WATER QUALITY ASSESSMENT AND

POLLUTION CONTROLWMA 509

Prof O. Martins, Dr O.Z. Ojekunle and Dr. G.O. OluwasanyaDr O.Z. Ojekunle and Dr. G.O. OluwasanyaDept of Water Res. Magt. & AgrometDept of Water Res. Magt. & Agromet

UNAAB. Abeokuta. Ogun StateUNAAB. Abeokuta. Ogun StateNigeriaNigeria

oojekunle@yahoo.com

PROPERTIES OF WATER

• Water is a chemical compound of oxygen and hydrogen and in the gaseous state can be represented by the molecular formula H2O. The isotopes of hydrogen and three isotopes of oxygen exist in nature, and if these are taken into account, 33 varieties of water are possible.

• The physical properties of liquid water are unique in a number of respects, and these departure from what might be considered as normal for such a compound are of the greatest importance with respect to both the existence of life on earth and the operation of many geochemical processes.

• The boiling point and freezing point of water are both far higher than would be the theoretically expected, considering the low molecular weight of the compound, and the range of temperature over which water as a liquid is wider than might be expected.

MOLECULAR STRUCTURE OF WATER

• Molecular and crystalline structures are often studied by the use of models, in which spheres of various sizes represent the atoms out of which the structures is built. Much information has been obtained, especially through the science of crystallography, as to the distances that separate the ions in crystals, and the effective size of the ions themselves

• The bonds connecting the hydrogen’s to oxygen describe an angle of 105o, so that the two hydrogen are relatively close together on one side of the molecule.

MOLECULAR STRUCTURE OF WATER (Cont)

• The molecule has dipolar properties because the positive charge associated with the hydrogen are connected on one side of the molecule, leaving a degree of negativity on the opposite side. Forces of attraction thus exist between hydrogens of one molecule and the oxygen bonds.

• Hydrogen bonds still remain an important force but their arrangement is continually shifting. From Ice to Liquid to Gas

• The attraction between molecules of a liquid is shown at a liquid surface by the phenomenon called Surface tension. When in contact with surface to which the liquid particles are attracted, water is drawn into the small openings with a force many times that of gravity, induced by the surface tension of the fluid.

PROPERTIES OF WATER

• Chemical Constitution of Water• Ionic and Non Ionic• Ionic

• Anion• Cations

– Major Anions» Bicarbonate, Chloride, Sulphate

– Major Cations» Sodium, Potassium, Calcium, Magnesium

• Non-Ionic• SiO2, Dissolved gases, oily Substance, Synthetic

detergent,

Properties which Affect Quality of Water

• All these impart certain quality characteristic of water, which are called its properties.

• Hardness• Carbonate (Temporary) Hardness CaCO3

• Non Carbonate (Permanent) Hardness CaSO4

• Concentration of Hydrogen-ion, which are expressed in pH units. It is the—Log10H+

• Specific Electrical Conductance• - Increases with temperature: values must therefore be related to the same

temperature (2%)• Colour• Alkanity: Ability to neutralize acid; due to the presence of OH-, HCO3

-, CO32-,

• Acidity: Water with pH 4.5 is said to have acidity; caused by the presence of free mineral acids and carbonic acids

• Turbidity: Measure of transparency of water column; indirect method of measuring ability of suspended and colloidal materials to minimize penetration of light through water.

• Dissolved gasses: O2, N2, CO2, H2S, CH4, NH3, etc.

PHYSICAL CHEMICAL PARAMETER

• Since water is not found in its pure in nature, it is important to determine its combined

• physical, • chemical and • biological characteristics. • This is done through monitoring of water for its

quality.• Physical chemical parameter analyzed in natural

environments; Atmosphere (rainfall), hydrosphere (river, lakes, and oceans) and Lithosphere (Groundwater) are similar-

PHYSICAL CHEMICAL PARAMETER (Cont)

• Temperature: Measurement is relevant • For Aquatic life• Control of waste treatment plants• Cooling purposes for industries• Calculation of solubility of dissolved gases• Identification of water source• Agriculture Irrigation• Domestic uses (Drinking, bathing)• Instrument of measurement is thermometer

PHYSICAL CHEMICAL PARAMETER (Cont)

• pH: Controlled by CO2/HCO3-/CO3

2- Equilibria in natural water. Its values lie between 4.5 and 8.5. It is important

• Chemical and biological properties of liquid• Analytical work• Measurement is done in the field. Most

common method of determination is the electrometric method, involving a pH-meter. It is important to calibrate the meter with standard pH buffer solutions

PHYSICAL CHEMICAL PARAMETER (Cont)

• Dissolved Oxygen: Water in contact with the atmosphere has measurable dissolved oxygen concentration. It values depends on

• Partial pressure of O2 in the gaseous phase

• Temperature of the water

PHYSICAL CHEMICAL PARAMETER (Cont)

• Concentration of salt in the water (the higher the salt content in water, the lower the concentration of dissolved oxygen and the other gases). Measurement is important in

• Evaluation of surface water quality• Waste-treatment processes control• Corrosivity of water• Septicity• Photosynthetic activity of natural water

Effect of Some Physical Parameter and their Measurement

• Temperature: Temperature affects the density of water, the solubility of constituents (Such as oxygen in water), pH, Specific conductance, the rate of chemical reactions, and biological activity of water. Continuous water quality sensor measure temperature with thermistor, which is a semiconductor having resistance that changes with temperature. Modern thermistor can measure temperature to plus or minus 0.1 degree celcius (oC).

Effect of Some Physical Parameter and their Measurement (Cont)

• Specific Conductance: Electrical conductivity is a measure of the capacity of water to conduct an electrical current and is a function of the types and quantities of dissolved substance in water. As concentration of dissolved ions increase, conductivity of the water increases. Specific conductance is the conductivity expressed in units of Microsiemen per centimeter at 25oC. Specific conductances are a good surrogate for total dissolved solids and total ions concentrations, but there is no universal linear relation between total dissolved solids and specific conductance.

• Specific conductance sensors are of 2 types: contact sensors with electrodes and sensor without electrodes

• Multiparameter monitoring systems should contain automatic temperature compensation circuits to compensate specific conductance to 25oC.

Effect of Some Physical Parameter and their Measurement (Cont)

• Salinity: Although Salinity is not measured directly, some sondes include the capability of calculating and recording salinity based on conductivity measurement. Conductivity has long been a tool of estimating the amount of chloride, a principle component of salinity in water. Salinity is commonly reported using the Practical Salinity Scale (PSS), a scale developed to a standard potassium-chloride solution and based on

• conductivity,• temperature and • barometric pressure measurement• Salinity in practical salinity units is nearly equivalent to

salinity per thousand.

Effect of Some Physical Parameter and their Measurement (Cont)

• Dissolved Oxygen: Sources of DO in surface waters are primarily atmospheric reaeration and photosynthetic activity of aquatic plants. DO is an important factor in chemical reactions in water and in the survival of aquatic organisms. In surface water, DO concentration typically range from 2-10mg/l. DO saturation decreases as water temperature increases, and DO saturation increases with increased atmospheric pressure.

• The DO Solubility in saline environments is dependent on • salinity • as well as temperature and • barometric pressure • The technology most commonly used for continuous water quality sensors

is the amperometric method, which measures DO with temperature compensated polarographic membrane-type sensor.

• The newest technology for measuring DO is the Luminescent sensor that is based on dynamic fluorescence quenching.

• The sensor has light emitting diode (LED) to illuminate a specially designed oxygen-sensitive substrate that, when excited, emits a luminescent light with a lifetime that is directly proportional to the ambient oxygen concentration

Effect of Some Physical Parameter and their Measurement (Cont)

• pH: In more technical terms, pH is defined as the negative logarithm of the hydrogen ions concentration. For example in pure water, the numerical value of hydrogen ions concentration 10-7 The logarithm, (or exponent) is -7, and the negative of that is 7

• Because the pH scale is based on logarithms to the base 10, each unit change in pH actually represents a tenfold change in the degree of acidity or alkalinity of a solution. For instance, a solution with a pH = 5 is ten times more acidic than the solution with a pH = 6, likewise a solution with a pH = 4 is 100 times more acidic than the solution with pH = 6.

• Dissolved gases such as carbon dioxide, hydrogen sulphide and ammonia, apparently affect pH. Dagasification (for example, loss of carbon dioxide) or precipitation of a solid phase (for example, calcium carbonate) and other chemical, physical, and biological reactions may cause the pH of a water sample to change appreciably soon after sample collection.

• The electrometric pH-measurement method, using a hydrogen-ion electrode, commonly is used in continuous water-quality pH sensors

Effect of Some Physical Parameter and their Measurement (Cont)

• Turbidity: Turbidity is defined as an expression of the optical properties of a sample that cause light rays to be scattered and absorbed, rather than transmitted in straight lines through a sample. ASTM further describe turbidity as the presence of suspended and dissolved matter, such as clay, silt, finely divided organic matter, plankton, other microscopic organisms, organic acids, and dyes. Implicit in this definition is the fact that colour, either of dissolved materials or of particles suspended in the water also can affect turbidity.

• Turbidity sensors operate differently from those for temperature, specific conductance, DO, and pH, which convert electrical potentials into the measurement of constituent of interest.

Effect of Some Physical Parameter and their Measurement (Cont)

• Most commercially available sensors report data in Nephelometric Turbidity Units (NTU)/ with a sensor range of 0-1000 and an accuracy of -+5 percent or 2NTU, whichever is greater. Some sensors can report values reliably up to about 1500 NTU.

WATER SAMPLING/ WATER POLLUTION

Part 1a

• Grab Sampling

• Composite Sampling

WATER POLLUTIONWater, Water Everywhere: Nor Any Drop to Drink---Samuel Taylor

Coleridge

• If pure water does not exist, outside of a chemist’s laboratory, how can a distinction are made between polluted and unpolluted water? Infact, the distinction depends on the type and the concentration of impurities as well as on the intended use of the water.

• In general terms, water is considered to be polluted when it contains enough foreign materials to render it unfit for a specific benefit use, such as for drinking, recreation, or fish propagation. Actually, the term pollution usually implies that human activities are the cause of the poor water quality.

CLASSIFICATION OF WATER POLLUTANTS

• First, a pollutant can be classified according to the nature of its origin as either a point sources of a Dispersed (Non Point) sources pollutant

CLASSIFICATION OF WATER POLLUTANTS (Cont)

• Point Sources pollutant are easies to deal with than are dispersed sources pollutant; those from a point source have being collected and convened to a single point where they can removed from the water in the treatment plant and the point discharges from treatment plant can easily be monitor by regulatory agencies.

• Pollutants from dispersed sources are much more difficult to control. Many people think that sewage is the primary culprit in water pollution problems, but dispersed sources cause a significant fraction of the water pollution in Nigeria. The most effective way to control the dispersed sources is to set appropriate restriction on land use.

CLASSIFICATION OF WATER POLLUTANTS (Cont)

• In addition to being classified by there origin, water pollutant can be classified into group of substances base primarily on there environmental or health effect. e.g., the following lists identify 9 specific types of pollutants.

• -Pathogenic organism, -Oxygen-demanding substances, -Plant nutrients -Toxic organics, -Inorganic chemicals, -Sediments, -Radioactive substances, -Heat, -Oil

WATER QUALITY EXPRESSION

• EXPRESSING CONCENTRATION• The properties of solutions, suspensions

and colloids depend to large extent on their concentrations. Since concentrations need to be expressed quantitatively, instead of qualitatively terms like dilute or strong, concentration are usually expressed in terms of mass per unit volume, part per million or billion, or percent.

MASS PER UNIT VOLUME:

• One of the common types of concentration is milligram per liter (mg/L).

• If 0.3g of salt is dissolved in 1500mL of water, then the concentration is expressed as 300mg/1.5L=200mg/L, where 0.3g = 300mg and 1500mL = 1.5L (1g=1000mg/L; 1L=1000mL).

• For example, a concentration of 0.004mg/L is preferably written as its equivalent 4g/L. Since 1000g=1mg, e.g., a concentration of 1250g/L is equivalent to 1.25mg/L.

• In air, concentrations of particulate matter of gases are commonly expressed in terms of micrograms per cubic meter (g/m3).

PART PER MILLION:

• One liter of water has a mass of 1kg. But 1kg is equivalent to 1000g or 1 million mg. therefore, if 1 mg of a substance is dissolved in 1 L of water, we can say that there is 1 mg of solute per million mg of water. In other words, there is one part per million (1 ppm)

• 1mg/L=1ppm. • MICROg/L is preferred over its equivalent of

ppb.

PERCENTAGE CONCENTRATION:

• Concentrations in excess of 10000mg/L are generally expressed in terms of percent, for conveniences. For practical purposes, the conversion of 1 percent = 10000 mg/L be used even though the density of the solutions are slightly more than that of pure water (10000mg/L = 10000mg/1000000mg = 1 mg/100 mg = 1 percent).

• A concentration expressed in terms of percent may be also computed using the following expression. Percent = (Mass of Solute (mg)/ Mass of Solvent (mg)) X 100

Work out

• EXAMPLE: A 500-mL aqueous solution has 125mg of salt dissolved in it. Express the concentration of this solution in terms of (a) mg/L, (b)ppm, (c)gpg (d) Percent and (e) lb/mil gal

• Solution• (125mg/500mL)X1000mL/L = 250mg/L• 250mg/L = 250 ppm• (250 mg/L X 1gpg)/17.1 mg/L = 14.6 gpg• Applying this equation Percent = (Mass of Solute (mg)/

Mass of Solvent (mg))X 100• Percent = 0.125g/500g X 100 = 0.025 percent Or divide

250mg/L by 10,000 to get 0.025 percent• 250 mg/L X 8.34 = 2090 lb/mil gal

PHYSICAL PARAMETERS

• Turbidity• Temperature• Colour• Taste and Odour

CHEMICAL PARAMENTER OF WATER QUALITY.

• DISSOLVED OXYGEN• BIOCHEMICAL OXYGEN DEMAND• CHEMICAL OXYGEN DEMAND• NITRATE• PHOSPHATE• IRON• MANGANESE• COPPER• ZINC• TDS• TSS etc

CHEMICAL PARAMENTER OF WATER QUALITY.

• The amount of oxygen used to completely decompose or stabilize all the biodegradable organics in a given volume of water is called Ultimate BOD,

• The BOD is a function of time. At the very beginning of a BOD test, or time = 0, no oxygen will have been consumed and the BOD = 0. As each day goes by oxygen is used by the microbes and the BOD increase. Ultimately, the BODL is reached and the organics are completely decomposed. A graph of the BOD versus time has the characteristic shape called the BOD Curve.

• The BOD curve can be expressed mathematically by the following equation:

• BODt = BODL X (1 – 10-kt)• Where BODt = BOD at any time t. mg/L• BODL = Ultimate BOD, mg/L• k = constant representing the rate of BOD reaction• t = time, d

CHEMICAL PARAMENTER OF WATER QUALITY.

• Example: A sample of sewage from a town is found to have a BOD after 5 d (BOD5) of 180mg/L. Estimate the Ultimate BOD (the BODL) of the sewage assuming that k = 0.1/d for this waste water.

• Solution• BODt = BODL X (1 – 10-kt)• 180 = BODt = BODL X (1 – 10-kt) , It implies that

180 = BODL X (1-10-0.1X5)• Therefore 180 = BODL X (1- 0.316) ; 180 = BODL

X 0.684• Rearranging terms to solve for BODL gives• BODL = 180/0.684 = 260 mg/L Rounded off.

Measurement of BOD5

• The traditional BOD test is conducted in the standard 300-mL glass BOD bottles. The test for 5-d BOD of water sample involves taking two DO measurements: an initial measurement when the test begins, at time t = 0, and a second measurement, at t = 5, after the sample has been incubated in the dark for 5 d at 20oC. The BOD5 is simply the difference between the two measurements.

• For example consider that a sample of water from a stream is found to have an initial DO of 8.0 mg/L. It is placed directly into a BOD bottle and incubated for 5 d at 20oC. After the 5 d, the DO is determined to be 4.5mg/L.The BOD is the amount of oxygen consumed, or the difference between the two DO readings. That is, BOD5 = 8.0 – 4.5 = 3.5mg/L.

SOLIDS:

• Solids occurs in water either in solution or in suspension. These 2 types of solid are distinguish by passing the water sample through a glass-fibre filter. By definition, the Suspended Solid are retain on top of the filter and the Dissolved Solid pass through the filter with the water.

• If the filtered portion of the water sample is placed in a small dish and then evaporated, the solid in the water remain as a residue in the evaporating dish. This material is usually called Total Dissolved Solid TDS. The concentration of TDS is expressed in term of mg/L. it can be calculated as follows.

• • Where A = equal to weigh of dish plus residue. Mg• B = Weight of empty dish• C = Volume of sample filtered mL.

C

XBATDS

1000)-(≡

• Example: The weight of an empty evaporating dish is determined to be 40.525g. After a water sample is filtered, 100mL of the sample is evaporated from the dish. The weight of the dish plus dried residue is found to be 40.545g. Compute the TDS concentration 200mg/l

C

XBATDS

1000)-(≡ 100

1000)525.40-545.40( XTDS ≡

• In drinking water, dissolved liquid may caused taste problems. Hardness, corrosion, or aesthetic problem may also accompany excessive TDS concentration. In wastewater analysis and water pollution control, the suspended retained on the filtered are of primary importance and are referred to as TOTAL SUSPENDED SOLID TSS.

• The TSS concentration can be computed using the TDS equation,

• where A represent the weight of the filtered plus retained solid

• B represent the weight of the clean filter• C represent the volume of the sample filtered

TOXIC AND RADIOACTIVE SUBSTANCES:

• A wide variety of toxic inorganic and organic substances may be found in water in very small or trace amount. Even in trace amounts, they can be a danger to public sources, but many come from industrial activities and improper management of hazardous waste

• A toxic chemical may be a poison, causing death, or it may cause disease that is not noticeable until many years after exposure. A carcinogenic substance is one that causes cancer; substances that are mutagenic cause harmful effects in the offspring of exposed people.

• Some heavy metals that are toxic are Cadmium, Cd, Chromium, Cr, Lead, Pb, Mercury, Hg, and Silver, Ag. Arsenic, as, Barium, Bar, and Selenium, Se, are also poisonous in organic elements that must be monitored in drinking water.

RADIATION:

• The emission of subatomic particles or energy from unstable nuclei of certain atoms, referred to as radiation, poses a serious public health hazard. Obviously, the consumption of radioactive substances in water is undesirable, and maximum allowable concentrations of radioactive materials have been established for public water supplies. Potential sources of radioactive pollutants in water include wastes from nuclear power plants, from industrial or medical research using radioactive chemicals, and from refining of uranium ores. Radon sometimes occurs naturally in groundwater.

BIOLOGICAL PARAMETERS OF WATER QUALITY

• The presence or absence of living organisms in water can be one of the most useful indicators of its quality. In the streams, river, and lakes, the diversity of fish and insect species provide a measure of the biological balance or health of the aquatic environment. A wide variety of different species of organisms usually indicates that the stream or lake is polluted. The disappearance of certain species and overabundance of other groups of organisms is generally one of the effects of pollution.

examples

• BACTERIA: • ALGAE:• PROTOZOA:• VIRUESE: • COLIFORM:

AEROBIC AND ANAEROBIC DIGESTION AND

TYPES OF DECOMPOSITION

• Microorganisms , like all living things, require food for growth . Biological sewage treatment consists of a step-by-step, continuous, sequenced attack on the organic compounds found in wastewater and upon which the microbes feed.

• In the following sections we will look at the processes of aerobic and anaerobic digestion and the decomposition of waste in each process.

Aerobic Digestion

• Aerobic digestion of waste is the natural biological degradation and purification process in which bacteria that thrive in oxygen-rich environments break down and digest the waste.

• During oxidation process, pollutants are broken down into carbon dioxide (CO2), water (H2O), nitrates, sulphates and biomass (microorganisms). By operating the oxygen supply with aerators, the process can be significantly accelerated. Of all the biological treatment methods, aerobic digestion is the most widespread process that is used throughout the world.

• Aerobic bacteria demand oxygen to decompose dissolved pollutants. Large amounts of pollutants require large quantities of bacteria; therefore the demand for oxygen will be high.

Advantages of Aerobic Digestion

• Aerobic bacteria are very efficient in breaking down waste products. The result of this is; aerobic treatment usually yields better effluent quality that is obtained in anaerobic processes. The aerobic pathway also releases a substantial amount of energy. A portion is used by the microorganisms for synthesis and growth of new microorganisms.

• Path of Aerobic Digestion

Aerobic Decomposition

• A biological process, in which, organisms use available organic matter to support biological activity.  The process uses organic matter, nutrients, and dissolved oxygen, and produces stable solids, carbon dioxide, and more organisms.  The microorganisms which can only survive in aerobic conditions are known as aerobic organisms.  In sewer lines the sewage becomes anoxic if left for a few hours and becomes anaerobic if left for more than 1 1/2 days.  Anoxic organisms work well with aerobic and anaerobic organisms.  Facultative and anoxic are basically the same concept.

Anoxic Decomposition

• A biological process in which a certain group of microorganisms use chemically combined oxygen such as that found in nitrite and nitrate.  These organisms consume organic matter to support life functions.  They use organic matter, combined oxygen from nitrate, and nutrients to produce nitrogen gas, carbon dioxide, stable solids and more organisms. 

Anaerobic Digestion

• Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of microorganisms that require little or no oxygen to live. During this process, a gas that is mainly composed of methane and carbon dioxide, also referred to as biogas, is produced. The amount of gas produced varies with the amount of organic waste fed to the digester and temperature influences the rate of decomposition and gas production.

Anaerobic digestion occurs in four steps:

• •  Hydrolysis : Complex organic matter is decomposed into simple soluble organic molecules using water to split the chemical bonds between the substances.

• •  Fermentation or Acidogenesis: The chemical decomposition of carbohydrates by enzymes, bacteria, yeasts, or molds in the absence of oxygen.

• •  Acetogenesis: The fermentation products are converted into acetate, hydrogen and carbon dioxide by what are known as acetogenic bacteria.

• •  Methanogenesis: Is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria.

• The acetogenic bacteria grow in close association with the methanogenic bacteria during the fourth stage of the process. The reason for this is that the conversion of the fermentation products by the acetogens is thermodynamically only if the hydrogen concentration is kept sufficiently low. This requires a close relationship between both classes of bacteria.

• The anaerobic process only takes place under strict anaerobic conditions. It requires specific adapted bio-solids and particular process conditions, which are considerably different from those needed for aerobic treatment.

Path of Anaerobic Digestion

Advantages of Anaerobic Digestion

• Wastewater pollutants are transformed into methane, carbon dioxide and smaller amount of bio-solids. The biomass growth is much lower compared to those in the aerobic processes. They are also much more compact than the aerobic bio-solids.

Anaerobic Decomposition

• A biological process, in which, decomposition of organic matter occurs without oxygen.  Two processes occur during anaerobic decomposition.  First, facultative acid forming bacteria use organic matter as a food source and produce volatile (organic) acids, gases such as carbon dioxide and hydrogen sulfide, stable solids and more facultative organisms.  Second, anaerobic methane formers use the volatile acids as a food source and produce methane gas, stable solids and more anaerobic methane formers.  The methane gas produced by the process is usable as a fuel.  The methane former works slower than the acid former, therefore the pH has to stay constant consistently, slightly basic, to optimize the creation of methane. 

DEMINERALIZATION

• Water, even if it is occurring in nature, consists of lot of minerals which is harmful for both humans and animals. The consumption of these harmful minerals can be avoided by using demineralizer. The minerals present in water are normally calcium, magnesium, sodium, alkalinity, chlorides, sulfates, nitrates, and silica which can also harm industrial pipes, boilers etc by causing corrosion, scale building, spotting on finished surfaces, precipitation in chemical products and other related problems. The demineralizer is designed to sort out these problems.

DEMINERALIZATION

• Dissolved ionic material from water is removed by demineralization process. This process is followed to obtain pure water. Demineralization takes place in an ion exchange unit called as demineralizer or deionizer that consist of cation bed, an anion bed and a mixed bed in series. The mixed bed consist of both cation and anion resins and is called as polisher. The mixed bed provides the highest ion removal efficiency.

Positively charged ions like calcium, magnesium and sodium are removed by the cation bed whereas negatively charged ions like sulfate and chloride are removed by the anion beds.

• It was only after the production of different types of resins demineralization on large scale began. The popular resin used for demineralization purpose are:

• Strong Acid Cation• Weak Acid Cation• Strong Base Anion• Weak Base Anion • Water softening process can be summarized in following chemical

equation:

Softening Process

WHAT IS DESALTING

• Water desalting, or desalination, has long been utilized by water-short nations worldwide to produce or augment drinking water supplies. The process dates back to the 4th century BC when Greek sailors used an evaporative process to desalinate seawater.

• Today, desalting plants worldwide have the capacity to produce over 6.0 billion gallons a day – enough water to provide over 15 gallons a day for every person in the United States.

• About 1,200 desalting plants are in operation nationwide. Most United States plants are used for brackish (moderately salty) ground water treatment for softening and organics removal, or to produce highly purified water for industrial use.

Uses of Desalting

• The conversion of salt water to drinking water is the most publicly recognized desalting use. Desalting processes are also used in home water treatment systems, to treat industrial wastewater, to produce high-quality water for industrial purposes, to improve the quality of drinking water from marginal or brackish sources, and for the treatment and recycling of municipal wastewater.

The Process

• Water desalting is a process used to remove salt and other dissolved minerals from water.

• Other contaminants, such as dissolved metals, radionuclides, bacterial and organic matter may also be removed by some desalting methods. In addition, desalting processes are used to improve the quality of hard waters (high in concentrations of magnesium and calcium), brackish waters (moderate levels of salt), and seawater.

• Desalting separates saline water into two products: fresh water and water containing the concentrated salts, or brine. Such separation can be accomplished by a number of processes. The two most widely used are thermal processes and membrane processes.

Thermal (Distillation) Processes

• Nature, through the hydrologic cycle, provides our planet with a continuous supply of fresh, distilled water. Water evaporates from the ocean (salt water) and other water bodies, accumulates in clouds as vapor, and then condenses and falls to the Earth’s surface as rain or snow (fresh water).

Thermal (Distillation) Processes

• Distillation desalting processes work in the same way. Over 60% of the world’s desalted water is produced by heating salty water to produce water vapor that is then condensed to form fresh water. Since heat energy represents a large portion of overall desalting costs, distillation processes often recover and reuse a portion of the heat required to decrease overall energy requirements. Boiling in successive vessels, each operated at a lower temperature and pressure can also significantly reduce the amount of energy needed.

• Depending on the plant design, distilled water produced from a distillation plant has salt concentrations of 5 to 50 parts per million (ppm) Total Dissolved Solids (TDS). Between 25 and 50% of the source water is recovered by most distillation methods.

Membrane Processes

• Both the electrodialysis (ED) and reverse osmosis (RO) processes use membranes to separate salts from water. No one desalting process is “the best.” A variety of factors come into play in choosing the appropriate process for a particular situation. These factors include the quality of the source water, the desired quantity and quality of the water produced, pretreatment, energy and chemical requirements, and concentrate disposal.

ELECTRODIALYSIS

• Electrodialysis is an electrochemical process in which the salts pass through the membrane, leaving the water behind. It is a process typically used for brackish water. Because most dissolved salts are ionic (either positively or negatively charged) and the ions are attracted to electrodes with an opposite electric charge, membranes that allow selective passage of either positively or negatively charged ions can accomplish the desalting. Freshwater recovery rates for this type of unit range from 75 to 95% of the source water.

REVERSE OSMOSIS

• In reverse osmosis, salt water on one side of a semi-permeable plastic membrane is subjected to pressure, causing fresh water to diffuse through the membrane and leaving behind a concentrate solution, containing the majority of the dissolved minerals and other contaminants. The major energy requirement for reverse osmosis is for pressurizing the source, or “feed,” water. Depending on the characteristics of the feed water, different types of membranes may be used. Because the feed water must pass through very narrow passages in the membrane, larger suspended solids must be removed during an initial treatment phase (pretreatment).

Wastewater Treatment Process

Conclusion

• Nanofiltration plants typically operate at 85 to 95% recovery. Brackish water RO plants typically recover 50 to 80% of the source water and seawater RO recovery rates range from 30 to 60%.