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BrineDis:

Environmental planning, prediction and management of brine discharges from desalination plants

Funded by MEDRC (Middle East Desalination Research Center)

MEDRC

Final report and results

Background

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)

 

Problem

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 discharges)

 

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 with CORMIX.

 

Objectives

  1. Identification of environmental impacts, regulatory frameworks and public concerns regarding brine effluent discharges with emphasis on MENA (Middle East, North African) and Mediterranean countries.
  1. Elaboration 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.
  1. Development of hydrodynamic model interfaces for predicting brine effluent concentrations of key parameters in the marine environment by coupling a near-field mixing model CORMIX for outfall design optimization with a far-field transport model for optimized outfall site.
  1. Model application and validation for typical case studies for the compilation of design recommendations with parallel improvement of design oriented input/output features.
  1. Management 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. Tobias Bleninger, Prof. Gerhard H. Jirka, Ph.D.)

  • 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

MixZon Inc. (Prof. Robert L. Doneker )

  •  Code implementation, pre- and postprocessing, quality control, software support

ARSU GmbH,  (Sabine Lattemann)

  • Expertise in Marine Ecology, Environmental assessment of pressures and impacts
 

 

 

Project Director: Prof. Gerhard H. Jirka, Ph.D.
Research Associates: Dr.-Ing. Tobias Bleninger

20.12.2010

 

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