La dessalinització. The Tordera desalination plant

Transcription

1 La dessalinització The Tordera desalination plant 2002

2 Tordera ITAM Installation of Treatment of Marine Water of the Tordera Antecedents: description of the Tordera delta and aquifer exploitation The river Tordera, which is approximately 65 km long, flows into the sea between Blanes and Malgrat de Mar. Along its course, the river collects water from numerous torrents and brooks, amongst which those of Arbúcies and Santa Coloma are worth mentioning. The mouth of the Tordera is characterised by a delta covering an area of approximately 8 km 2. The Tordera Delta reaches a maximum width of 6,400 metres at the coast and a length of almost 3,000 metres along the river axis.

3 Description of the different types of exploitation of the Tordera aquifer Urban uses There are currently three different local authorities that supply drinking water by exploiting wells in the Tordera aquifer: 1. The drinking-water plant of Palafolls, managed by El Maresme Regional Council, which supplies drinking water to the municipalities of El Maresme North. 2. The drinking-water plant of Tossa-Lloret, managed by the Costa Brava Consortium, which supplies drinking water to the villages of Tossa and Lloret. 3. The Blanes Town Council, using a purifying plant run by the company Aigües de Blanes, supplies water to its own municipality. The volume of water supplied to the aforementioned towns for domestic use is around 25 hm 3 a year. The volume of water, far from being distributed uniformly throughout the year, has a marked seasonal nature. That is, it varies greatly depending on the time of year under analysis. Industrial uses Unlike urban uses, consumption for industrial uses is largely stable throughout the year, varying only during holiday seasons. Consumption for industrial use is around 7 hm 3 per year. Tordera ITAM

4 Tordera ITAM Thus, in comparison, the volume of water consumed by domestic use is far greater than that consumed by industry. Comparison of industrial and domestic use Agricultural uses In terms of agricultural uses, the volume of water consumed is 9hm3/ year and, just as with domestic use, is also characterised by clear seasonal changes, coinciding with the traditional irrigation periods of the area s crops. Reasons for and repercussions of desalination plant construction Causes of aquifer overexploitation Up to now, all the water required to meet the demand for urban, industrial and agricultural use has been extracted from the Tordera aquifer by wells. If only the most important of these wells are plotted, the points form a constellation that clearly illustrates the extent to which the Tordera aquifer is being exploited

5 The contribution of the desalination plant The Tordera desalination plant will supply an annual volume of 10 hm3 in it is initial phase, although it is calculated to reach up to 20 hm3/year in subsequent phases. These 10 hm3/year will be distributed between the three existing drinking-water plants, so that the basic needs can be covered. Thus the Alt Maresme drinking-water plant will receive 5.5 hm 3, the Tossa-Lloret plant 2.5 hm³ and the Blanes plant 2 hm 3 a year. The desalinated water is distributed to the treatment plants through pipes that go from the desalination plant itself to the treated-water tanks at each of the drinking-water plants. From there the water is pumped to the different municipalities through the existing upstream distribution network. Tordera ITAM

6 Tordera ITAM Causes and effects on the aquifer Evolution simulation of the aquifer with the desalination plant in operation shows that, although it does not stop the saltwater intrusion of the aquifer completely on the short term, on the medium term the saltwater wedge does stabilise and its progression inland halted. In this case, the 10 hm³ from the desalination plant, and not from the aquifer, mean that more remains in the aquifer to counteract the effects of the seawater wedge, thus halting its progress. The photos show the evolution of the salinity of the aquifer over 5 years under the conditions prior to the construction of the desalination plant and from a starting point coinciding with the autumn of That is, they show how the Tordera Delta would have evolved if the desalination plant had not been built. The results obtained show how the seawater wedge would have progressed, with the consequent degradation of the delta and the salinisation of the drinking-water catchment wells. This situation would have clearly been unsustainable. Thus, the desalination plant is a key point for the future management programme for the recovery of the Tordera Delta and constitutes a step forward in achieving the sustainable exploitation of the area s water resources.

7 Supply locations and systems Municipalities supplied El Maresme North: Arenys de Mar, Arenys de Munt, Calella, Canet de Mar, Malgrat de Mar, Palafolls, Pineda de Mar, Sant Cebrià de Vallalta, Sant Iscle de Villalta, Sant Pol de Mar, Santa Susanna. Southern Costa Brava: Blanes, Tossa de Mar, Lloret de Mar. Tordera ITAM

8 Desalination Desalination Desalination is a process that enables most of the salts present in seawater to be separated from it to produce fresh water of optimum quality, fit for human consumption. This process can be carried out by means of various technologies, but in the Mediterranean, the process most commonly used is reverse osmosis. The process of treatment of the desalinisation has the following phases 1. Water inflow 2. Inlet tank and chlorination 3. Pre-treatment and filtering 4. The process of osmosis 5. Final treatment 6. Treatted water outflow Water inflow Catchment wells The desalination process begins with seawater catchment. Using ten deep wells located parallel to the coastline, seawater, not fresh water, is collected from the aquifer. The wells are approximately 150 metres deep. The seawater enters through the permeable pipe located between 100 and 150 m deep and climbs to a height of about sea level, to the entrance of water-filling units. This makes it unnecessary that the pump be located at a depth of 100 metres. (1) The first 100 metres are covered by an impermeable tube which prevents fresh water from being collected. (2) The last 50 metres are in a marine water area. The impermeable coating is slotted to allow the water to pass through whilst retaining solid particles. These particles thus act as the first filter, which improves the water quality. Once the pumps begin to pump water to the desalination plant, the level in the wells drops according to the volume of flow extracted. For this reason the pumps are installed at a depth of 40 metres.

9 Desalination Pressure pipes The water is collected from the wells in two 600 mm pressure pipes; each one collects the water from 5 wells. These pipes go to the well control house. From there, two 800 mm pipes - buried under the levee on the left bank of the Tordera - run approximately 2 km to the seawater inlet tank in the desalination plant. This point marks the beginning of the actual desalination process. Inlet tank and chlorination The seawater inlet tank has a capacity of 1,000 m 3. The demand in stage one is a daily volume of flow 64,000 m 3 and 128,000 m 3 in stage two. In this tank, an initial chlorination of the water takes place to ensure that there is no growth of bacteria as the water passes through the plant that could impede the plant s operation.

10 Desalination Pre-treatment and filtering From the inlet tank, the water is pumped to pre-treatment. Pretreatment is an essential part of the process of osmosis membrane desalination. Semipermeable osmosis membranes require specific physical and chemical water conditions in order to function correctly. Sand Filters: once other chemicals have been added to the water to adjust certain parameters, the water goes through sand filters. In stage one, four sand filters were installed, which would later be increased to 8 in the extension phase. Cartridge filters: the water then goes through microfilters that offer finer filtration than the sand filters, with a grade filtration of 20 absolute microns. In this first stage, three units are installed parallel to each other, and two more additions are planned. Each filter contains 12 cartridges, each 1,524 mm long. Control Equipment: At the end of the pretreatment several control instruments are installed, which ensure that on entering the osmosis process the water has suitable qualities. The redox potential indicates the oxidation power of the water. This oxidation power must be controlled so that it does not affect the membranes.

11 Osmosis process High-pressure pumping The osmosis process basically consists of pumping water at high pressure against a semi-permeable membrane, thus retaining the salts dissolved in the water. In this first phase of the project, the high-pressure pumping equipment is made up of 5 turbo pumps in this first stage, with another four to be added in the expansion phase. The turbo pumps increase the water pressure to 70 kg/cm 2. This is the equivalent of a 700-metre-high water column. Each pump supplies a flow rate of 667 m 3 /h with a differential pressure of 68 kg/cm 2. The input power is 1,482.8 kw with an output of 85%. Turbo pumps Membranes Membranes The turbo pumps inject pressured water into the membrane racks. There are currently four racks installed. Each rack consists of 80 membrane modules, each module formed by 7 membrane packs. The membranes are rolled in a spiral around a central pipe. Each pack consists of a rectangular sheet of semi-permeable membrane, folded in half so that the active layer faces outwards. A sheet of fabric is placed between the two halves to collect the osmosed water that crosses the membrane and to carry it to the central collection pipe. Desalination

12 Desalination The water pumped from the turbo pump to the membrane racks divides into two: osmosed water, which is that which crosses the membranes and contains a minimum of salts, and the rejected water, which does not cross the membrane and has a greater concentration of salts. The proportion of water product to pumped seawater is approximately 45%. Energy recovery The reject water leaves the racks at a slightly lower pressure than when it entered the membranes. This means that it has a high energy content, which can be recovered. This energy recovery means reduced energy consumption. Energy recovery takes place physically with the installation of turbines. In the case of the Tordera desalination plant, 5 Pelton turbines have been installed, integrated with the high-pressure split casing pumps. In this type of turbine, also called impulse turbines, the pressurized energy of the rejected water is converted into kinetic energy in the form of a high-speed jet of water. The roller connects to the shaft of the high-pressure pump, reducing the energy required to pump. Altogether, 3.06 kwh are required for every m 3 of osmosed water. The reject water, which has lost almost all its energy, is sent to a tank with a capacity of 340 m 3 and from there to the seabed via an ocean outfall.

13 Final treatment The water that crosses the osmosis membranes must undergo a treatment process to ensure that it is fit for human consumption. The osmosed water has a low ph, which means that it is slightly acid. Furthermore, its mineral content is insufficient for it to be considered drinking water. For this reason, lime and carbon dioxide are added to the osmosed water to modify its hardness and acidity and make it drinkable. Finally, the water runs into the product water tank. Here the drinking water is chlorinated again to ensure that it remains disinfected on its journey to the tap. This action guarantees the quantity and quality of the supply to the municipalities of Alt Maresme and southern Costa Brava. Treated water outflow Distribution to drinking-water plants (ETAP) The drinking water that leaves the desalination plant is distributed to the mains supply of El Maresme North and southern Costa Brava through the stations. Desalination

14 Desalination To balance a variable daily demand with a continuous daily production, new regulation tanks with a capacity of 18,000 and 9,000 m 3, respectively, have been built at the entrance to the Palafolls and Tossa-Lloret drinking-water plants. These regulation tanks enable any excess to be stored when demand is low and to be supplied when demand exceeds production. Water quality The limits for the different parameters that characterise drinking water are included in Royal Decree 1138/1990 of 14 September, entitled Technical-Sanitary Regulation (RTS), on the supply and quality control of public drinking water. The main difference between the composition of seawater and that of drinking water is its salt content. Thus, drinking water is obtained by extracting the salt from seawater. The water extracted from catchment wells is seawater. Some of the characteristics of this water are its conductivity, at around 50,000 microsiemens per centimetre (µs/cm), a concentration of chlorides of 23,000 milligrams per litre (mg/l) and a sodium concentration of 12,000 mg/l. The maximum concentrations of chlorides and sodium stated in the RTS are of 200 mg/l and 150 mg/l, respectively.

15 Desalination Comparison of mean values for the desalination plant (September and October 2002) with supply wells in RTS: Technical-Sanitary Regulation The desalination plant reduces the conductivity to 520µS/cm, the concentration of chlorides to 115 mg/l and the sodium concentration to 50 mg/l, complying with the maximum values defined in the Technical- Sanitary Regulation and producing water of excellent quality.

16 Osmosis Osmosis Osmosis is a natural process in which a liquid passes across a semipermeable membrane that allows a larger proportion of solvent than solute to pass through. Imagine a receptacle containing two solutions with the same constituents, formed by a solvent such as water and a solute such as salt, with a greater concentration of salt in the right half than in the left. When these solutions come into contact with each other, a phenomenon called diffusion takes place, which equals the concentration of the two solutions. This means that the salt diffuses from the stronger solution to the weaker solution, while the water does the same in the opposite direction.

17 Osmosis Now imagine that we separate the two solutions with a membrane that lets only the water through. The water will continue to pass from the weaker solution to the stronger solution. This leads to a rise in the water level, which will stop when the pressure generated by the increase in the level counteracts the pressure that causes the water to cross the membrane. If the weaker solution is in fact pure water, the difference in height will be what is called osmotic pressure. Reverse osmosis consists in inverting this process. If we apply pressure greater than the osmotic pressure, the diffusion of water will take place in the opposite direction and the salt will still be unable to cross the membrane. This way, the stronger solution becomes more concentrated and the weaker solution more diluted. This basic process is followed in all reverse osmosis desalination plants. In the industrial process, a pump sends salt water against a semipermeable membrane, maintaining a constant pressure. Part of the water crosses the membrane along with a small quantity of salts, and another part containing a much larger quantity is rejected. The rejected water leaves the process at high pressure, and therefore with high energy content, which is recovered by a turbine.