6 Salinity, temperature & density
6 Deaeration & oxygen scavengers
6 Chlorine
6
Heavy metals
6 Antiscalants
6 Coagulants
6 Antifoamings
6 Cleaning chemicals

Potential Impacts of Seawater Desalination

The sea as a source of drinking water seems to be unlimited, but pollution by land-based activity often impairs the quality of coastal water bodies that serve as feedwater for desalination plants. 'Hot spots' of marine pollution are typically near centres of intense human activity such as cities, harbours and industrial areas, which are also the areas where desalinated water is most needed for socio-economic development.

The process of desalination is not per se environmentally friendly and seawater desalination plants also contribute to the wastewater discharges that affect coastal water quality. This is mostly due to the highly saline brine that is emitted into the sea, which may be increased in temperature, contain residual chemicals from the pretreatment process, heavy metals from corrosion or intermittently used cleaning agents. The effluent from desalination plants is a multi-component waste, with multiple effects on water, sediment and marine organisms. It therefore affects the quality of the resource it depends on.



Large 'industrial' power and desalination facility (photo: Thomas Höpner)

 

 

Salinity, temperature and density
Salinity is increased in the waste stream of all processes, but elevated temperature values are characteristic of distillation plant effluents only. The RO brine has a higher density than seawater as a result of its increased salinity and will mostly affect benthic communities, while distillation plant discharges tend to float on the surface and interfere with productivity in the pelagic community. The positive buoyancy of distillation plant discharges is mostly due to the discharge of large volumes of cooling water, which are blended with the brine.
 
Deaeration and oxygen scavengers
With increasing temperature and salinity, oxygen becomes less soluble in seawater. However, oxygen levels are deliberately reduced in distillation plants by physical deaeration and addition of oxygen scavengers like sodium bisulfite to inhibit corrosion. Oxygen depletion is also a problem of the RO brine, as sodium bisulfite is commonly used as a neutralizing agent for chlorine. The lack of dissolved oxygen could be toxic to marine organisms and aeration is recommended prior to oceanic discharge.
 
Chlorine
One major pollutant of distillation processes is chlorine, which is added to the desalination plant feedwater to prevent biofouling on heat exchanger surfaces. In RO plants, chlorine is also a common biocide but modern plants often operate on polyamide membranes, which are sensitive to oxidizing chemicals such as chlorine. Neutralization is typically required before the feedwater enters the RO unit and it can be assumed that the brine is free from chlorine, too. Chlorine is a strong oxidant and highly effective biocide. Residual levels in the discharge may therefore be toxic to marine life in the discharge site. The use of chlorine also leads to the formation of oxidation by-products such as halogenated organics. These compounds are usually rather persistent in the marine environment and sufficient evidence exists that some of them are carcinogenic to animals. Due to environmental and health problems caused by residual chlorine and disinfection by-products, several alternative pretreatment methods have been considered to replace chlorine in desalination plants. Alternative biocides include for example ozone and monochloramine, while disinfection with ultraviolet light may be used instead of biocides to eliminate micro-organisms.
 
Heavy metals
The waste brine often contains low amounts of heavy metals that pass into solution when the plant's interior surfaces corrode. Brine metal composition depends on the use of different construction materials in distillation and reverse osmosis plants: Copper contamination is a major problem of distillation plants, as copper-nickel alloys are common materials for heat exchanger surfaces. In contrast, non-metal equipment and stainless steels are typically used in RO plants. The RO brine may therefore contain traces of iron, nickel, chromium and molybdenum, but contamination levels are generally low. Heavy metals tend to enrich in suspended material and finally in sediments, so that areas of restricted water exchange and soft bottom habitats could be affected by heavy metal accumulation. Many benthic invertebrates feed on this suspended or deposited material, with the risk that metals are enriched in their bodies and passed on to higher trophic levels. It is therefore recommended that limits are established for heavy metal concentrations in the brine discharges.
 
Antiscalants
Scaling on heat exchanger surfaces, inside tubes, or on RO membranes impairs plant performance. Antiscalants are commonly added to the feedwater in both distillation and RO plants to prevent scale formation. The main representatives of antiscalants are organic, carboxylic-rich polymers such as polyacrylic acid and polymaleic acid. Acids and polyphosphates are still in use at a limited scale but on the retreat. As antiscalants have a low toxicity, the acute environmental risk associated with their release into the marine environment is relatively low. Due to a poor degradability, however, dispersal and relatively long residence times must be expected, during which interference with element cycles of trace metals is a possible risk.
 
Coagulants and coagulant aids
Coagulants like ferric- or aluminum chloride are used to improve filtration of suspended material from the RO feedwater. Coagulant aids (organic substances with high molecular masses that bridge particles further together) and pH control are supplementary methods to enhance coagulation. The filter backwash can be discharged to the sea, as toxic effects are not expected by coagulants and coagulant aids. However, possible impacts such as reduced primary production or burial of sessile organisms by increased turbidity in the discharge should be anticipated. For impact mitigation, the backwash could be diluted, e.g. by continuous blending with the brine, or alternatively it could be removed from the filters and transported to a landfill.
 
Antifoaming agents
To reduce foaming in distillation plants, antifoaming agents like polyglycols are added to the feedwater, which are not toxic but poorly biodegradable. Adverse effects are not to be expected with regard to a low dosage level and sufficient dilution following discharge.
 
Cleaning chemicals
Cleaning intervals have to be established for each desalination plant individually and are typically three to six months depending on the quality of the plant's feedwater.
In RO plants, alkaline cleaning solutions (pH 11-12) are used for removal of silt deposits and biofilms, whereas acidified solutions (pH 2-3) remove metal oxides and scales. Further chemicals are often added to improve the cleaning process of RO membranes, such as detergents, oxidants, complexing agents or biocides for membrane disinfection.
In distillation plants, cleaning is typically very simple: Copper-nickel heat exchanger surfaces are washed with acidified warm seawater to remove alkaline scales. The acidic solution often contains a chemical inhibitor which is added to protect the plant from corrosion.
Most of the named cleaning and disinfection chemicals may be hazardous to aquatic life, so that disposal to the ocean should be strictly regulated. Neutralization of the extremely alkaline or acidic solutions and treatment of additional cleaning agents is recommended before discharge to the ocean to remove any potential toxicity.

 

 

 




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