If conventional resources are not usually available to cover excess or peak demands, non-conventional resources are employed. Among natural non-conventional water resources several possibilities are considered, but the usual ones are runoff water, natural brackish water, or desalinated seawater (Georgopoulou et al. 2001). Non-conventional water resources coming from the anthropic water cycle can also be used; reclaimed wastewater will be considered (Asano & Levine 1998).
In the Mediterranean basin, a lot of streams carry water only after rain events; this is due to high rain intensity, the small size of coastal basins, and the imperviousness of important parts of the basin. Due to the disappearance of coastal lagoons, marsh and wetlands (the natural ways to control excess flows), and sometimes to the transformation of free-flowing streams in pipeline systems, there is an increase in the quantity of runoff per unit time.
Because this runoff is concentrated in short periods of time and does not recharge groundwater at all, it can be used to increase water supplies if appropriate treatments are performed. This is the case, for example, for Palma de Majorca, where occasional runoff is collected, treated and distributed as tap water for the city (Terrassa & Cadenas 2000). There is an economic consideration here: is it worth building treatment infrastructures for a scarce amount of resources? The answer is not easy and demands good economic assessments.
When there is sufficient space to install constructed wetlands using marginal lands or areas without any specific use such as motorway surroundings, rainwater can be diverted to such areas until it percolates and recharges groundwater using the soil/subsoil system (vadose zone) for additional treatment. In other cases, small or medium-sized storage pools or dams can be used to retain such waters if there is enough space.
If there is no other water available, water containing high amounts of salts can be used. The salt contents can be eliminated using several technologies, but mainly membrane-related ones (Medina 2000). Several crops can use waters containing certain amounts of salts during the growing period. Sometimes this improves the quality of the crop, because it forces the plant to produce more sugar or other substances.
Apart from evaporation, which consumes excessive amounts of energy for Mediterranean standards, reverse osmosis (RO) and electrodialysis reversal (EDR) are being used nowadays for water desalination. RO is preferred when using seawater as affluent, while for salt contents up to 10 g/l EDR seems to be the best technology. As the cost of water desalination could exert a strong influence on the choice of technology even at the planning stage, decision-support systems should be employed in order to choose the best system for specific circumstances.
Water from RO cannot be used directly, but needs to be mixed with other water containing salts or directly with salts (e.g. lime). The RO water can be assimilated with distilled water, which is not adequate for direct human consumption or for irrigation. There is an additional problem in relation to RO and other desalting technologies, which is the salt generated during the separation processes. If salt is in a solid state the problem is not so complex; nevertheless, the brine generated by EDR must be disposed of. The usual destination has been the sea, through outfalls; however, there is increasing concern about the impact of such brines in the sea biotope, because it seems that Posidonia and other components of the marine ecosystem suffer from this excess of salts.
When working with integrated management of water in areas where desalination is being practised, reuse is an imperative because after the first use water has still a low content of salts, and can be easily employed after advanced treatment for irrigation. This is a way to spare energy and resources.
Reclaimed wastewater is the most commonly used non-conventional water resource. As wastewater reclamation and reuse have been studied extensively since the 1920s, and great amounts of treated wastewater are usually available at the coastline and in inland towns and industries, reclaimed wastewater should be fully studied and employed.
Reclaimed water is defined as wastewater purposely treated for reuse. However, it seems that nowadays 'water recycling' is the preferred term for generic water reclamation and use in view of the acceptance and success of other urban recycling programmes (AATSE 2004). As with the term 'biosolids' for sludge to be reclaimed, 'water recycling' could be a marketing definition, because of the disappearance of the term 'waste' and the appearance of the term 'recycling', which has positive connotations.
Apart from the conventional wastewater from urban areas, which consists mainly of domestic wastewater with small amounts of industrial waste-water, there is the possibility to separate different types of wastewaters at their source: the houses. Ledin et al. (2001) describe the possibility to reuse/infiltrate so-called 'diluted', 'light' or 'grey' wastewater. All three terms refer to wastewater produced in households, office buildings and schools as well as some types of industries, where there is no contribution from toilets or heavily polluted process water. Grey wastewater is waste-water from baths, showers, hand-basins, washing machines and dishwashers, laundries and kitchen sinks. This type of wastewater has been estimated to account for about 73% of the volume of combined residential sewage. In general terms, grey wastewater has lower concentrations of organic matter, some nutrients (e.g. nitrogen) and microorganisms than combined wastewater. However, the concentration of phosphorus, heavy metals and xenobiotic organic pollutants are around the same level.
Other non-conventional resources are water transported through non-classical methods (ships, railways), and water obtained from condensation; usually, these resources will not offer significant amounts of water and just serve for specific situations (see Table 1), where localized problems can be solved in this way.
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