Sanitary sewage engineering :
The wastewater convey in system consisting of large ground of
Pipes with different diameters range between ( 15 cm – 3 m) with use of on open channels in
some necessary cases to its locations of collection via trunk line which terminates to
wastewater treatment plant. The pipes lines and the open channel deal with discharge works
is called "sewerage system " while the pipes of conveyance is called " sewers" the liquid
wastewater is called "sewage"
Quantity of sewage :
It is necessary to determine the quantity of sewage before implementation of design of
sewage pipes net . the sewage comprises on residential , in dustrial ,rain water , and
infiltration / inflow (infiltration is the groundwater that enters sewers through service
connections , cracked pipes , defective joints , and defective pipes and manhole walls . inflow
is the surface runoff may enter through manhole cover , roof and area drains and crossconnections from sewers and combined sewers
1-domistic wastewater (sewage) :
the domestic or residential wastewater is the portion of wastewater produced in homes .
(bathrooms , kitchens , toilets , washing basins) in addition to service buildings , restaurants ,
hotels , schools , ….. etc. this type of sewage contains on some percentage of organic and
inorganic like salts , sugar , starch , oils , and etc.
2-industrial wastewater :
industrial wastewater produced from different stage of industrial activities, disposed from
productive factories like paper factories , pharmaceutical , textile ,sugar , tanning , canning ,
and food production factories these sewage different in its contents of organic materials and
inorganic from factory to another . some industries have high percentage of organic materials
as liquid waste produced by foods, and sugar factories another part of industrial wastewater
contains on chemicals materials in high degree ( toxic waste) like wastewater in high degree(
toxic waste) like wastewater issued from pharmaceutical and , and lentils in dustiest .there is a
type of industrial wastewater contain low percentage of impurities like wastewater cooling
water .
3-rain –water or storm sewage :
Rain water contains on soils , sands and organic materials due to its flow on roads , surfaces ,
areas , and open regions exposed to rain failing . the sewage produced from street washing ,
gardens irrigation , fires extinguishing considered added quantities to rain water that
discharged to sewage system via manholes presented in streets .
1
4- infiltration water as mentioned above
Liquid wastewater Quantity :
The percentage of liquid wastewater is usually specified by ( 60 – 80 )% of drinking water
supplied to one person (capita)
Through are day . it is reality that the percentage of domestic wastewater is estimated by
(65%) from daily water consumption the industrial wastewater is estimated by (75%) from
daily water consumed in industry .
Average domestic sewage= 65% *Q * pop. = 0.65 * Q*p
Where :
Q= water consumed by one person for one day , and
P= population .
M1 =
*2
Where : M1 = ration of maximum flow ( Q max) to average flow (Q avg.)
P = population in thousands.
N= seasonal variation coefficient its values
From 1.2 to 1.4 the normal recommended value of n is 1.20 therefore
M1 =
M2 = 0.2 * p 1/6
Where : m2 = ratio of minimum flow (Q min ) to average flow ( Qavg)
P= population in thousands.
Ex: determine the upper and lower limits , and the daily average flow rates for city of
population of ( 200 000 ) capita. The daily water consumption per capita is ( 250 l).
Sol.
1-average domestic sewage = 0.65 * 250 * 200 000 = 32.5 * 106 l/d
2-max. domestic sewage = M1 * average domestic sewage =
= 1.984*32.5 106 = m64.48 * 106 l/d
2
* 32.5 106
3-Min . domestic sewage =M2 * average domestic sewage = 0.2 ( 200) 1/6 * 32. 106 = 0.483 *
32.5 * 106 =
15.75 * 106 l/d.
Sewage collection and disposal system:
There are three type of collection and disposal system of sewage :
1)separate system :The this type of collection it was used tow separate sewage systems one system is for
collection and conveyance of rain water the second system is specified to collect and convey
residential , and industrial liquid wastes . Usually this system is called dry weather flow rate .
this first system convey rain-water directly to rivers , channels , and drains, while the second
system convey liquid waste to the treatment plant.
ِAdvantages:
1-this system is required small diameter of pipes due to small quantities of residential , and
wastewater which separated from rain water .
2-redyction of load are treatment plant , and leads to improve its performance which reduces
pollution of river water that received wastewater .
Disadvantages:1-it is probable to clog pipes and difficulty of cleaning of pipes because small diameters used.
2-high expensive cost needed because using of tow systems .
3-the rain water system may be exposed to clog because throwing of solid wastes especially at
dry seasons of year .
3-combined system :
Advantages:
1-it is easy to clean and maintain this system because big size of its diameters
2-it does not occupied large areas of constructed roads
3-dillution is happened and thus reduced concentration of sewage due to mixing with
rainwater .
Disadvantages:
1-it is difficult to load and convey pipes because their have big weights and big diameters .
2-combination of sewage and rainwater causes big operation effort on units of treatment
plant.
3
3-it is probable to happen flooding of pipes of system and thus leads to high level of pollution
4-in dry seasons of year it happens large lack of sewage quantity and such lack has bad effects
on operational efficiency of units of treatment plant.
5-in dry seasons of year it is probable to settle inorganic materials presence in sewage due to
decrease of flow velocity
3-partially combined system
In this system it was used one sewage net comprises on working for the tow system of
separate and combined .parts of the city may be served by combined system while other parts
may be served by separate system. The sharing occurred in some points to transfer increasing
of sewage quantity due to strong storm on the city.
Advantages
1-the diameters of pipes in the system have moderated sizes which reduse the problem of
sedimentation of organic materials .
2-it is easy to discharge rain water collected in houses , and service buildings .
Disadvantages :
1-some parts of this system facing the problem of relative low velocity of flow especially at dry
seasons of year .
2-the costs of design and construction of this system is high . also , it needs to high level of
accuracy .
3-the rainwater increases the quantity of flow and thus leads to happen big operation efforts
on pumps and the units of treatment plants .
In Iraq , the client who responsible
For design and construction of sewage system decided to use separate system because it is
most suitable for topography and climatic condition of Iraq . also because of continuous large
Urban expansions in all cities in Iraq .
Sewage characteristics :1-phisical characteristics
The fresh wastewater has soapy or oily o dour , and it contains solid materials of large
diameter , the stale sewage has o dour of hydrogen sluphide , its color is dark gray or black ,
the characteristics of wastewater changes from fresh to state during period of
(6-7) hrs depending on organic matter concentration .
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2-chemical characteristics :
The chemical quality of wastewater is expressed in terms of organic and inorganic and
inorganic constituents , domestic wastewater generally contains 50 percent organic and 50
percent in organic matter . major components of organic matter in domestic wastewater are
carbohydrates , proteins ,are easily biodegradable . fats , oils and grease are more stable and
are decomposed slowly by micro-organisms . the table below shows the values which might
be expected in domestic sewage.
Typical domestic sewage characteristics ,mg/l
parameter
Total suspended
solids
Volatile suspended
solids
BOD
COD
TOC
Ammonia – N
Organic – N
Po4 - p
Strong
Wear
medium
100
200
350
75
135
210
100
175
100
5
8
7
200
300
200
10
20
10
400
600
400
20
40
20
Bacteria in wastewater
By its nature , domestic wastewater contains enormous quantities of micro-organisms .
depending on its ago and the quantity of dilution water , bacterial counts in row sewage may
be expected to range from 500000 to 5000000 per ml
Viruses , protozoa , worms , and other forms are also found, but their presence is seldom
important enough to require measurement
Anaerobic processes
Anaerobic bacteria oxidize organic matter utilizing electron acceptors other than oxygen in
carrying out their metabolic processes they produce Co2 ,H2O , H2S , CH4 , NH3 , N2 , reduced
Anaerobic bacteria
organics , and more
bacteria .
Organic materials
Co2 , H2O , H2S , CH4 , NH3 , N2, ….
Aerobic processes
Aerobic bacteria Utilize free oxygen as an electron acceptor . the end products of aerobic
activity are CO2 , H2o SO-24 , NO-3 , NH3 , and more bacteria
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Organic materials
CO2 , H2O , SO-24, NO-13, NH3
New bacteria
Other micro organisms
Other micro organisms of occasional importance in wastewater treatment include algae ,
protozoa , fungi, and rotifers.
Biochemical Oxygen Demand BOD.
Bacteria placed in contact with organic material will utilize it as a food source . in this
Utilization , the organic material will eventually be oxidized to stable and products such as CO2
and water . the amount of oxygen used in this process is called the "biochemical oxygen
demand " and is considered to be a measure of the organic content of the waste . it also
represents , to some extent , the amount of oxygen which would be required to stabilize the
waste in a natural environment such as a stream or lake .
The BOD determination has been standardized and measures the amount of oxygen utilized
by microorganisms in the stabilization of wastewater during 5 days at 20 co . for ordinary
domestic sewage the 5-day value , or BOD5 represents approximately two-thirds of the
demand which would be exerted if all the biologically oxidizable material were in fact
oxidizable material were , in fact , oxidized .
The BOD of wastewater is not a single point, but rather a time - dependent variable . A 5-d
incubation has become
A standard practice
It takes approximately 20 d to stabilize all bio degradable organic matter and nitrogen
contained in a wastewater for sample the carbonaceous oxygen demand is called first – stage
BOD , while the nitrogenous oxygen demand is called second-stage .
BOD
BOD ,mg/l
exerted
= - K.L
Where : L is the BOD ( or
BOD , remaining
Lt
L = organic matter concentration ) remaining at time t.
t
Time , days
K1= constant ( reaction rate constant at 20 co , d-1 ( for domestic wastewater K = 0.2 to 0.3 d-1 )
t=time
6
By inter age of Eq .(1) one abtains:
Lt =L * e-K1.t
....................(2)
Lt = BOD concentration at any time . t
L= BOD concentration at ( t=0) , alsc it called ultimate BOD . BODt = L – Lt …………………..(3)
( exerted)
Then , BODt = L (1-e-K1.t) …………………….(4)
BOD t : exerted BOD at time t.
Eq. ( 4) may be written in the following form :
BOD t = L ( 1- e-K.t) …………………………..(5)
The constants ( K1 , varies with temperature in accordance with the following correction
formulas :
K1(T) = K1 (20c0) . (1.047 ) T-20
L(T) = L(20coT + 0.6 )
Where K1 (20co) is the value determined at 20 co .
K1 ( T) is the value determined at any temperature ,
L ( 20 c0) is the ultimate BOD concentration at 20co , and
L ( T) is the ultimate BOD concentration at any temperature , T
Ex: the biochemical oxygen demand of wastewater during five days at temperature of 20 co is
190 mg/l , if the value of
K1= 0.25 d-1 . determine the biochemical oxygen demand through one day , then through five
days under temperature degree of 13 c0 .
Sol.
BODt = L ( 1-e-k1.t) 190 = L ( 1- e-0.25 * 5 ) L = 266 mg/l
L (T) = L (20) ( 0.02 T + 0.6 )
L(13) = 266 ( 0.02(13)+.06) = 229 mg/l
K1(T) = K1 (20) (1.04)T-20 K1(13) = 0.25 (1.047)13-20 :. K1(13) = 0.181 d-1
BOD1 = 229 b(1 - e-0.181*1) = 37.9 mg/l
BOD5 = 229 ( 1 – e-0.181*5)= 136.3 mg/l .
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EX: determine the ratio of BOD10 to BOD5 if the rate of reaction is 0.20 d-1
Sol:
BODt = L( 1 - e-K1.t) BOD10 = L(1 - e-0.2*10)
BOD10 = L ( 1 – e-2)
BOD5= L ( 1- e-0.2*5)
:.
BOD5 = L(1 –)
=
=
=1.367
Determination of L, and K,
Thomas noted the similarity between the first terms for the series expansion of 1 – e-k1.t and
k1.t[1 + k1.t/6]
And developed the approximate formula
-3
BOD = L . K1 .t (1+
(
1/3
= ( K1. L )-1/3 + (
which can linear zed as
t)
By plotting (t / BOD)1/3 versus t ,a straight line may be obtained with intercept (K1 . L)-1/3 at
t=0 and slope K12/3 / 6L1/3
Ex. The following data are to be used to determine the value of K1 and L for the waste testes
1/2
1
2
3
4
5
7
10
15
5
20
90
160
200
220
260
285
320
350
300
(15 , 220)
250
200
150
100
50
0
(15 , 220)
300
250
(7 , 260)
(5 , 220)
(1-e-0.295(t-0.8) )
150
100
(4 , 200)
(1 , 90)
Time 1
days
BOD , mg/l
(3 , 160)
(1 , 20)
5
10
Time days
0
15
BOD exerted versus time
8
50
0
The time case it appears that the time of beginning which best the dota is t0 = 0.8 days . the
time at which the BOD date were taken are thus corrected by this amount
Time days
0.50
1.0
2.0
3.0
4.0
5.0
7.0
10.0
15.0
1/3
(t/BOD) is plotted versus t.
Corrected
Time . days
............
0.20
1.20
2.20
3.20
4.20
6.20
9.20
14.20
BOD
Mg/L
5
20
90
160
200
220
260
285
320
(t/BOD)1/6
0.22
0.24
0.24
0.25
0.27
0.29
0.32
0.35
0.40
(t/ BOD)1/3
K12/3
6 L1/3
0.30
0.20
0
5
10
15
Time days
Graphic determination of K1 and L
From figure , (K1 . L)-1/3= 0.22 and
K12/3/6L1/3 = 0.0108 , then solving simultaneously
Gives K1 = 0.295 d-1 . and
L= 319 mg/L
The derived BOD equation is thus BOD = 319 [1-e-0.295(t-0.8)]
Treatment of wastewater
The wastewater contains impurities which include organic and inorganic matter . the main
goal of treatment processes in to remove these impurities . the organic and inorganic will be
suspended or dissolved or floating matters . the wastewater pass through different stages of
treatment :
1)preliminary treatment system :9
The purpose of this stage of treatment is to remove suspended and floating solids matters like
sand particles which have quick sedimentation , as well as to remove oil , and fats . this stage
includes :
a-screens:
to remove dead animals , rages, cinders , metals , plastic, and plant pieces ….. etc . As well as
to remove coarse solid matters .
b-Grit chambers :
to remove suspended particles ( sand) which have fast sedimentation that pass through
screens.
c-skimming tanks :
the purpose of skimming tanks is to remove oil , and fats and also to remove small floating
particles .
2)primary treatment systems :
This includes passing of wastewater through sedimentation tanks to remove fine suspended
particles which can rot be removed by grit chambers.
The sedimentation is always done by tow stages . the first stage is done before biological
treatment stage and its called primary sedimentation while the second stage is done after
biological treatment and its called secondary treatment . if the sedimentation has adding of
some chemicals (chemical coagulants ) to wastewater , then its called chemical sedimentation
. before the primary treatment stage the wastewater should be passed through parshal flume
in order to measure its flow rate .
Secondary treatment system
In this stage the removal of colloidal and soluble organic matter which remain after primary
treatment stage this stage is called biological treatment and which is done by one of these
following ways :
a-Trickling filters .
b Aeration lagoons -
For small cities
c-Activated sludge tanks
For big cities
d-Aerated Oxidation ponds
4-chlorination of sewage :
In order to kill harmful becteria it is necessary to disinfect wastewater before it disposing to
the river . the wastewater should be passed through flume before it discharging to the river
5- sludge treatment system
10
The sludge can be
11