Methodologies for abstraction from coastal aquifers for supplying desalination plants in the south-east of Spain

Supplying seawater to desalination plants using boreholes or drains can be less expensive than direct water intakes, since the silt density index (SDI) tends to be much lower. This approach is dependent on the presence of coastal aquifers with good hydraulic connection to the sea. Moreover, conventional boreholes are not suitable for pumping water on a long-term basis, since seawater is very aggressive. Hence, this type of abstraction requires the application of special technologies, from borehole location to borehole lining and special operations. These include the selection of the most appropriate drilling method, which depends on the lithology to be penetrated and its structure. This study focuses on Horizontal Directional Drilling (HDD), which encompasses very advanced technology. Various examples of work done in south-eastern Spain are described, along with the keys to their success.     

Material specification and the availability and life of desalination equipment in both Saudi Arabia and the Arabian Gulf

The author describes operational experience with various materials of construction in both low temperature polyphosphate treated evaporators and also in high temperature evaporators using acid treatment.The author references the repeated use of unsuitable materials and proposes a solution by the development of more detailed and positive specifications for plant and equipment. A problem faced by many manufacturers is the fact that as a result of broad, open specifications they are forced to use inferior materials in order to remain competitive. The author discusses the use of both corrosion resistant linings and operating experience. In summary, the paper correlates the operating conditions with selection of materials.     

Energy and exergy use in the utility sector of Saudi Arabia

We present an analysis of energy and exergy utilization in the utility sector of Saudi Arabia by considering the sectoral energy and exergy flows for the years 1990-2001. Energy and exergy analyses were conducted for its two subsectors, namely power-only plants and power/distillation plants, and hence the energy and exergy efficiencies were obtained for comparison. The power/distillation plant subsector appeared to be more energy/exergy efficient compared to the conventional power-only plant subsector for the particular reference conditions assumed in the analysis. A comparison of the overall energy and exergy efficiencies of Saudi Arabian utility sector with the Turkish utility sector is also presented for the year 1993. Although the sectoral coverage is different for each country, it is useful to illustrate the situation of how energy and exergy efficiencies vary. The turkish utility sector appeared to be more efficient for that particular year. Power/distillation makes a significant contribution to Saudi Arabia's overall power generation in the utility sector.     

Characterization of uncertainties in the operation and economics of the proposed seawater desalination plant in the Gaza Strip

In the Gaza Strip, the available freshwater sources are severely polluted and overused. Desalination of seawater through reverse osmosis (RO) has become the most realistic option to meet a rapidly growing water demand. It is estimated that the Gaza Strip will need to develop a seawater desalination capacity of about 120,000 m³/d by the year 2008, and an additional 30,000 m³/d by the year 2016 in order to maintain a fresh water balance in the coastal aquifer and to fulfill the water demand for different uses in a sustainable manner. Cost and reliability of a large RO facility are still subject to much uncertainty. The cost of seawater desalination by RO systems varies with facility size and lifetime, financing conditions, intake type and pre-treatment requirements, power requirements, recovery rate, chemicals cost, spare parts cost, and membrane replacement cost. The permeate salinity is a function of feed water temperature, recovery rate, and permeate flux. The quantity of water produced depends mainly on plant size, recovery rate, and operating load factor. Many of these parameters are subject to a great deal of uncertainty. The objective of this work is to develop a probabilistic model for the simulation of seawater reverse osmosis processes using a Bayesian belief network (BBN) approach. This model represents a new application of probabilistic modeling tools to a large-scale complex system. The model is used to: (1) characterize the different uncertainties involved in the RO process; (2) optimize the RO process reliability and cost; and (3) study how uncertainty in unit capital cost, unit operation and maintenance (O&M) cost, and permeate quality is related to different input variables. The model utilizes information from journal articles, books, expert opinions, and technical reports related to the study area, and can be used to support operators and decision makers in the design of RO systems and formulation of operational policies. The structure of the model is not specific to the Gaza Strip and can be easily populated with data from any large-scale RO plant in any part of the world.     

Environmental assessment of natural radionuclides and heavy metals in waters discharged from a lignite-fired power plant

Gross alpha, gross beta and ²²⁶Ra activities as well as the concentration of trace metals (V, Cr, Mn, Ni, Cu, Zn, Mo and Pb) in the discharge waters of the major lignite-fired power plant in Greece were measured during the period October 2004 to May 2006. Gross alpha activity of particulate matter in the discharge waters was 0.75 ± 0.40 Bq g⁻¹ (0.3-1.2 Bq g⁻¹) while the beta activity was 1.54 ± 0.50 Bq g⁻¹ (1.2-1.7 Bq g⁻¹). The ranges of water gross alpha, beta and ²²⁶Ra activities were 0.062-0.268 Bq L⁻¹, 0.064-0.268 Bq L⁻¹ and 0.021-0.062 Bq L⁻¹, respectively. The mean concentration of ²²⁶Ra in the discharge waters was at least one order of magnitude higher than in natural water bodies. Soil samples were collected from fields irrigated with discharge waters and 29.2 ± 2.2 Bq kg⁻¹ of ²³⁸U, 1.2 ± 0.2 Bq kg⁻¹ of ²³⁵U, 26.8 ± 0.8 Bq kg⁻¹ of ²²⁶Ra, 36.8 ± 1.5 Bq kg⁻¹ of ²³²Th and 492.6 ± 25.8 Bq kg⁻¹ of ⁴⁰K were determined. The concentration values of dissolved metals in the discharge waters were higher than those usually observed in water streams near coal-fired power plants or rivers due to metal leaching from lignite or/and by-products. However, the leaching at high pH’s as those observed in the discharged waters does not raise the concentration of the studied metals to values higher than the criteria maximum concentrations and criterion continuous concentration (CCC) values of the US EPA water quality criteria. Statistical analysis was further applied to reveal the correlations between the different water components. Hierarchical cluster analysis revealed four different clusters: the first cluster was primarily composed of radioactivity and physicochemical parameters; the second cluster consisted of Cu, Ni and Zn, the third of Mn, Fe, Mo and Pb and the fourth of V and Cr. This clustering agrees with the associations suggested for elements in most coals or with the Goldschmidt classification.     

Feasibility study and performance assessment for the integration of a steam-injected gas turbine and thermal desalination system

This study proposes a systematic approach for retrofitting a steam-injection gas turbine (SIGT) with a multi-effect thermal vapor compression (METVC) desalination system. The retrofitted unit's product cost of the fresh water (RUPC) was used as a performance criterion, which comprises the thermodynamic, economic, and environmental attributes when calculating the total annual cost of the SIGT–METVC system. For the feasibility study of retrofitting the SIGT plant with the METVC desalination system, the effects of two key parameters were analyzed using response surface methodology (RSM) based on a central composite design (CCD): the steam air ratio (SR) and the temperature difference between the effects of the METVC system (?T_METVC) on the fresh water production (Q_freshwater) and the net power generation (W_net) of the SIGT–METVC system. Multi-objective optimization (MOO) which minimizes the modified total annual cost (MTAC) and maximizes the fresh water flow rate was performed to optimize the RUPC of the SIGT–METVC system. The best Pareto optimal solution showed that the SIGT–METVC system with five effects is the best one among the systems with 4–6 effects. This system under optimal operating conditions can save 21.07% and 9.54% of the RUPC, compared to the systems with four and six effects, respectively.     

Experimental investigation of a baffled shell and tube desalination column using the humidification-dehumidification process

The humidification-dehumidification process is an interesting technique that has been adapted for water desalination. Most previous work performed the humidification-dehumidification desalination process in two separate columns, which increases the complexity of the system and limits the humidification effect of the carrier gas as well as the thermal efficiency of the process. In this work, a baffled shell and tube desalination unit was built to perform humidification and dehumidification simultaneously at the tube and shell side of the column, respectively. The effects of several operating conditions on the production and thermal efficiency of the process were investigated, including water flow rate, inlet water temperature, carrier gas flow rate and external steam flow rate. The results show that the productivity and thermal efficiency of the process both increase with increasing inlet water temperature. Suitable flow rates exist for both the water and carrier gas, which are 6–30 kg h⁻¹ and 4–20 kg h⁻¹ for the present column, respectively. The increase of external steam flow rate increased the productivity but decreased thermal efficiency of the process. A further performance comparison with a previous unbaffled desalination unit indicated that the baffle plates significantly enhance the productivity of the column, which is usually 3–6 times that of the unbaffled one. Meanwhile, the salinity of the produced water was determined to be in the range of 20–30 mg/L with the feed water containing 10,000 mg/L NaCl.     

Nitrogen use efficiency by maize as affected by a mucuna short fallow and P application in the coastal savanna of West Africa

Maize is the primary food crop grown by farmers in the coastal savanna region of Togo and Benin on degraded (rhodic ferralsols), low in soil K-supplying capacity, and non-degraded (plinthic acrisols) soils. Agronomic trials were conducted during 1999–2002 in southern Togo on both soil types to investigate the impact of N and P fertilization and the introduction of a mucuna short fallow (MSF) on yield, indigenous N supply of the soil, N recovery fraction and internal efficiency of maize. In all plots, an annual basal dose of 100 kg K ha^–1 was applied to the maize crop. Maize and mucuna crop residues were incorporated into the soil during land preparation. Treatment yields were primarily below 80% of CERES-MAIZE simulated weather-defined maize yield potentials, indicating that nutrients were more limiting than weather conditions. On degraded soil (DS), maize yields increased from 0.4 t ha^–1 to 2.8 t ha^–1 from 1999 to 2001, without N or P application, in the absence of MSF, with annual K application and incorporation of maize crop residues. Application of N and P mineral fertilizer resulted in yield gains of 1–1.5 t ha^–1. With MSF, additional yield gains of between 0.5 and 1.0 t ha^–1 were obtained at low N application rates. N supply of the soil increased from 10 to 42 kg ha^–1 from 1999 to 2001 and to 58 kg N ha^–1 with MSF. Application of P resulted in significant improvements in N recovery fraction, and greatest gains were obtained with MSF and P application. MSF did not significantly affect internal N efficiency, which averaged 45 kg grain (kg N uptake)^–1. On non-degraded soils (NDS) and without N or P application, in the absence of MSF, maize yields were about 3 t ha^–1 from 1999 to 2001, with N supply of the soil ranging from 55 to 110 kg N ha^–1. Application of 40 kg P ha^–1 alone resulted in significant maize yield gains of between 1.0 (1999) and 1.5 (2001) t ha^–1. Inclusion of MSF did not significantly improve maize yields and even reduced N recovery fraction as determined in the third cropping year (2001). Results illustrate the importance of site-specific integrated soil fertility management recommendations for the southern regions of Togo and Benin that consider indigenous soil nutrient-supplying capacity and yield potential. On DS, the main nutrients limiting maize growth were N and probably K. On NDS, nutrients limiting growth were mainly N and P. Even on DS rapid gains in productivity can be obtained, with MSF serving as a means to allow farmers with limited financial means to restore the fertility of such soils. MSF cannot be recommended on relatively fertile NDS.