Impacts Of Seawater Desalination On Marine Environment

Impact of seawater desalination on marine environment is a new andimportant topic. With exponentially growing of seawater desalination andthe nation’s requirement for sustainable development, impact of seawaterdesalination has attracted more interest in both water production andenvironment protection. This thesis explores several aspects of seawaterdesalination that have significant impacts on marine environment.Firstly, once concentrated seawater from desalination is dischargedto the sea, it will cause salinity increase of surrounding seawater, andthen affect the survival of marine organisms. In this paper fourmicroalgae: Prorocentrum micans, Alexandrium tamarense, Skeletonemacostatum and Chlorella are chosen to study the changes of living andreproduction abilities under high salinity conditions. Meanwhile,changes in cell morphologies of Alexandrium tamarense and Skeletonemacostatum are studied in order to explore the mechanism of the salinityimpacts on the growth and reproduction of microalgae. It reveals thatSkeletonema costatum and Chlorella have certain tolerance to salinityincrease, which maintains considerable growth and reproductive abilitiesunder high salinity conditions. Prorocentrum micans and Alexandriumtamarense are salinity sensitive microalgae. Cell morphology ofAlexandrium tamarense has obvious changes when salinity reaches45.0,while cell structure of Skeletonema costatum is damaged in salinity of53.6. According to the results obtained from the study, brine dischargewith superhigh salinity will have negative effects on marinephotoplankton.Secondly, brine from seawater desalination plants contains a largeamount of Cu²⁺. If it is discharged directly to the ocean, the marineenvironment will be greatly affected. Chapter3of this thesis explored the impacts of Cu²⁺dosing on the growth, reproduction and moving abilities,as well as the changes of cell morphologies of Alexandrium tamarense andProrocentrum micans. The results reveal that the increase of Cu²⁺concentration restricts the growth and reproduction ability, while it haslittle impact on the cell morphology of Alexandrium tamarense. Cu²⁺hasstimulating effect on the moving ability of Alexandrium tamarense in shortperiod, but restricting effect in the long term. Addition of Cu²⁺can damagethe cell morphology of Prorocentrum micans in a short time. However, astime goes on, Cu²⁺will not do harm to the other healthy cells, but promotethe growth and reproduction of microalgae. Cu²⁺has different effects onAlexandrium tamarense and Prorocentrum micans. This is a comprehensivereflection of Cu²⁺toxicity and self-adaptive mechanisms of microalgae.If birne is discharged without post-treatment, heavy metals in the brinewill have certain influence on marine microalgae.Thirdly, chapter4of this thesis analyzes the distribution of CaCO³and acid-insoluble sulfides （AIS） in the ambient areas of outlet pointof San Pedro and Valdelentisco desalination plants, and discusses theimpacts of brine discharge on the marine ecosystem. It shows that the mostimportant influence factors are terrigenous input and man-made pollutions.The brine discharge from the desalination plants also has obvious effectson part of the study area. As the amount of brine discharge is varied indifferent seasons, it will cause seasonal variation of CaCO³and AIScontents in the sediments. The brine affects the contents of CaCO³andAIS in two ways. On the one hand, the special chemical characteristicsof brine can influence the formation of CaCO³and AIS. On the other hand,brine discharge can do damage to marine organisms in the ambient seawater,and indirectly affect the contents of CaCO³and AIS.Finally, in chapter5, the quality of intake seawater and brinedischarge of three desalination plants with different intake systems is studied. The three intake systems discussed in this paper, deep-sea openintake, beach wells and horizontal directional drilling, all can obtaingood seawater quality with less than1mg/L suspended solids. However, thethree different intake systems also have great influence on the detailedintake seawater parameters, such as temperature, pH, conductivity,dissolved oxygen and total organic carbon. The contents of metal elementsare very different in intake seawater of three intake systems. There arebasically two reasons. Primarily, beach wells and horizontal directionaldrilling make full use of the filtration function of sediments and seabed,which will greatly reduce the contents of metal. On the other hand, theconstruction, operation and transport process through the pipe caninevitably introduce some metal elements to the intake seawater.Parameters of the brine are mainly affected by the desalination progress.The content of some mental elements are very high in the brine. If it isdischarged to the ocean directly, its impacts on the marine environmentmust be taken into consideration. These findings provide some usefulreferences for the researches of seawater desalination and environmentprotection.