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YANGON TECHNOLOGICAL UNIVERSITY

DEPARTMENT OF CIVIL ENGINEERING

CE – 5018

ENVIRONMENTAL ENGINEERING (II)

SAMPLE QUESTIONS AND SOLUTION

Question 1

Define the following terms

(a) Aeration period

(b)

BOD loading

(c) F/M ration

(d) eturn sludge rate

!olution

(a) Aeration period

Aeration period in an acti"ated sludge process is e#ual to "olume of the aeration basin

di"ided b$ flow of raw water% !ince biological filters do not contain a li#uid "olume& h$draulic

loading is presented as the amount of waste water applied per unit of surface area& for e'ample& cubic

meters per s#uare meter per da$%

(b) BOD loading

Organic loading on biological treatment units are stated in terms of ilograms of applied

da$ BOD% *oading on an aeration basin is commonl$ e'pressed as grams BOD applied per cubic

meter of tan "olume per da$% A biological filter loading uses the same units e'cept the "olume

refers to the #uantit$ of media rather than li#uid "olume%

(c) F/M ration

+he food to micro,organism ration (F/M) is a wa$ of e'pression BOD loading with regard to

the microbial mass in the s$stem% +he F/M "alue as grams of BOD applied per da$ per gram of

M*!! in the aeration tan% F/M ration as an e'pression of BOD loading relates to the metabolic state

of the biological s$stem rather than to the "olume of aeration basin%

(d) eturn sludge rate

+he rate of return studge from the final clarifier to the aeration basin is e'pressed in

percentage of the raw waste water influent% -f the return acti"ated sludge rate is . percent and the

raw waste water flow into the plant is 1% m /s& the recirculated flow e#ual %. m /s& BOD remo"ed. .

in aeration and subse#uent setting b$ the raw BOD entering%

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Question

2'plain briefl$ the effects of organic pollution on stream%

!olution

Man$ waste discharges from municipalities and industries contain organic compound that

decompose using dissol"ed o'$gen% +he rate of biological stabili3ation is a time,temperature

function with deo'$genation increasing as the temperature rises% O'$gen is replenished in the

stream water primaril$ b$ reaeration from the atmosphere% +hus& #uantit$ of flow& time of passage

down the ri"er& water temperature and reaeration and reaeration are the four ma4or factors that

go"ern self,purification from organic wastes%

A stream polluted from a substantial point source of organic matter e'hibits four fairl$ well

defined 3ones% +he 3one of degradation& immediatel$ following the sewer outfall& has a progressi"e

reduction of dissol"ed o'$gen used up in satisf$ing BOD% +he 3one of acti"e decomposition e'hibits

the characteristics of significant pollution% Dissol"ed o'$gen is at a minimum le"el and often

anaerobic decomposition of bottom mud results in offensi"e odors% 5igher forms of life& particularl$

fishes& find the en"ironment of these polluted ones undesirable% Bacteria and fungi thri"e on the

decomposition of organic decreasing the BOD and increasing ammonia nitrogen% -n the 3one of

reco"er$& reaeration e'ceeds the rate of deo'$genation and the le"el of dissol"ed o'$gen increases

slowl$% Ammonia nitrogen is con"erted biologicall$ to nitrate% otifer crustaceans and tolerant fish

species reappear% Algae thri"e on the increase in inorganic nutrients that result from the stabili3ation

of the matter% +he 3one of clean water supports a wide "ariet$ of a#uatic plants and animals and

more sensiti"e fishes% Dissol"ed o'$gen returns to its original "alue& and the BOD has been nearl$

eliminated% +he permanent changes in water #ualit$ prior to the waste discharge and the clear water

3one include an increase in inorganic compounds& such as nitrate& phosphates& and dissol"ed salts%

+hese nutrient produce and support higher algal population other en"ironmental conditions of

sunlight& p5 and temperature are ade#uate%

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Question .

6hat is waste stabili3ation pond7 2'plain briefl$ about facultati"e ponds%

!olution

!tabili3ation ponds& also called lagoons or o'idation ponds are generall$ emplo$ed as

secondar$ treatment in rural areas% 8onds are classified as facultati"e& tertiar$& aerated& and anaerobic

according to the t$pe of biological acti"it$ that taes place in them%

Facultati"e 8onds are the most common lagoons emplo$ed for stabili3ing municipal waste

water% +he bacterial reactions include both aerobic and anaerobic decomposition and& hence& the term

facultati"e pond% 6aste organics in suspension are broen down b$ bacteria releasing nitrogen and

phosphorus nutrients& and carbon dio'ide% Algae use these inorganic compounds for growth& along

with energ$ from sunlight& releasing o'$gen to solution% Dissol"ed o'$gen is in turn taen up b$ the

bacteria& thus closing the s$mbiotic c$cle% O'$gen is also introduced b$ reaeration through wind

action% !ettleable solids decomposed under anaerobic conditions on the bottom $ield inorganic

nutrients and odorous compounds& for instance& h$drogen sulfide and organic acids% +he latter are

generall$ o'idi3ed in the aerobic surface water thus pre"enting their emission to the atmosphere%

Bacterial decomposition and algal growth are both se"erel$ retarded b$ cold temperature%

During winter when pond water is onl$ a few degrees abo"e free3ing& the entering waste organics

accumulate in the frigid water% Microbial acti"it$ is further reduced b$ ice and snow co"er that

pre"ents sunlight penetration and wind reaeration% 9nder this en"ironment& the water can become

anaerobic causing odorous conditions during the spring thaw& until algae become reestablished% +his

ma$ tae se"eral wees depending on climatic conditions and the amount of waste organics

accumulated during the cold weather%

Operating water depths range from %: to 1% m& with %;m of die freeboard abo"e the high

water le"el% +he minimum %: m depth is needed to pre"ent growth of rooted a#uatic weeds& but

e'ceeding a depth of 1% m ma$ create e'cessi"e odors because of anaerobiosis on the bottom%

Facultati"e ponds treating onl$ domestic waste water normall$ operate odor free e'cept for a

short period of time in the spring of the $ear% On the other hand& lagoons treating municipal waste

water that include industrial wastes can produce persistent obno'ious odors% Often this is the result of

organic o"erload from food,processing industries or a result of the odorous nature of the industrial

wastes itself or both% +he best solution is to re#uire pretreatment of the offending waste waters prior

to discharge to the sewer s$stem%

Facultati"e ponds are best suited for small towns that do not anticipate industrial e'pansion&

and where e'tensi"e land area is a"ailable for construction and effluent disposal% +he ad"antages of

low initial cost and ease of operation& as compared to a mechanical plant& can be offset b$

operational difficulties% +he e$ problems are poor assimilati"e capacit$ for industrial wastes&

odorous emission& and meeting the minimum effluent standards for disposal in surface waters%



Question <%

2'plain briefl$ about e'tended aeration%

!olution

+he most popular application of this process is in treating small flows from schools&

subdi"isions& trailer pars& and "illages% Aeration basins ma$ be cast,in,place concrete or steel tans

fabricated in a factor$% =ontinuous complete mi'ing is either b$ diffused air or mechanical aerators&

and aeration periods are < to .: h% Because of these conditions& as well as low BOD loading& the

biological process is "er$ stable and can accept intermittent loads without upset% For e'ample& a unit

ser"ing a school ma$ recei"e waste water during a 1 h period each da$& for onl$ fi"e da$s a wee%

=larifiers for small plants are conser"ati"el$ si3ed with low o"erflow rates& ranging from > to

<m. /m% d& and long detention times% !ludge ma$ be returned to the aeration chamber through a slot

opening b$ emplo$ing an air,lift pump% A slot return re#uires periodic cleaning to pre"ent plugging

b$ settled solids% Although satisfactor$ performance can be achie"ed& returning settled solids b$

pumping pro"ides a more positi"e process control% !ludge that floats to the surface of the

sedimentation chamber is returned to the aeration tan b$ either h$draulic action or through a

simming de"ice attached to the air,lift pump return%

+here is usuall$ no pro"ision for wasting of e'cess acti"ated sludge from small e'tended

aeration plants% -nstead& the mi'ed li#uor is allowed to increase in solids concentration o"er a period

of se"eral months and then is discharged directl$ from the aeration basin% +his is performed b$

allowing the suspended solids to settle in the tan with the aerators off& and then pumping the

concentrated sludge from the bottom into a "ehicle for hauling awa$% +he M*!! operating range

"aries from a minimum of 1& mg/l to a ma'imum of about 1& mg/l% -n treating domestic

waste water under normal loading& the mi'ed li#uor concentration increases at the rate of

appro'imatel$ mg/l suspended solids per da$%

*arger e'tended aeration plants consisting of an aeration basin& clarifier and aerobic digester

are used b$ small municipalities% +he basin ma$ be a concrete tan with diffused aeration& a lined

earth basin with mechanical aerators& or a race,trac,shaped o'idation ditch% Final clarifiers are

generall$ separate circular concrete tans with mechanical sludge collectors% -n most cases& the BOD

loading of these s$stems is at the upper limit of g/m , d of BOD for e'tended aeration& and the.

aeration period is as low as 1h% +he treatment plants& in addition to lighter loading& are distinguished

from con"entional and step aeration processes b$ appl$ing unsettled waste water directl$ to aeration

without primar$ settling%

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if nutrients from point sources are reduced% -n part& this results from the long turno"er time (detention

time) which reduces the rate of flushing%

Macronutrients for plant growth are carbon dio'ide& inorganic nitrogen& and phosphateC a

"ariet$ of trace elements& such as iron& are also needed for growth% +he e$ to controlling rate of lae

eutrophication lies in limiting plantnutrients% atural waters contain sufficient carbon in the

bicarbonate alalinit$ s$stem to pro"ide carbon dio'ide in e'cess of growth needs% At present&

emphasis is being placed on phosphorus reduction to control the e'tent of plant growth in laes%

emo"al of algae has been suggested as a means for reducing nuisance species and

withdrawing nutrients% =opper sulfate is commonl$ used for control of algae in water suppl$

reser"oirs% +his algicide has se"eral shortcomings as a eutrophication control de"ice% =opper sulfate

poisons fish when used in e'cessi"e concentrations and has been demonstrated to accumulate in

bottom muds of laes following application o"er a period of se"eral $ears% -n fertile laes& the copper

sulfate must be applied at inter"als throughout the growing season to ensure effecti"e algal

controlCblooms must be anticipated and treated before the$ occur% +his process is "er$ e'pensi"e in

both man,power and chemical costs% A large number of herbicides are a"ailable to pro"ide relief

from a#uatic weeds& both preemergent and emergent%

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Question :

5ow to determine the performance e"aluation of treatment plants7

!olution

=omprehensi"e studies are needed to determine treatment efficienc$ and economical

operation in waste,water processing% As,built drawings of all treatment units pro"ide dimensions of

tans and interconnecting piping% From these& a process flow diagram can be setched showing

normal plant operation% =hanges in plant operation to meet unusual flow and load conditions should

be noted% 8h$sical facilities for flow,measuring and sampling at "arious points are essential% +he

influent par shall flume should be checed accurac$& since significant error in raw waste,water flows

precludes satisfactor$ results% Flow measuring at "arious points within the plant ma$ be

accomplished b$ calibrating pump discharges& or b$ installing temporar$ weirs in flow channels%

!ampling points must be carefull$ selected to insure collection of representati"e portions for

composting% Often& a lac of ade#uate flow,measuring facilities and accessibilit$ for sampling in,

plant 4eopardi3es or pre"ents the stud$ of indi"idual unit operations% 8h$sical modifications ma$ be

needed in some plants to permit e"aluation% Finall$& a laborator$ facilit$ is needed to perform at least

routine tests& such as total and suspended solids& BOD& p5& fecal coliforms& and chlorine residual%

Metropolitan plants re#uire additional e#uipment for anal$ses of =OD& grease& alalinit$& phosphates&

"arious forms of nitrogen& sulfides& "olatile acids& gas anal$sis& sludge filterabilit$& and biological

o'$gen uptae% -n addition it ma$ be desirable to ha"e testing facilities for hea"$ metals and total

organic carbon%

+he person super"ising a plant e"aluation must be thoroughl$ familiar with each unit

operation and how it fits into the o"erall plant process% =haracteristics of the raw waste water must

be completel$ defined b$ flow patterns& waste strength parameters& t$pes of industrial waste& and

infiltration,inflow #uantities% athering these data relies on a $ear,round testing and sampling

program of influent waste water and monitoring of industrial wastes discharged to the sewer s$stem%

+o insure that the degree of treatment is satisfactor$& the superintendent should be familiar with local&

state& and federal water #ualit$ and effluent standards% A complete stud$ also includes a re"iew of

maintenance procedures and accurate records of operating costs%

+he process diagram for a t$pical treatment plant in figure indicates the minimum testing

program for e"aluation% 2ffluent standards re#uire dail$ monitoring of a"erage BOD and suspended

solids concentrations& p5& and fecal coilform counts% -n some locations& tests for chlorine residual&

presence of hea"$ metals& phosphates& and ammonium nitrogen content ma$ also be specified b$

regulator$ agencies% 8ercentage of organic matter remo"al is traditionall$ calculated b$ comparing

influent and effluent BOD and suspended solids%



2'amination of indi"idual unit operations within a treatment plant re#uires testing of all

influent and effluent flow streams% Also& loading parameters should be calculated for each unit

process to determine whether the s$stem is being stressed be$ond its intended design capacit$%

=omplete and accurate records of all phases of plant operation and maintenance are essential%

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Question E

Define the terms of solid waste& municipal waste& ha3ardous waste and its source%

!olution

!olid wastes are all the wastes arising from human and animal acti"ities that are normall$

solid and that are discarded as useless or unwanted . -ndustrial wastes are those wastes arising from

industrial acti"ities and t$picall$ include rubbish& ashes& demolition and construction wastes& special

wastes& and ha3ardous wastes% 6astes that pose a substantial danger immediatel$ of o"er a period of

time to human& plant& or animal life are classified as ha3ardous wastes% A waste is classified as

ha3ardous it if e'hibits an$ of the following characteristics (1) ignitabilit$& () corrosi"it$& (.)

reacti"it$& or (<) to'icit$%

-n the past& ha3ardous wastes were often grouped into the following categories (1)

radioacti"e substances& () chemicals& (.) biological wastes% (<) Flammable wastes& and ()

e'plosi"es% +he chemical categor$ includes wastes that are corrosi"e reacti"e& or to'ic% One lie

principal sources of ha3ardous biological wastes are hospitals and biological research facilities%

5a3ardous wastes are generated in limited amounts throughout most industrial acti"ities% -n

terms of generation& the concern is with the identification of the amounts and t$pes of ha3ardous

wastes de"eloped at each source& with emphasis on those sources where significant waste #uantities

are generated% 9nfortunatel$& "er$ little information is a"ailable on the #uantities of ha3ardous

wastes generated in "arious industries%

+he spreading of ha3ardous wastes b$ spillage must also be considered% +he #uantities of

ha3ardous wastes that are in"ol"ed in spillages usuall$ are not nown% After a spill& the wastes

re#uiring collection and disposal are often significantl$ greater than the amount of spilled wastes&

especiall$ where an absorbing material& such as straw& is used to soa up li#uid ha3ardous wastes or

where the soil into which a ha3ardous li#uid waste has percolated must be e'ca"ated% Both the straw

and (lie li#uid and the soil and the li#uid are classified as ha3ardous wastes)%

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Question >

6rite down the sampling procedure for anal$3ing solid waste%

!olution

8erhaps the most difficult tas facing an$one concerned with the design and operation of

solid,waste management s$stems is to predict the composition of solid wastes that will be collected

now and in the future% +he problem is complicated because of the heterogeneous nature of waste

materials and the fact that unpredictable e'ternalities such as world oil prices can affect the long,

term abundance of the indi"idual waste components%

+he load,count and the mass,"olume methods of anal$sis are recommended% +he following

techni#ue is recommended to assess the indi"idual components within a gi"en waste categor$ (e%g%&

domestic wastes)%

1% 9nload a trucload of wastes in a controlled area awa$ from other operations%

% Quarter the waste load%

.% !elect one of (lie #uarters and #uarter that #uarter%)

<% !elect one of the #uartered #uarters and separate all of the indi"idual components of

the waste into pre,selected components%

% 8lace the separated components in a container of nown "olume and tarce mass and

measure the "olume and mass of each component% +he separated components should

be compacted tightl$ to simulate the conditions in the storage containers from which

the$ were collected%

6. Determine the percentage distribution of each component b$ mass and the discarded

densit$% +$picall$& from 1 to g ( to < lb) of waste should be sorted to

obtain a representati"e sample% +o obtain a more representati"e distribution of

components& samples should be collected during each season of the $ear% =learl$& no

matter how man$ samples are anal$3ed& common sense is needed in selecting the

loads to be sorted& in anal$3ing the data& and in preparing pro4ections%

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Question ;

6rite down the sources of air pollutant%

!olution

All air contains natural contaminants such as pollen& fungi spores& salt spra$& and smoe and

dust particles from forest fires and "olcanic eruptions% -t contains also naturall$ occurring carbon

mono'ide (=O) from the breadown of methane (=5 )C h$drocarbons in the form of trepans from<

pine treesC and h$drogen sulfide (5 !) and methane (=5 ) from the anaerobic decomposition of <

organic matter%

-n contrast to the natural sources of air pollution there are contaminants of anthropogenic

origin% +he use of fossil fuels for heating and cooling& for transportation& for industr$& and for energ$

con"ersion& and the incineration of the "arious forms of industrial& municipal& and pri"ate waste all

contribute to the pollution of the atmosphere% !o do the handling and processing operations of

"arious and sundr$ industries% +he sources of these pollutants are so numerous and "aried that the$

ha"e been categori3ed into four main groups,mobile transportation% (i%e%& motor "ehicles& aircraft&

railroads& ships& and the handling and/or e"aporation of gasoline) stationar$ combustion (i%e%&

residential& commercial& and industrial power and heating& including steam,powered electric power

plants)& industrial processes (i%e%& chemical& metallurgical& and pulp,paper industries and petroleum

refineries)& and solid,waste disposal (i%e%& household and commercial refuse& coal refuse& and

agricultural burning)%

-t will be noted that while transportation was the single largest source of air pollution& fuel

combustion in stationar$ sources (for power and heating) was the second ma4or contributor% 8ower

generation and heating accounted for about > percent of the o'ides of sulfur and 1 percent of the

o'ides of nitrogen emitted to the ambient air& while industrial processes contributed percent of the

h$drocarbons%

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Question 1

2'plain the effect of acid rain%

!olution

+he effects of acid deposition "ar$ according to the sensiti"it$ of the ecos$stems upon which

the deposits fall% -n some highl$ buffered areas acidic compounds could be deposited for $ears

without causing an$ appreciable increase in Gsoil or surface,water acidit$& but the same deposition

could cause sharp increases in acidit$ in poorl$ buffered areas% Acid rain has cause considerable

damaged to buildings and monuments in highl$ industriali3ed areas& but damage is not limited to the

immediate area of industriali3ation% +all stacs disperse pollutants into the upper reaches of the

troposphere where the$ ma$ remain for da$s& often being carried long distances% 8ollutants that are

generated in one countr$ and deposited in another ha"e become a matter of international concern and

of international negotiation%

Other changes in the atmosphere as a whole ma$ not be #uite so ob"ious% For e'ample& the

twentieth centur$ has seen side spread use of radioacti"e materials& and concern o"er the long,range

effects of release of these substances into the atmosphere has led to in"estigation of possible methods

of safe disposal b$ deep burial in the earth or ocean%

+he o3one (O ) la$er in the stratosphere is being depleted as o3one reacts with chlorine.

released from the fluorocarbons used as aerosol spra$ propellants% !ince the O in the atmosphere.

reduces the ultra"iolet radiation that reaches the earth ?s surface& and since ultra"iolet radiation at high

le"els can damage plants and animals& loss of O represents a potentiall$ serious problem% -n light of .

this danger& some industriali3ed nations ha"e banned the use of fluorocarbons%

+he amount of tropospheric carbon dio'ide (=O ) is reported to be increasing at a rate of 1%>

mg/m per $ear& a process that ma$ not be re"ersible% Furthermore& this increase has been.

accompanied b$ an e#ui"alent decrease in atmospheric o'$gen (O )% =urrentl$& there is more than

E billion tons of carbon in the form of =O in the atmosphere% 2ach $ear this figure increases b$

%. billion tons& the e#ui"alent of a . percent increase e"er$ decade%

Fossil fuel consumption and agricultural& forestr$ and land use practices of "arious t$pes

contribute to the =O build up% =O strongl$ absorbs long wa"e (infrared) terrestrial radiation& and

continued =O buildup could lead to a significant enough rises in earth ?

s surface temperatures to melt

the Arctic ice pac% -f the warming trend can be confirmed and positi"el$ lined to =O buildup&

then global action such as reforestation ma$ e"entuall$ ha"e to be pursued to remo"e =O from the

atmosphere%

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Question 11

6rite down the carbon mono'ide in air and discuss the sources of carbon mono'ide%

!olution

=olorless& tasteless& and odorless& carbon mono'ide gas is chemicall$ inert under normal

conditions and has an estimated atmospheric mean life of about Hmonths% +he total emission of

=O on a mass basis in 1;EE accounted for slightl$& o"er all (. percent) of all the anthropogenic air

pollutants%

=arbon mono'ide at present ambient le"els has little if an$ effect on propert$& "egetation& or

materials% At higher concentrations& it can seriousl$ affect human aerobic metabolism& owing to its

high affinit$ for hemoglobin& the component of the blood responsible for the transport of o'$gen%

=arbon mono'ide reacts with the hemoglobin (5b) of blood to gi"e carbo'$hemoglobin(=O5b)%

+hus reducing the capabilit$ of the blood to carr$ o'$gen% !ince the affinit$ of hemoglobin for

carbon mono'ide is more than times as great as i ts affinit$ for o'$gen& =O can seriousl$ impair

the transport of O & e"en when present at low concentrations% As =O5b le"els increase& effects

become more and more se"ere%

+he absorption of =O b$ the bod$ increases with =O concentration& e'posure duration& and

the acti"it$ being performed% =arbon mono'ide concentrations are especiall$ high in congested

urban areas where traffic is hea"$ and slow,mo"ing%

=arbon mono'ide sources are both natural and anthropogenic% . Hbillion tones of =O are

produced in nature $earl$ b$ the o'idation of methane gas from deca$ing "egetation% !till another

source is human metabolism% +he e'halations of a resting person contain appro'imatel$ 1 ppm =O%

Applied to the entire nation& this would total about %; tones of =O produced each da$%

Iet this production is still a great deal less than the estimated :;%1 million tones produced in

1;> b$ transportation sources primaril$ gasoline,powered internal combustion engines% +he

#uantities of =O emission from the four ma4or groups transportation& fuel combustion in stationar$

sources (power& heating)& industrial purposes and solid waste disposal% On a weight basis& the total

estimated emission of =O from transportation in 1;:>& 1;E& 1;E& 1;EE& and 1;> was about E>%.

percent of the total =O emitted b$ all sources combined% -n 1;>& the ne't largest source of

anthropogenic carbon mono'ide was solid,waste disposal and miscellaneous causes& which included

forest fires& structural fires& coal refuse& and agricultural burning%

On a mass basis& the emissions of carbon mono'ide from anthropogenic sources ha"e

dropped from 1.E million tones in 1;:> to > million tones in 1;>% +his reduction has taen place

mainl$ in the automoti"e area& owing to the initiation in 1;:> of pollution control de"ices% 2"en at

present le"els of emission& were it not for the natural processes of remo"al& the =O content of the

atmosphere would be increasing at the rate of about % ppm $earl$%

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Question 1

Discuss con"entional and step aeration briefl$%

!olution

+hese processes are similar to the acti"ated sludge s$stems that were constructed for

secondar$ treatment of municipal waste water% +he aeration basin is a long rectangular tan with air

diffusers along one side for o'$genation and spiral flow mi'ing% -n a con"entional basin& the air

suppl$ is tapered along the length of the tan to pro"ide greatest aeration at the head end where raw

wastewater and return acti"ated sludge are introduced% Air is pro"ided uniforml$ in step aeration

while wastewater is introduced at inter"als& or steps& along the first portion of the tan% Fine,bubble

air diffusers are set at a depth of %m or more to pro"ide deep mi'ing and ade#uate o'$gen transfer%

+he air header is connected to a 4ointed arm so that diffusers can be swung out of the tan for

cleaning and maintenance% A "ariet$ of diffusers are maretedC two popular t$pes are s$nthetic cloth

tubes& easil$ remo"ed for laundering& and diffuser no33les that can be detached from the air header

pipe%

8lug,flow pattern of long rectangular tans produces an oscillating biological growth pattern%

+he relati"el$ high food ,to,microorganism ratio at the head of the tan decreases as mi'ed li#uor

flows through the aeration basin% !ince the aeration period& is : to > h& and can be considerabl$

greater during low flow& the microorganisms mo"e into the endogenous growth phase before their

return to the head of the aeration basin% +his wea star"ing microbial population must #uicl$ adapt

to a renewed suppl$ of waste organics% +he process has few problems of instabilit$ where waste,

water flows are greater than & m . /dC howe"er& because of wide hourl$ "ariations in waste loads

from small cities& the con"entional plug,flow s$stem can e'perience serious problems of biological

instabilit$% +his phenomenon was a ma4or factor contributing to the de"elopment of complete mi'ing

aeration for handling small flows%

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Question 1.(a)

+wo rapid sludge return final clarifiers following high rate aeration are 1>m in diameter with

a %E m side water depth% +he effluent weir is an inboard channel set on a diameter of 1:% m% For a

total flow of 1. m. /d& calculate the o"erflow rate& detention time& and weir loading%

!olution

Q Q 1.V J J J J %< m / m , d.

O"erflow rate A pd p K 1>

K K < <

p d p K 1>

K 5 K K %E K L < <Detention time t J < K J < K J < K J %< hQ Q 1.

Q Q 1.6eir loading J J J J E%<E m / m,d.

weir length pd K K p K1>K

+ricling Filter

*ow rate +/F 7 no Q & BOD loading J g/ m ,d.

!ingle stage +/F

5igh rate +/F 7 Allow Q

+wo stage +/F

5igh BOD loading

E g/ m. ,d

Question 1.(b)

6hat is 5$drocarbons7 6rite down the t$pes of 5$drocarbons%

!olution

Organic compounds containing onl$ carbon and h$drogen are classified as h$drocarbons%

Most of the ma4or chemicals in gasoline and other petroleum products are h$drocarbons& which are

di"ided into two ma4or classifications, aliphatic and aromatic%

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Question 1<

A tricling filter plant has the following a primar$ clarifier with a 1:%> m diameter& %1 m

side water depth& and single peripheral weirC a :% m diameter tricling filter with a %1 m deep

roc,fill setting tan with a 1% m diameter& %1 m side water depth& and single peripheral weir% +he

normal operating recirculation ratio is % with return to the wet well from the bottom of the final

during periods of low influent flow% +he dail$ waste,water flow is m /d with a"erage BOD of .

1> mg/l& essentiall$ all domestic waste% =alculate the loadings on all of the units& and the

anticipated effluent BOD at = and 1: =%

!olution

8rimar$ !ettling +an

Q Q O"erflow rate V J J J J .% m / m , d (6ithout Q ).

A pd p K1:%>

< <

Q N Q N K %V J J J .%. m / m ,d (6ith Q ).

A p K1:%>

<

p d p K1:%>

K 5 K %1L < <Detention time t J < K J < K J < K J %1< hQ Q

Q Q 6eir loading J J J J;>%; m / m , d

.

weirlength pd p K1:%>

BOD remo"al efficienc$ J .

emaining BOD J : of 1> mg/l J %: ' 1> J 11E mg/l

+ricling Filter

Q K !ettled BOD K 11EBOD loading J J J <E%EE g/m , d.

pLolK : K %1

<

Q N Q N K %5$draulic loading J J J1<%E m / m , d .

A p K:

<

BOD remo"al efficienc$ at = J E> (+able ,1E)

BOD remo"al efficienc$ at 1: = J :> (+able ,1;)

emaining BOD at = J of 11E mg/l J % ' 11E J %E< mg/l

emaining BOD at 1: = J . of 11E mg/l J %. ' 11E J .E%<< mg/l



Final =larifier

Q Q O"erflow rate V J J J J >%<< m / m , d (6ithout Q ).

A pd p K1%

< <

Q N Q N K %V J J J <.%1 m / m ,d (6ith Q ).

A p K1%

<

p d p K1%

K 5 K %1L < <Detention time t J < K J < K J < K J 1%E hQ Q

Q Q 6eir loading J J J J1;%.1 m / m , d.

weirlength pd p K1%

O"erall +reatment 8lant efficienc$ at =&

1> , %E<(O)2 J K1 J >%E

1>

. 2 . E>%2 J1 ,1 1, 1, J1 ,1 1 , 1, J>%E

1

1 1 1 1

O"erall +reatment 8lant efficienc$ at 1: =&

1> , .E%<<(O)2 J K1 J E;%

1>

. 2 . :>%2 J1 ,1 1, 1, J1 ,1 1 , 1, J E;%1

1 1 1 1

0000000000000000000000000



Question 1

+he design flow for a two,stage tricling filter process is << m . /d with an a"erage BOD

concentration of < mg/l% =alculate the unit loadings and treatment plant efficienc$ of waste,water

temperature of 1E =%

8rimar$ tan surface area J m

First,stage filter area J < m

Lolume J >. m .

-ntermediate clarifier area J 1< m

!econd,stage filter is identical to first stage%

Final clarifier area J 1< m

ecirculation pattern return to wet well is 11 m . /d under flow of each clarifier for total

PQ Jm. /d direct recirculation around each filter

Q is ;. m . /d%

!olution

8rimar$ =larifier

Q <<O"erflow rate V J J J m / m , d.

A

BOD remo"al J .

emaining BOD J : of < mg/l J %: ' < J : mg/l

First,stage +/F

Q K ! ettled BOD << K :BOD loading J J J 1.E>%. g/m , d.

Lol >.

PQ N Q N Q << N N ;.5$draulic loading J J J 1%1> m / m , d.

A <

Q N QP N ;.ecirculation atio J 1%1E

J J J

Q <<

BOD remo"al efficienc$ at = J E

BOD remo"al efficienc$ at 1E = J :

emaining BOD J . of : mg/l J %. ' : J ;1 mg/l

-ntermediate =larifier

Q N Q << N11O"erflow rate V J J J .; m / m , d.

A 1<

!econd,stage +/F

Q K !ettled BOD << K ;1BOD loading J J J <>%<1 g/m ,d.

Lol >.



Q N Q N QP << N 11 N ;.5$draulic loading J J J1>%E. m / m , d .

A <

Q N QP 11 N ;.ecirculation atio J %;

J J J

Q <<

Actual BOD loading <>%<1Ad4usted BOD loading J J J .;.>%<

1 , 2 1 , :

1

1 1

BOD remo"al efficienc$ at = J >

BOD remo"al efficienc$ at 1E = J .

emaining BOD J <E of ;1 mg/l J %<E ' ;1 J <%EE mg/l

Final =larifier

Q <<O"erflow rate V J J J .1%<. m / m , d.

A 1<

O"erall +reatment 8lant efficienc$ at 1E =&

< , <%EE(O)2 J K1 J >;%.

1>

. 2 2 . :% .2 J1 ,1 1, 1, 1, J1 ,1 1, 1, 1, J >;%.1

1 1 1 1 1 1

0000000000000000000000000000



Question 1:

+he design flow for a two,stage tricling filter process is << m . /d with an a"erage BOD

concentration of > mg/l% =alculate the unit loadings and treatment plant efficienc$ of waste,water

temperature of 1E =%

8rimar$ tan surface area J m

First,stage filter area J < m

Lolume J >. m .

-ntermediate clarifier area J 1< m

!econd,stage filter is identical to first stage%

Final clarifier area J 1< m

ecirculation pattern return to wet well is 11 m . /d under flow of each clarifier for total

PQ Jm. /d direct recirculation around each filter

Q is ;. m . /d%

!olution

8rimar$ =larifier

Q <<O"erflow rate V J J J m / m , d.

A

BOD remo"al J .

emaining BOD J : of > mg/l J %: ' > J 1> mg/l

First,stage +/F

Q K ! ettled BOD << K 1>BOD loading J J J ;<%> g/m , d.

Lol >.

PQ N Q N Q << N N ;.5$draulic loading J J J 1%1> m / m , d.

A <

Q N QP N ;.ecirculation atio J 1%1E

J J J

Q <<

BOD remo"al efficienc$ at = J E

BOD remo"al efficienc$ at 1E = J :>

emaining BOD J . of 1> mg/l J %. ' 1> J >%< mg/l

-ntermediate =larifier

Q N Q << N11O"erflow rate V J J J .; m / m , d.

A 1<

!econd,stage +/F

QK !ettled BOD << K>%<BOD loading J J J .>%E< g/m , d.

Lol >.



Q N Q N QP << N 11 N ;.5$draulic loading J J J1>%E. m / m , d .

A <

Q N QP 11 N ;.ecirculation atio J %;

J J J

Q <<

Actual BOD loading <>%<1Ad4usted BOD loading J J J .1%<

1 , 2 1 , :>

1

1 1

BOD remo"al efficienc$ at = J :

BOD remo"al efficienc$ at 1E = J :

emaining BOD J << of >%< mg/l J %<E ' >%< J %:. mg/l

Final =larifier

Q <<O"erflow rate V J J J .1%<. m / m , d.

A 1<

O"erall +reatment 8lant efficienc$ at 1E =&

< , %:.2 J K1 J >;%.

1>

00000000000000000000000000000000



Question 1E(a)

A high,rate tricling filter is a 1 m diameter and %1 m depth% +he raw waste,water flow is

. m /d with 1. mg/l BOD% -ndirect recirculation is during low flow period is 1 m /d and. .

direct recirculation is .> l/s% =alculate the BOD and h$draulic loadings& recirculation ratio& BOD

remo"al efficienc$ and effluent BOD at a temperature of 1: =%

!olution

Q K !ettled BOD .. K 1.J J .:%1; g/m , d.

BOD loading J Lol p K 1

K %1<

PQ N Q N Q . N 1 N .>.%5$draulic loading J J J %<E m / m , d

.

A p K1

<

PQ N Q 1 N .>.%ecirculation atio J 1%;

J J J

Q .

BOD remo"al efficienc$ at =&

1N 1N1%;F J J J1%;.

(1N %1) (1N%1K1%;)

1 12 J J J >1

BOD load %.: % %

1 N %<.. 1 N %<..F 1%;.

BOD remo"al efficienc$ at 1: = J 2 ' 1%. +, =

J 2 ' 1%. J >1%E ' 1%. J E%:<1:, 1:,

1: =

emaining BOD effluent J ; of 1. mg/l J %; ' 1. J .E%E mg/l

Question 1E(b)

ame the four ma4or la$ers of the atmosphere%

!olution

Four ma4or la$ers of the atmosphere are(i) troposphere

(ii) stratosphere

(iii) mesosphere

(i") thermosphere

000000000000000000000000000



Question 1>(a)

+wo rapid sludge return final clarifier following high rate +/F are 1> m diameter and %E m

side water depth% +he effluent weir is inboard channel set on a diameter of 1:% m for raw w/w flow

of 1. m. /d& with 1 mg/l BOD% =alculate the loading on both +/F and F/=%

(a) ecirculation J %

(b) BOD emo"al effluent in primar$ clarifier J .

(c) BOD emo"al efficienc$ in +/F J >

(d) Area of +/F J : m and depth is % m

!olution

+ricling Filter

!ettled BOD J : of 1 mg/l J %: ' 1 J 1.:%

QK !ettled BOD 1. K1.:%BOD loading J J J 1; g/m , d.

Lol : K %

Q N Q 1. N 1. K %5$draulic loading J J J . m / m , d.

A :

emaining BOD J 1 of 1.:% mg/l J %1 ' 1.:% J %<> mg/l

Final =larifier

Q 1.O"erflow rate d

V J J J %< m /m ,.

A p K1>

<

p d p K1>

K K 5 K K %EL < <Detention time t J < K J < K J < K J %< hQ Q 1.

Q Q 1.6eir loading J J J J :%E m / m ,d.

weir length pd K K p K1%K

O"erall +reatment 8lant efficienc$ at =&

1 , %<>2 J K 1 J ;%

1

Question 1>(b)

6hat are the ma4or components of the troposphere7

!olution

-n troposphere the air which we breathe& consists b$ "olume of about E> of nitrogen& 1

of o'$gen& 1 of argon and %. of carbon dio'ide%

0000000000000000000000000



Question 1;(a)

2stimate the design flow and BOD load for a high,rate filled tricling filter plant with the

following si3ed units%

Diameter (m) Area (m ) Depth (m)

8rimar$ =larifier 1> < %<

+ricling Filter E E. %1

Final =larifier 1< 1< %1

!olution

Final =larifier

Assume L J . m . /m ,d

L J Q/A

Q J L ' A J . ' 1< J <;> m /d.

=hecing in 8rimar$ !ed tan (without Q )

Q <;>O"erflow rate (1: . m /m ,d). V J J J ;%1 m / m , d.

A <

Q N Q <;> N <;> K %(1: . m /m ,d). V J J J ;%1 m / m ,d.

A <

L < K%<Detention time t J h

<K J <K J %;E (R% hrs)Q <;>

+ae Q J <;> m /d.

BOD loading on +ricling Filter J E g/m. ,d

Q K!ettled BODBOD loading J

Lol

EKLol EKE.K%1

BOD (mg/l) J mg/l J J1>.%1.

Q <;>

1>.%1.BOD in aw w/w J J >1%E< mg/l

%:

Question 1;(b)

6hat is acid deposition7

!olution

Acid rain or acid deposition results when gaseous emissions of sulfur o'ide& nitrogen o'ide

interact with water "apor and sunlight are chemicall$ con"erted to strong compounds such as sulfuric

acid and nitric acid% +hose compounds along with other organic chemicals are deposited on the earth

as aerosols and particulates (dr$ deposition) or are carried to the earth b$ raindrops& snowflaes& fog

or dew (wet deposition)%

0000000000000000000000000



Question

+he following are a"erage operating data from a con"entional acti"ated sludge secondar$%

6aster,water flow J ; m /d ecirculated sludge flow J 1 m /d. .

L J > m . 6aste sludge #uantit$ J m . /d

-nfluent total solid J ;; mg/l !uspended solids in waste sludge J ;> mg/l

-nfluent suspended solids J 1 mg/l t J 7

-nfluent BOD J 1E. mg/l BOD loading J 7

2ffluent total solids J <;E mg/l FJ 7

M2ffluent !%! J mg/l

!%!& BOD remo"al efficienc$ J 72ffluent BOD J mg/l

M*!! J mg/l

!olution

L >Aeration 8eriod t J h

< K J <K J E%.Q ;

Q K ! ettled BOD ; K 1E.BOD loading J J J ;%< g/m ,d.

Lol >

;; , <;E+/! remo"al efficienc$ J K1 J 1E%.

;;

1 ,

!/! remo"al efficienc$ J K1 J E>1

1E. , BOD remo"al efficienc$ J K 1 J >>%<<

1E.

F QK BOD ; K1E. g/d of BODJ J J %.:

M L KM*!! >K g of M*!!

F 1BOD sludge age J J J <%< da$s

M %.:

g M*!!!%! sludge age J sludge)

g/d (!! in effluent N !!in waste

g M*!! J ' > J 1% ' 1 g:

!%! in effluent J ' ; J :.%> ' 1 < g/d

!%! in waste sludge J ;> ' J 1%;: ' 1 : g/d

1%K1:

( )!%! sludge age J J>%1> da$s

:.%>K1 K1%;: K1< :

Q 1

eturn sludge ratio J %.J J

Q ;

00000000000000000000000



Question 1

2stimate the design flow and BOD loading for a step aeration acti"ated,sludge plant based on

the following data%

8rimar$ clarifier A J > m & Depth J %< m

Aeration basins& L J 1E m .

Aeration capacit$& L J 1 m of air per second N stand b$ compressor .

Final =larifiers A J ; m & Depth J %< m

!olution

Final =larifier

+r$ L J < m /m ,d (S. m /m ,d). .

L J Q/A

Q J L ' A J < ' ; J 1: m /d ( m /d) ot OT . .

+r$ L J > m /m ,d.

Q J L ' A J > ' ; J > m /d ( m /d to : m /d) ot OT . . .

+r$ L J . m /m ,d.

Q J L ' A J . ' ; J ::>> m /d (R : m /d). .

=hec detention time

L ; K %<t J hr

<K J < K J1%> hr U O Q ::>>

-n aeration tan

=hec

L 1EAeration period t J com

mmon)<

K J <K J :%1hr (:, E% O Q ::>>

8rimar$ !edimentation

Q ::>>O"erflow rate V J J J .%>; m / m , d.

A >

L > K%<Detention time t J hr)

<K J <K J %<1 hr ( RQ ::>>

Design flow Q J ::>> m. /d

BOD loading in Aeration +an J 7

; m. of air 7 1g BOD

1 m /s 1.

1 m. /s of air 7 BO

D1

K J g/s; m ;.

1 1KBOD loading (g/m. ,d) J K1 K < .: K J :<1%;> g/m ,d. .

; 1E

1 1KBOD concentration (mg/l) J K1 K < .: K J 1:.%1> g/m J mg/l. .

; ::>>

i%e% : of raw BOD (. remo"al in primar$ sedimentation)

raw BOD J 1:.%1>/%: J 1% mg/l



Question (a)

A con"entional acti"ated,sludge basin is E%. m wide& . m long and has < m li#uid depth%

+he influent flow is .<<o m /d containing < g BOD% =ompute the BOD loading and aeration.

period% +he operating M*!! is mg/l and the settled sludge "olume in the !L- test is . ml/l%

=ompute the F/M ratio& !L-& recommended sludge recirculation rate and solids concentration in the

return sludge%

!olution

<K1.

BOD loading J J1.%E g/m , d.

( )

.KE%.K<

L E%.K. K<Aeration period t J hr

<K J <K J:%11Q .<<

F QK BOD < K1 g/d of BOD.

J J J %..( )M L KM*!!

E%.K. K< g of M*!!

L K 1 . K 1!L- J J J 1<% ml/gm

M*!!

L K Q . K Qecommended sludge recirculation rate Q J J J 1E%. m /d.

1 , L 1 , .

Q 1E%.

atio %

;;

J J J Q .<<

1 1: :

!olid concentration return sludge J ;:<%>

J J!L- 1<%

Question (b)

6hat are primar$ and secondar$ pollutants7

!olution

According to their origin& pollutants are considered as either primar$ or secondar$

contaminants% 8rimar$ pollutants such as sulfur o'ide& nitrogen o'ide and h$drocarbons are those

emitted directl$ to the atmosphere and found there in the form in which the$ were emitted%

!econdar$ pollutants such as o3one and pero'$acet$l (8A) nitrate are those formed in the

atmosphere b$ a photochemical reaction of h$drol$sis or o'idation%

000000000000000000000000000000



Question .(a)

A step,aeration acti"ated,sludge secondar$ is operating under the following conditions

influent waste,water flow J > m /d& a"erage influent BOD J 1: mg/l& "olume of aeration basin.

is < m . and M*!! concentration mg/l% Determine the following% BOD loading& aeration

period& F/M ratio& and estimated effluent BOD assuming good operation%

!olution

Q K ! ettled BOD > K 1:BOD loading J J J :1:%:: g/m , d.

Lol <

L <Aeration period t J hr

<K J <K J <%;Q >

F QK BOD > K1: g/d of BODJ J J %

M L KM*!! < K g of M*!!

For good operation

BOD remo"al efficienc$ J > to ;

2ffluent BOD J to 1 of raw BOD

J % ' 1: J :%. mg/l to %1 ' 1: J 1>%; mg/l

Question .(b)

6hat are the important factors that to be considered in the solid waste management7

!olution

+o access the management possibilities it is important to consider

(i) materials flow in societ$

(ii) reduction in raw materials usage

(iii) reduction in solid waste #uantities

(i") reuse of materials

(") materials reco"er$

("i) energ$ reco"er$

("ii) da$,to,da$ solid waste management

00000000000000000000000000



Question <(a)

A 1E m. complete mi'ing aeration basin treats 1.. m . /d with an a"erage BOD of mg/l%

+he operating M*!! is < mg/l& the effluent suspended solids concentration e#uals < mg/l& and

waste sludge solids a"erage > g/da$% =ompute the BOD loading& aeration period& F/M ratio& BOD

and !! sludge age%

!olution

Q K ! ettled BOD 1.. K BOD loading J J J1:<%E1 g/m , d.

Lol 1E

L 1EAeration period t J hr

<K J <K J .%EQ 1..

F QK BOD 1.. K g/d of BODJ J J %.;1

M L KM*!! 1E K< g of M*!!

M 1BOD sludge age J J J %: da$s

F %.;1

g M*!!!%! sludge age J sludge)

g/d (!! in effluent N !!in waste

g M*!! J < ' 1E J :> ' 1 g.

!%! in effluent J < ' 1.. J . g/d

!%! in waste sludge J > ' 1 J > ' 1 . g/d

:>K1

.

( )!%! sludge age J J%1da$s

.K >K1.

Question <(b)

Describe the effect of sulfur o'ide on human health%

!olution

!ulfuric acid& sulfur o'ide and sulfate salts tend to irritate the mucous membranes of the

respirator$ tract and foster the de"elopment of chronic respirator$ diseases particularl$ bronchitis

and pulmonar$ emph$sema% -n a dust$ atmosphere& !O is particularl$ harmful because both sulfur

dio'ide and sulfuric acid molecules paral$3e the hairlie cilia which line the respirator$ tract%

6ithout the regular sweeping action of the cilia& particulates are able to penetrate to the lungs and

settle there% +hese particulates usuall$ carr$ with them concentrated amounts of !O thus bringing

this irritant into direct& prolonged contact with delicate lung tissues% +he !O particulate combination

has been cited as cause of death in se"eral air pollution tragedies%

000000000000000000000000



Question (a)

2stimate the design flow and BOD loading for an e'tended aeration s$stem with floating

mechanical aerators% +he units si3es are "olume of aeration basins J ; m . & O'$gen transfer

capacit$ of aerators J 1 g/h at % mg/l DO and .% m deep final clarifiers with a surface area of

.1 m %

!olution

Final =larifier

+r$ L J 1 m. /m ,d L J Q/A

Q J L ' A J 1 ' .1 J .E: m . /d (to m. /d) ( :)

+r$ L J < m /m ,d.

Q J L ' A J < ' .1 J E<< m /d (R : m /d) O . .

=hec detention time

L .1 K.t J hr

<K J < K J. hr R O Q E<<

=hec aeration period

L ;Aeration period t J hr

<K J <K J .%: (< .:hr) O Q E<<

Design flow J E<< m. /d

BOD loading J 1 g/hr

1 g O J 1g BOD

1 g O J 1 g BOD

1BOD loading J 1 g/hr K 1 K K < J .E>%; g/m , d. .

;

( ) 1BOD concentration J

1 g/hr K1 K < K J <>.%>E mg/l.

E<<

Question (b)

6hat are the ma4or components of the troposphere7

!olution

-n troposphere the air which we breathe& consists b$ "olume of about E> of nitrogen& 1

of o'$gen& 1 of argon and %. of carbon dio'ide%

0000000000000000000000000



Question :

A waste,water effluent of : l/s with a BOD J mg/l& DO J .mg/l and temperature of

.V= enters a ri"er where the flow is %> m /s with BOD of <% mg/l& DO J >% mg/l and temperature.

of 1EV=% From laborator$ BOD testing& of the waste is %1 per da$ V=% +he ri"er downstream has1

an a"erage "elocit$ of %1>m and depth of 1% m% =alculate the minimum dissol"ed o'$gen le"el and

its distance downstream b$ using the o'$gen sag e#uation%

!olution

At discharging pt&

= Q N = Q < K %> N K %:BOD& = J J J11%:E mg/l1 1

Q N Q %> N %:1

>% K %> N .K%:DO& = J J E%..E mg/l

%> N %:

1E K%> N . K%:+& = J J 1>V=

%> N %:

1 , t J log 1 , D 1

, *c

1 1 1

DO J DO sat G DO mi' J ;% G E%.. J %1E mg/l

BOD J * (1, 1 ),1t

11%:E J * (1, 1 ),%1 '

* J 1E%1 mg/l

& + J & V= ' 1%<E+,

1 1

& 1>V= J %1 ' 1%<E J %;1 /da$1>,

1

& + J & V= ' 1%1+,

& 1>V= J %.11 ' 1%11>, J %. /da$

L %1> J % K J % K J %.11/da$

5 1% 1 %. . 1% . .

1 , 1 %. %. , %;1t J log 1 , D J log 1 , %1E J 1%E da$s 1

, * %. ,%;1 %;1 %;1 K 1E%1c

1 1 1

1%>

( ) ( ) *D J

1 ,1 N DO 11

, t , t , t1 c c c

, c

1

( ) ( )%;1K1E%1J

1 ,1 N %1E 1 J .%E mg/l,%;1 K1%> ,%. K1%> ,%. K1%>

%., %;1

Min DO conc J DO sat G D J ;% G .%E J %;. mg/lc

7 J L ' t J %1> ' 1%> ' < ' .: ' 1 J E%;; m,.c c

0000000000000000000000000000000000



Question E(a)

+he following treated effluent is discharged to a streamC Q J . l/s& DO J % mg/l& da$

BOD J <% mg/l& J %1/da$ and + J V=% 9pstream from the outfall the water course has the1

following characteristicsC Q J %E m . /s& DO J >% mg/l& da$ BOD J % mg/l and + J V=% +he

stream channel has a J %./da$% =alculate the critical dissol"ed o'$gen concentration

downstream and the distance from the outfall to this point assuming a mean "elocit$ of %: m/s in

the ri"er% !aturation DO at V= is ;% mg/l%

!olution

At mi'ing pt&

= Q N = Q > K %E N K %.DO& = J J J E%< mg/l1 1

Q N Q %E N %.1

K%E N< K%.BOD& = J J %> mg/l

%E N %.

+& = J V=

DO J DO sat G DO mi'ing J ;% G E%< J 1%> mg/l

BOD J * (1, 1 , t )

1

%> J * (1, 1 ,%1 ' )

* J >%<> mg/l

1 , 1 %. %. , %1t J log 1 , D J log 1 ,1%> J1%1; da$s 1

, * %. , %1 %1 %1 K >%<>c

1 1 1

( ) ( ) *D J

1 ,1 N DO 1

, t , t , t1 1 c c c

, c

1

( ) ( )%1K>%<>J

1 ,1 N1%> 1 J %1 mg/l,%1 K1%1; ,%. K 1%1; ,%. K1%1;

%., %1

Min DO conc J DO sat G D J ;% G %1 J E% mg/lc

7 J L ' t J 1%1; ' %: ' < ' .: ' 1 ,. J :1%:; mc c

Question E(b)

6hat are primar$ and secondar$ pollutants7

!olution

According to their origin& pollutants are considered as either primar$ or secondar$

contaminants% 8rimar$ pollutants such as sulfur o'ide& nitrogen o'ide and h$drocarbons are those

emitted directl$ to the atmosphere and found there in the form in which the$ were emitted%

!econdar$ pollutants such as o3one and pero'$acet$l (8A) nitrate are those formed in the

atmosphere b$ a photochemical reaction of h$drol$sis or o'idation%



Question >

+he following treated effluent is discharged to a streamC Q J < l/s& DO J % mg/l& da$

BOD J <% mg/l and + J V=% 9pstream from the outfall the water course has the following

characteristics with saturated DOC Q J . m . /s& da$ BOD J .% mg/l and + J 1;V=% From laborator$

BOD testing of the waste is %1/da$ at V=% +he ri"er downstream has an a"erage "elocit$ of 1

%:m/s and depth of 1% m% =alculate the minimum dissol"ed o'$gen le"el and its distance

downstream b$ using the o'$gen sag e#uation% !at DO at 1;V= J ;%. mg/l%

!olution

= Q N = Q . K . N < K %<BOD& = J J J .% mg/l1 1

Q N Q . N %<1

;%. K. N K%<DO& = J J ;%1; mg/l

. N %<

1;K . N K %<+& = J J1;V=

. N %<

DO J DO sat G DO mi' J ;%. G ;%1; J %11 mg/l

BOD J * (1, 1 , t )

1

.% J * (1, 1,%1 ' )

* J %1; mg/l

& + J & V= ' 1%<E+,

1 1

& 1;V= J %1 ' 1%<E J %;: /da$1;,

1

& + J & V= ' 1%1+,

& 1;V= J %E: ' 1%11;, J %E> /da$

L %: J % K J % K J %E:/da$

5 1% 1 % .. 1% ..

1 , 1 %E: %E: ,%;:t J log 1 , D J log 1 , %11 J 1% da$s 1

, * %E: ,%;: %;: %;: K %1;c 1 1 1

( ) ( ) *D J

1 ,1 N DO 11

, t , t , t1 c c c

, c

1

( ) ( )%;:K%1;J :

1 ,1 N %11 1 J %<; mg/l,%;: K1%

, %E: K1% , %E: K1%

%E: , %;1

Min DO conc J DO sat G D J ;%. G %<; J >%>1 mg/lc

7 J L ' t J %: ' 1% ' < ' .: ' 1 J :<%> m,.c c



Question ;

A stream with BOD . mg/l and saturated with DO has a normal flow of %> m . /s and recei"e

a sewage effluent also saturated with DO of %; m /s with BOD . mg/l% Determine the DO deficit.

o"er the ne't da$s and hence plot the DO sag cur"e% =alculate the critical DO deficit throughout

saturation DO at <V= >%< mg/l% at V= is %1 per da$& at V= is %. per da$%1

!olution

= Q N = Q . K %> N . K%;BOD& = J J J1%E> mg/l1 1

Q N Q %> N %;1

DO& = J sat& >%< mg/l

+& = J <V=

DO J mg/l

J * (1, 1 ),1tBOD

1%E> J * (1, 1 ),%1 '

* J 1%EE mg/l

( ) ( ) *D J

1 ,1 N D 1, t , t , t1 1

, t

1

& + J & V= ' 1%<E+,

1 1

& <V= J %1 ' 1%<E<, J %1 /da$

1

& + J & V= ' 1%1+,

& <V= J %. ' 1%1<, J %. /da$

( ) ( ) *D J

1 ,1 N DO 1, t , t

, t1 1

, t

1

( )%1K1%EEJ :

1 ,1 N J %1< mg/l, %1 K ,%. K

%., %1

1 , 1 %. ( )t J log 1 , D J log

1 , J %1. da$s 1

, * %. ,%1 %1c

1 1 1

( ) ( ) *D J

1 ,1 N DO 1

, t , t , t1 1 c c c

, c

1

( )%1K1%EEJ :

1 ,1 N J .%> mg/l, %1 K %1. ,%. K %1.

%. , %1



( ) ( ) *D J

1 ,1 N DO 1 (for table)1 , t , t

, t1

, t

1

( )%1K1%EEJ :

1 ,1, %1 Kt , %. Kt

%., %1

DO conc J DO sat G D J >%< , Dt t

+ime (da$s) DO deficit DO conc

>%<

1 %: %E

.%> %1

. .%; %.1

< %:< %E:

%1< :%:

: 1%:; :%E1

E 1%.1 E%;

> 1%1 E%.;

; %EE E%:.

1 %; E%>1

11 %< E%;

1 %.< >%:1. %: >%1<

1< % >%

1 %1 >%

DO conc

;

>

E

:

O'$gen sag cur"eDO conc

<

.

1

1 . < : E > ; 11111.1<11:



Question .

6h$ sewer maintenance and program is necessar$%

!olution

!ewer maintenance re#uires a through nowledge of the la$out and appurtenances used in

collection s$stems& waste water flows& infiltration& and inflow% 8rere#uisite information regarding the

function of "arious unit operations and how the$ relate to each other is re#uired%

!erious and e'pensi"e sewer problems can result from improper design or poor construction%

Ade#uate slopes to maintain self cleaning "elocit$ is essential to minimi3ing maintenance% !election

of a suitable pipe 4oint is "ital to pre"ent penetration of roots and e'cessi"e infiltration% =utting of

tree roots from sewer lines can be an e'pensi"e and reoccurring cleaning process% round water

entering 4oints carries with it soil from around the pipe which ultimatel$ causes structural failure% -n

addition to re"iew of new design and super"ision of construction& building permits should re#uire

careful inspection of all ser"ice connections before bacfilling% -t is important to mae certain that

unused ser"ice lines are properl$ capped when buildings are demolished%

A successfull$ maintenance program operates on a planned schedule and re#uires eeping

effecti"e records% Maps are used to show location of manholes& flushing inlets& ser"ice connections

and the appurtenances% ecords should be ept on maintenance performed with particular stress on

troublesome lines that are nown to re#uire more fre#uent inspection or cleaning% 6hile large sewer

on ade#uate slopes ma$ ne"er re#uire flushing or cleaning& others must be placed on regular schedule

that ma$ range e"er$ month to once a $ear% +he number of emergenc$ sewer blocages can be

materiall$ reduced b$ such pre"enti"e maintenance%

!ewer stoppages are caused chiefl$ b$ sand& grease materials& stics& stones and tree roots%

+he latter are most troublesome% =ommon cleaning techni#ues are flushing with water& scraping with

mechanical tools& h$draulic scoring with high pressure 4ets and addition of chemicals% 8eriodic

flushing helps to eep lines clear and is often performed in association with inspection% +he usual

procedure for de"eloping scouring "elocit$ is to inert a fire hose into the sewer through manhole%

+his is most ad"antageous in cleaning lines in residual sections that do not ha"e sufficient

connections to pro"ide cleaning flow of waste water% Flushing has limitations& since it merel$ mo"es

debris from one section of a sewer into anotherC it is assumed that flows in downstream pipes are

sufficient to suspend the solids and eep them mo"ing%

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