unit ii. part -c. water resources management

7/31/2019 Unit II. Part -c. Water Resources Management

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Unit II

a. Pollution engineering: Types of pollution, Air pollution: sources, effects, technology to combat air pollution and air quality standards. Technologyto combat soil pollution: bioremediation, organic farming. 02 Hrs

b. Waste water engineering: Waste water characteristics, primary water treatment technologies, Secondary water treatment technologies: tricklingfilter, rotating biological contactor, activated sludge process, aeration pond and Tertiary waste water treatment technologies: nitrogen removal,

phosphorous removal and disinfection. 02 Hrs

c. Water Resources Management: water recharging, water conservation and management. Drinkingwater standards, water purification technologies: flash evaporation, electrodialysis and reverse osmosis,

production of mineral water. 01 Hrs

1. Water RechargingVarious kinds of recharge structures are possible which can ensure that rain water percolates in the

ground instead of draining away from the surface. At many places, existing features like wells, pits and

tanks can be modified to be used as recharge structures.

a) Borewells/dug wells. If a borewells are used for recharging, then the casing of the bore well should

preferably be a perforated pipe so that more surface area is available for the water to percolate.

b) Recharge pits: A recharge pit is a pit of 1.5m to 3m wide and 2 m to 3m deep. The pit is lined with brickwall with openings at regular intervals.

) Recharge Trenches: Recharging through recharge trenches, recharge pits are simple compared to

recharge through wells. A recharge trench is a continuous trench excavated in the ground and refilled withporous media like pebbles, brickbats. It can be 0.5m to 1m wide and 1m to 1.5 m deep.

2. Water conservation and Management

Following are some of the strategies adopted in the conservation of water

Surface water resources:Canal water: the best way to reduce seepage losses in canal is by providing lining of different

materials such as precast blocks of cement, brick lining, polythene-cum-brick lining etc.

Run off water: The measures to control the sediment erosion are the establishment of vegetative

barriers, contour trenches, contour furrows etc.

Use of surface and ground water: 1) control of waterlogging and salinization 2) use of saline

water 3) control of salt intrusion in coastal aquifers

Management of groundwater resources: following methods are available for artificiallyrecharging the aquifers

1) Water spreading 2) recharging 3) wells and shafts 3) pumping to induce recharge

from surface water bodies

Rain water Harvesting: water harvesting refers to collection and storage of rain water and alsoother activities aimed harvesting surface and groundwater

Methods of harvesting water

1) Storing rainwater for direct use. It is practiced throughout the world. In some cases, the

rooftop harvesting system is little more than a split pipe or bamboo directing run off fromthe roof into an old oil drum placed near the roof.

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2) Recharging ground water aquifers. Various kinds of recharge structures are possible

which can ensure that rain water percolate in the ground instead of draining away from the

surface. Some structures promote percolation of water through the soil strata at shallowerdepth; others conduct water to greater depths from where it joins the ground water.

Following are kinds of recharge structures a) bore wells b) recharge pits c) soak ways d)

recharge trenchers e) permeable surfaces

Watershed Management: Watershed is simply the land that water flows across on its to a commonstream, river or lake. Water shed can be very large or very small, such as a 20 acre watershed that drains to

a pond. Water shed management involves the exploration and development of the complex

interrelationship between the resources of watershed and the people of the area.

3. Drinking water standards,

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4. Water purification: Reverse osmosis, electrodialysis and flash evaporationA paradox faced by many coastal communities is that of having access to a practically inexhaustible

supply of saline water but having no way to use it. Although some substances dissolved in water, such as

calcium carbonate, can be removed by chemical treatment, other common constituents, like sodium

chloride, require more technically sophisticated methods, collectively known as desalination. In the past,the difficulty and expense of removing various dissolved salts from water made saline waters an

impractical source of potable water. However, starting in the 1950s, desalination began to appear to be

economically practical for ordinary use, under certain circumstances.There are two types of membrane processes used for desalination: reverse osmosis (RO), electrodialysis

(ED) and flash evaporation.

4.1 Reverse osmosis

Osmosis may be described as the physical movement of a solvent through a semi-permeable membrane.

A beaker of water as shown in figure 1 is divided through the center by a semi-permeable membrane. The

black dotted line represents the semi-permeable membrane. We will define this semi-permeable membraneas lacking the capacity to diffuse anything other than the solvent, in this case water molecules.

Now we will add a little common salt (NaCl) to the solution on one side of the membrane (see figure 2).The salt water solution has a greater chemical potential (concentration) than the water solution on the other

side of the membrane. In an effort to equilibrate the difference in chemical potential or concentration,

water begins to diffuse through the membrane from the water side to the salt water side. This movement isosmosis. At equilibrium, the force of gravity operating on the Salt water side (salt solution) exactly

balances the tendency of water to diffuse through the membrane into the salt side. The osmotic pressure is

the pressure that must be applied to return the level of the salt solution to the level of water in the beaker.

By exerting a hydraulic pressure greater than the osmotic pressure difference we can cause water to diffuse

in the opposite direction (Figure 3), into the less concentrated solution. This is reverse osmosis. The

greater the pressure applied, the more rapid the diffusion. Using reverse osmosis we are able to

Figure 2. The osmosis process atwork

Figure 3. Hydraulic PressureCausing Reverse Osmosis

Figure 1. Osmosis

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concentrate various solutes, either dissolved or dispersed, in a solution. Alternatively we can obtain water

free from dissolved solutes i.e., desalination.

RO Process:

A reverse osmosis system consists of four major components/processes: (1) pretreatment, (2)

pressurization, (3) membrane separation, and (4) post-treatment stabilization. Figure 16 illustrates thebasic components of a reverse osmosis system.

Pretreatment: The incoming feedwater is pretreated to be compatible with the membranes by removing

suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused byconstituents such as calcium sulphate.

Pressurization: The pump raises the pressure of the pretreated feedwater to an operating pressure

appropriate for the membrane and the salinity of the feedwater. For brackish water desalination the

operating pressures range from 250 to 400 psi, and for seawater desalination from 800 to 1000 psi.

Separation: The permeable membranes inhibit the passage of dissolved salts while permitting the

desalinated product water to pass through. Applying feedwater to the membrane assembly results in a

freshwater product stream and a concentrated brine reject stream. Because no membrane is perfect in

its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains inthe product water. Reverse osmosis membranes come in a variety of configurations. Two of the most

popular are spiral wound and hollow fine fiber membranes. They are generally made of celluloseacetate, aromatic polyamides, or, nowadays, thin film polymer composites. Both types are used for

brackish water and seawater desalination, although the specific membrane and the construction of the

pressure vessel vary according to the different operating pressures used for the two types of feedwater.

Stabilization: The product water from the membrane assembly usually requires pH adjustment and

degasification before being transferred to the distribution system for use as drinking water. The

product passes through an aeration column in which the pH is elevated from a value of approximately

5 to a value close to 7. In many cases, this water is discharged to a storage cistern for later use.

Elements of the Reverse Osmosis Desalination Process.

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4.2 ElectrodialysisElectrodialys is a membranebased separation processes where the separation of ions is brought about by

differences in membrane permeability under a potential difference. The main advantage of membranebased processes is feasibility of a substance separation without phase changes, usually at ambient

temperature.

When the electrodialysis is running, the direct current field affects the flow of dissociated salt componentsin water solution in such a way that cations moving towards the cathode pass through the cation exchange

membranes and cannot go through the anion exchange membranes, while the anions drawn to the anode

pass through the anion exchange membrane but stop at the cation exchange membranes. Using the rightcombination of anion exchange and cation exchange membranes we can separate the ions in the inlet

solution and create a desalted flow called diluate, and concentrated flow called concentrate. Electrodialysis

takes place in electrodialyzer, i.e. an equipment comprising tightening boards with electrodes and a bundleconsisting of ion exchange membranes and space. The overall result of the electrodialysis process is an ion

concentration increase in the concentrate stream with a depletion of ions in the diluate solution feed

stream.

The major application of electrodialysis is the desalination of brackish water or seawater as an alternative

to RO forpotable waterproduction and seawater concentration for salt production (primarily in Japan). Innormal potable water production RO is generally believed to be more cost-effective when total dissolved

solids (TDS) are 3,000parts per million (ppm) or greater, while electrodialysis is more cost-effective forTDS feed concentrations less that 3,000 ppm or when high recoveries of the feed are required.

CM-cathode Membrane

AM-Anode Membrane

4.3 Flash evaporationFlash evaporation is the processes involve series of evaporation followed by condensation of the resultantsteam and are also known as multiple-effect evaporation and vapor-compression distillation.

This widely used method involves heating seawater and pumping it into lower-pressure tanks, where thewater abruptly vaporizes (flashes) into steam. The steam then condenses and is drawn off as pure water.

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To enhance the efficiency flash-distillation is performed in several steps. This is called multi-stage flash

evaporation or MSF. This is where the preheated liquid passes through a series of stages or chambers with

each successive stage at a lower vapour pressure so some of the liquid will flash at each stage.

Distillation plants having high capacities and using combustible fuels employ various devices to conserve

heat. In the most common system a vacuum is applied to reduce the boiling point of the water, or a sprayor thin film of water is exposed to high heat, causing flash evaporation; the water is flashed repeatedly,

yielding fresh distilled water. This multistage flash distillation method is used in more than 2,000

desalination plants, including one in Saudi Arabia that produces 250 million gallons of freshwater per day.

5. The Bottled Water Purification ProcessBottled water, often called drinking water, is usually bottled at the source and sealed in safe drinkingcontainers. There are many types of bottled water, held inside many types of unique shaped bottles. It

seems the fancier the bottle, the more expensive the water inside.

Kinds of bottled water available are:

--Spring water: this comes from an underground formation and must flow naturally to the earth's surface or

through a sanitary borehole.--Purified drinking water: this type of water has been processed to remove chlorine and a majority of

dissolved solids, such as magnesium. The source is not required to be named unless it is untreated publicsource of water.

--Purified Mineral water: typically from a spring, this contains dissolved solids like calcium, magnesium,

sodium, potassium, silica and bicarbonates.Spring water and Purified water

--Seltzer Water: the FDA regulates this as a soft drink, which means rules are less strict than those for

bottled water. They are similar to Indian sodas.

Manufacturing Process:

1st stage filtration using sand filter to remove suspended solids

2nd stage filtration using activated granular carbon filter to remove odors, colour and other organiccontaminants.

pH adjustment to make water slightly acidic by adding HCl, so that all the salt shall dissolve in.

3rd stage of filtration using RO system to remove dissolved solids.

pH re adjustment to make it neutral.

Disinfection, UV Sterilization/Ozonization - Used to remove any chemical and microbiological

contaminants, including Cryptosporidium.

Packaging - After the bottles have been cleaned and sterilized by an automated process that uses

sanitizing solutions and precise heat, they are automatically filled with spring water and then

capped by automatic bottling lines.

Water is tested by an outside laboratory regularly to verify that it meets and exceeds all central andstate regulations.

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unit ii. part -c. water resources management

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