Sea water characteristics
The efficiency of a desalination process largely depends upon the water nature to be submitted to this treatment. Hence, the greater its contents on salts, the worse quality the final effluent will have.
The water sources for a desalination plant can be different: Specially there are mainly two big types of water sources:
a) Brackish and sea water. Mainly its difference dwells on either salts contents
b ) Water from sewage plant.They come from a sewage plant which salts content is so high that require a further desalination process in order to be used in gardening irrigation or other uses.
Within the quality of water to be treated, different parameters can be defined:
1.- Physical parameters
1.- a) Smell and taste: None of these parameters can be easily measured as a buffer substance does not exist in order to identify the threshold smell and taste Nevertheless, there definitely is a relationship between these parameters and their belonging to one group or another throughout the sewage water group.
1.-b) Colour: The colour can be measured by a buffer substance that defines the absolute zero from a ray of light that crosses the water inside a sample. Their units are Co-Pt Units units.
1.-c) Turbidity: Measured in NTU (Nephelometric Turbidity Units) represents the resistence of a ray of light after incidir reflected from a sample of water to be measured. There are other but rarely used colour units.
1.-d) Conductivity and salinity: Both properties are closely linked between each other and related to the total disolved salts in a water sample.
The relationship between them is approximately as follows:
TDS (mg/L)= EC (uS/cm)*0.72.
1.-e) Temperature: This is an important datum for the desalination plant design as the salt passage depends upon it producing, as a consequence, an increase of 3% approximately for each Celsius degree, and requiring a lesser working pressure when the temperature increases.
2.- Chemical parameters
2.- a) pH: The great or tiny water acidity and its chemically neutral value 7.0 referred to the balanced equilibrium between H+ and OH- ions although the Water Healthy and Technical Regulation stablishes and normal average between 6.5 and 9.5.
2.- b ) Colloids: Their presence is closely related with the water turbidity and its elimination is achived by means of a Coagulation-Flocculation or by means of Ultrafiltration.
3 .- Solved Solids: Its a measure that indicates a main parameter for the membrane processes design so that, it is very important determining not only the working pressure but also the conversion.
4 .- Suspension Solids: Their presence can lead to the membranes fouling so this is the main reason for them to be removed by means of a filtering bed or using a microfiltration process.
5.- Total solids: It is the addition of total solved solid plus suspended solids.
6 .- Cations: They carry the positive electric ions involved in the inlet stream to the system.
The most important ones are:
a ) Sodium and Potasium
Sodium is the most abundant of the alkaline metals and the only one that turns up in significant quantities within the sewage and sewerage waters to the system. STR stablishes an average of 200 mg/L.
On the other hand, potasium reacts chemically with the sodium and forming soluble salts Por el contrario, el potasio reacciona químicamente con el sodio formando sales solubles y no provocando problemas importantes en las instalaciones que incluyen los procesos de desalación.
b ) Calcium and Magnesium
They are responsibles of the water hardness in accordance to the Sodium Adsortium Ratio (SAR) value. This value is determined by the semiaddition of the two ions above mentioned which values must be given in mequivalents per liter.
Another important parameter for the water hardness is the hidrogencarbonates ions concentration.
c) Ferrum: It can turn up as a ferrous or ferric iondepending on the pH value, chemical composition and organic matters within the water sample.
STR stablishes an average of 200 ug/L.
Three kinds of anions can be distinguished, chlorides, carbonates and hidrogencarbonates(all of them in the same group) and also sulphates.
a) Chlorides: They usually are the most abundant anions. For potable water their concentration is less than 250 mg/L.
b ) Sulphates: They form low or medium solubility salts with calcium, magnesium and/or barium. They can be considered factor converters.
c) Hidrogencarbonates y carbonates: The relationship between them determines the pH value but not only have they influence on the pH value but also the conversion and surfactants.
8.- Minor elements:
a) Ferrum: From a concentration of 0.5 ppm on can appear in both forms: In ferrous state (Fe++) rather soluble, but not in ferric state (Fe+++) in which it precipitates.
b ) Manganesum: Its behaviour is similar to the irons but its contents must not be higher than 50 ug/L.
c) Sílicon: It can turn up as silicic acid and colloid matter and can reach a non soluble concentration (110-130 mg/L) according to temperature and pH.
d) Pollutant Metals : Arsenic, cadmium, plumbium, chromium, barium and selenium usually dwell in polluted waters mainly due to some salts low solubilty
e) Nitrates: Same as previous substances, their presence is mainly due to the pollution. Membranes do not normally have rejection to this type of salts.
f) Boron: The membrane manufacturers are trying to make their utmost in order to eliminate it from the product water. Some ion exchange resins have been tested in order to eliminate the boron contents from the product water. Another possibility is adding a second stage system and incorporate a second pass within the second stage with brackish water Boron high rejection membranes.
All these techniques are valid in order to reduce the Boron final concentration up to 0.5 ppm in the effluent in accordance to the Israelite Authorities.