This is the old United Nations University website. Visit the new site at http://unu.edu


Contents - Previous - Next


3.4 Experimental seawater reverse-osmosis desalination


3.4.1 Background

As has been indicated, Kuwait has been producing fresh water by distilling seawater since the 1950s. The multi-stage flash desalting process, which has been used exclusively in the Arabian peninsula, has proved to be very simple and reliable, but it requires extensive materials and energy. The MSF system reached its maturity with very few improvements. It seems, however, that the race for the second generation of seawater desalters will be won by reverse osmosis and low-temperature multi-effect horizontal-tube evaporators (Darwish and Jawad 1989a). Both systems are characterized by their low energy requirements, as compared with MSF. Energy consumption is the largest single cost item in desalination. Intensive efforts were made in the 1980s to evaluate the feasibility of RO desalination of seawater, including the installation of a pilot RO plant in Doha, where a cost analysis was made to compare the costs of the experimental RO and the existing MSF systems.

3.4.2 The Doha experimental RO plant

This pilot plant, with an installed capacity of 3,000 m per day, was installed in 1984 to evaluate membrane and operating systems. It has three lines, equipped with different types of modules, namely spiral wound, hollow fine FIBRE, and plate-frame (Darwish and Jawad 1989b):

  1. line 1: first stage, spiral-wound (UOP-PA 1501); second stage, spiral-wound (UOP-PA 8600);
  2. line 2: first stage, hollow fine fibre (Dupont B10); second stage, hollow fine fibre (Dupont B9);
  3. line 3: first stage, plate and frame (Enro + Scheicher & Schuell Film Tec.); second stage, spiral-wound (Hydronautics 8040B).

The feed seawater to the RO plant usually contains high concentrations of inorganic salts and foreign materials that can foul membranes and decrease their productivity. The main foulants associated with feed seawater are due to biological slime formation, suspended solids, colloids, metal oxide, and scale formation. Pre-treatment is essential to control the life of the membranes. Different methods of conventional pre-treatment were examined in each line.

Since the beginning of the plant's operation, pre-treatment has been running satisfactorily, with an availability of more than 96%. Most of the time, it has been successfully controlled to give a silt density index of less than 4, but in some cases it has failed to produce an acceptable quality, owing to clogging of the dual-media filters, absence of or overdosing with FeClSO4, breakdown of the destabilizer mixer, and climatic conditions such as temperature, dust storms, and wind.

3.4.3 Cost evaluation

The cost effectiveness of the membrane (RO) process can be assessed by comparison with the cost of the predominant thermal (MSF) process. These costs have two major components: (1) direct capital cost, and (2) operation and maintenance costs. The cost of equipment forms a major part of the capital cost, while the cost of the energy and chemicals consumed forms a major part of the operating and maintenance cost.

An evaluation was undertaken to compare the cost per unit of water produced by large-scale MSF and RO plants of typical design, each with an installed capacity of 27,300 m (6 mig) per day. The feed water is assumed to be of the quality of seawater in the Arabian Gulf, with concentrations per litre of 45,000 mg of TDS, 800 mg of Ca++, 1,700 mg of Mg++, 12,500 mg of Na+, 500 mg of K+, 3,600 mg of SO4--, 24 mg of CO3--, 24,000 mg of Cl-, 180 mg of HCO3 ,12 mg of Sr++, and 0.04 mg of Ba++. The evaluation further assumes the unit electric energy cost to be US$0.07/kWh, the rate of replacement of the membranes 20% per year, twenty years' plant life, a 90% load factor, and an interest rate of 10% a year. The results of the comparison were as follows (Darwish and Jawad 1989b):

FACILITIES. The seawater intake size and flow rate of the MSF unit are twice those of the RO unit.

The volume of the MSF unit is about three times that required for the RO permeators. The land area required for the MSF unit is at least four times that required for the RO permeators.

Extensive and heavy materials are used in the MSF unit, which are more than ten times those required for the RO unit. The heavy weight of the MSF unit requires heavy foundations and extensive civil engineering work.

ENERGY CONSUMPTION. Thermal energy is consumed only by the MSF unit, and amounts to 89 MW. This thermal energy can be very expensive if it is obtained directly from boilers (not extracted from steam turbines).

The energy consumption for pumping seawater to the pre-treatment system and the high-pressure feed pump in the RO plant was estimated to be 0.25 and 7.98 kWh/m respectively, or a total of 8.23 kWh/m, which is about 25% more than that required for the MSF unit. However, the pumping energy for RO can be decreased about 30%, from 8.23 to 5.9 kWh/m, by installing an energy-recovery unit such as a reversed centrifugal pump or Pelton wheel.

The average energy consumption per cubic metre of the product water for the MSF unit was 15.27 kWh/m, which was about three times as high as the rate of 5.9 kWh/m for the RO plant.

UNIT COST OF PRODUCT WATER. From the above analysis, the on-site unit water costs of seawater desalination were estimated to be US$2.7/ m by MSF and US$1.7/m by RO. These are about twice as high as the costs of US$1.00/m for municipal water supply and of US$0.95/m for waste-water treatment in Japan (MFJ 1991), which are often used as a world standard for comparison.

CONJUNCTIVE USE PLAN FOR MSF AND RO. Introducing RO seawater desalting plants in Kuwait does not mean phasing out older desalting units. A combination of new RO and existing MSF units could be cost effective in a water-supply plan, as illustrated in fig. 3.10 in section 3.6 below.


3.5 Experimental brackish -ground water reverse -osmosis desalination


3.5.1 Background

The groundwater in Kuwait is mostly brackish, with total dissolved solids of the order of 2,000-8,000 mg/l, Brackish groundwater with salinity two to eight times as high as the WHO standard maximum allowable level for drinking water (TDS = 1,000 mg/l) is being blended with permeate from MSF distillation plants or used to irrigate garden crops. No direct use of brackish groundwater for drinking purposes is possible without desalination. Reverse osmosis is the best means of demineralizing brackish waters and has been in use in the United States since the 1970s. Brackish-groundwater desalination is usually three to five times less expensive than seawater desalination (Burgs 1989) and has the following advantages:

Thirteen skid-mounted mobile brackish-water RO units designed to supply fresh water for emergency purposes, equipped with standby power-generating units and with an installed capacity of 0.25 mig (1,137 m) per day each, have been installed since 1987 in various locations in Kuwait-the Labour Institute for Juveniles, the Shuwaikh storage area, two army camps, and nine hospitals. Kuwait is the first country to use such mobile RO units for the desalination of brackish groundwater. The technical and cost feasibilities are reviewed here before examining the concept of a hydro-powered RO desalination system in section 3.6.

3.5.2 The experimental RO unit

The first of the thirteen skid-mounted RO units began a one-year test operation in 1988 and ran continuously for 8,260 hours according to specifications. During the test operation, no membrane unit was added or replaced, but frequent changes of cartridge filter elements were needed to avoid big-fouling. The test operation was successfully com pleted without encountering any significant problems (Malik et al. 1989).

THE RO SYSTEM UNIT. The RO unit is housed in two standard containers. The first, the operation container, includes the membranes, highpressure pumps, cartridge filter, flushing/cleaning tank, transfer pump, dosing stations, control panel, electrical switch board, etc. The second contains two dual-media filters, the feed pump, a backwash air blower, and associated pipes and valves.

Brackish water is supplied to the feed-water tank (227 m) through the existing brackish-water pipeline network.

The pre-treatment system consists of dual-media filters (hydroanthracite/fine sands) and cartridge filters (5-m size). Sulphuric acid (5 mg/l) and antiscalant flocon (6 mg/l) are added before the cartridge filters and sodium bisulphate (2 mg/l) at the suction of the feed pump.

After the feed water passes through the cartridge filter, the pressure is increased up to the operating level of 15-25 kg/cm by centrifugal pump.

The RO unit consists of eight pressure vessels. The membrane is a lowpressure type, spiral-wound BW-8040 composite module, 8 inches in diameter.

The permeate from the modules flows to the flushing/cleaning tank. A neutral pH value is achieved in the final product water by dosing with caustic soda (5-10 mg/l) Sodium hypochlorite (1 mg/l as Cl2) is injected to sterilize the product water.

BIO-FOULING PROBLEMS. The brackish feed water contains bacteria with count-concentrations of the order of 60-400 CFU/ml, which was the main cause of blockage in the cartridge filter elements. Cleaning or replacement of the elements was needed every 300-400 hours, which is five times as frequent as the standard rate of 1,500-2,000 hours. To combat this problem the feedwater tanks and sand filters were disinfected with 5 mg of chlorine per litre, which improved the rate of filter replacement up to 700-800 hours. Shockchlorinating the cartridge filter periodically with 5 mg of chlorine per litre further significantly improved the replacement time to about 1,800 operating hours.

MEMBRANE CLEANING. Cleaning the membrane every 1,000 hours with a solution of NaOH (0.1%) and EDTA (0.1%) and replacing the cartridge filter every 1,800 hours was found to be more economical and safer than any other method, such as increasing the chlorine dose rate (from 0.2 to 2 mg/l), in the feed water.

Fig. 3.8 One-year test operation of brackish-groundwater RO desalination in Kuwait (Source: Malik 1989)

3.5.3 Technical performance

One year of test operation (8,260 hours) of the brackish-water reverse osmosis desalination was successfully completed in 1989. The skid mounted RO unit can operate continuously with an availability of 94.3%. The average discharge of product water was 46.83 m/hour, which was 98.9% of the designed value of 47.4 m/hour (fig. 3.8). quality OF FEED WATER AND PRODUCT WATER. The salinity of the brackish feed water varied from 3,134 to 3,874 mg of TDS per litre, with an average of 3,407 mg/l, The salinity of the product water averaged 73.5 mg/l with a minimum of 62 mall and maximum of 122 mg/l, The feed-water temperature was between 26C and 37C. The pH of the feed water was 7.87 on average, with a minimum of 7.65 and a maximum of 8.0.

OPERATING PRESSURE AND POWER CONSUMPTION. The operating pressure varied from a minimum of 15 kg/cm to a maximum of 21 kg/cm. The average power consumption during the 8,260 hours of operationincluding all auxiliary functions such as air conditioners, lights, mixers, etc.was 2 kWh per m of product water.

RECOVERY AND SALT REJECTION. The recovery of fresh water was 59.86% on average, ranging between 56% and 64%. The average salt rejection was 98.4%, with a minimum of 98%.

3.5.4 Cost performance

This case study on a very small-scale brackish-water RO desalination unit was aimed at evaluating the cost-effectiveness of supplying fresh water to remote or isolated towns and for emergency needs by using a skid-mounted unit. The cost estimates for the initial capital cost and operation and maintenance costs assumed the following:

The initial capital cost comprised: mechanical equipment, 49.8%; membranes, 19.5%; electric generators, 17.5%; instrumentation equipment, 5.0%; training, 4.5%; and civil, 3.5%. The operation and maintenance (O&M) cost, comprising labour, chemicals, spare parts, energy/electricity, and membrane replacements, was estimated to be KD 0.160 (US$0.48) per m. O&M is the most important cost item in a small-scale plant and is five times as high as the capital cost. The energy costs and labour costs are the dominant elements in O&M, accounting for 31.7% and 27.0% respectively. Other costs account for less than half of O&M, including 14.6% for membrane replacement, 14.4% for chemicals, and 12.3% for spare parts. The unit cost of the product water from this particular small-scale skid-mounted mobile system was estimated to be as high as KD 0.726 (US$2.18) per m (Malik et al. 1989).


3.6 Hydro-powered brackish-groundwater reverse-osmosis desalination: A new proposal


In this study I propose the use of hydro-powered reverse-osmosis desalination to minimize the cost of energy consumption, which is the largest single cost element in desalination engineering. Hydro-powered desalination makes effective use of the hydro-potential energy in a water pipeline system carrying brackish groundwater from a wellfield to the terminal reservoir where the differential head is 200 m or more. This section examines this new concept for the existing Shigaya groundwater development project in Kuwait to evaluate its cost feasibility.

3.6.1 Brackish groundwater wellfield

The proposed wellfield is located in the potential wellfields of West Shigaya and a part of North Shigaya, about 100 km west of Kuwait city (fig. 3.9). The ground elevation is 200-300 m above sea level. The Damman limestone at this point has a thickness of about 150 m. The piezometric levels, however, are rather low, being in the range of 50-100 m above sea level. The groundwater is brackish, with a salinity of 2,500-7,000 mg of TDS per litre. The potential yield has been estimated to be 68,000 m per day in each potential wellfield (Abusada 1988). For this study, the potential yield is assumed to be 45 million m per year (123,400 m per day), with the wellfield within an area with an elevation of more than 200 m. It is estimated that 46 production wells would be required, assuming a unit rate of 2,700 m per day per well.

3.6.2 Pressure pipeline system and pre-treatment plant

The proposed hydro-powered scheme would use the piezometric head difference between the collecting reservoir (elevation, 230 m) and the Jahara RO plant (elevation, 20 m). The pre-treatment plant would be sited immediately east of the collecting reservoir, where the feed water gravitates to the Jahara plant. A ductile iron pressure pipe 750 mm in diameter and 60 km long, with a pressure limitation of 25 bar maximum, would carry the feed water to the Jahara plant. The design discharge and velocity in the pressure pipe are 1.42 m/sec and 3.2 m/see, respectively.

3.6.3 Estimate of hydro-potential energy in the trunk main

The head difference between the collecting reservoir (230 m) and the RO plant (20 m) is 210 m. The energy loss would consist mainly of friction loss in the pressure pipe, together with other losses, and is estimated to be 10 m of water head, which is only 5% of the total head of 210 m. From the effective head of water at 200 m, or 20 kg/cm, the theoretical hydro-potential of the scheme is estimated to be 2,780 kW.

Fig. 3.9 Proposed layout of hydro-powered RO desalination system in Kuwait

The following equations were used to estimate the theoretical hydropotential (Pth) and installed capacity (P), both in kW, and the potential power generation (Wp) in kWh per year:

Pth = 9.8 x Ws x Q x He,
P = Pth x Ef,
Wp = 365 x 24 x Gf x P.

where

Ws = specific weight of water ( = 1.0),
Q = flow discharge (m/sec),
He = effective difference head of water (m),
Ef = synthesized efficiency (assumed to be 0.80),
Gf = generating efficiency (assumed to be 0.68).

In the design of the water supply system, the hydraulic pressure in the trunk main is to be broken at 20-25 kg/cm to prevent the mechanical failure of the pipe. The flow discharge (Q) of 1.18 m/sec at a differential head of water of 210 m, or effective head (He) of 200 m, has a potential yield of generated electric power (Wp) of 11 million kWh per year.

3.6.4 Hydro-powered reverse-osmosis desalination system

The application of hydro-potential energy, which is a typically clean energy, is the key to hydro-powered reverse-osmosis desalination, to minimize energy consumption and operating costs.

The potential energy in the trunk main can be used more effectively to provide hydraulic pressure in the pressure-pumping unit of an RO system than in generating electricity, owing to the direct use of hydropotential energy as hydraulic pressure rather than through a turbine and generator. The energy losses in a turbine and generator are generally 16% and 5% respectively, which is in total 20% of the theoretical hydro-potential energy. The energy requirement of the pressure pumping system is a major cost factor in operating an RO plant. Hydro-powered reverse-osmosis desalination would have the great advantage of avoiding two stages of energy conversion, to electricity and then to hydraulic pressure. It would also have the advantages of low initial capital cost, compact design, short construction time, and minimal energy requirements and costs.

The brackish feed water would be pumped from the Damman limestone aquifer into a collecting reservoir at an elevation of 235 m above sea level (fig. 3.10). The feed water is estimated to have an average salinity of 4,000 mg of TDS per litre, a temperature of between 26C and 37C, and an average pH of 7.87, ranging from 7.65 to 8.0 (Malik et al. 1989). The following design criteria were assumed:

Fig. 3.10 Schematic profile of proposed hydro-powered RO system

The reverse-osmosis unit would be in two parts. The first would be a pretreatment unit sited immediately east of the collecting reservoir, with dualmedia filters (hydro-anthracite and fine sands) and cartridge filters (5-pm size) and with sulphuric acid (5 mg/l, and antiscalant flocon (6 mg/l) added before the cartridge filters. Sodium bisulphate (2 mg/l, would be added at the suction of the feed pump.

After passing through the cartridge filter, the feed water would enter a pressure pipeline (trunk main) to sustain a hydraulic pressure head of 20 kg/cm, which would be used directly to overcome the osmotic pressure to permeate the membrane.

The 8-inch diameter RO module would have a low-pressure, spiralwound composite-type membrane. The specifications of the module would be as follows:

A unit line of the RO vessel would consist of a circuit with six modules in series. Recovery is estimated to be 70% of the feed water, yielding 31.5 million m of permeate per year with TDS at 500 mg/l and 8.0 million m of brine reject per year with TDS at 17,700 mg/l, The membrane will be cleaned every 1,000 hours using a solution of NaOH (0.1%) and EDTA (0.1%), and the cartridge filters will be replaced every 1,800 hours.

The effective pressure of the brine reject is estimated to be 17 kg/ cm, assuming a friction loss of 3 kg/cm in the RO circuit. The potential energy recovery of the RO brine reject is preliminarily estimated to be 333 kW, assuming the total efficiency of the turbine and generator to be 80%, which would generate 1.98 million kWh of electricity per year with a load factor of 65%.

The permeate from the modules would then flow to the flushing/ cleaning tank. A neutral pH value would be achieved in the final product water by dosing with caustic soda (5-10 mg/l Sodium hypochlorite (1 mg/l as Cl2) would be injected to sterilize the product water.

3.6.5 Cost effectiveness

The investment cost of the proposed hydro-powered desalting plant is preliminarily estimated to be US$94,065,000 in total, with an annualized capital cost of US$5,656,000, comprising US$74,488,000 of capital cost and US$19,577,QOO of design and construction supervision. The capital cost would include the following major cost elements:

Financial expenditure is estimated to be US$24,428,700, based on 1990 prices with 8% interest during three years' construction.

The annual cost of operation and maintenance is estimated to be US$5,653,600, made up of the following main items:

The costs of source water and benefits from energy recovery are not included in this cost estimate. The above cost estimates are based on the following assumptions:

The unit water cost of the hydro-powered reverse-osmosis desalination for 31.5 million m of product water per year is estimated to be US$0.40/m, which is lower than the cost of such other methods as seawater desalination by MSF (US$2.70/m), seawater desalination by RO (US$1.70/m), and brackish-groundwater desalination by RO without the use of hydro-power (US$0.60/m), as shown in fig. 3.11. Such an application of hydro-potential energy recovery in a pipeline system is likely to have a strategic value in saving fossil-fuel energy and the global environment in addition to minimizing costs in desalination engineering in many parts of the world where there are groundwater resources at an appropriate elevation.

Fig. 3.11 Unit water cost of desalination in Kuwait (Sources: Darwish 1989; Murakami 1991; Murakami and Musiake 1991)


3.7 Development alternatives and a conjunctive-use plan


3.7.1 Development alternatives

The predominant multi-stage flash (MSF) desalination facilities in Kuwait, which consume extensive materials and more energy than RO, will be replaced in stages after the completion their plant life of about 15-20 years. RO seawater desalting plants will replace the old MSF desalter units. The unit cost of RO brackish-groundwater desalination is also much lower than that of seawater desalination, implying lower energy consumption and less capital investment. Hydro-powered RO desalination is the method with the lowest cost, minimizing both energy consumption and capital cost. The development alternatives include the following:

>> MSF distilling of seawater (existing plant; highest cost);

>> RO desalination of seawater (experimental stage now being completed);

>> RO desalination of brackish groundwater (existing skid-mounted RO units);

>> Hydro-powered RO desalination of brackish groundwater (as proposed here; the lowest-cost method).

3. 7.2 Conjunctive-use plan

A number of old MSF plants in Kuwait are going to be phased out by the year 2000. Seawater RO will replace the old MSF in stages, but almost pure water from the existing MSF system will also be blended with RO product water with a salinity of about 500 mg of TDS per litre to obtain water of a quality suitable for drinking.

The salinity ranges of the product water from the various processes are as follows:

These may be compared with the WHO drinking-water standards of 250-500 mg of TDS per litre for Europe, the United States, and Japan and 500-1,000 mg/l, for the Middle East.

In a hybrid RO/MSF seawater desalination system, a seawater reverseosmosis plant is combined with either a new or an existing MSF co-generation plant, with the following advantages:

>> Both the capital and operating cost for the RO system are reduced.

>> A single-stage RO process can be used, and the life of the RO membrane can be extended by reducing the water-quality specification for the permeate.

>> The temperature of the MSF product water is reduced by blend ing the hot product water from MSF distillation with the RO permeate.

The combination of the new RO system with the existing MSF will be the key approach to developing the Kuwait water supply system in the 1990s.

Combining the proposed hydro-powered RO desalination system with the present MSF system would make more effective use of brackish-groundwater desalination at the lowest cost, taking into account the limited potential of brackish-groundwater resources and the unlimited potential source of seawater. The salinity of the permeate from brackish-groundwater RO desalination can be controlled in the range 100-500 mg of TDS per litre, while the brine reject water has a salinity as high as 10,000 mg/l or more. The salinity of the product water from MSF distillation is as low as 25-50 mg/l, Brackish-groundwater RO desalination will contribute (1) a direct supply of good quality drinking water to meet WHO standards and (2) an indirect supply by blending brine reject water from RO with almost pure water from the existing MSF system. The conjunctive-use plan suggests the following priority uses of the water and energy elements:

>> RO product water (permeate)-31.5 million m per year with 500 mg of TDS per litre-for direct use for drinking-water supply;

>> RO brine reject water-8 million m per year with 17,700 mg of TDS per litre-for indirect use to blend with MSF product water;

>> MSF product water-632,000 m per day maximum with 25-50 mg of TDS per litre-to be blended with RO brine reject water;

>> MSF brine reject water->45,000 mg of TDS per litre-to be safely disposed of offshore in Kuwait Bay;

>> energy recovered from the RO process-1.98 million kWh of electricity per year-to be used to supply electricity for treatment and/ or pumping.

A tentative conjunctive-use plan is illustrated as a flow diagram in fig. 3.10.

3.7.3 Remarks on future development planning

The desalination of saline water by a membrane separation with low energy requirements will play an increasingly important role in the water-resources planning of arid states in the twenty-first century. Reverse osmosis is the least costly process today, but it may not be the optimum solution, which may be neither reverse osmosis nor thermal desalination. Membrane desalination, however, will be a key application for water-resources planning in the twentyfirst century. A new desalination system using reverse osmosis either with or without the use of hydro-power will be incorporated in the existing MSF system in Kuwait by stages to make a reality of energy-saving desalination technology.


Contents - Previous - Next