Environmental Engineering Reference
In-Depth Information
impacts. Electricity consumption is the main indirect impact. Plants that use reverse osmosis
need 5 to 7 kWh of electricity per cubic meter (Cheremisinoff, 2001). Distillation plants, on
the other hand, need much less electricity, but they burn large amounts of natural gas or other
fuel, which causes direct emissions with global impact (greenhouse gases [GHGs]) and
regional impact (e.g., nitrogen oxides, sulfur dioxide, particle matter, and volatile organic
compounds). Emissions of carbon dioxide per cubic meter of desalinized water for a fossil
fuel base plant are 23.41kg for a multistage plant and 18.05 for a multiple effect plant;
whereas if they are driven by waste heat, emissions go down to 1.98 and 1.11 kg/m 3 , respec-
tively (Micale et al., 2009).
If desalination plants are run on fossil fuels, then they are not sustainable in the long run.
Of the total environmental impact throughout the life of a desalination plant integrated with
energy production, 90 percent goes to operating the plant and 10 percent or less takes on plant
construction, maintenance, and decommission (Raluy et al., 2004).
Another problem with no global impact as the consumption of fossil fuels is the disposal of
concentrate, which has important local effects. One of the by-products of the desalination
process is a water stream with a concentration of salt between 46 and 80 g/L (in contrast,
concentration of the salt in seawater is 35 g/L). In distillation plants, the temperature of the
concentrate is significantly higher than the intake water, has little dissolved oxygen, and con-
tains chemicals from pretreatment and chemicals used to clean equipment (California Coastal
Commission, 2004). Some alternatives can mitigate the impact of returning the concentrate to
the sea. For more information on this, consult Younos (2005).
Water intake to feed desalination plants is not impact free either. The volume of water used
by these plants is large, so water suction at the intake points interferes with local ecosystems,
kills marine life at intake screens and during the process (impingement and entrainment), and
gets in the way of local currents. Instead of direct water uptake from the sea, beach wells or
infiltration galleries eliminate these problems. However, these intake techniques may limit the
capacity of water drawn (California Coastal Commission, 2004).
Toward a water crisis
The overexploitation of water resources, pollution in water sources, changes in regional weather
patterns, and an increasing demand of water as a result of population growth are creating the right
conditions for a water crisis in the near future. According to the World Bank, about 700 million
people live in parts of the world experiencing water stress or water scarcity, and it is projected
that by 2035 this number will rise to 3 billion (“Water Resources Management,” n.d.). Areas are
considered to be under water stress when the annual supply per person drops below 1,700 m 3 ,
under water scarcity when the supply drops below 1,000 m 3 , and under absolute scarcity when
the supply falls below 500 m 3 (United Nations Development Programme [UNDP], 2006).
Water scarcity is often associated with places on Earth that have been traditionally dry and
without access to sources of water. But now it is happening in places such as California, which
has benefited from a secure water supply for many years and at this time is facing a water
crisis. The reason is a combination of excessive use, reduced precipitation, recession of the
snowpack in the Sierra Nevada Mountains, and deterioration of the Sacramento-San Joaquin
River Delta (Wilkinson and Rounds, 1998).
The High Plains Aquifer in the US Midwest, which provides water to irrigate 27 percent of
the irrigated land in the United States and covers eight states, is showing signs of remission.
According to readings taken by the US Geological Survey in 2000, the total average amount
of water in the aquifer had decreased by 6 percent in relation to the initial level with larger
declines in Texas, 27 percent, and Kansas, 16 percent (McGuire, 2003; McGuire et al., 2000).
Search WWH ::




Custom Search