9 Seawater Desalination Plants: Heavy Coastal Industry
95
To understand the importance of these emissions a detailed discussion on chemical
nature and environmental toxicities is necessary which goes beyond the limitations of
this chapter. One of the discharges (9) should be discussed briefly, however. Apart from
additives and reaction products, desalination plants discharge the same load of seawater
constituents as taken in. The only difference is the concentration. According to Morton
et al. (1996) a typical product recovery is 10% of the amount of raw water taken in.
Then the salinity of the concentrate is 1.1 times higher than the raw water salinity. This
means, for instance, discharge of a concentrate of 44 units of practical salinity (psu)
into a seawater of 40 psu (to choose conditions which are typical of the shallow southern coast of the Gulf).
On the one hand, it is widely accepted that a marine biocoenosis tolerates only salinity
changes of 1 psu (EPRI 1994, cited in Mickley 1995), and conservative discharge recommendations follow this line (Del Bene et al. 1994). On the other hand, in hot and arid
zones evaporation produces elevated and changing salinities exceeding this tolerance
by far. Salinity increases by evaporation are generally larger the lower the water depths.
Local salinity changes with depth and time and depends in addition on solar irradiation, wind, tidal regime, water exchange between shallows and offshore waters and other
influences. Hence, the relevance of salinity increased by a concentrate outfall has to be
examined with careful regard for the local conditions, and usually it is little or absent.
To summarise the emissions of a thermal desalination plant, all emissions have been
applied to a production of 1 000 000 m 3 freshwater (Fig. 9.1). This is not an unrealistic
figure. At some places it is reached within 2 or 3 days, and along the Gulf coasts every
day about 6 000 000 m 3 of freshwater are produced. The flow of matter scheme is a preliminary model using the data of Wang nick (pers. comm.). It needs further theoretical
and empirical research. However, at present it is the only scheme available.
The emissions affect the marine ecosystem as a whole and they affect peculiar subsystems. In some cases (e.g. antifoaming additives) the biological effects are unknown.
Metal ions are expected to accumulate in the sediments. As a rule, concentrations are of
less importance than loads. The extent of the effects depends largely on the sensitivity
of the target environment.
9.6
A Case Study: Copper
Thermal desalination plants discharge copper, nickel, iron, chromium, zinc and other
heavy metals depending on the alloys present in the process line. In terms of concentrations, copper and iron are highest. The lowest copper concentration value recorded
by Oldfield and Todd (1996) is 0.02 ppm (20 ppb, effluent of the Al Khobar desalination
plant). To understand that this is critical requires the comparison with natural background concentrations in seawater. Laane (1992) gives 0.07 ppb. This is an oceanic concentration. Data from the Gulf are not available (and it is questionable whether a "natural background" can be expected anywhere in the Gulf, see Sect. 9.10). Hence, copper
concentrations in desalination effluents are 200-fold (and more) higher than natural
copper concentrations in oceanic seawater.
Assuming 0.02 ppm (20 ppb) copper in the brine (a very low value), a capacity of
500000 m 3 product per day (Khobar has 576000 m 3 ), and a water conversion of 10%,
then 100 kg copper will be discharged with 5 000 000 m 3 brine every day at this site. In
95
To understand the importance of these emissions a detailed discussion on chemical
nature and environmental toxicities is necessary which goes beyond the limitations of
this chapter. One of the discharges (9) should be discussed briefly, however. Apart from
additives and reaction products, desalination plants discharge the same load of seawater
constituents as taken in. The only difference is the concentration. According to Morton
et al. (1996) a typical product recovery is 10% of the amount of raw water taken in.
Then the salinity of the concentrate is 1.1 times higher than the raw water salinity. This
means, for instance, discharge of a concentrate of 44 units of practical salinity (psu)
into a seawater of 40 psu (to choose conditions which are typical of the shallow southern coast of the Gulf).
On the one hand, it is widely accepted that a marine biocoenosis tolerates only salinity
changes of 1 psu (EPRI 1994, cited in Mickley 1995), and conservative discharge recommendations follow this line (Del Bene et al. 1994). On the other hand, in hot and arid
zones evaporation produces elevated and changing salinities exceeding this tolerance
by far. Salinity increases by evaporation are generally larger the lower the water depths.
Local salinity changes with depth and time and depends in addition on solar irradiation, wind, tidal regime, water exchange between shallows and offshore waters and other
influences. Hence, the relevance of salinity increased by a concentrate outfall has to be
examined with careful regard for the local conditions, and usually it is little or absent.
To summarise the emissions of a thermal desalination plant, all emissions have been
applied to a production of 1 000 000 m 3 freshwater (Fig. 9.1). This is not an unrealistic
figure. At some places it is reached within 2 or 3 days, and along the Gulf coasts every
day about 6 000 000 m 3 of freshwater are produced. The flow of matter scheme is a preliminary model using the data of Wang nick (pers. comm.). It needs further theoretical
and empirical research. However, at present it is the only scheme available.
The emissions affect the marine ecosystem as a whole and they affect peculiar subsystems. In some cases (e.g. antifoaming additives) the biological effects are unknown.
Metal ions are expected to accumulate in the sediments. As a rule, concentrations are of
less importance than loads. The extent of the effects depends largely on the sensitivity
of the target environment.
9.6
A Case Study: Copper
Thermal desalination plants discharge copper, nickel, iron, chromium, zinc and other
heavy metals depending on the alloys present in the process line. In terms of concentrations, copper and iron are highest. The lowest copper concentration value recorded
by Oldfield and Todd (1996) is 0.02 ppm (20 ppb, effluent of the Al Khobar desalination
plant). To understand that this is critical requires the comparison with natural background concentrations in seawater. Laane (1992) gives 0.07 ppb. This is an oceanic concentration. Data from the Gulf are not available (and it is questionable whether a "natural background" can be expected anywhere in the Gulf, see Sect. 9.10). Hence, copper
concentrations in desalination effluents are 200-fold (and more) higher than natural
copper concentrations in oceanic seawater.
Assuming 0.02 ppm (20 ppb) copper in the brine (a very low value), a capacity of
500000 m 3 product per day (Khobar has 576000 m 3 ), and a water conversion of 10%,
then 100 kg copper will be discharged with 5 000 000 m 3 brine every day at this site. In