In Spain, and particularly in the Valencia Region, the scarcity of water resources means that water resource exploitation must be optimized. In this light, reusing the large amounts of treated wastewater is a top priority, especially in agriculture, urban use and the irrigation of golf courses. Rincón de León wastewater treatment plant–water reclamation plant (Alicante, Spain) supplies reclaimed flow to a number of users according to the guidelines stated in the Royal Decree 1620/2007. Reclamation treatment includes: coagulation + flocculation + filtration (sand bed), ultrafiltration, ultraviolet disinfection and desalination (reverse osmosis). By combining these processes, three tertiary treatment alternatives were configured, and for each of them the quality of effluents, treatment costs, energy consumption and the uses of treated water were analysed. The results show that the quality of the water treated using the three alternatives is suitable for different uses. Moreover, the costs resulting from the tertiary treatment processes, their energy consumption and the final price of the treated water paid by farmers have been obtained.

INTRODUCTION

The reuse of reclaimed wastewater in Mediterranean European countries is of increasing potential. Spain, in particular, shows the highest projected reuse potential with the likelihood of it being three to six times higher than in 2005 (Hochstrat et al. 2005a, 2005b; Bixio et al. 2006; Angelakis & Durham 2008; Iglesias et al. 2010). In Spain, the reuse of wastewater is carried out mainly at the Mediterranean coast and on the islands. Valencia and Murcia reuse 57% of all the treated wastewater while the islands (Canaries and Balearic) reuse 23% (Downward & Taylor 2007; Iglesias et al. 2010; Pedrero et al. 2010).

The growth in water reuse presents challenges as a result of inefficiencies in the legal framework. To date, there are no common supra-national regulations on water reuse in Europe. The Spanish legal framework dates back to 2007. Royal Decree 1620/2007 sets out the legal framework governing this field, including authorized and prohibited uses, as well as the quality conditions required for each use.

Notwithstanding, it is necessary to conduct a thorough analysis comparing the social and financial costs and benefits involved in water reuse (Wade Miller 2006; Molinos-Senante et al. 2010). This should, of course, be a comprehensive study not only of the inherent costs of the activity, but of the intrinsic social and environmental outputs and opportunity costs (Hernández et al. 2006). Moreover, the rise in energy costs is one of the greatest concerns, meaning that more energy efficient technologies are paramount (Bixio et al. 2008; Salgot 2008).

Among numerous treatment technologies, membrane processes are considered to be the most advanced for wastewater reclamation, of which there are a number of prominent schemes worldwide (Wintgens et al. 2005). There are many publications on the use of membrane bioreactors and membrane applications as prior tertiary treatment for the reuse of wastewater (Santos et al. 2011; Lin et al. 2012; Cartagena et al. 2013; Raffin et al. 2013; Roccaro et al. 2013; Guo et al. 2014; Young et al. 2014). However, very few provide information about full-scale facilities that integrate the use of activated sludge (organic matter removal) with microfiltration or ultrafiltration (UF) membranes (disinfection, turbidity and micropollutant removal) as well as reverse osmosis (RO) (salinity removal) (Al-Rifai et al. 2011; García et al. 2013).

Treated wastewater is most widely used for irrigation. In the final report on wastewater reuse prepared by EMWIS, the Euro-Mediterranean Information System on know-how in the Water sector (http://www.emwis.org/topics/WaterReuse/Final_report.doc), it is stated that: ‘The total volume of reused treated wastewater in Europe is 964 Mm³/yr, which accounts for 2.4% of the treated effluent. Spain accounts for largest proportion of this (347 Mm³/yr); Italy uses another 233 Mm³/yr. In both countries, agriculture absorbs most of the treated wastewater.’

State policies in watershed planning (the Júcar River Basin Plan: http://www.boe.es/boe/dias/2014/07/12/pdfs/BOE-A-2014-7371.pdf), as well as other regional policies (the Master Plan of Sanitation and Purification: www.docv.gva.es/datos/2003/10/08/pdf/2003_10716.pdf), are currently favouring water reuse as a key solution to water stress issues. Rincón de León wastewater treatment plant–water reclamation plant (WWTP-WRP), and the corresponding management model discussed in this article, are part of this strategy.

This paper presents a real case of desalinated wastewater reuse, carried out in the Rincón de León WWTP-WRP, in Alicante (Valencia Region, Spain). The objectives of the work are, on the one hand, to analyse different alternatives of tertiary treatment for wastewater reuse and, on the other hand, to define treatment costs, energy consumption and the cost recovery results.

Tertiary treatments studied include various combinations of coagulation + flocculation + filtration (CFF), UF, ultraviolet (UV) disinfection and desalination by RO.

SCENARIO ANALYSIS

Rincón de León WWTP-WRP

Rincón de León WWTP-WRP (38° 20′ 7″ N, −0° 31′ 26″ E) is one of the three treatment facilities operating in the city of Alicante (38° 20′ 43″ N, 0° 28′ 52″ W) and nearby municipalities (Figure 1). The plant is designed to treat 75,000 m3/d. The operator is EMARASA (Joint Venture Corporation for Wastewater Treatment in Alicante). During 2012 it treated an average flow of 52,644 m3/d (EPSAR 2012).

Figure 1

Rincón de León WWTP-WRP (EPSAR).

Figure 1

Rincón de León WWTP-WRP (EPSAR).

The wastewater treatment includes (Figure 2): pre-treatment (screening and grit removal and flow equalization), primary treatment (settling), secondary treatment (activated sludge), tertiary treatment (UF and RO), sludge treatment (thickened, anaerobic digestion, centrifugation and sludge storage), and cogeneration (combustion in biogas engines to obtain electricity and heat recovery).

Figure 2

Schematic flow diagram of Rincón de León WWTP-WRP (water train).

Figure 2

Schematic flow diagram of Rincón de León WWTP-WRP (water train).

Treated wastewater uses

To date, no regulation of wastewater reuse has been passed in the EU. Section 12 of the European Wastewater Directive 91/271/EEC (EU 1991) only states that: ‘Treated wastewater shall be reused whenever appropriate.’ In Spain, wastewater reuse was first regulated by an amendment in the Water Act (BOE 2001), and was fully regulated by Royal Decree 1620/2007 (BOE 2007). According to this legal framework, the quality criteria for reused water distinguish 14 different patterns of use classified under five headings: (1) urban, (2) agricultural irrigation, (3) industrial, (4) recreational and (5) environmental.

Effluent from Rincón de León WWTP-WRP is used for urban, agricultural and recreational uses. Common quality criteria for these uses are shown in Table 1.

Table 1

Quality criteria for water reuse (Royal Decree 1620/2007).

Use Suspended solids (mg/L) Turbidity (NTU) E. coli (cfu/100 mL) Intestinal nematode eggs 
Urban uses 
 Quality 1.1 Residential: (a) private garden watering; (b) discharge from sanitary appliances 10 1 egg/10 L 
 Quality 1.2 Urban services: (a) watering of urban green areas (parks, sports grounds, etc.); (b) hosing down streets; (c) fire-fighting systems; (d) industrial car wash 20 10 200 1 egg/10 L 
Agricultural uses 
 Quality 2.1: (a) irrigation of fresh food crops for human consumption, through water application systems allowing for direct contact of regenerated water with edible parts 20 10 100 1 egg/10 L 
 Quality 2.2: (a) irrigation of crops for human consumption, through water application systems without avoiding direct contact of regenerated water with edible parts, but not for consumption as fresh food since there is subsequent industrial treatment; (b) irrigation of pastureland for milk or meat-producing animals; (c) aquiculture 35 No limit set 1,000 1 egg/10 L 
 Quality 2.3: (a) localized irrigation of ligneous crops impeding contact of regenerated water with food for human consumption; (b) irrigation of ornamental flowers, greenhouses and nurseries with no direct contact of regenerated water with crops; (c) irrigation of industrial crops, greenhouses, fodders stored in silos, cereals and oleaginous seeds 35 No limit set 10,000 1 egg/10 L 
Recreational uses 
 Quality 4.1: (a) irrigation of golf courses 20 10 200 1 egg/10 L 
Use Suspended solids (mg/L) Turbidity (NTU) E. coli (cfu/100 mL) Intestinal nematode eggs 
Urban uses 
 Quality 1.1 Residential: (a) private garden watering; (b) discharge from sanitary appliances 10 1 egg/10 L 
 Quality 1.2 Urban services: (a) watering of urban green areas (parks, sports grounds, etc.); (b) hosing down streets; (c) fire-fighting systems; (d) industrial car wash 20 10 200 1 egg/10 L 
Agricultural uses 
 Quality 2.1: (a) irrigation of fresh food crops for human consumption, through water application systems allowing for direct contact of regenerated water with edible parts 20 10 100 1 egg/10 L 
 Quality 2.2: (a) irrigation of crops for human consumption, through water application systems without avoiding direct contact of regenerated water with edible parts, but not for consumption as fresh food since there is subsequent industrial treatment; (b) irrigation of pastureland for milk or meat-producing animals; (c) aquiculture 35 No limit set 1,000 1 egg/10 L 
 Quality 2.3: (a) localized irrigation of ligneous crops impeding contact of regenerated water with food for human consumption; (b) irrigation of ornamental flowers, greenhouses and nurseries with no direct contact of regenerated water with crops; (c) irrigation of industrial crops, greenhouses, fodders stored in silos, cereals and oleaginous seeds 35 No limit set 10,000 1 egg/10 L 
Recreational uses 
 Quality 4.1: (a) irrigation of golf courses 20 10 200 1 egg/10 L 

Maximum values permitted for urban, agricultural and recreational uses.

The key users of the effluent at Rincón de León WWTP-WRP are the Alicante Irrigation Association (AGRICOOP) and the High Vinalopó Irrigation Association (ARALVI). Part of the reclaimed water also irrigates the median strip of a highway and a public park (1,000 m3/d, Alicante Palm Tree Grove Park). Other users are members of the Monforte del Cid Irrigation Association; they use a mixture of wastewater (35–40%) and fresh water (part of the treated wastewater comes from WWTP-Elda). Irrigation associations hold concessions allowing them to reuse wastewater granted by the Watershed Authority.

AGRICOOP was founded as an irrigation association in early 1996. This association uses treated wastewater for agricultural irrigation as well as for watering a golf course. The total area of irrigated land is 1,104 ha. The prevailing irrigation system today is drip irrigation. The main crops are: almonds (530 ha), citrus fruits (94 ha), tomatoes (450 ha) and pomegranate and olive trees (30 ha). El Plantío golf course is 800,000 m2. The field requires spray irrigation, while the trees are drip irrigated.

ARALVI spans a number of municipalities (San Vicente del Raspeig, Mutxamel, Alcoraya, Rebolledo, Bacarot) and it also waters a golf course. The irrigated agricultural area spreads over 2,040 ha. The main crops are almonds (70%), grapes (8%), nectarines (5%), oranges (2%) and olives (1%). The remaining 15% of the land is not currently being farmed. The soil has low organic matter content, which facilitates controlling the risk of salinization and alkalinization. Alenda Golf Course has a total area of 1,331,617 m2. In summer it requires 1,500 m3/d of water.

Reused flows by ARALVI and AGRICOOP in 2012 were, respectively, 3,063,033 m3/yr and 3,467,035 m3/yr (889,728 m3/yr from water tank 1 + 2,577,307 m3/yr from water tank 2), which represents a total of 6,530,068 m3/yr. Figure 3 shows the monthly evolution of reused flows during 2012.

Figure 3

Monthly evolution of reused flows during 2012 (data provided by ARALVI and AGRICOOP).

Figure 3

Monthly evolution of reused flows during 2012 (data provided by ARALVI and AGRICOOP).

It can be seen that the summer months are those with the highest demand for water reuse, while in winter demands significantly decrease. As there is not enough storage capacity to keep a fixed pattern in the production of treated water throughout the year, tertiary treatment processes experience frequent stops. This leads to increasing maintenance costs in tertiary treatment facilities.

Tertiary treatment options

Tertiary treatment aims to achieve the quality required for reuse (BOE 2007). Current treatment got underway in summer 2006, with UF and RO. In 2010, operations incorporated an equalization tank, a CFF stage and UV disinfection.

The various uses of treated water demand different water qualities. The corresponding qualities are obtained by mixing treated water from three treatment options.

  • Alternative A: tertiary treatment is initiated in the homogenization tank (8,500 m3), which homogenizes the changes in influent quality. Water is pumped to two rapid mixing chambers where ferric chloride is dosed as coagulant (thereby improving the subsequent filtration and removing some of the dissolved phosphorus in water). The flocculated water then passes into a filtration process. There are six filtration lines, each one with a capacity of 10,000 m3/d (each line has 10 silica sand filters, a grain size of 1–2 mm and a filtration rate of 7.88 m/h). Conversion in the filtration process is 93%. Part of the filtered water (up to 8,000 m3/d) goes to a UV process, UV disinfection, and then to a mixing receptacle where it mixes with ultrafiltered water.

  • Alternative B: the remaining filtered wastewater enters three self-cleaning 500 μm filters (to protect the UF membranes). Then, water is ultrafiltered in six parallel channels with six modules, each with UF submersible hollow fibre membranes (57 Zenon UF modules ZeeWeed model 1000 V3). The specific rate of operation is 20.55 L/m2h, with a yield of 90% filtration. The maximum ultrafiltered water flow is 42,063 m3/d. Part of the treated effluent is mixed with water resulting from filtration and is supplied to irrigators.

  • Alternative C: UF water passes into five 5 μm filter cartridges (to protect the RO membranes). RO is performed in a facility configured as a double desalination stage (with booster pump between stages), reaching 73% conversion. It has five racks, quantifying 2,016 membranes (most being Dow Chemical model DOW FILMTEC(TM) BW30XFR-400/34i) with a total filtration area of 69,955 m2. A maximum flow of 25,675 m3/d of desalinated water with a conductivity of 100 μS/cm can be achieved. The osmotic water is also supplied to irrigators.

Osmotic and ultrafiltered water flows are passed to a distribution and regulation chamber. The flow of each type of water is regulated according to the conductivity conditions demanded by irrigators. Pipelines of 630 and 350 mm in diameter are used to, respectively, carry ultrafiltered water and osmotic water to several water tanks owned by the irrigators.

In Figure 4, tertiary treatment as well as the three treatment alternatives are schematically shown.

Figure 4

Tertiary treatment alternatives.

Figure 4

Tertiary treatment alternatives.

RESULTS AND DISCUSSION

Reclaimed water quality and performance of the different treatment alternatives

Tables 25, respectively, show water quality parameters from the secondary sedimentation, and from UV disinfection (alternative A), UF (alternative B) and RO (alternative C), corresponding to the monthly average data of 2012. For each parameter the average, minimum, maximum and standard deviation is indicated. These values were obtained by statistical analysis of the monthly average values supplied by EMARASA (the Joint Venture Corporation for Wastewater Treatment in Alicante) (Ordóñez 2013).

Table 2

Quality of water from the secondary sedimentation of the Rincón de León WWTP (compiled using data provided by EMARASA)

Parameter Mean Minimum value Maximum value Standard deviation 
pH 7.43 7.26 7.52 0.07 
Suspended solids, SS (mg/L) 17.0 11.3 23.8 3.8 
Conductivity 20 °C (μS/cm) 2,338 2,065 2,542 161 
Turbidity (NTU) 4.80 3.18 6.57 1.16 
Chemical oxygen demand, COD (mg/L) 52.4 42.9 63.8 7.0 
Biochemical oxygen demand, BOD (mg/L) 12.7 4.0 22.0 5.1 
Total nitrogen, (mg/L) 39.9 29.5 45.5 4.7 
Total phosphorus (mg/L) 4.6 2.3 6.1 1.1 
Chlorides (mg/L) 512 480 588 49 
Escherichia coli (cfu/100 mL) 1.8 exp + 5 2.1 exp + 4 4.0 exp + 5 1.4 exp + 5 
Parameter Mean Minimum value Maximum value Standard deviation 
pH 7.43 7.26 7.52 0.07 
Suspended solids, SS (mg/L) 17.0 11.3 23.8 3.8 
Conductivity 20 °C (μS/cm) 2,338 2,065 2,542 161 
Turbidity (NTU) 4.80 3.18 6.57 1.16 
Chemical oxygen demand, COD (mg/L) 52.4 42.9 63.8 7.0 
Biochemical oxygen demand, BOD (mg/L) 12.7 4.0 22.0 5.1 
Total nitrogen, (mg/L) 39.9 29.5 45.5 4.7 
Total phosphorus (mg/L) 4.6 2.3 6.1 1.1 
Chlorides (mg/L) 512 480 588 49 
Escherichia coli (cfu/100 mL) 1.8 exp + 5 2.1 exp + 4 4.0 exp + 5 1.4 exp + 5 
Table 3

Disinfection effluent quality (alternative treatment A = CFF + UV) (compiled using data provided by EMARASA)

Parameter Mean Minimum value Maximum value Standard deviation 
pH 7.33 7.11 7.49 0.11 
SS (mg/L) 11.3 8.3 15.7 2.30 
Conductivity 20 °C (μS/cm) – – – – 
Turbidity (NTU) 3.20 1.92 5.11 0.98 
COD (mg/L) 41.9 33.6 51.5 5.72 
BOD (mg/L) 6.9 3.0 10.0 2.71 
Total nitrogen (mg/L) 37.0 26.5 43.5 5.55 
Total phosphorus (mg/L) 4.04 2.00 5.35 1.00 
E. coli (cfu/100 mL) 73.5 6.25 138.5 42.00 
Legionella spp. (cfu/100 mL) 
Parameter Mean Minimum value Maximum value Standard deviation 
pH 7.33 7.11 7.49 0.11 
SS (mg/L) 11.3 8.3 15.7 2.30 
Conductivity 20 °C (μS/cm) – – – – 
Turbidity (NTU) 3.20 1.92 5.11 0.98 
COD (mg/L) 41.9 33.6 51.5 5.72 
BOD (mg/L) 6.9 3.0 10.0 2.71 
Total nitrogen (mg/L) 37.0 26.5 43.5 5.55 
Total phosphorus (mg/L) 4.04 2.00 5.35 1.00 
E. coli (cfu/100 mL) 73.5 6.25 138.5 42.00 
Legionella spp. (cfu/100 mL) 
Table 4

UF effluent (alternative treatment B = CFF + UF) (compiled using data provided by EMARASA)

Parameter Mean Minimum value Maximum value Standard deviation 
pH 7.37 7.19 7.48 0.09 
SS (mg/L) 0.91 0.31 2.39 0.72 
Conductivity 20 °C (μS/cm) 2,311 1,920 2,487 187.79 
Turbidity (NTU) 0.43 0.33 0.50 0.06 
COD (mg/L) 27.1 23.5 29.8 2.27 
BOD (mg/L) 3.08 1.00 7.00 1.93 
Total nitrogen (mg/L) 35.0 20.5 45.0 6.89 
Total phosphorus (mg/L) 3.54 1.70 4.95 1.03 
Chlorides (mg/L) 499 387 571 44.32 
E. coli (cfu/100 mL) 33.74 6.75 54.25 16.24 
Intestinal nematodes (eggs/L) 
Parameter Mean Minimum value Maximum value Standard deviation 
pH 7.37 7.19 7.48 0.09 
SS (mg/L) 0.91 0.31 2.39 0.72 
Conductivity 20 °C (μS/cm) 2,311 1,920 2,487 187.79 
Turbidity (NTU) 0.43 0.33 0.50 0.06 
COD (mg/L) 27.1 23.5 29.8 2.27 
BOD (mg/L) 3.08 1.00 7.00 1.93 
Total nitrogen (mg/L) 35.0 20.5 45.0 6.89 
Total phosphorus (mg/L) 3.54 1.70 4.95 1.03 
Chlorides (mg/L) 499 387 571 44.32 
E. coli (cfu/100 mL) 33.74 6.75 54.25 16.24 
Intestinal nematodes (eggs/L) 
Table 5

RO effluent (alternative treatment C = CFF + UF + RO) (compiled using data provided by EMARASA)

Parameter Mean Minimum value Maximum value Standard deviation 
pH 6.65 6.33 6.86 0.19 
SS (mg/L) 0.33 0.00 2.10 0.63 
Conductivity 20 °C (μS/cm) 57.09 39.32 76.57 13.22 
Turbidity (NTU) 0.20 0.15 0.24 0.03 
COD (mg/L) 3.43 0.80 7.30 1.75 
BOD (mg/L) 0.92 0.00 2.00 0.51 
Total nitrogen (mg/L) 3.60 1.75 8.60 2.25 
Total phosphorus (mg/L) 0.20 0.00 0.55 0.16 
Chlorides (mg/L) 15.6 11.5 22.7 3.67 
E. coli (UFC/100 mL) 
Intestinal nematodes (eggs/L) 
Legionella spp. (cfu/100 mL) 
Parameter Mean Minimum value Maximum value Standard deviation 
pH 6.65 6.33 6.86 0.19 
SS (mg/L) 0.33 0.00 2.10 0.63 
Conductivity 20 °C (μS/cm) 57.09 39.32 76.57 13.22 
Turbidity (NTU) 0.20 0.15 0.24 0.03 
COD (mg/L) 3.43 0.80 7.30 1.75 
BOD (mg/L) 0.92 0.00 2.00 0.51 
Total nitrogen (mg/L) 3.60 1.75 8.60 2.25 
Total phosphorus (mg/L) 0.20 0.00 0.55 0.16 
Chlorides (mg/L) 15.6 11.5 22.7 3.67 
E. coli (UFC/100 mL) 
Intestinal nematodes (eggs/L) 
Legionella spp. (cfu/100 mL) 

Comparing the average values of the various flows, Table 6 shows the operational performance of some parameters with the different treatments.

Table 6

Performance of the different treatments (% elimination efficiency)

Parameter Alternative A Alternative B Alternative C 
SS 33.5 94.6 98.1 
Conductivity 20 °C (μS/cm) – 1.16 97.6 
Turbidity 33.3 91.0 95.8 
COD 20.0 48.2 93.5 
BOD 45.7 75.7 92.8 
Total N 7.2 12.3 91.0 
Total P 12.2 23.0 95.6 
Chlorides – 2.5 97.0 
E. coli 99.96 99.98 100 
Parameter Alternative A Alternative B Alternative C 
SS 33.5 94.6 98.1 
Conductivity 20 °C (μS/cm) – 1.16 97.6 
Turbidity 33.3 91.0 95.8 
COD 20.0 48.2 93.5 
BOD 45.7 75.7 92.8 
Total N 7.2 12.3 91.0 
Total P 12.2 23.0 95.6 
Chlorides – 2.5 97.0 
E. coli 99.96 99.98 100 

As can be observed, with alternative A, Escherichia coli bacteria is almost completely eliminated. In addition, suspended solids (SS), biochemical oxygen demand (BOD) and turbidity are significantly reduced. Chemical oxygen demand (COD) and phosphorus concentrations are also partially reduced. Total nitrogen is reduced to a very small proportion. With alternative B, E. coli is also almost completely eliminated, whereas turbidity and SS decrease by more than 90%. A very high proportion of BOD is also reduced, with COD reducing to a lesser extent. Phosphorus and nitrogen are reduced in smaller proportions. With alternative C, E. coli is removed entirely while removal for the other parameters was over 90%.

Production costs of reclaimed water

The construction cost of the reclamation facilities amounted to a total of €20,676,893, of which €15,800,878 corresponded to the initial installation (2006): UF and RO (€10,970,550 on equipment and €4,830,328 on civil works), and the remaining €4,876,015 on the extension (2010): equalization tank, CFF and UV disinfection (€3,657,011 on equipment and €1,219,004 on civil works).

According to information provided by CADAGUA (the company that managed the tertiary treatment from the start of the operations in 2007 until February 2012), the most relevant operating costs are electricity, staff and reactants, in that order. Staff costs cannot be segregated for the different treatments. With regards to the other two concepts, it is possible to estimate the corresponding costs according to the following approximate distribution of the effluent flow in 2011: CFF = 13,000,000 m3/yr, disinfection (UV) = 2,900,000 m3/yr, UF = 9,100,000 m3/yr and RO = 3,500,000 m3/yr. The cost distribution of energy and reactants is shown in Figure 5.

Figure 5

Distribution costs of energy and reactants.

Figure 5

Distribution costs of energy and reactants.

The average energy consumption for each unit of treatment is as follows: CFF = 0.047 kWh/m3, UV = 0.056 kWh/m3, UF and RO = 0.236 kWh/m3 and 0.869 kWh/m3, respectively. Maintenance costs, overheads and business profit must be added on. As a result, the final average operation cost, taking only variable costs into account, of the effluents from each unit of treatment is: CFF = €0.0142/m3, UV = €0.0067/m3, UF = €0.0337/m3 and RO = €0.2098/m3.

Economic considerations and cost recovery

The reuse of treated water helps to increase the available amount of water resources at a relatively low marginal cost. In addition, it creates positive environmental outputs since there is no need to use fresh water. From an economic efficiency point of view, a key feature is that the treated effluent quality can be adapted to the users' needs. However, this flexibility can be lost in part when the number of users and destinations increases. In addition, destinations for treated wastewater being located closer to each other immediately leads to relevant savings in infrastructure and transport costs (Hermanowicz et al. 2001). For Rincón de León WWTP-WRP, effluent quality at an affordable price is achieved by mixing water of three different qualities and therefore three different production costs. This is a good strategy for optimizing production as long as the quality required is variable.

However, operation is strongly related to demands. Demands are communicated at short notice. Therefore, there are frequent stops and starts in operation, which makes it more expensive (it leads to damaged membranes and increased quantities of reactant for cleaning, etc.). Users would be well advised to plan their long-term needs and increase their storage structures to allow the plant to operate on a more regular basis.

The principle of cost recovery was established in the European Water Framework Directive (WFD) (Directive 2000/60/EC). Its implementation must be applied throughout, although socio-economic or physical circumstances (geographical, environmental and climatic) could mean exemptions or limit its enforcement. The water user must bear the full cost of water production, transport and distribution. Cost recovery also concerns tertiary treatment including desalination. It was introduced into the Spanish legal system by means of an amendment to the 1985 Water Act, included in Act 62/2003 (BOE 2003).

Operation costs before tertiary treatment are charged to every user of the urban water supply. For this purpose, Wastewater Treatment Regional Act 2/1992 (DOGV 1992) established a particular tax in the Valencia Region. This tax is calculated according to the total operational cost for primary and secondary treatment. Urban users must pay in accordance with the quantity of municipal water they use. It is legally assumed that every water consumer generates pollution, and hence the tax is objectively estimated and imposed on all users regardless of the actual pollution loads. The tax is included as part of the water bill together with the sewage tax and the water supply tariff. This ensures almost 100% of the revenue.

Another relevant factor to consider is the payment that the user of treated waters has to pay to be able to benefit from it. According to the agreements between EPSAR and the irrigation associations, the latter are responsible for transporting water to its destination. In 2011, a tariff of €0.124/m3 for water resulting from the mixture of UV and UF processes (alternative 1 and alternative 2) was agreed with ARALVI, and €0.165/m3 for water resulting from UF + RO processes (alternative 3). Sale prices are updated in accordance with the consumer price index. In 2013, the price of desalinated water was €0.19/m3. However, including energy, staff, transportation and infrastructure costs (€0.17/m3), ARALVI farmers are charged a total price of €0.36/m3.

AGRICOOP users pay €0.28/m3 on average for drip irrigated land (60% of the total irrigated land) and €0.23/m3 for flood irrigated land (40%).

The price is affordable for farmers, even though they have to pay a substantially higher amount than the average charged for surface water or groundwater for agricultural use in Spain. Water stress makes cheaper water resources unavailable, and therefore makes wastewater reuse financially sustainable and its prices acceptable for users. Given that the cost of tertiary treatment, transportation and distribution is directly charged to the farmers, it can be assumed that the system meets all the requirements of the WFD full recovery cost principle. The costs of wastewater treatment prior to tertiary treatment are obviously charged to urban consumers, who are in fact the pollutant agents.

CONCLUSIONS

Tertiary treatment in Rincon de León WWTP-WRP comprises three alternatives: alternative A = CFF + UV, alternative B = CFF + UF and alternative C = CFF + UF + RO. Treated water is used for urban uses, agricultural irrigation and golf course irrigation. With reference to the parameters considered in Spanish law (Royal Decree 1620/2007), the results allow us to conclude that the quality of water treated with alternative C is suitable for all uses referred to in this study, i.e., urban uses (residential and urban services), agricultural irrigation (all agricultural uses) and golf course irrigation (recreational use). On the other hand, water treated with alternative B is suitable for all applications except for residential, while water treated with alternative A is suitable for all uses except for residential and irrigation of fresh food for human consumption. Nevertheless, drinking water use is strictly forbidden under Spanish law.

Regarding energy consumption, the unitary process that requires most energy is RO (0.869 kWh/m3). It represents more than triple that of UF (0.236 kWh/m3), which is the second largest consumer. The CFF method is the unitary process that demands least power (0.047 kWh/m3).

In terms of variable production costs (2012), the RO process is the highest (€0.2098/m3), around 600% higher than the second most expensive process, which is UF (€0.0337/m3). UV disinfection is the lowest cost unitary process (€0.0067/m3).

The price finally charged to ARALVI farmers in 2013, including the cost of reclaimed desalinated water (€0.19/m3), as well as energy, staff and infrastructure costs (€0.17/m3), amounts to €0.36/m3. The price finally charged to AGRICOOP farmers in 2013 amounts to €0.28/m3 on average for drip irrigated land and €0.23/m3 for flood irrigated land.

The total volume of reused water supplied from Rincon de León WWTP-WRP in 2012, for agriculture and golf course irrigation, exceeded 6 million cubic metres.

ACKNOWLEDGEMENTS

This study was partially financed by the Ministry of Education via the projects ‘Treatment of superficial water and wastewater by membrane technologies to obtain high quality effluents' (CTM2010-15348) and ‘Treatment and wastewater reuse for a sustainable management’ (CONSOLIDER) (CSD200644), as well as by the Ministry of Science and Innovation via the project ‘Quality of Aquifers and agricultural impacts' (DER2011-27765), and the Council for Education, Formation and Occupation of the Government of Valencia (ACOMP 2012/136). We wish to thank the companies EMARASA and CADAGUA, and in particular Ms Barbara Escalante Sánchez, for the valuable information provided about the operation of the plant and operating costs.

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