WATER PURIFICATION-DESALINATION WITH MEMBRANE TECHNOLOGY SUPPLIED WITH RENEWABLE ENERGY
Massimo Pizzichini, Claudio Russo
Italian National Agency for New Technology Energy and the Environment
C.R. Casaccia, Via Anguillarese 301, 00060 Roma Italy
Fax +39630483327. e.mail
Abstract
Chronic water shortages, water pollution and growing economies all over the word drive many Countries to consider water purification and desalination as a solution to their water supply problems.
Membrane separation technologies offer a large spectrum of applications for water purification, particularly in order to maximise the purified water recovery. Technologies such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are largely applied all over the world for wastewaters purification and for seawater potabilization.
The renewable energy sources (RES), such as photovoltaic (PV) and wind turbine (WT) are important to drive membrane technology toward recovery of purified water from polluted fresh water sources or from brackish and seawater.
The energy required for water purification depends on the chemical composition of feed water and, consequently on the purification technologies suitable to reach the final water specification.
For example the potabilization of freshwater polluted from pathogen us bacteria can be obtained with MF-UF that requires low energy consumption (1.2 kWh/m3).
On the contrary, the potabilization of seawater requires a devoted plant (pre-treatment section, RO unit, refining section) with energy consumption in the range of 3-15 kWh/m3.
RO offers the chance to treat wastewaters, brackish water and seawater in small (< 500 m3/day) and big size plants (>100.000 m3/day). The energy required for water purification by RO depends on salts content of feed water, and plant size. For a seawater desalination process, the energy consumption ranges between 15 and 3 kWh/m3 respectively for small and big plants.
PV energy system requires a high investment cost (60% of the total installation) and, consequently, a high cost of energy production, about 0,5 €/kWh. The WT electricity generation presents lower investment costs in comparison with the PV system and can provide the energy needed by small and medium RO desalination plants. In several areas the price of WT is lower than those obtained using an electric grid source.
This paper reports a comparison analysis of energy costs for 1 m3 of potable water, obtained with different renewable energy sources. In order to optimise the performance of the membrane system a continuous energy source must be guarantee through an energy storage apparatus, or different energy generation systems (diesel, grid connection, etc.).
1. Introduction
Seawater, which represents 97% of the total water amount, is too salty for agriculture, industry and municipal uses (1). Most of the remainder is locked up either in polar ice or in inaccessible deep ground water, leaving less than 0.5% to meet the needs of human population that now exceeds 6 billion people. The critical level for water resources is estimated to be 1.000 m3/head/year. At this level a country has sufficient water resources to maintain its agricultural development, but could be exposed to a critical situation in adverse weather conditions causing drought over several agricultural seasons. Irrigated agriculture is by far the leading use of water, exceeding 90% in some countries and averaging 86% across the Middle East and North Africa (MENA) (2,3).
In many parts of the world, water resources are insufficient to meet local demand mainly for two reasons: water resource reduction and population increase, as reported in figure 1.
Figure 1 Available water resources as function of time and population increase
Half of this critical level, 500 m3/head/y, is considered the level at which a country is obliged to limit its agricultural development, in order to avoid substantial problems in providing water for domestic and industrial uses. In many countries of MENA water shortage is worrying especially for the future, as reported in table 1.
Table1. Water availability in some MENA countries (FAO data)
Countries / Water availability(m3/head/y)
1960 / 1990 / 2025
Morocco / 2.650 / 1.185 / 651
Algeria / 1.704 / 737 / 354
Tunisia / 1.036 / 532 / 319
Libya / 538 / 154 / 55
Egypt / 2.251 / 1.112 / 645
Malta / 100 / 75 / 75
Israel / 1.024 / 467 / 311
Jordanian / 529 / 224 / 91
Lebanon / 2.000 / 1.407 / 809
Syria / 1.196 / 439 / 161
Saudi Arabia / 537 / 156 / 49
Emirates / 3.000 / 189 / 113
Yemen / 481 / 214 / 72
Oman / 4.000 / 1.333 / 421
Qatar / n.d. / n.d. / n.d.
Iraq / 14.706 / 5.285 / 2.000
Iran / 5.788 / 2.152 / 1.032
In order to have sufficient water supply it is necessary to recycle all types of wastewater, but for human needs, in particular for potabilization, the choice of desalination of seawater seems to be the best solution. Membrane separation technologies offer a large spectrum of applications for water purification, particularly in order to maximise the purified water recovery. Technologies such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are largely applied all over the world for wastewaters purification and for seawater potabilization. In this field, RO seems to be economically competitive, with seawater at medium salts content, in comparison with traditional thermal processes based on evaporation and condensation.
In table 2 the world’s water production by thermal processes and RO is reported (4).
Table 2. Seawater desalination technologies
Country
/ Installedpotentiality
(10³ m³/day) / World
percentage (%) / Thermal process
(%) / RO
(%)
Saudi Arabia / 5.253 / 20,3 / 66 / 32,3
USA / 4.328 / 16,7 / 11 / 74,5
Emirates / 2.891 / 11,2 / 94 / 5,5
Kuwait / 1.615 / 6,2 / 96,5 / 3,3
Spain / 1.234 / 4,8 / 8,5 / 84,3
Japan / 945 / 3,6 / 6,2 / 85,9
Libya / 701 / 2,7 / 75,7 / 15,9
Italy / 581 / 2,2 / 63,8 / 21,7
Qatar / 573 / 2,2 / 98,3 / 1,8
Bahrain / 473 / 1,8 / 71 / 26,9
Oman / 378 / 1,5 / 92,3 / 7,6
Over 43% of world water production from seawater is concentrated in the Persian Gulf Countries, having large availability of fossil fuels and a total number of users of about 30 millions people.
The market of RO is rapidly increasing in the last decade. In figure 2 is reported a comparison of installed desalination plants in the Mediterranean Basin. RO plants show a higher rate of installation in comparison with thermal technologies (MSF and VC).
Figure.2. Production capacities (103 m3/day) of different plants in operation since 1970
2. Membrane technologies
Membrane technologies are based on the particular use of special semi-permeable filters (membranes) in which the feed tangentially sweeps powered by a high-pressure pump. The filtered stream is called "permeate". The unfiltered one is called "concentrate" or "retentate", because it carries all the elements rejected by the membrane. The process is called "cross flow" or "tangential flow." because feed and concentrate flow parallel to the membrane instead of perpendicular. Depending on the membrane pores dimension, cross flow filters are effective in the range of RO, NF, UF and MF. Cross flow membrane filtration allows continuous removal of contaminants, which under normal filtration would very rapidly "blind" (cover up) or plug the membrane pores. MF membranes are specially used for separation of suspended solids and bacteria; UF membrane to remove water-soluble macromolecules such as proteins, in the molecular weight range between 1.0 and 60 kdalton (5). NF membranes are specialised in the retention of bivalent ions, and RO membranes are suitable to remove all kind of water solutes, especially mineral salts such as sodium chloride. Membrane costs have reduced of more than 10% per year in the past 20 years, and are expected to diminish of about 29% within the next five years. Table 3 reports the energy consumption from an electric grid necessary to supply different membrane processes operating on wastewater feed solutions with low salts concentration (MF,UF,NF) and salts concentrations in the range of 15-30 g/l (RO).
Table 3. Energy consumption of membrane processes for wastewater treatment
MembraneProcess / Operative Pressure
(bar) / Energy Consumption
(kWh/m3) / Energy cost
(€/m3 )*
MF / 1-2 / 1.0 / 0.088
UF / 3-5 / 2.0 / 0.176
NF / 15-20 / 3.2 / 0.281
OI / 30-60 / 7.2 / 0.634
* 1kW from grid=0.088 Euro
Nowadays the energy cost to obtain potable water from seawater with RO is about 0.6 €/m3. This cost, obtained using the electric grid, is lower than mean water price now present in the European Countries, which is estimated in 2.5 €/m3.
3. RO desalination process
Reverse osmosis, also known as hyper filtration, is used to purify water and remove salts and other impurities in order to improve the colour, taste or properties of the water. The most common use of RO is the production of purified water.
RO uses a semi-permeable membrane, allowing the fluid to be purified to pass through it, rejecting the contaminants. RO process requires a high driving force to push the fluid through the membrane. The most common force used is pressure provided by a pump.
The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions carrying a charge, such as salts, are more likely rejected by the membrane than those without charges, such as organic molecules. The larger the charge and the larger the particle, more likely it will be rejected, but the risk of membrane fouling should be always considered.
For seawater potabilization, RO requires to operate at high pressure in order to counterbalance and exceed the osmotic pressure. RO membrane selection depends on the quality and chemical composition of the row water being processed (6).
Seawater desalination process with RO is strongly conditioned by two parameters: temperature and water salinity. High temperatures (30-40°C) increase the productivity. On the contrary, a high water salinity feed, which does not affect the thermal desalination process, requires more energy consumption to obtain the same permeate salts concentration. In RO special attention must be devoted not only to sodium chloride concentration, but also to other salts like carbonates, sulphates, silica, etc. responsible of membrane fouling.
Desalination of seawater with RO requires the optimisation of three main sections of the plant: seawater pre-treatment, RO process section, and the final water refining focused to meet the potability specifications. Figure 3 reports a layout of the desalination process.
Figure 3. Layout of a RO desalination process
Generally seawater pre-treatment consists in mechanical prefilters coupled with sandy filters to remove bacteria, algae, colloidal and particular matters. Special chemical reagents reduce the formation of insoluble salts on the membrane surface (7). UF processes replace conveniently both prefiltration system and antiscalant chemicals (chloride, sodium bisulphate, inorganic flocculants, scale inhibitor, etc.). In addition UF process reduces chemicals costs and increases RO performance and durance (8). The refining section requires a microbial sterilization with ozone or with UV ray. It’ better to avoid chloride dosages for the formation of toxic compounds as alometane. Water recovery (percentage of permeate volume against feed volume) in RO processes, has to be maintained at a level of 30-35% for each membrane units of the plant. This condition minimizes membrane fouling and consequently facilitates membrane cleaning. Low-pressure membranes are used to produce potable water from brackish water, with a total dissolved solid (TDS) concentration < 10 g/l. High pressure RO membranes are required for the production of drinking water, if the TDS value is higher than 10 g/l. For seawater, the TDS is about 35 g/l, consequently is needed an operative pressure at least of 65 bar. The most important parameter, directly influencing energy consumption of RO plants, is the feed pressure. The permeate production (amount of produced potable water per hour) strongly depends on feed pressure, which generally ranges between 45 and 65 bar. The energy required by RO plants ranges between 7-12 kWh/m3 of potable water produced, for small size plants (lower than 500 m3/day), and 2-3 kWh/m3 for big plants with more than 100,000 m3/day. To lower the cost investments, energy can be recovered by pressure-exchange systems (turbines) installed in the retentate stream. This solution allows reducing the energy consumption of about 50%. Moreover the turbines are only in use for large RO seawater desalination plants (9).
4. Renewable energies
Renewable energy sources offer the opportunity to supply the energy needed by membrane technologies in remote areas not served by electric grid (10). This solution allows supplying fresh water for small communities located in arid regions or in islands, in which underground water is not easily available, electric grid and petrol resources are absent.
4.1 Photovoltaic energy
PV energy depends on climatic conditions, as irradiation, which favours the production, and temperature, which reduces it. In the Mediterranean area, PV systems with a nominal power of 1 kW potentially produce 1800 kWh/year, with a real power of 1500 kWh/year. That means, in 20 years of panel life, a fossil fuel reduction of about 7.500 kg.
In terms of water production with PV, about 500 m2 of panels, like a roof surface, provide 40 kW for 7 hours. This power, considering an energy consume of 7 kWh/m3 of water produced, can supply a RO plant that produces 40 m3, sufficient for a body of users of 265 inhabitants which consume 150 L/day. In general the PV energy cost remain in the range of 0.36-0.5 €/kWh.
However PV system is yet too expensive in comparison with other renewable sources, but often it is the only available source (11,12) to supply potable water to populations in arid Regions.
Using direct current, instead of alternative current, the energy efficiency increases and consequently the desalination process cost is reduced.
The connection of PV, which produces energy during the day, with the electric grid, if present, or with other energy sources (diesel generator and battery storage) for the supply during the night, allows managing RO technologies in a better way. Using PV systems for the energy production, gives other advantages:
§ PV doesn’t produce pollutant substances, in particular greenhouse gases