Unit-4
Objectives of water treatment
Q1) What is the difference between unit operations and unit processes?
A1)
Unit Operation
Unit Process
Q2) Write a note on different types of screens used in primary treatment of wastewater.
A2)
It is the first operation in wastewater treatment to remove most of the bigger and longer visible objects such as trees, branches, sticks, rags, boards, animals etc present in raw water of surface water sources as the screens protect pumps and other mechanical equipment’s and to prevent clogging of valves and other appurtenances.
4.2.1.1 Types of Screens
Screens are of three types depending upon the size of opening-
a) Coarse screens
b) Medium screens
c) Fine screens
a) Coarse Screens - They are also known as racks. Spacing between bars is 50 mm or more than this. These screens help in removing large floating objects from sewage. The material separated by coarse screens consists of rags, woods, sticks and paper etc. which will putrefy and must therefore be disposed of by incineration, burial or dumping.
Coarse screens consist of parallel iron rods placed vertically or at a slight slope of about 2 to 10 cm centre to centre. The coarse screens are also now normally kept inclined at about 45-60° so as to increase the opening area to reduce the flow velocity and thus making the screening more effective. Velocity of wastewater through screen should not be more than 0.8 to 1 m/sec. The material which is collected on the upstream side of screens is removed either manually or mechanically. In mechanically cleaned screens, a rack traverses the front of the screen either continuously or intermittently. Mechanical cleaning is done at large plants with mechanically operated rakes while manual cleaning is done at small plants with hand operated rakes.
b) Medium Screens - In this type of screen, spacing between the bars is 6-40 mm. These screens will ordinarily collect 30-90 litres of material per million litre of Sewage. Screenings (material removed by screens) usually contain some quantity of organic material which may putrefy and become offensive, therefore, be disposed of by incineration or burial (not by dumping).
Rectangular shaped coarse and medium screens, made up of steel are now-a-days widely used at sewage treatment plants. These are fixed parallel to one another at desired space on a rectangular steel frame and are called bar screens which are set in screen chamber. Now-a-days, these screens are generally kept at about 30-60° to the direction of flow so as to increase the opening area and to reduce the flow velocity and thus making the screen more effective. Screens can be either fixed or movable depending upon whether the screens are stationary or capable of motion. Fixed screens are permanently set in position while movable can be moved bodily for the purpose of cleaning.
c) Fine Screens - In this type of screen, spacing between bars is 1.5 mm to 3 mm size. It removes about 20% of suspending solids from sewage. These screens are made up of fine wire, brass or bronze plates with openings less than 1 cm. Metal used should be resistant to rust and corrosion. They may be disc or drum type and operated continuously by electric motors. These screens often get clogged and are to be cleaned frequently. They are, therefore, used for treating industrial wastewater or for municipal waste containing industrial wastewater. Cleaning frequency is governed by head loss through the screen. More the screen openings clogged more will be the head loss.
Q3: How many types of grit chambers can be used in primary treatment process?
A3)
Grit removal is done in grit chambers, channels/basins. Inorganic solids such as pebbles, sand, silt eggshells, glass and metal fragments, heavier organics such as bone chips, seeds etc. when collected together, constitute grit, are removed from wastewater to prevent damage to pumps and to prevent their accumulation in sludge digester. Grit chambers are in fact sedimentation tanks designed to separate heavier inorganics by sedimentation due to gravitational forces and to pass forward the lighter organic material. It may be placed either after or before the screen.
1. Channel type
These are horizontal flow grit chambers in which the horizontal velocity is maintained at approximately 0.3 m/sec. Even a 25% increase in horizontal velocity may result in washout of grit whereas 25% decrease may result in retention of non-target organics. So, horizontal velocity must be artificially controlled. It is installed at small plants.
Grit from this type of grit chamber, may contain a sizable fraction of biodegradable organics that must be removed by washing or must be disposed off quickly to avoid nuisance problems. Grit containing organics must either be placed in a sanitary landfill or incinerated along with screenings to a sterile ash for disposal.
2. Aerated rectangular basin
It is installed at large plants. In this, injection of compressed air creates turbulence and keeps lighter organic matter in suspension while the heavier grit falls to bottom. Here roll velocity is more important rather than horizontal velocity as it separates the non-target organics from the grit so artificial control of horizontal velocity is not required. Adjustment of air quantity provides settling control.
Q4) What is the principle lying behind sedimentation?
A4)
The screens and the grit chambers remove most of the floating materials like paper, rags, cloth, wood and tree branches etc and the heavy inorganic settleable solids from the sewage. However, a part of the suspended organic solids which are too heavy to be removed as floating matters and too light to be removed by grit chambers are generally removed by the sedimentation tanks. This process is called
sedimentation. The sedimentation tanks are thus designed to remove apart of the organic matter from the sewage effluent coming out from the grit chambers.
In a complete sewage treatment, the sedimentation is in fact carried out twice - once before the biological treatment and once after the biological treatment.
Principle of Sedimentation
Most of the suspended impurities (organic matter) present in water have a specific gravity greater than that of water. In still sewage, these impurities will therefore, tend to settle down by gravity whereas in a flowing sewage, they are kept in suspension because of turbulence in water. Hence as soon as the turbulence is retarded by offering storage to sewage, these impurities tend to settle down at the bottom of the tank offering such storages. This is the main principle behind sedimentation.
Theory of sedimentation
The settlement of a particle by gravity in liquid, when brought to rest, is opposed by the following factors:
1. Velocity of flow: It carries the particle horizontally. Greater the flow area, lesser is the velocity and hence more easily the particle will settle down.
2. Viscosity of water: In this particle travels. Viscosity is inversely proportional to temperature. Warm water is less viscous and therefore, offers less resistance to settlement.
3. Size, shape and specific gravity of the particle: Greater the specific gravity, more readily a particle will settle. Size and shape of the particle also affect the settling rate e.g. the weight and volume of a spherically shaped particle varies with the cube of its diameter and its area varies with the square of the diameter.
4. Sedimentation tanks: The clarification of sewage by the process of sedimentation can be affected by providing conditions under which the suspended material present in sewage can settle out. This is brought about in specially designed tanks called sedimentation tanks.
Out of the three forces which control the settling tendencies of the particles, two forces i.e., the velocity of flow, and the shape and size of the particles, are tried to be controlled in these settling tanks. The third force i.e. viscosity of sewage or temperature of sewage is left uncontrolled, as the same is not practically possible.
Q5) Design a rapid sand filter to treat 10 million litres of raw water per day allowing 0.5% of filtered water for backwashing. Half hour per day is used for backwashing. Assume necessary data.
A5)
Total filtered water = 10.05 x 24 x 106 = 0.42766 Ml / h
24 x 23.5
Let the rate of filtration be 5000 l / h / m2 of bed.
Area of filter = 10.05 x 106 x 1 = 85.5 m2
23.5 5000
Provide two units. Each bed area 85.5/2 = 42.77. L/B = 1.3; 1.3B2 = 42.77
B = 5.75 m; L = 5.75 x 1.3 = 7.5 m
Assume depth of sand = 50 to 75 cm.
Under drainage system:
Total area of holes = 0.2 to 0.5% of bed area.
Assume 0.2% of bed area = 0.2 x 42.77 = 0.086 m2
100
Area of lateral = 2 (Area of holes of lateral)
Area of manifold = 2 (Area of laterals)
So, area of manifold = 4 x area of holes = 4 x 0.086 = 0.344 = 0.35 m2
\ Diameter of manifold = (4 x 0.35 /p)1/2 = 66 cm
Assume c/c of lateral = 30 cm. Total numbers = 7.5/ 0.3 = 25 on either side.
Length of lateral = 5.75/2 - 0.66/2 = 2.545 m.
C.S. area of lateral = 2 x area of perforations per lateral. Take dia of holes = 13 mm
Number of holes: n p (1.3)2 = 0.086 x 104 = 860 cm2
4
\ n = 4 x 860 = 648, say 650
p (1.3)2
Number of holes per lateral = 650/50 = 13
Area of perforations per lateral = 13 x p (1.3)2 /4 = 17.24 cm2
Spacing of holes = 2.545/13 = 19.5 cm.
C.S. area of lateral = 2 x area of perforations per lateral = 2 x 17.24 = 34.5 cm2.
Diameter of lateral = (4 x 34.5/p)1/2 = 6.63 cm
Check: Length of lateral < 60 d = 60 x 6.63 = 3.98 m. l = 2.545 m (Hence acceptable).
Rising wash water velocity in bed = 50 cm/min.
Wash water discharge per bed = (0.5/60) x 5.75 x 7.5 = 0.36 m3/s.
Velocity of flow through lateral = 0.36 = 0.36 x 10 4 = 2.08 m/s (ok)
Total lateral area 50 x 34.5
Manifold velocity = 0.36 = 1.04 m/s < 2.25 m/s (ok)
0.345
Wash water gutter
Discharge of wash water per bed = 0.36 m3/s. Size of bed = 7.5 x 5.75 m.
Assume 3 troughs running lengthwise at 5.75/3 = 1.9 m c/c.
Discharge of each trough = Q/3 = 0.36/3 = 0.12 m3/s.
Q =1.71 x b x h3/2
Assume b =0.3 m
h3/2 = 0.12 =0.234
1.71 x 0.3
\ h = 0.378 m = 37.8 cm = 40 cm
= 40 + (free board) 5 cm = 45 cm; slope 1 in 40
Clear water reservoir for backwashing
For 4 h filter capacity, Capacity of tank = 4 x 5000 x 7.5 x 5.75 x 2/ 1000 = 1725 m3
Assume depth d = 5 m. Surface area = 1725/5 = 345 m2
L/B = 2; 2B2 = 345; B = 13 m & L = 26 m.
Diameter of inlet pipe coming from two filter = 50 cm.
Velocity <0.6 m/s. Diameter of wash water pipe to overhead tank = 67.5 cm.
Air compressor unit = 1000 l of air/ min/ m2 bed area.
For 5 min, air required = 1000 x 5 x 7.5 x 5.77 x 2 = 4.32 m3 of air.
Q6) Write a note on different types of filters?
A6)
1. Activated Carbon Filters
These are also known as carbon filters or pre-filters and are generally responsible for removing larger particles like sediment and silt from your water. They work by attracting and absorbing these particles so they’re no longer present in the liquid that comes out of your faucet.
An activated carbon filter will also make sure the end result has less in the way of odor and tastes much better. This is because they reduce the amount of chlorine and other contaminants that can make your water smelly or just darn right unpleasant to drink.
2. Reverse Osmosis
This type of filter is incredibly popular mainly because it has the ability to remove all sorts of contaminants that can be a danger to your health, as well as making sure the end result is clear and odor free.
I know this sounds a little vague, so if you want to find out more, feel free to take a look at my section reverse osmosis water filtration.
3. Alkaline/Water Ionizers
These filters use a process known as electrolysis. What this means is the water is passed over plates which are electrically charged, and it’s separated into two streams. One is alkaline and the other is acidic.
Not only do you get softer water as a result, water that’s low in acidity is much better for your skin as well.
4. UV Filters
These types of filters are possibly one of the newest technologies on the market. When ultraviolet radiation is used to treat water, it has the ability to destroy various bacteria that can be damaging to your health.
If you want a more environmentally friendly way of purifying your water, this filter may well be the answer because it doesn’t need any chemicals or additional heat to be effective.
5. Infrared Filters
As with alkaline filters, this technology is used to help soften your water, so if you live in a hard water area infrared technology will help. Much like alkaline filters, infrared uses heat and light to negatively charge the water, and give it a softer feel.
Q7) What are the requirements of an ideal disinfectant?
A7)
Q8) What are the various forms of chlorine that is used as a disinfectant in water treatment process?
A8)
Chlorine may be applied to water in one of the following forms.
1- Bleaching powder- also called calcium hypochlorite Ca (OCL) 2 is a chlorinated lime containing 33.5% of chlorine .it is therefore used for small installations or under emergency conditions. Commercial compounds such as HTH, pitticide, hoodchlor etc. are used instead of bleaching powder.
2- Chloramines- These compounds of ammonia and chlorine. In this test ammonia is added to water just before the chlorine is applied. The usual proportions are 1part of ammonia and 4.5 parts of chlorine by weight. The following reactions takes place-
H20 +CL2= HCL+HOCL
NH3+HOCL=H20+NH2CL (MONOCHLORAMINE)
NH2CL+HOCL=H20+NHCL2 (DICHLORAMINE)
NHCL2+HOCL=H20+NCL3 (TRICHLORAMINE)
3- Free chlorine- It is in gaseous or in liquid form. Gaseous chlorine is a greenish yellow poisonous substance with a typical odour and is 2.5 times heavier than air. Liquid chlorine is amber colored oily liquid and is about 1.44 times as heavy as water. When chlorine is subjected to a pressure of 7kg/cm2 then it is converted into liquid.
4- Chlorine dioxide- Its bacterial properties is greater than chlorine. It is unstable and it is produced at the point of use by passing the chlorine gas through sodium chlorite solution. The following reaction takes place.
2NACLO2+CL2=2NACL+2CLO2
Q9) What are the various methods of chlorination?
A9)
There are various methods of chlorination
1- Plain chlorination
2- Pre chlorination
3- Post chlorination
4- Double chlorination
5- Break point chlorination
6- Super chlorination
7- Dichlorination
1- Plain chlorination
It is application of chlorine which is applied to raw water supply as it enters the distribution system. It also includes the chlorination of raw water as it enters into reservoir to check the growth of weeds, organic matter algae, bacteria. It also removes the color and odour from water. The normal dose is between 0.5 ppm to 1 ppm.
2- Pre chlorination
It is application of chlorine to water before its treatment such as filtration, sometimes chlorination is done before raw water enters into sedimentation tanks. It reduces the quantity of coagulants required. It eliminates taste and odour.
3- Post chlorination-
It is the application of chlorine to water after its treatment. This is the standard form of chlorination in which chlorine is added to water as it leaved the slow sand filters or rapid filters before it enters the distribution system. The dose of chlorine is 0.1 to 0.2 ppm.
4- Double chlorination-
It is also called multiple chlorination. It is the application of chlorine in which chlorine is applied to just before water enters the sedimentation tanks and just it after leaves the filter plants. This is done specially when raw water is highly contaminated and contains large number of bacteria and other organic matter.
5- Break point chlorination-
When chlorine is added to water, it first reacts with ammonia. Breakpoint chlorination is the point where the demand for chlorine has been fully satisfied in terms of chlorine addition to water. When chlorine is added to water, a reaction is produced in the compounds present in the water. These compounds utilize the chlorine, resulting in zero chlorine residual. Once chlorine has been added to water, it is consumed by a type of chemical reaction that has a net effect of increased concentration of chlorine. For typical addition of chlorine, the rate of reaction instantly speeds up, reducing the concentration of chlorine. This is because chlorinated compounds acquire more chlorine.
6- Super chlorination-
Super chlorination is most commonly used when water has very high bacteria content and generally comes from river sources or where some form of pollution has occurred. It is also an important part of swimming pool maintenance because it keeps chlorine content at the right level to effectively kill off bacteria and other contaminants. Super chlorination is also known as shocking.
7- Dichlorination-
Dichlorination is the process of removing residual chlorine from disinfected wastewater prior to discharge into the environment. Sulphur dioxide is most commonly used for Dichlorination. Some Dichlorination alternatives include carbon adsorption, sodium metabisulfite, sodium bisulfite, and hydrogen peroxide. Sodium metabisulfite and sodium bisulfite are mainly used in small facilities because these materials are more difficult to control compared to sulfur dioxide.
Q10) Design a rectangular sedimentation tank to treat 2.4 million litres of raw water per day. The detention period may be assumed to be 3 hours.
A10) Raw water flow per day is 2.4 x 106 l. Detention period is 3h.
Volume of tank = Flow x Detention period = 2.4 x 103 x 3/24 = 300 m3
Assume depth of tank = 3.0 m.
Surface area = 300/3 = 100 m2
L/B = 3 (assumed). L = 3B.
3B2 = 100 m2 i.e. B = 5.8 m
L = 3B = 5.8 X 3 = 17.4 m
Hence surface loading (Overflow rate) = 2.4 x 106/100 = 24,000 l/d/m2 < 40,000 l/d/m2 (OK)
