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Solution for Environmental Engineering -2

December 2015

Index

Q.1)

a) ………………………………………………………………………….2-3

b) ………………………………………………………………………….3-4

c) ………………………………………………………………………….4

d) ………………………………………………………………………….N.A

Q.2)

a) …………………………………………………………………………. 5-6

b) ………………………………………………………………………….6-7

c) ………………………………………………………………………….7-8

Q.3)

a) …………………………………………………………………………. 9-10

b) ………………………………………………………………………….10-11

c) …………………………………………………………………………. N.A

Q.4)

a) …………………………………………………………………………. 12-13

b) ………………………………………………………………………….N.A

Q.5)

a) …………………………………………………………………………. 14-15

b) ………………………………………………………………………….N.A

c) ………………………………………………………………………….15-18

Q.6)

a) …………………………………………………………………………. 19

b) ………………………………………………………………………….20-22

c) ………………………………………………………………………….N.A

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Q1)

(a) State principles governing design of building drainage. (5 mks)

Solution:

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(b) List various control measures of air pollution. (5 mks)

Solution:

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(c) Compare one pipe and two pipe system. (5 mks)

Solution:

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(d) The CO content of an air sample is 90µ/m3 at 0oC and 1 atm. Pressure

.Convert it to PPM .Assume necessary data. (5 mks)

Ans: N.A

Q2)

(a) Draw a neat sketch of automatic flushing tank and explain the

operation of flushing. (5 mks)

Solution:

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(b) Design imhoff tank for 25000 people and sewage flow lpcd.Assume

data . (10 mks)

Solution: Similar Sum

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(c) Explain the biological action and processor of sludge

digestion. (5 mks)

Ans: Anaerobic digestion is a collection of processes by

which microorganisms break down biodegradable material in the absence

of oxygen The process is used for industrial or domestic purposes to manage

waste and/or to produce fuels. Much of the fermentation used industrially to

produce food and drink products, as well as home fermentation, uses anaerobic

digestion.

Anaerobic digestion occurs naturally in some soils and in lake and oceanic basin

sediments, where it is usually referred to as "anaerobic activity". This is the

source of marsh gas methane as discovered by Volta in 1776.

The digestion process begins with bacterial hydrolysis of the input materials.

Insoluble organic polymers, such as carbohydrates, are broken down to soluble

derivatives that become available for other bacteria. Acidogenic bacteria then

convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia,

and organic acids. These bacteria convert these resulting organic acids

into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide.

Finally, methanogens convert these products to methane and carbon

dioxide. The methanogenic archaea populations play an indispensable role in

anaerobic wastewater treatments.

It is used as part of the process to treat biodegradable waste and sewage sludge.

As part of an integrated waste management system, anaerobic digestion reduces

the emission of landfill gas into the atmosphere. Anaerobic digesters can also be

fed with purpose-grown energy crops, such as maize.[8]

Anaerobic digestion is widely used as a source of renewable energy. The

process produces a biogas, consisting of methane, dioxide and traces of other

‘contaminant’ gases.[1] This biogas can be used directly as fuel, in combined

heat and power gas engines or upgraded to natural gas-quality biomethane. The

nutrient-rich digest state also produced can be used as fertilizer.

Process stages

The four key stages of anaerobic digestion

involve hydrolysis, acidogenesis, acetogenesis and methanogenesis.[17] The

overall process can be described by the chemical reaction, where organic

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material such as glucose is biochemically digested into carbon dioxide (CO2)

and methane (CH4) by the anaerobic microorganisms.

C6H12O6 → 3CO2 + 3CH4

Hydrolysis

In most cases, biomass is made up of large organic polymers. For the bacteria in

anaerobic digesters to access the energy potential of the material, these chains

must first be broken down into their smaller constituent parts. These constituent

parts, or monomers, such as sugars, are readily available to other bacteria. The

process of breaking these chains and dissolving the smaller molecules into

solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-

weight polymeric components is the necessary first step in anaerobic

digestion.[18] Through hydrolysis the complex organic molecules are broken

down into simple sugars, amino acids, and fatty acids.

Acetate and hydrogen produced in the first stages can be used directly by

methanogens. Other molecules, such as volatile fatty acids (VFAs) with a chain

length greater than that of acetate must first be catabolised into compounds that

can be directly used by methanogens.[19]

Acidogenesis

The biological process of acidogenesis results in further breakdown of the

remaining components by acidogenic (fermentative) bacteria. Here, VFAs are

created, along with ammonia, carbon dioxide, and hydrogen sulfide, as well as

other byproducts. The process of acidogenesis is similar to the way milk sours.

Acetogenesis

The third stage of anaerobic digestion is acetogenesis. Here, simple molecules

created through the acidogenesis phase are further digested by acetogens to

produce largely acetic acid, as well as carbon dioxide and hydrogen.

Methanogenesis

The terminal stage of anaerobic digestion is the biological process

of methanogenesis. Here, methanogens use the intermediate products of the

preceding stages and convert them into methane, carbon dioxide, and water.

These components make up the majority of the biogas emitted from the system.

Methanogenesis is sensitive to both high and low pHs and occurs between pH

6.5 and pH 8. The remaining, indigestible material the microbes cannot use and

any dead bacterial remains constitute the digestate.

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Q3)

(a) Why de-watering of sludge is necessary? Explain the method of de –

watering the sludge on sludge drying beds. (8 mks)

Ans: Water content of sludge may be reduced by centrifugation, filtration,

and/or evaporation to reduce transportation costs of disposal, or to improve

suitability for composting. Centrifugation may be a preliminary step to reduce

sludge volume for subsequent filtration or evaporation. Filtration may occur

through underdrains in a sand drying bed or as a separate mechanical process in

a belt filter press. Filtrate and cent rate are typically returned to the sewage

treatment process. After dewatering sludge may be handled as a solid containing

50 to 75 percent water. Dewatered sludge with higher moisture content are

usually handled as liquids.

THE SLUDGE TREATMENT PROCESS

Thickening - Dewatering] [Digestion] [Drying] [Destruction]

De watering the sludge on sludge drying bed method

A sludge drying bed is a common method utilized to dewater sludge via

filtration and evaporation. Perforated pipes situated at the bottom of the bed are

used to drain seepage water or filtrate. A reduction of about 35% or less in

moisture content is expected after drying. Sludge drying beds are usually

situated beside treatment plants to readily receive and treat incoming sludge

coming from primary or secondary treatment facilities. The basic design

components of the drying bed are composed of (i) concrete structure for bed and

walls, (ii) sand and gravel to be used as filter media, (iii) splash block, (iv)

underdrain, and (v) inlet.

Applicability

Sludge drying beds are suitable for treatment plants serving a population

ranging from 1,000 to 20,000. These facilities exhibit reliability and good

process flexibility. However, during the wet season, its efficiency decreases.

Performance

In terms of its efficiency, while dried sludge is not fully disinfected, the solid

content is increased to 50%–70% of total solids.

Cost

Among the available sludge dewatering methodologies, investment cost for

sludge drying beds is considered the lowest. For operation and maintenance, the

only item to be considered is the labour cost.

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Advantages

• Sludge drying beds are simple to operate and energy-efficient. • It presents the

least cost technology option for dewatering sludge.

Disadvantages

• Treatment is required for seepage water. • Solar power is required. • The beds

are prone to odour and insect problems.

(b) What are the basic difference between aerobic and anaerobic

processes. (6 mks)

Ans: Anaerobic Digestion

Anaerobic compositing is an expensive process to complete. It requires

continual introduction of large quantities of feedstock in order for the process to

work efficiently. This is one of the reasons that it generates large quantities of

methane gas as the food waste decomposes. That methane gas is not only highly

combustible, methane gas is one of the most potent greenhouse gasses on the

planet. Further, as this gas builds up within the system, the pressures within

make it highly explosive and a safety hazard that must be monitored closely.

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Additionally, as the compost is broken down by anaerobic digestion, it creates a

sludge-like material that is even more difficult to break down. This requires

time and considerable amounts of energy to accomplish. As a matter of fact, it

can take up to a year before an anaerobic composter can fully break down the

raw material into a viable compost.

Aerobic Digestion

The process of aerobic digestion that takes place within in-vessel

aerobic composters is very similar to the process that occurs without any human

assistance in nature. However, instead of taking place on the forest

floor beneath the pitter patter of hooves and the like, the process takes place in a

container that is easily monitored and maintained.

As aerobic digestion within the in-vessel composter takes place, the by-

products are simply heat, water, and carbon dioxide (CO2). While CO2 is a

greenhouse gas, it is at least 1/20th as potent as methane. To minimize the

impact on the environment, the CO2 gas can be safely collected via a gas

collection system that will prevent the gas from seeping out into the

environment.

Naturally, one of the most important benefits of aerobic compositing is that the

heat which is produced during the decomposition process is great enough that it

kills harmful bacteria and pathogens within the pile. This is not the heat of

Hades or Phoenix in July, but rather it ranges between 55F and 140F, and it

usually lasts for just a few days or so. While this heat is killing the harmful

bacteria, it is also facilitating the growth of beneficial bacteria species

including psychrophilic, mesophilic, and thermophilic bacteria which thrive at

the higher temperature levels. These bacteria facilitate a healthy biomass that

plants feed on and thrive in.

(c) If 5 days 200 c BOD of waste water is 350 mg/l .What will be the its’s

7 days ,25 o C BOD ? K20 = 0.1 /d .Assume data if required. (6 mks)

Ans: N.A

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Q4)

(a) Draw the flow chart of the municipal sewage treatment plant by using

activated Sludge process and tricking filter. (10 mks)

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(b) Determine the size of high rate trickling filter for the following data:

(i) Sewage flow 4 M.L.D (10 mks)

(ii) Recirculation ratio = 1.5

(iii) BOD5 of raw sewage : 280 mg/l

(iv) BOD removel in PST = 25%

(v) Final effluent BOD5 desired = 30 mg/l

Ans: N.A

Q5)

(a) Explain in detail with sketch sedimentation tank and its design

parameters. (8 mks)

Ans: Sedimentation is a physical water treatment process using gravity to

remove suspended solids from water. Solid particles entrained by the turbulence

of moving water may be removed naturally by sedimentation in the still water

of lakes and oceans. Settling basins are ponds constructed for the purpose of

removing entrained solids by sedimentation. Clarifiers are tanks built with

mechanical means for continuous removal of solids being deposited by

sedimentation. Suspended solids (or SS), is the mass of dry solids retained by

a filter of a given porosity related to the volume of the water sample. This

includes particles of a size not lower than 10 μm.

Colloids are particles of a size between 0.001 µm and 1 µm depending on the

method of quantification. Because of Brownian motion and electrostatic forces

balancing the gravity, they are not likely to settle naturally.

The limit sedimentation velocity of a particle is its theoretical descending speed

in clear and still water. In settling process theory, a particle will settle only if:

1. In a vertical ascending flow, the ascending water velocity is lower than

the limit sedimentation velocity.

2. In a longitudinal flow, the ratio of the length of the tank to the height of

the tank is higher than the ratio of the water velocity to the limit

sedimentation velocity.

Removal of suspended particles by sedimentation depends upon the size

and specific gravity of those particles. Suspended solids retained on a filter may

remain in suspension if their specific gravity is similar to water while very

dense particles passing through the filter may settle. Settle able solids are

measured as the visible volume accumulated at the bottom of an Inhofe cone

after water has settled for one hour.

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Some of the key parameters that has to be considered while designing the

sedimentation tank are as follows:

Estimate your settling velocity according to the characteristics of the solids. you can use Stoke law for this Vs=((g*(P1-P)*D1^2)/18u

Estimate your overflow rate (flow/area)

and some general rules:

basin floor area of 41 Lpm per m2 of flow.

250 to 410 Lpm per m width of weir for outflow.

submerge inlet weir 15% of basin water depth.

use 25 cm wide weirs and use rounded edges .

some general characteristics (for circular tanks):

diameter: 12-45 m

height: 3-5 m

slope: 80 mm/m

(b) Find the volume of digester for population equivalent -7000,loading

rate -0.09 kg/capita/day ,volatile solids in raw sludge -70% ,moisture

content of digested sludge -92% ,storage period required for digested

sludge -90 days. (6 mks)

Ans: N.A

(c) What is biological treatment process? Explain aerobic and anaerobic

process in detail. (6 mks)

Ans: Biological treatment process is a process that seems simple on the surface

since it uses natural processes to help with the decomposition of organic

substances, but in fact, it’s a complex process at the intersection of biology and

biochemistry — a process not completely understood.

Biological treatments rely on bacteria, nematodes, or other small organisms to

break down organic wastes using normal cellular processes. Wastewater

typically contains a buffet of organic matter, such as garbage, wastes, and

partially digested foods. It may also contain pathogenic organisms, heavy

metals, and toxins.

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The goal of biological wastewater treatment is creating a system in which the

decomposition results are easily collected for proper disposal. Scientists have

been able to control and refine both aerobic and anaerobic biological processes

to achieve the optimal removal of organic substances from wastewater.

These types of treatments are used worldwide because they are effective and

economical compared to many mechanical or chemical processes.

Biological wastewater treatment is often a secondary treatment process, used to

remove any material remaining after primary treatment. In the primary water

treatment process, sediments or substances such as oil are removed from the

wastewater.

Wide Range of Biological Processes

The biological processes used to treat wastewater include subsurface

applications, such as septic or aerobic tank disposal systems; a wide variety of

types of aeration, including surface and spray aeration; activated sludge

processes; ponds and lagoons; trickling filters; and anaerobic

digestion. Constructed wetlands and various types of filtration are also

considered biological treatment processes

These types of wastewater treatment methods can generally be divided into

anaerobic and aerobic processes. “Aerobic” refers to a process in which oxygen

is present, while “anaerobic” describes a biological process in which oxygen is

absent.

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Aerobic Wastewater Treatment

Aerobic wastewater treatment processes include treatments such as activated

sludge, oxidation ditches, trickling filter, lagoon-based treatments, and aerobic

digestion. Diffused aeration systems, for example, help maximize oxygen

transfer and minimize odours as the wastewater is treated. Aeration is one of the

first treatment stages as the helpful bacteria and other organisms need oxygen to

decompose organic substances in the wastewater being treated.

A good example of an aerobic treatment method is the activated sludge process.

This is a proven biological wastewater treatment widely used for the secondary

treatment of both domestic and industrial wastewater. It is well suited for

treating waste streams high in organic or biodegradable content and is used to

treat municipal sewage; wastewater generated by pulp and paper mills or food-

related industries such as meat processing; and the wastes of industries

producing waste streams containing carbon molecules.

Anaerobic Treatment

By contrast, anaerobic treatment uses bacteria to help organic material

deteriorate in an oxygen-free environment. Lagoons and septic tanks are among

the various anaerobic treatment methods. The best known anaerobic treatment is

anaerobic digestion, which is used for treating food and beverage manufacturing

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effluents, as well as municipal wastewater, chemical effluent, and agricultural

waste treatment.

Anaerobic digestion also can produce biogas, which is an increasingly

important and valuable wastewater treatment adjunct. It lets users create a

source of income from waste.

The type of biological treatment selected for wastewater treatment — whether

aerobic or anaerobic — depends on a wide range of factors, for example,

compliance with environmental regulations related to the composition of water

discharged into surface water such as streams, rivers, or lakes.

Other Treatments

Other treatments such as chlorination and carbon adsorption are typically used

as adjunct treatments with biological treatments. Membrane-based technologies,

such as reverse osmosis and ultrafiltration, can be used in conjunction with

different types of biological treatments.

There can be instances in which biological treatments can contribute to

pollution. This can occur when the process does not remove enough organic

material from the wastewater. When this nominally treated water is discharged,

it provides nutrients — such as nitrogen or phosphorous — to naturally-

occurring microorganisms that allows them to consume too much oxygen from

the surrounding environment, which contributes to eutrophication, a condition

harmful the environment and which can lead to algal blooms and fish kills.

Researchers continue investigating and experimenting with augmentations to

conventional biological-based wastewater treatment methods in order to further

optimize aspects of the process. For example, Finnish researchers added iron

sulphate to the wastewater prior to biological treatment in order to reduce

phosphorous in tough-to-treat pulp mill wastewater. Other researchers

used ultraviolet light to remove challenging substances such as chemical

residues and pharmaceutical compounds.

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Q6)

(a) Explain intercepting trap in detail (4 mks).

Ans: An intercepting trap is often provided at the junction of a house sewer and

a municipal sewer, so as to prevent entry of foul gases from of the municipal

sewer into the house drainage system. It has a high water seal of 100 mm depth.

Fig. shows the intercepting trap.

Advantages of Interception traps:

Foul gases of larger municipal sewers are prevented from entering house

granage system.

Harmful pathogens are not entered in house drains.

Well designed and constructed interceptors can quickly remove fou

matter of house drain to municipal sewers.

Loss of trap seals:

If a trap seal losses smell from the sanitary applicances would enter the building

.Therefore the watre seal in the trap must be maintained under all

circumstances.

1) Evaporation : When trap is not being used ,the rate of water evaporation

depends upon the relative humidity of the air in the room.

2) Capillary attraction: Is another rare occurance which happens in ‘S’ trap

when a piece of porous material being caught over the bend of a trap

absorbs water and deposits it down the waste discharge pipe.

3) Momentum: This caused bu a sudden discharge of water from a bucket

.Due to velocity water is discharged and it shoots around the trap bend

and goes down the waste pipe, leaving no seal.

4) Wavering out: This caused by the effect of the wind which passes over

the top of the ventilation pipe and thus causes pressure fluctuations.

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(b) Explain construction of sewers and steps involved on laying

of it .(8 mks)

Ans:

The sanitary sewer facilities shall be designed with the future maintenance of

the sewer facilities being considered. The sewer lines shall be sized

appropriately for the development, taking into account any future developments

that could be served by the sewer system. Where future maintenance of the

sewer lines would be difficult due to depth or surface congestion, there may be

additional requirements from what is identified in the Manual. Any special

considerations or requirements will be identified during the review process.

1) Contiguous Areas :

The design engineer shall consider any additional land that is located

within the drainage basin that can drain through the new sewer line and

shall design and provide for future connections to the sewer system. Stub-

outs or other means of future connections to the sewer system must be

provided, which may involve easements and other means to allow for the

least disruptive means for future connections.

2) Pipeline Velocities :

The minimum flow velocity (or scour velocity) in the sewer shall be 2 fps

and the maximum flow velocity (or critical velocity) shall be 15 fps. The

minimum slopes for sewer lines specified in Section 3.4.4 of this Manual

account for the required scour velocity for gravity flow. When calculating

the critical flow velocity for a sewer line, a minimum Manning “n” value

for gravity flow shall be 0.013 for cement lined ductile iron pipe, and

0.009 for PVC or HDPE pipe.

3) Hydraulic Analysis:

Manholes shall be designed so that the flow transitions smoothly across

the invert and turbulence is minimized. The outgoing pipe invert

elevation shall be lowered as necessary to maintain a smooth energy

gradient across the manhole. Where multiple lines combine in a single

manhole, each line shall be analyzed separately to determine the

appropriate exit elevation. All manholes shall have an elevation drop

across the invert. At a minimum there shall be a 0.10 foot drop where the

manhole does not include a break in direction greater than 22 degrees and

a 0.25 foot drop if the manhole includes a break in the direction greater

than 22 degrees for a standard 48 inch diameter manhole.

4) Capacity Estimation

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Criteria Sanitary sewers shall be designed using commonly accepted design

standards, using per-capita flow rates. Sewers shall be designed to flow half

full at peak flow. If requested, the engineer must provide the calculations and

assumptions used in sizing sanitary sewer lines.

5) Lower floor elevation:

Gravity sanitary sewer mains shall be designed so that the lowest floor

elevation of any structure that is served by public sewer is at least 12 inches

above the rim elevation of the connection manhole or the nearest upstream

manhole. If the lowest floor elevation is not 12 inches above the manhole

rim elevation, the customer shall install a private pump station for the sewer

service or an approved backwater valve on the private gravity service line

capable of preventing sewer backflows into the structure.

6) Sewer size and material :

The minimum sewer size for a public gravity sewer main shall be 8

inches. Solid-wall PVC pipe SDR 35 or SDR 26 may be used for sewer

mains, provided that the depth of cover is between 3 feet and 12 feet, and

there are no unusual loadings or concerns. The cover requirements for

PVC pipe will not be changed with increased pipe thickness

classification. High-density polyethylene (HDPE) pipe DR 9 is also

allowed on a case-by-case basis for installations between depths of 3 feet

and 12 feet. Gravity sewer mains with less than 3 feet of cover shall be

ductile iron Pressure Class 350 or Thickness Class 51. Gravity sewer

mains installed with more than 12 feet of cover shall be ductile iron, of

the appropriate Thickness Class for the installation.

Steps involved on laying of sewer

The construction methodology of laying sewerage pipes is first, the pipes

should be laid in a straight line from point to point with a fall meaning at

predetermined angle and a predetermined depth. Two feet must then be

added for every fall. Next, excavation of trench need to be construct using

an excavator so that working space and bedding around the pipes are

allowed. Trench that are more than 1200mm depth must be properly

shored up. The trench should be dug out from the site so that the pipes

can be laid with a fall and the main tapped straight out from the building

and should be at least 15 feet long so that a full length of pipe can be laid

in the trench. The first pipe that needs to be laid first is the pipe from the

curb to the main. The pipes must be placed between the curb and the

main before the water is turned on. If there are any leaks, the pipes need

to be repaired. Pipes should not be covered p until they are tested and

approved for water-tightness. Beams are installed between the intervals to

avoid landslide occurring at both sides of the trench excavation and to

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protect the underground pipes. In refilling the trench, sand or fine gravel

should be placed in first and compactly around the pipe without

disturbing the joint. Then, the trench will covered with good ashes or

gravel. When the trench is refilled, concreting should be done and carried

up minimum of half the height of the pipes, so that these may be securely

bedded in it and also at least 6 inches thick all around.

On the left: Bedding detail for rigid pipes (Clayware) Class B bedding

On the right: Bedding detail for flexible pipes (Plastic)

There are two types of bedding pipes that are for vitrified clay (rigid) pipes and

ABS (flexible) pipes. The function of bedding is to cover the pipes from soil,

large stones or other materials. Rigid pipe materials include clayware, concrete

and cast iron while flexible pipe materials include plastics comprise those

manufactured from PVC, polyethylene and polypropylene. The performance of

each polymer is different depend on the pipe stiffness and the creep ratio. These

different beddings require varying degrees of support to the pipe and the

compaction of the material. It also depends on the type of pipe for permanent

protection against mechanical damage. The bedding factor is the ratio of the

failure load in a crushing machine.

(c) A 30 cm dia sewers having an invert slope of 11: 200 was flowing

full.What would be velocity of flow and discharge ? (n=0.013) .Is the

velocity self-cleaning ? What would be velocity and discharge, when

the same is flowing 0.2 and 0.8 of its full depth? (8 mks)

Porportionate depth

(d/D)

Porportionate velocity

(v/V)

Porportionate

discharge (q/Q)

0.2 0.615 0.088

0.4 0.902 0.3364

0.6 1..072 0.6711

0.8 1.140 0.9781

Ans: N.A