UNIT-4

Water Technology

4.1 Determination of hardness of water by EDTA method

Water is a chemical compound consisting of two hydrogen atoms and one Oxygen atom. The name water typically refers to the liquid state of the compound. The solid phase is called as ice and the gas phase is called as steam. Under specific conditions, water also forms a supercritical fluid. Water is the main compound found in living organisms. Approximately 62 percent of the human body contains water. The word "water" comes from the Old English word water or from the Proto-Germanic water or German Wasser. All of which mean "water" or "wet." The boiling point of water is 99.98 degrees C (211.96 degrees F; 373.13 K).

Water hardness can be readily determined by titration method with the chelating agent, EDTA (ethylenediaminetetraacetic acid) (Greek χηλή, chelè, meaning claw). This reagent is a weak acid that can lose four H (in bold) on complete neutralization;

The four acid oxygen sites and the two nitrogen atoms have unshared pair of electrons, which can form bonds with a metal ion forming a complex ion or coordination compound. The complex that is formed is quite stable, and the conditions of its formation can easily be controlled so that it is ready for selection for a particular metal ion.when a titration to determine the concentration of a metal ion is carried out, the added EDTA quantitatively combines with the cation to form the complex. The endpoint is reached when essentially all of the cation has reacted. In this analysis a solution of EDTA will be standardize by titration against a standard solution made from calcium carbonate, CaCO3. The EDTA solution can then be used to determine the hardness of an unknown water sample. Since both EDTA and Ca2+ are colourless, therefore a special indicator is used to detect the end point of the titration. The indicator mostly used is called Eriochrome Black T, which forms a very stable wine-red complex, MgIn–, with the magnesium ion. A small amount of this complex will be present in the solution during the titration. As EDTA is added, the complex free Ca2+ and Mg2+ ions, leaving the MgIn– complex alone until essentially all of the calcium and magnesium are converted to chelates. At this point EDTA concentration will increase marginally to displace Mg2+ from the complex indicator; the indicator reverts to its uncombined form, which is sky blue, thus establishing the end point of the titration. The titration is carried out at a pH of 10, in a NH3/NH4 + buffer, the equations for the reactions which occur during the titration are:

Titration reaction: HY3–(aq) + Ca2+(aq)                   CaY2–(aq) + H+ (aq) (also for Mg2+) End point reaction: HY3–(aq) + MgIn– (aq)                  MgY2–(aq) + HIn2–(aq)

As the indicator requires a trace of Mg2+ to operate properly, a little magnesium ion will be added to each solution. The effect of the added Mg2+ can be subtracted by titrating a blank.

A simple way to calculate the hardness in water.

A sample of hard water contains 1mg cacl2 and 1 mg mgcl2 per

4.2 Potable water

Potable water is the safe and clean water that is suited for all living beings, it is fit for consuming by all forms of life. It is also called drinking water, in a reference to its intended use. Naturally water can be portable as in springs or they may be treated to make it safe for consumption. However, water is tested and treated before it is used for many purposes.

4.3 Municipal water treatment

The following are the most common processes used for municipal drinking water treatment:

1. Filtration

Water is first treated through filters, they are usually large and contain gravel, sand and charcoal, this helps in removing dissolved particles like bacteria, viruses and chemicals.

Here Rapid sand filters are used, which helps the water tomove vertically through sand this layer of rapid sand has a layer of coal or activated carbon or above it. The top layer helps in removing organic compounds, most of the particles are trapped in the sand while a few that remain are get trapped in pore spaces or adhere to sand particles.

In case of slow sand filters, a large amount of area is required to carry out the process and they mainly depend on the biological processes rather than filtration. The filters consist of graded layers of sand where the coarse sand is at the bottom and the finest sand settles at the top. Filtration depends on the thin biological layer (or biofilm) on the surface of the filter, which provides effective purification by absorbing and metabolizing contaminants.

2. Disinfection

A multi-barrier approach is required to reduce the water contamination as it reaches the taps.More than one disinfection treatment is used for treatment of water, thedisinfectants include ozone, UV radiation, chlorine, carbon and ion exchange, and sometimes reverse osmosis also to remove very minute particles that are dissolved in water.

At the minimum, chlorine or chloramine (a combination of chlorine and ammonia) is added to disinfect the water.  This disinfection method is very common, as the chlorine remains in water till it reaches the consumers, the presence of chlorine also protects the water from further contamination by microorganisms. However, a major problemremains that few people who are sensitive to the smell of chlorine may become aware of occasional changes in chlorine levels in their tap water.

3.Sterilization and Disinfection

These are two processes that deal with decontamination, disinfection mainly is a process that eliminates or reduces harmful microorganisms from surfaces and inanimate objects. On the other hand, Sterilization deals with the process of killing all microorganisms. That is the main difference between sterilizing and disinfecting.  Is that in Sterilization process the viable spores of different organisms that may be present in liquids, or on surfaces or in compounds like culture media. However, such conditions like extreme decontamination is required for places like surgery, or in hospitals, industries or labs. It is more practical to use disinfection in everyday life.

Disinfection is a process that involves disinfectants, some of them are effective as they show a broad spectrum of activity, capable of killing a wide range of However, other disinfectants have a narrow range of activity and is easy to use, they are also inexpensive and nontoxic

Sterilization can be done by three methods: physical, chemical and physiochemical. Physical method of sterilization includes heat, radiation, and filtration. Chemical methods involve using liquid and gaseous chemicals. Physiochemical is a combination of physical and chemical method.

4.4 Boiler Feed water

Boiler feedwater becomes an important part during boiler operations. The boiler feed water is fed into the steam drum, in the steam drum, the water turns into steam from the heat it receives, after the formation of steam it is carried to the main condenser, from the condenser it is pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feed water is never open to the atmosphere. This cycle is known as a closed system. 

Since water impurities cause a lot of boiler problems, good water quality check is a must when water is taken for generating steam. The unique property of water is that, it absorbs more water than any other common inorganic substance. Water is known to expand around 1600times and forms steam at atmospheric pressure. The steam that is formed is capable of carrying large amounts of heat. All these properties of water make it an ideal raw material and also used in power generating processes.

The boiler water that is fed into drum must have a composition, such that the composition of the impurities must be of reasonable number., keeping the limits of the boiler design, if the limits are however exceeded the water must be pre-treated to remove the impurities. The impurities need not be completely removed in all cases, however, since chemical treatment inside the boiler can effectively and economically counteract them.

4.5 Broiler troubles

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1. The absence of heat or hot water.
2. The Leaking and Dripping of the broiler units.
3. Kettling.
4. On occasions the Pilot Light might go off.
5. Boilers that show low pressure performance
6. Sometimes the pipes are Frozen andCondensate.
7. Thermostat Issues.
8. Strange Banging, Whistling or Gurgling Noises.
9. Radiators Not Heating Up.
10. Boiler Keeps Switching Itself Off.

4.6 Foaming and Priming

Foaming:

This occurs inside the boiler Foaming is the formation of thick layer of steam bubbles on the top of water surface inside the boiler due to:

• High concentration of Impurities
• Presence of vegetable or animal fats in feed water (Carryover from Oil Heaters)
• Increase in level of dissolved & suspended solids (TDS level)
• Increase in Boiler water level

4.7 Prevention of Foaming

To prevent foaming, scum or surface blow down shall be carried out frequently to expel any floating impurities from boiler. Also, no lube oil (or oil) shall be allowed to enter the boiler.

Priming:

Priming is the condition of Boiler in which large amount of water is carried along with steam into the steam line. It is Caused by:

• Excessive Foaming
• Insufficient steam space
• Because of sudden rush of steam (This may happen if steam stop valve is opened suddenly)

Prevention of Priming

• Never keep boiler water level too high &
• Open steam stop valve slowly

Actions to be taken during Priming & Foaming of Boiler

• Scum blow down
• Reduce boiler burner firing rate
• Check weather boiler chemical added in excess?
• Detection of source of contamination present in boiler feed water by oil.

4.8 Scale and Sludge formation

Boilers are used for steam generation. When hard water is evaporated, progressively the concentration of dissolved salt is increased. When their saturation points are reached, the dissolved salts of calcium and magnesium along with other soluble impurities are precipitates on the inner walls of boilers and in due course of time adhere to the metal surface in the form of scales and sledges. The sludge is generally form by the substance, which have greater solubility in hot water than cold water. So, sludge is formed in colder parts of boiler.

Hard Scale (Scale)

- Forms in steam boiler

-Has the appearance of a white and brown concentration

-smooth texture

Soft Scale (Sludge)

-Forms in the hot water heaters

-Appears as a thick, brown or black sludge

4.9 Corrosion

Corrosion is the disintegration of a metal due to the chemical reactions between the metal and the surrounding environment. Both the types of metal and the environmental conditions, particularly gasses that come in contact with the metal, determine the form and rate of the corrosion.

All metals can corrode. Some metals, like pure iron, deteriorate very fast. Stainless steel, and, metals that combines with iron and other alloys, is slower to corrode and is therefore used more efficiently.

All small group of metals, are called the Noble Metals, and show much less reaction than others. As a result, they deteriorate rarely.

The main factors which affect corrosion are

1. The more the metal shows its reactivity, the possibility of the metal getting     corrode increases.

2. The corrosion occurs faster when the impurities help in setting up voltaic cell.

3. The rate of corrosion is also affected by the presence of electrolytes in water.

4. The rusting of Iron is increased if the amount of carbon dioxide is present in more amounts in water.

5. The rate of corrosion can however be reduced when the surface of Iron is coated with layers of metal that are actually more active than the Iron itself.

6. An increase in temperature (within a reasonable limit) also increases the rate of corrosion.

4.10 Caustic Embrittlement

This phenomenon occurs in boilers, the caustic materials accumulate in boiler materials. The phenomena can also be described as the cracking of mild steel riveted boiler plates. The process occurs at atemperature of 200°-250°C resulting in the local deposition of concentrated hydroxide.

In Caustic embrittlement the main things that are focussed are the parts of the boiler like bends, cracks, rivets and joints. At high temperature and pressure sodium hydroxide is formed from residual sodium carbonate which undergoes hydrolysis.

Caustic embrittlement is also known as stress corrosion cracking.

There are many causes of caustic embrittlement, including the combined action of the following three components:

• A susceptible material
• A given chemical species
• Tensile stress

Caustic embrittlement can be prevented through several methods, including:

• Controlling the temperature and potential
• Controlling the stress levels and hardness
• Materials are used that do not crack when placed in a given environment.
• Alkali is avoided wherever necessary
• Softening agents such as sodium sulphates can be used in place of sodium carbonates
• Hairlines can be prevented by adding lignin, tannin or sodium sulphate they also prevent infiltration of sodium hydroxide into the areas

4.11 Turbine Deposits

These are deposits, where silica saturation occurs and silica also condenses from the steam in the areas of the turbine. Part of the silica that has been deposited gets evaporated, while the other portions of the silica gets dissolved in the continuous flow of steam. Deposits therefore remain in the turbine.

The deposits on the turbine causes corrosion in blades, discs and rotors. The turbine efficiency, malfunction of valves and its MW generating capacity are also reduced. Itis evaluated that industrial deposits have around 50% deposits on the turbine that are harmful.  It is almost impossible to evaluate turbine deposition without an expensive and time-consuming turbine disassembly.  Without an analysis of the deposits, evaluation of sources of impurities is tough. During inspection when the turbines are opened, it may be too late sometimes as the damage done by the deposits is huge.

Brackish water has more dissolved solids than freshwater, but generally less than seawater. Besides sodium chloride, other prominent dissolved solids include manganese, iron, sodium sulfate/bicarbonate, calcium sulfate/bicarbonate, and occasionally naturally occurring radioactive material (e.g. Radium). The compositions can vary extensively and may even change with time.

Brackish groundwater can be extracted from suitable locations, often below freshwater aquifers. It is increasingly used as an alternative to freshwater. For many purposes brackish water is desalinated to lower its salinity and/or total dissolved solids (TDS). Advanced membrane treatment technologies that are used to do this include reverse osmosis (RO) and ultrafiltration (UF). However, since many brackish water resources are inland, consideration of brine minimization and management for these plants is essential.

95% of the water on Earth is salty, in the face of fresh water shortages, many regions have no choice but to use this resource, which is most often abundant, whether it comes from seawater or brackish water (marshes, ponds, underground aquifers affected by the saltwater wedge).
Desalination aims to remove the salt from salt or brackish water in order to make it potable and to allow its use for drinking, industrial or agricultural uses.

Reverse osmosis is a process of separation in liquid phase by permeation through semi-selective membranes under the effect of a pressure gradient, higher than the osmotic pressure of the water. Under the effect of pressure, water will pass through the barrier of the membrane, while dissolved solids (salts, etc.) will remain confined on the other side.

Applied to seawater or brackish water, the reverse osmosis retains ions and reduce the salinity of water by 99%.

4.14 Electrodialysis

Desalination of brackish water using electrodialysis: Effect of operational conditions as the most countries on the earth, shortage of drinking water is a major problem in Tunisia. One mean to obtain low cost drinking water is the desalination of brackish water. The desalination of brackish water by electrodialysis was investigated in this work. This technique is a membrane separation process based on the selective migration of aqueous ions through ion exchange membranes as a result of an electrical driving force. It represents one of the most important methods for desalting solutions as well as thermal process and reverse osmosis. The parameters which can influence the performance of process were studied. These parameters are: concentration of feed solution, flow rate, voltage and circulation mode. Experiments were carried out on sodium chloride solutions with known concentration. Working in continuous mode showed that the desalination rate does not exceed 55%. However, working in batch recirculation mode, more efficient results were obtained. An initial concentration of 3 g L-1 of salts is considered as the maximum recommended feed concentration.

4.15 Nanotechnology

Nanoscale means one billionth part of a meter or it is also referred to as the dimension of 10-9 meters.The term nanoscale refers to the dimension of 10-9 meters. It is the one billionth part of a meter. Therefore, the particles that have a dimension with respect to their external, internal and surface structure and lie in the range of 1nm to 100nm are called as Nanomaterials

These materials are not visible to the eye, the material that have a science approach to nanotechnology are considered nanomaterials. These materials have unique properties and compared to their molecular-scale behaviour, they show properties that include optical, electronic, quantum and mechanical properties.

A nanomaterial may include nano object or a nanostructured material. Nano objects are the discrete pieces of material, whereas, Nanostructured materials have their internal or surface structure in the nanoscale dimension.

Nanomaterials can occur naturallyor can be, artificially manufactured or incidentally formed. With the advance in the research and technology, nanomaterials are being commercialized and are being used as commodities.

Properties of Nano materials

The physical properties of the bulk are less known when compared to viewing its properties from a condensed range matter as unique properties are displayed, between the dimensions on an atomic scale and the normal dimensions. Some such peculiar properties are known, many properties are still to be discovered. Some known physical properties of nanomaterials are related to different origins: for example, (i)imperfections are reduced large fraction of surface atoms, (ii)  surface energy, (iii) spatial confinement, and (iv) surface atoms having large fractions. The following are just a few examples:

1. Nanomaterials, show huge fraction of surface atoms, from the total amount of atoms, they are known to significantly lower melting point or phase transition temperature and also lattice constants are reduced.
2. Mechanical properties of nanomaterials include strength that is higher than the single crystals in the bulk form, which may be a few magnitudes higher. The minimal absence of defects may be the reason for the enhancement in mechanical strength.
3. The properties that are optical in nature are markedly different from the nanomaterial and the bulk crystals. For example, for a given semiconductorthe optical absorption peak is shifted to a shorter wavelength, as the band gap shows an increase. Thecolour of metallic nanoparticles may change with their sizes due to surface plasmon resonance.

The Electrical conductivity of Nanoparticles shows a decrease with a reduced dimension that is caused due to increased scattering of surface.However, this feature of electrical conductivity in nanomaterials could also be improved, due to the better ordering in microstructure, e.g. In polymeric fibrils

Applications of Nanomaterials

Today the commercial value of nanomaterialsishaving increased.

• The commercial products of nanomaterial available in market include electronics, cosmetics, strain resistant textiles, sunscreens paint etc.
• Consumer products such as sports equipment’s, windows. And automobilesare produced by Nanocoating’s and nanocomposites.
• Nanocoating is applied to glass bottles, due to the damage cause by sunlight, nanocoating blocks the UV lights that protect the bottles.
• Composites of Nano clay are used to manufacture long lasting tennis balls. Nanoscale is used as a filler in dental fillings.
• The optical properties of the nanomaterials are used in photochemistry and also in biomedicine, they are also widely used to create displays solar cells detectors sensors etc.
• In microbial fuel cells, the electrodes are made up of carbon nanotubes. Nanocrystalline zinc selenide is used in the display screens to increase the resolution of the pixels forming High Definition TV sets and personal computers. In the microelectronic industry, miniaturizing of circuits such as transistors, diodes, resistors, and capacitors is emphasized.

References: Nanoparticles technology Handbook by Mindy Adams

Water chemistry Industrial and Power station Water treatment

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