|Managing Water for Peace in the Middle East: Alternative Strategies (UNU, 1995, 309 pages)|
|3. Hydro-powered reverse-osmosis desalination in water-resources development in Kuwait|
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 m³ per day in each potential wellfield (Abusada 1988). For this study, the potential yield is assumed to be 45 million m³ per year (123,400 m³ 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 m³ 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 m³/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/cm², the theoretical hydro-potential of the scheme is estimated to be 2,780 kW.
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.
Ws = specific weight of water ( = 1.0),
Q = flow discharge (m³/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/cm² to prevent the mechanical failure of the pipe. The flow discharge (Q) of 1.18 m³/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 26°C and 37°C, and an average pH of 7.87, ranging from 7.65 to 8.0 (Malik et al. 1989). The following design criteria were assumed:
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/cm², 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 m³ of permeate per year with TDS at 500 mg/l and 8.0 million m³ 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/ cm², assuming a friction loss of 3 kg/cm² 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 m³ of product water per year is estimated to be US$0.40/m³, which is lower than the cost of such other methods as seawater desalination by MSF (US$2.70/m³), seawater desalination by RO (US$1.70/m³), and brackish-groundwater desalination by RO without the use of hydro-power (US$0.60/m³), 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.