East Desalination Research Center)
Final report and results
Numerous large scale
desalination plants have been built and are being planned in arid zones
and water-scarce areas, where other drinking water sources are close to
depletion. For example, the available renewable water per person in the
Middle East and North Africa (MENA) region is only 20% of the rest of the
world (UNCSD, 1999).
In 2004, about 75% of
total world installed capacity of desalination plants was situated in the
MENA region (Goebel, 2005), where some states depend on desalinated water
for more than 50% for their domestic use.
The immediate need to
increase the production and supply of potable water is a key focus for
governments across the region, generating massive investment and creating
demand for global expertise plus the latest advanced systems and
technologies. In the period up to 2015, the countries of the MENA region
are expected to spend US$24 billion in desalination costs (www.middleeastelectricity.com).
Figure 1: Al Ghubrah desalination plant (biggest in
Oman, capacity: 191,000 m³/d) with brine discharge directly
at the coast via an open channel into the Golf of
Oman (Foto courtesy of Hamdi Al-Barwani)
The impacts of a desalination plant discharge on the
marine environment depend on the physical and chemical properties of the
desalination plant reject streams, and the susceptibility of coastal ecosystems
to these discharges depending on their hydrographical and biological features.
Therefore, a good knowledge of both the effluent properties and the receiving
environments is required in order to evaluate the potential impacts of
desalination plants on the marine environment.
The brine flows are considerably large, generally up to 40 % (for membrane based
technologies, like reverse osmosis, RO) and up to 90 % (for thermal technologies,
like multi-stage-flash, MSF, including cooling water) of the intake flowrate.
Thus either almost as large or even considerably larger flows than the required
freshwater water flow. Salinity and temperature directly influence the density
of the effluent. The various density differences between the brine and the
receiving water represented by the buoyancy flux causes different flow
characteristics of the discharge. The dense RO effluent flow has the tendency to
fall as negatively buoyant plume and spread as a density current on the
sea-floor (see figure 2 and 3). The effluent from thermal desalination plants is
distinguished by a neutral to positive buoyant flux causing the plume to rise
and to spread on the sea-surface (see figure 1 and 2).
Sea water desalination plants carry a number of
waste products into the coastal environment (Lattemann and Höpner, 2003):
concentrated salt brine that may also have an elevated temperature, often
containing anti-foulings and anti-scalents, and other substances. Modern, large
capacity plants require submerged discharges that ensure a high dilution in
order to minimize harmful impacts on the marine environment.
There is increased public concern and scientific
awareness on the environmental impact of desalination plants (Genthner, 2005).
Impacts become key issue for discharge permit (thus influence plant
commissioning date and eventual modifications).
New regulations demand for better pollution control
at the discharge point (“effluent standards”) as well as within the receiving
water (“ambient standards”). In order to meet these regulations, optimized high
efficiency mixing designs are needed for the discharges.
Discharge designs are often not optimized regarding
environmental impacts or operational needs. Especially for larger plants or
plant complexes there is a potential for recirculation to the plant intake,
reducing overall system efficiency. There exists no efficient planning tool to
assist desalination plant designers and plant managers (News-video about the importance of good planning for brine
Figure 2: Natural mixing is slow. Optimized mixing
device (e.g. submerged multiport diffuser) reduce local impacts considerably.
Optimized siting of outfalls allows for improved operational conditions and
better environmental protection. Negatively buoyant jet: e.g. RO-plant discharge,
where density effects strongly influence mixing characteristics. Positively
buoyant jet: thermal-desal-plant discharge (even brine with elevated temperature
is more dense than seawater, however when mixed with substantial volumes of
cooling water brine reaches neutral to positive density difference)
Figure 3: Left: Laboratory setup to visualize a
dense brine discharge resulting from a RO plant; Right: Modeled dense discharge
Identification of environmental impacts, regulatory frameworks and
public concerns regarding brine effluent discharges with emphasis on
MENA (Middle East, North African) and Mediterranean countries.
of easily applicable design
calculators and nomograms including the density dependence
on salinity and temperature as basis for the first screening process
within the assessment of brine effluents after discharge into the
receiving coastal waters.
of hydrodynamic model interfaces for predicting brine effluent
concentrations of key parameters in the marine environment by coupling a
near-field mixing model
for outfall design optimization with a far-field transport model for
optimized outfall site.
application and validation for typical case studies for the compilation
of design recommendations with parallel improvement of design oriented
and realization of capacity building on environmental planning,
prediction and management of brine discharges from desalination plants.
Partner and work packages
The project started January 2008 and ended 2010.
University Karlsruhe, Institut für Hydromechanik (Dr.-Ing.
Prof. Gerhard H.
- Project coordination, environmental
hydraulics, software development, design-studies
Sultan Qaboos University (SQU), Oman, Department of Mathematics and
Statistics (Prof. Anton Purnama, Prof. Hamdi Al-Barwani)
- Software development, modelling
issues, local/regional issues, case-studies
(Prof. Robert L. Doneker )
- Code implementation, pre- and
postprocessing, quality control, software support
- Expertise in Marine Ecology,
Environmental assessment of pressures and impacts