic aptitude of TW.
The future of water resource management policies involves sustainable wastewater reuse within a circular economy frame, because this is the only way to guarantee everyone solidarity access to water.