Real Risk

It is also important to note that effusions during the commis-

sioning, recommissioning and/or start-up of a desalination plant

following construction or periods of shutdown for regular main-

tenance procedures cause more biological risks than the effluent

regularly discharged when the plant is operating normally.

Risk of increased salinity in open seawaters

There has long been a misconception among some environmental-

ists that the concentrated salt content of the brine water disposed

of by seawater desalination plants poses significant risk to the open

near-shore marine environment. Fortunately, this has not been

substantiated in the Gulf region for one or more of 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• The amount of seawater withdrawn for desalination is relatively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 and nature of salts discharged with the brine are

identical to the salt content of the open sea

• The brine concentration factor increases on average by no more

than three-to-one. In most cases, brine is mixed with power

production cooling waters, reducing the concentration factor

from three to near-one

• In order to avoid the recirculation of plant effluents to the

intakes of desalination plants, the outlets are specifically

designed to discharge in coastal areas where hydrographic

currents can easily disperse and dilute the brine.

It is safe, then, to assume that the risk associated with the increased

salinity of open seawater in the vicinity of large seawater desalina-

tion plants has been overstated, and should not give reasons for

concern, particularly when near-shore hydrographic circulation

patterns are considered in the design of the plant’s discharge outlets.

Risk of increased salinity in semi-enclosed marine

environments

The discharge of brine water in shallow and relatively stagnant,

nearly-land-locked coastal areas such as bays and harbours in the

Gulf region might result in more pronounced risks. The unlimited

disposal of brine water in sheltered bays or harbours might pose

serious risks to local marine habitats, particularly to fauna and flora,

resulting in the near disappearance of a variety of marine organisms.

Semi-enclosed and shallow bays in the Gulf are naturally char-

acterized by a higher salt content due to the elevated rate of

evaporation, lack of freshwater discharges and restricted dispersion

and dilution. In some near-shore localities in the Gulf, the average

salinity in summer can reach some 48 parts per thousand (ppt),

compared to 37 ppt in winter. On the other hand, it is reasonable

to suggest that the indigenous biota has adapted to the naturally

prevailing environmental conditions characterized by its elevated

salinity. This high salt content, in addition to natural cycles of even

higher salinity, might have impacts several times the order of magni-

tude of discharges from desalination plants. Very limited

environmental risks were documented, associating natural cycles

of higher salinity with negative ecological implications. Hence, the

slight increase in salinity in the proximity of points of brine-water

discharge is projected to pose limited risk to the enclosed marine

ecology of the Gulf.

Environmental risks of thermal pollution from blow-down

The temperature of the brine water effluent resulting from thermal

desalination processes is typically five to eight degrees Celsius

above that of feed water. The degree of damage to the biota

present in the vicinity of the point of discharge is assumed to be

a function of its type, the temperature levels (levels of exposure)

and duration of thermal inputs (duration of exposure).

In the life of marine organisms, temperature elevations from

ambient values cause thermal stress that might result into an eco-

toxicological effect such as disturbed enzyme activity, water

balance and cellular chemistry. The buoyancy, locomotion and

respiration of some marine organisms have been found to be

disturbed as a result of temperature-induced changes in density,

viscosity and solubility of gases in the receiving waters. However,

if the near-shore is open and well mixed, then the risks will only

be noticeable to within 300 metres from the discharge point.

The impact of thermal pollution in enclosed areas might be

more significant. These could be manifested by changes in

community structure such as types of dominating organisms, and

by changes in the characteristics of the individual species such as

lower tolerance or adaptation. Thermal pollution will lower the

amount of DO, increase bacterial and aquatic invertebrate activ-

ity, increase the growth rate of microscopic plants and fish, and

increase the sensitivity of aquatic life to toxic elements. The

common practice in modern desalination plants in the Gulf region

is to ensure the minimization of these effects by selecting plant

sites and engineering designs for their discharge systems that will

expedite the dissipation of their thermal inputs in the receiving

near-shore marine environment.

Environmental risks of residual chlorine oxidants in

brine water

For well over half a century, chlorine has proven to be of immense

benefit in bio-fouling control, in power plants, and lately in desali-

nation plants. However, adverse toxic effects at trace levels have

become evident. The chemistry of seawater is unique when chlo-

rination is involved, owing to the presence of bromides in

seawater. When mixed with seawater, chlorine reacts immediately

with the bromide ions to form hypobromous acid.

The discharge of residual chlorine even at the very low fraction

of one part per million (ppm) is posing a real risk to the near-

shore marine environment, particularly to fish and invertebrates

that were found to be more sensitive to residual chlorine oxidants

than aquatic plants. Given these facts, it appears that the

discharge of trace levels of residual chlorine oxidants, either in

open or enclosed seawaters, is posing serious risks to aquatic life

in the near-shore marine environment.

Risk of trihalomethanes in brine water

The formation of trihalomethanes (THMs) in brine water is a direct

consequence of the chlorination process – free chlorine reacts with

the natural organics occurring in seawater and other organic pollu-

tants to form THMs. Some of the volatile THM species were found

to be mutagenic to humans and harmful to seafood.

Brominated species dominate the formation distribution, with

bromoform (CHBr3) accounting for more than 90 per cent of the

total THMs. Currently, there is growing concern within the scien-

tific community about possible damage to the near-shore marine

ecology into which chlorinated brine is discharged. Except in the

immediate vicinity of the brine water point of discharge, it is very

unlikely that the concentrations of THMs are significant enough

to pose any ecological threat. However, near the point of discharge

the relatively high concentrations may pose some risks.

Environmental risks of trace metals in discharged brine water

In thermal desalination plants, it is plausible to find corrosion

products in brine water resulting from the effect of water flow,

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