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ENVIRONMENTAL ASPECTS OF POWER-DESALINATION

ENVIRONMENTAL ASPECTS OF POWER-DESALINATION. I - AIR EMISSIONS.

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ENVIRONMENTAL ASPECTS OF POWER-DESALINATION

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  1. ENVIRONMENTAL ASPECTS OF POWER-DESALINATION

  2. I - AIR EMISSIONS • Steam electric generating power/desalination plants are designed to burn crude oil, gas-oil or heavy fuel oil. The products of combustion released by fossil fuel are ash particles, carbon dioxide (CO2), carbon monoxide (CO), water vapor, Sulfur dioxide (SO2), & nitrogen oxides (NOx).

  3. II- ENVIRONMENTAL IMPACTS OF PLANT’S INTAKE There are two types of intake effects associated with power-desalination operation. • Impingement effects: As the seawater going into the power-desalination plant is screened & filtered, aquatic organisms are removed from water. These organisms are either discharged or returned back to the water body. • Entrainment effects: Smaller organisms passing through filters find their way through the process they get exposed to chemicals, higher temperature or pressure, conditions which are endangering their existence. Both of these effects may cause increased mortality rates for planktons of all types as well as small fish. This in turn may result in reduced population & reduced biodiversity.

  4. III- ENVIRONMENTAL IMPACTS OF BRINE WATER DISCHARGE • Desalination plants separates saline water into two streams: 1- A low dissolved solid concentration stream (fresh water); and 2- brine reject (blow-down). • The environmental impact of brine discharge depends to a large extent on the physical, chemical & biological characteristics of the receiving near-shore marine environment. • Large & modern desalination plants have sound designs for their intakes & out-falls to ensure more efficient operation & to avoid the passage of discharged effluents to the feed water intakes.

  5. The environmental impact of brine discharged to the near-shore marine environment might be manifested in one or more of the following forms: • Physical Impact: Resulting from the discharge of hot brine from thermal desalination plants. • Chemical Impact: Resulting from chemical agents remaining in the brine water & added for the control of biofouling, control of calcium carbonate scale formation (sulfuric acid, polyphosphates & polyelectrolytes) & antifoaming agents. RO necessitates pretreatment of the feed water to avoid accelerated fouling & scaling of the RO membranes. • Biological Impact: Biological impact is the secondary effect of oxygen demand exerted by the natural & induced organics in the brine water. The impact of BOD associated with lower levels of DO in brine waters due to higher salt content & temperature will ultimately reduce the level of DO in seawater adjacent to the brine water out-falls.

  6. IV- Environmental Impacts of Increased Salinity on Open Seawaters There has always been a myth among some environmentalists that brine water discharged desalination plants inflicts damage to the open near-shore marine environment. This is not the case for the following reasons: • The amount of seawater withdrawn for desalination is relatively minute when compared to the water mass of the open sea. • The amount & nature of salts discharged with the brine are identical to the salt content of the open sea. • The concentration factor increases on the average by no more than three. In case the brine is discharged after blending with cooling waters, this ratio of three will significantly drop to near one. • In order to avoid re-circulation of plant effluents to the intakes, the outlets are specifically engineered to discharge in coastal areas where maximum hydrographic circulation disperse & dilute the brine.

  7. V- Impacts of Increased Salinity on Semi-Enclosed Marine Environment • The discharge of brine water in shallow & relatively stagnant nearly-land-locked coastal areas such as bays, khors, harbors, etc. result into a more pronounced impacts on the encircled marine environment. • The slight increase in salinity in the proximity of points of brine water discharge is projected to be of limited impact on the semi enclosed marine ecology of the Gulf.

  8. VI- Environmental Impacts of Thermal Pollution from Blow-down • The temperature of the brine water effluent is typically above the feed water temperature by 5 to 8 Co. • In the life of marine organisms, temperature elevations from ambient values causes thermal stress that result into an eco-toxicological effect. • According to the US-EPA regulations, the allowable increase in the weekly average temperature beyond 300 meters from the point of discharge is 1 Co. • The common practice is to ensure the minimization of these effects, by selecting the plant sites & the engineering designs of the discharge systems that expedite the dissipation of the thermal inputs to the receiving marine environment.

  9. VII- Environmental Impacts of Residual Chlorine Oxidants in Brine Water The bromide content of sea water ranges from 50 to 70 ppm, well in excess of any chlorine dose applied in the desalination operation. When chlorine mixes with seawater (pH=8.3), the chlorine reacts in <10 sec. with the bromide ions to form hypobromous acid HOCL + Br - -------> HOBr + Cl- • Fish were absent in water plumes having total residual chlorine > 0.05 ppm • Photosynthesis of marine phytoplankton at 0.04 ppm of residual chlorine was reduced. • Discharge of trace levels of residual chlorine oxidants either in open or enclosed seawaters will be very detrimental to aquatic life in the nearshore marine environment.

  10. IIX- Formation of Trihalomethanes (THMs) in Brine Water • The formation of THMs in brine water is a direct consequence of free chlorine reaction with natural organics occurring in seawater & other organic pollutants acting as precursors to form THMs. Some of the volatile THMs species were found to be carcinogenic & mutagenic to humans. • The problem is further complicated by the fact that low boiling point THMs can reach the desalination plant intakes & re-circulate within the system. Once in the intakes, THMs will evaporate then co-distill & concentrate in the potable water condensate. The possible appearance of THMs in desalinated water can pose a serious public health threat to consumers.

  11. IX- Environmental Impacts of Trace Metals in Discharged Brine Water • In thermal desalination plants, it is plausible to find corrosion products in brine waters resulting from the effect of water flow, dissolved gases and treatment chemicals (acids) on the alloys utilized in the construction of desalination pipes & equipment’s. The corrosion products may include harmful heavy metals such as Ni, Cu & Mo & less toxic metals such as Fe & Zn. Hg form chlorine production was a serious problem. • As conservative pollutants, metals will last in different compartments of the marine environment forever. The level of metals reflects the general status of the environment but it doesn’t necessarily reflect the biological availability of these metals.

  12. X- Environmental Impacts of Anti-scalants in Brine Water Alkaline scale forms in the desalination plants. The scales occur when the bicarbonate ion breaks down by heat. • The addition of H2SO4 breaks down the bicarbonate alkalinity & prevent the calcium carbonate scales from forming. Environmental impacts of acid additions is minimal due to the extremely large carbonate buffering capacity of sea water. • A scale inhibitor such as hexametaphosphate & surface active agents like lignin sulphonic acid derivatives & esters of polyalkyl glycols (Hagevap, Albrevap, Salvap) are added in desalination plants to hamper the growth of carbonate & sulfate crystals.

  13. X- Environmental Impacts of Anti-scalants in Brine Water For higher temperature scale inhibition, Belgard EV, Belgard EV2000, Flocon 247 are used. These are polymers of organic acids such as maleic anhydride & acrylic acids. The environmental impact of polyphosphate in reject brine lies in its nutritional value. When present phosphate causes a growth of plants. This excessive plant growth usually means a reduction in diversity of species, and result in an imbalance of food chain materials.

  14. XI- Impact of (VLHs) on the Near-shore Marine Environment • VLHs are defined as compounds with boiling points ranging between n-C6 & n-C14 normal & branched alkanes, monocycloalkanes, aromatics and alkyl – substituted analogues. It also include light aromatics such as benzene & toluene. VLHs are considered to be the most immediately toxic components of petroleum other than the carcinogenic PNAHs. • VLHs that can vaporize & consequently, co-distill during the desalination process.

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