Contents
List of figures ........................................................................................................................................... 1
List of tables ............................................................................................................................................ 1
List of symbols......................................................................................................................................... 2
1.
Executive summary ......................................................................................................................... 4
2.
Background of the report................................................................................................................ 5
3.
Introduction .................................................................................................................................... 9
4.
Overall Mass balance .................................................................................................................... 12
4.1 Component balance on the overall system ................................................................................ 14
4.1.1 Total suspended solids mass balance .................................................................................. 15
4.1.2 Chemical Oxygen Demand mass balance ............................................................................ 17
4.1.3
5.
Ammonia mass balance ................................................................................................ 18
Energy balance on a centrifugal pump ......................................................................................... 20
5.1 Flow diagram............................................................................................................................... 20
5.2 Piping system characteristics ...................................................................................................... 21
5.2.1 Valves and fittings ................................................................................................................ 21
4.2.2Pipe specification .................................................................................................................. 23
5.3 Pumping energy requirements to overcome loses. .................................................................... 24
5.3.1 Materials of construction ..................................................................................................... 26
5.3.2 Head loss due to friction and and pipe fittings .................................................................... 27
5.3.3 Power requirements ............................................................................................................ 28
6.
Heat transfer operation ................................................................................................................ 30
a.
Introduction .............................................................................................................................. 30
b.
Design procedure ...................................................................................................................... 31
c.
Design calculation ..................................................................................................................... 31
7.
Evaluation of mass transfer operation.......................................................................................... 34
a.
Background ............................................................................................................................... 34
Derivation of formula........................................................................................................................ 37
b.
Sample calculation .................................................................................................................... 37
8.
Conclusion ..................................................................................................................................... 40
9.
References .................................................................................................................................... 41
List of Figures
Figure 1: Map of Macassar...................................................................................................................... 5
Figure 2: Satellite Photographic Map ..................................................................................................... 6
Figure 3: Fine screen at inlet works ........................................................................................................ 7
Figure 4: Bio-Reactor .............................................................................................................................. 8
Figure 5: Process Flow Diagram ............................................................................................................ 10
Figure 6: Process and Instrumentation Diagram .................................................................................. 11
Figure 7: Pump Station Flow Diagram .................................................................................................. 20
Figure 8: Long Radius Elbow ................................................................................................................. 21
Figure 9: Short Radius Elbow ................................................................................................................ 21
Figure 10: Globe Valve .......................................................................................................................... 22
Figure 11: Gate Valve ............................................................................................................................ 22
Figure 12: Piping System Diagram ........................................................................................................ 24
Figure 13: Friction Factor Chart ............................................................................................................ 26
Figure 14: Pump curves of different speeds ......................................................................................... 29
Figure 15: Shell and tube heat exchanger ............................................................................................ 30
Figure 16: Shell and tube heat exchanger ............................................................................................ 31
Figure 17: Correction factor chart ........................................................................................................ 33
Figure 18: Vacuum Flask ....................................................................................................................... 34
Figure 19: Moisture Analyser ................................................................................................................ 35
Figure 20: Drying Curve......................................................................................................................... 36
List of Tables
Table 1: Stream composition ................................................................................................................ 14
Table 2: Miscellaneous loss calculation ................................................................................................ 23
Table 3: Variable speeds of the impeller .............................................................................................. 29
Table 4: Drying Experimental Results ................................................................................................... 36
Table 5: Suspended solids result........................................................................................................... 39
1
List of Symbols
V=
volume (m3)
A=
area (m2)
L=
length (m)
ρ = density (kg/m )
3
µ=
viscosity (Pa.s)
d=
diameter (m)
h=
height (m)
P=
pressure (kPa)
F=
flowrate (m3/hr)
Re = Reynolds number (dimension less)
f=
friction factor (dimension less)
P=
power (kW)
C=
concentration (kg/d)
k=
reaction constant
G=
generation term
G’ =
consumption term
R=
reaction rate (kg/d)
m=
mass (kg)
Rf =
relative rougness
U = overall heat transfer co-efficient (W/m2 oC
Cp = Heat capacity (kJ/kg oC)
Qh = Heat load (kW)
Q = flow rate (l/s)
v =
specific velocity (m/s)
ML = Mega Litres (l)
2
List of Abbreviations
ppm = parts per million
BOD = Biological Oxygen demand
TSS = Total Suspended Solids
MLSS = Mixed liquor suspended solids
RAS = Return Activated Sludge
3
1. Executive summary
Waste management became essential for the sake of environmental protection and nature
conservation, since water is the main substance to transport human waste to natural environmental
systems such as rivers and seas, waste water treatment process is used to purify that sewage. Waste
water treatment process implements science, technology and engineering to treat and improve
quality of the sewage before disposal to the natural environmental systems, to meet government
specifications and standards. Untreated waste water is particularly detrimental to a health of living
organisms as it contains millions of potentially dangerous pathogenic micro-organisms that can lead
to many diseases and unhealthy environment. Waste water treatment works is one of the most
important part of any town infrastructural planning as the sewerage from the residential areas is
directed by the network of pipelines to a waste water treatment works to be treated and purified.
Waste water treatment plant is design such that it will improve the quality of waste water to a
standard where it can be re-used in more useful utilities such as, irrigation, process line cooling or as
a raw material in some processes etc. good process control and operation determines the success
and failure of any waste water treatment plant, there for skills such as mass and energy balance
should be put into practice to guarantee the success service of treating water to the higher value of
quality. This can also ensure good operation of equipment there for the will be less maintenance and
operational cost. Series of experiments needs to be done in the plants daily to enable decision
making.
4
2. Background of the report
This report was written during the experiential training at City of Cape Town municipality in
Macassar waste water treatment works to express and illustrate the knowledge and experience
obtained in waste water treatment process during training period. The orientation received at the
beginning of training through various divisions of the department of water and sanitation, was to
outline the significance and importance of co-operation between different divisions that monitor,
supervise and control the purification of waste water that originates from residential areas and
industries, throughout the Western Cape. The objective is to meet and exceed government
specification and standards regarding the environmental protection towards untreated waste water,
as it are detrimental to the environment.
Macassar waste water treatment plant was built in 1984. It is located between Macassar and Strand,
but aside from the community, it has a sea view and is very close to the Easter river in so much that
its final effluent is discharged to the downstream of Easter River that goes straight to the sea. It
serves both communities Macassar and Strand, by treating the sewage that originates from these
communities. It is a middle size plant of capacity 35ML per day. It uses biological processes to
remove biodegradable nutrients from the raw sewage with the aid of micro-organisms in the
activated sludge. The plant consists of two biological reactors of volume 24500 m 3 each. Each
reactor has six aerators to facilitate aerobic process. The plant has six secondary settling tanks and is
design to handle the sludge from both reactors. Plant purpose is to produce final effluent that
complies with government specifications and standards’ regarding the disposal of waste water to
the environment, this plant achieves this objective by treating the sewage through three stages
termed primary, secondary and tertiary treatment processes.
Figure 1: Map of Macassar
5
Figure 2: Satellite Photographic Map
Primary treatment stage is when the incoming raw sewage has to be pre-treated to remove large
debris such as rags, papers, plastics and wood sticks before it enters the process stream. They
cannot be degraded by the micro-organisms in the biological process, and this also serves to protect
the equipment of the downstream process from being damaged. This is achieved by using fine
screens that will only allow sewage to pass through and block any large materials. The screenings are
removed by automated grab bar rakes which pick up the materials to a screw conveyer that
transport the screening materials to skips. Fine screens are followed by a grit trap chamber that
removes inorganic particles like sand and small stones to protect the downstream process
equipment from being damaged. Equipment such as pumps and small pipe diameter can easily block
out by settled out grit inside piping system.
6
Figure 3: Fine screen at inlet works
Secondary treatment is a biological process stage that uses micro-organisms to convert finely
divided and dissolved nutrients and organic matters in waste water into flocculent that can easily
settle out from suspension. The main objective is to reduce the organic content and nutrients in
waste water which is measured as Biological Oxygen Demand (BOD) and Chemical Oxygen Demand
(COD) that determines the strength of the raw sewage. An activated sludge is used as the source of
supplying active micro-organisms, mainly bacteria and protozoa that can digest nutrient and organic
matter which makes up the composition of a raw sewage. Components such as proteins,
carbohydrates, fats, ammonia and phosphates, that renders the sewage to be septic and dangerous
for disposal to the environment. Activated sludge is the concentrate underflow of suspended solids
from the secondary settling tank, which is returned to the biological reactor in order to mix with the
raw sewage forming mixed liquor, which gets aerated by surface aerators for an adequate contact of
raw sewage with micro-organisms in the activated sludge. Aeration process in a biological reactor
simply creates large surface area of contact between mixed liquor and oxygen in the air which is
needed by micro-organisms in order to be alive and be capable of decomposing nutrients and
organic matter in waste water (aerobic digestion). Aerators also help to direct the flow to exit the
reactor as an effluent called mixed liquor suspended solids (MLSS).
7
Anaerobic
zones
Aerobic
zones
Flow direction
MLSS
Aerators
Mixed
feed
Figure 4: Bio-Reactor
During protein degradation under anaerobic digestion methane (CH4), ammonia (NH3) and hydrogen
sulphide gas (H2S) is produced and depending on the pH NH3 will be in equilibrium with NH4+ in
waste water. At pH = 7, 95% of NH3 is found in the form NH4+ hence the effluent pH of waste water
must be greater than 8 to reduce NH3 in the solution. High molecular fats and carbohydrates
produce biogas anaerobically as follows.
CH3COOH
CH4 + CO2
…………………………………………
Equation 1
CO2 + 4H2
CH4
…………………………………………
Equation 2
+ 2H2O
Presence of ammonia in waste water effluent in a form of NH4+ is poisonous to aquatic species at
concentrations above 0.2 ppm hence the removal of ammonia is essential in the waste water
treatment process before disposal to natural environmental system, the process of nitrogen content
removal in waste water is called Nitrification and De-nitrification process. Bacteria responsible for
Nitrification process is Nitrosomonas in the presence of oxygen, the reaction is as follows
NH4+ + 1.5O5
NO2- +
H2O + H+
………………………………
Equation 3
……………………………………
Equation 4
NO2- is oxidised by nitrobacter to NO3- as follows.
NO2- + 0.5O5
NO3-
Facultative anaerobic bacteria uses nitrate ( NO3- ) to respire as follows.
8
4NO3- + 5C + 4H+
5N2 + 5CO2 + 2H2O.
………………………..
Equation 5
Nitrogen which is the main component in the formation of ammonia will be relies in the form of a
gas to the atmosphere, this will decrease the concentration of ammonia in waste water.
Micro-organisms they continuously re-produce themselves in a process of braking down organic
constituent in waste water, which can lead to a sludge bulking that is unacceptable. Sludge wasting
is the only solution for prevention of sludge bulking where by a portion of sludge concentrate is
removed from the system, de-watered and transported to landfill sites.
A portion of underflow concentrate of activated sludge is completely removed from the process
after de-watering it in a belt pressing machine. It is initially flocculated with the polyelectrolyte
flocculent, for complete gravimetric separation between sludge and clear water, belt press filter is
used to recover most of water from the flocculated sludge, dry sludge of total solid 14% is produced
from the filtering belt process, all recovered water returns back to the process as a filtrate. Sludge
cake from the belt press is wasted to the landfill sites.
Chlorine is used as a disinfection agent of the final effluent, this is classified as a tertiary treatment
process step. This is for removal of pathogens and some nutrients which were not completely
removed from the preceding treatment steps. This effluent goes to four maturation ponds which are
wild shallow dams of about 1 meter depth to allow sunlight penetration up to the bottom of the
pond and promote aerobic conditions as the oxygen from the atmosphere will be able to dissolve as
much because of large surface area of contact between water and the air. The loading on the
maturation ponds is calculated on the assumption that 80% of BOD has been removed in the
preceding treatment process step. Before water can be discharged from the treatment plant as a
final effluent further disinfection process is implemented after maturation ponds, where by chlorine
is dosed to reduce the level of bacteria in the effluent. After dosing water is passed through
detention period, with reduced flow rate that allows enough time of contact between chlorine
dosed and the bacteria that still exist in the water. Maximum permitted level after disinfection is 400
organisms per 100ml of water.
3. Introduction
Mass balance is a good practice to be carried out. On-going mass balance in a waste water process it
works for good as it is in any other chemical process. It is the basis for process modelling and
optimization, as the solids and any other toxic components that entering a process plant and those
who are generated in the process are handled by mass balance operation. It enables wise and
efficient operation of equipment in the plant to suit operation requirements. This section of the
report will cover the mass balance of total suspended solids, chemical oxygen demand, ammonia
and chlorine concentration over entire process. This also serves to evaluate the operation of a
process, enables decisions to be made in terms of process alterations and plant expansion projects.
Determination of the amount of sludge to be wasted from the process per unit time is based on
mass balance calculation, as it will track exactly how much sludge is being produced per unit time.
9
Skip bins
Landfill
Filtrate
Screenings
Grit and
sand
Sludge cake
Grit
chamber
Bar screens
Raw sewage
De-watering
Biological
reactors
MLSS
RAS
Chlorine
Chlorine
contact
chamber
Maturation
ponds
Clear
wate
r
Secondary
settling
tanks
Final effluent
Figure 5: Process Flow Diagram
10
Solid waste to landfil
P-4
P-64
E-13
Biological Reactor 1
P-49
Screenings
P-65
P-49
P-58
P-5
P-29
Bins
P-8
Chlorine Tank
P-37
P-28
P-33
P-31
Final Effluent
P-30
P-7
V-2
Grit
P-9
P-50
P-49
P-32
E-6
E-9
P-36
P-10
P-35
P-11
P-34
P-69
P-38
P-22
E-2
Raw sewage
P-48
E-11
E-10
P-51
P-56
Fine screens
P-19
P-39
filtrate
P-39
Chlorine contact tank
P-38
V-1
P-23
P-18
P-24
P-45
P-46
E-8
E-7
P-63
P-26
Sldge cake
P-38
E-12
Belt press
P-42
Maturation pond 1
P-43
P-44
P-3
P-52
P-12
Maturation pond 2
P-53
Biological Reactor 2
Maturation pond 3
Sludge truck to landfil
P-54
P-55
Maturation pond 4
Figure 6: Process and Instrumentation Diagram
11
According to theory the amount of water coming into the plant per unit time for purification must
be equal to the amount of the effluent leaving the plant per unit time. Although in practice that is
not really taking place due to some losses inside the treatment works that caused by leakages and
spilling in parts of the process. Overall mass balance calculation of this report is based on the
assumption that the process is at steady state, meaning that there is no accumulation of mass inside
the plant.
4. Overall Mass balance
F5ρ5
F 2ρ 2
Macassar WWTW
F1 ρ 1
F 3ρ 3
F4ρ4
F1 = Raw inflow (m3/d)
F2 = Final Effluent (m3/d)
F3 = Irrigation water to Strand (m3/d) (Located at Heldarberg)
F4 = Irrigation water to Macassar plant (m3/d)
F5 = Water that goes with the sludge to hoppers (m3/d)
Ρ1 = Raw inflow density (kg/m3)
12
P2 = Final Effluent density (kg/m3)
P3 = Density of irrigation water to strand (kg/m3)
P4 = Density of irrigation water to macassar (kg/m3)
P5 = Density of water in the sludge (kg/m3)
 change _ of _ mass   all _ that _ adds _ to
  all _ that _ removes _ from 

 
 

 of _ the _ systerm    the _ mass _ of _ the _ systerm   the _ mass _ of _ the _ systerm

 
 

time
time
time

 
 


 
 

dM
  M in   M out
dt inlets
outlets
............................................................... Equation 6
d  v 
  1 F 1  2 F2   3 F3   4 F4   5 F5
dt
Simplifying assumption
 is constant
There for 

 1   2  3   4  5
dv
  ( F1  F2  F3  F 4  F5 )
dt
dV
 F1  F2  F3  F4  F5
dt
dV
 F1  F1  F3  F4  F5 
dt
Assume that the volume is constant
dV
0
dt
F 1 F2  F3  F4  F5
27949 kg/d = 25000 kg/d + 2109.52 kg/d + 314.93 kg/d + 60.5 kg/d
27949 kg/d = 27484.95 kg/d
13
4.1 Component balance on the overall system
Component balance is basically the evaluation of raw composition and the final effluent, to actually
track the process as some of the components will be consumed, some will be generated during the
process, there for there will be a change of composition within the system. The composition of raw
and final effluent is tabulated in the table below.
Table 1: Stream composition
PARAMETER
UNITS
RAW FLOW (Ml/d)
/07/2011
/08/2011
28
30.1
Raw composition
Total Suspended Solids
mg/l
92
300
Volatile Suspended Solids
mg/l
0
0
Settleable Solids
mg/l
1
14
COD
mg/l
411
617
Ammonia
mg N/l
54.6
47.1
Total Phosphorus
mg P/l
7.9
9.5
Ortho-Phosphate
mg P/l
6.4
5.8
7.7
7.6
pH
Chloride
mg/l
129
134
Alkalinity
mg CaCO3/l
381
403
Total Suspended Solids
mg/l
5
7
Total Dissolved Solids
mg/l
0
48
COD
mg/l
25
22
Ammonia
mg N/l
0.9
3.4
Nitrate/Nitrite
mg N/l
4.5
7.6
Ortho-Phosphate
mg P/l
4
2.7
8
7.8
Final Effluent
pH
Chloride
mg/l
137
125
Alkalinity
mg CaCO3/l
126
146
E.coli
per 100ml
4
1
14
4.1.1 Total suspended solids mass balance
The in-coming raw sewage there is organic and inorganic solid that does not dissolve in water, these
solids are entrained by the flow, forming up suspension with water. These solids are termed to be
suspended solids since they form part of the fluid. During processing some solids are being
generated in a biological reactor, as the micro-organisms degrade the organic components that
make up the sewage, they gain more energy and turn to cling together forming un-dissolving solid
that adds to the total amount of suspended solids.
F5CTSS
F2CTSS
Macassar WWTW
F1 CTSS
F3CTSS
F4CTSS
.
CTSS = Concentration of Total Suspended Solids in a stream to which is located (mg/l)
15
 change_ in _ TSS
  all _ that _ adds _ to   TSS _ generated
  all _ that _ removes_ from   TSS _ consumed


 
 
 
 

within
_
the
_
systerm
TSS
_
of
_
the
_
systerm
within
_
the
_
systerm
the
_
TSS
_
of
_
the
_
systerm
within
_
the
_
systerm







 
 
 
 

time
time
time
time
time

 
 
 
 


 
 
 
 

d (CTSSV )
  CTSS F  GTSS   CTSS F  GTSS
dt
inlets
outlet
........................... Equation 7
dV
0
dt
No total suspended solids are being consumed in the system.
V
dCTSS
 CTSS F1  GTSS  CTSS F2  CTSS F3  CTSS F4  CTSS F5
dt
V
dCTSS
 CTSS F1  GTSS  CTSS F2  CTSS F3  CTSS F4  CTSS F5 
dt
Assume that there are no solids on the final effluent and the irrigation water.
V
dCTSS
 CTSS F1  GTSS  CTSS F5
dt
dCTSS
F G
F
 CTSS 1  CTSS 5  TSS
dt
V
V
V
 2794m 3

 60.5 m 3 
dCTSS

d   7.26  103 kg 
d   GTSS
 7392kg 

3
d
d  28  103 
dt
V
28  103 m 
d



 2794m 3

 60.5 m 3
 kC 28  103 m 3


dCTSS

d
d   TSS
d
 7392kg 
 7.26  103 kg 

3

3
3
d
d
3
3
dt
 28  10 m 
28  10 m
 28  103 m

d
d

d

dCTSS
 7379kg  15.7 kg  kCTSS
d
d
dt
dCTSS
 7363.3 kg  kCTSS
d
dt
16
4.1.2 Chemical Oxygen Demand mass balance
3
Bio-reactor
1
Inlet
works
Irrigation
water pump
station
SST’s
4
2
5
1
2
3
4
5
= Raw inflow stream
= Final Effluent stream
= Stream of irrigation water to strand
= Stream of irrigation water to Macassar
= Sludge stream to de-watering
 Change_ in _ COD
  All _ that _ adds _ to
  COD _ generated  All _ that _ removes   COD _ removed 

 
 
 
 

 with _ in _ the _ systerm   COD _ of _ the _ systerm   in _ the _ systerm    COD _ in _ the _ systerm   in _ the _ systerm

 
 
 
 

time
time
time
time
time

 
 
 
 


 
 
 
 

dCCODV
  CCOD Fin  GCOD   CCOD Fout  GCOD
dt
inlets
otlets
............................. equation 8
dCCODV
'
 CCOD F1  GCOD  CCOD F2  CCOD F3  CCOD F4  CCOD F5  GCOD
dt
C
dV
0
dt
V
dCCOD
'
 CCOD F1  GCOD  CCOD F2  CCOD F3  CCOD F4  CCOD F5  GCOD
dt
V
dCCOD
'
 CCOD F1  GCOD  (CCOD F2  CCOD F3  CCOD F4  CCOD F5 )  GCOD
dt
No COD produced during reaction there for GCOD is eliminated.
17
V
dCCOD
'
 CCOD F1  (CCOD F2  CCOD F3  CCOD F4  CCOD F5 )  GCOD
dt
Concentration of COD on stream 2,3,4,5 will be the same since all other streams 3,4,5 are extracted
from final effluent stream 2 after disinfection stage.
'
F  F3  F4  F5  GCOD
dCCOD
F
 C COD 1  C COD 2

dt
V
V
V
F  F3  F4  F5   kCCODV
dCCOD
F
 CCOD 1  CCOD 2
dt
V
V
V
F  F3  F4  F5   kC
dCCOD
F
 CCOD 1  CCOD 2
COD
dt
V
V
 27949m 3

 27485m 3

dCCOD




kg
kg
d
d
 20104
 1232



  kCCOD
3
3
d
d
dt
 28  103 m

 28  103 m

d
d


dCCOD
 20067kg  1209.34 kg  kCCOD
d
d
dt
dCCOD
 18858kg  kCCOD
d
dt
dCCOD
 18858kg  20104kg k
d
d
dt
4.1.3
Ammonia mass balance
Removal of ammonia in a waste water treatment process is of vital importance as it facilitates
purification. The process of ammonia removal is termed nitrification and de-nitrification process
whereby oxygen is supplied for conversion of ammonia to nitrate, undergoing a three step process.
18
F5CNH4
F2CNH4
Macassar WWTW
F1 CNH4
F3CNH4
F4CNH4
CNH4 = Concentration of ammonia in a stream
 change _ of _ ammonia   all _ that _ adds _ to 
 All _ that _ removes_ from 

 



 in _ the _ systerm
   ammonia
   ammonia _ generated   ammonia _ of _ the _ systerm   ammonia _ consumed 
 


 
 
 
time
time
time
time
tine



 




 



dCNH4 V
dt
dCNH4 V
dt
C
V
V
  C NH4 Fin  G NH4   C NH4 Fout  G NH4
inlets
otlets
............................ Equation 9
'
 C NH4 F1  G NH4  C NH4 F2  C NH4 F3  C NH4 F4  C NH4 F5  G NH
4
'
 C NH4 F1  G NH4  C NH4 F2  C NH4 F3  C NH4 F4  C NH4 F5  G NH
4
dV
0
dt
dCNH4
dt
dCNH4
dt
'
 C NH4 F1  G NH4  (C NH4 F2  C NH4 F3  C NH4 F4  C NH4 F5 )  G NH
4
19
F  F3  F4  F5   GNH4
F GNH4
 C NH4 1 
 C NH4 2
V
V
V
V
'
dCNH4
dt
dC NH 4
 2480.8 kg
dt
dC NH 4
 2480.8 kg
dt
dC NH 4
dt
d
 k C  43.9 kg
d
 2437.71kg
 27484.95 
 k 'C
d  28  103 
 k C  43.09 kg
d
d
 k 'C
 k C  k 'C
5. Energy balance on a centrifugal pump
The design of a pump cannot be isolated from the piping system since the energy requirements will
be the function of static head due to elevation and pressure differences between suction and
discharge, and the dynamic head due to frictional losses on valves, fittings and bends.
5.1 Flow diagram
Pump station
De-watering
unit
Pump 1
Bio-reactor
Sludge
cake
Pump 2
Filtrate
Figure 7: Pump Station Flow Diagram
20
5.2 Piping system characteristics
5.2.1 Valves and fittings
Figure 8: Long Radius Elbow
Figure 9: Short Radius Elbow
21
Figure 10: Globe Valve
Figure 11: Gate Valve
22
4.2.2Pipe specification
Pipe diameter = 200mm
Pipe length = 243m
Table 2: Miscellaneous loss calculation
Equivalent
length (m)
Quantity
Total length
(m)
long radius bends
4.6
5
23
short radius bends
6
5
30
T-pies from leg
12
3
36
T-pies into leg
18
1
18
fully opened valves
1.5
3
4.5
Half opened valve
40
1
40
sum
151.5
type
23
5.3 Pumping energy requirements to overcome loses.
P2= 0 pKa
P1 = 0 pKa
DE-WATERING
h2=5.6m
PLANT
BIOLOGICAL
REACTOR
h0=1.8 m
Underground
height (assumed)
Ground level
h1=0.3m
SLUDGE PUMP STATION
TYPE: Centrifugal pump
Figure 12: Piping System Diagram
24
Assume fluid velocity to be 1 m/s
Cross sectional area of a pipe (A)
A  r 2
= (3.14)(0.1)2
=0.0314 m2
Reynolds number (Re)
Re 

ud

1000 1 0.2
0.000894
 2.23 105
25
Point of intersection
Figure 13: Friction Factor Chart
5.3.1 Materials of construction
Type: carbon steel pipe
Absolute roughness (e) is 0.06 mm (assumed average)
Relative roughness is the absolute roughness divided by internal diameter of the pipe
Rr = 0.06 mm/200mm
= 0.0003
Frictional factor from the chart (f) = 0.004
26
5.3.2 Head loss due to friction and pipe fittings
Equivalent pipe length to compensate frictional losses is approximately (151.5m) obtained from pipe
diameter (m) multiply by miscellaneous losses (dimension less). The length obtained will be added to
actual pipe length in order to calculate a pressure drop.
Pipe length = 243 m
Equivalent pipe length = 151.5 m
Total length used to calculate pressure drop required in a pipe line.
Total length = 243 m + 151.5 m
= 394.5 m
 L  u
Pf  8 f 

 di  2
2
................................................................. Equation 10
 394.5  1000 1
 8(0.0065)

2
 0.2 
2
= 51285 N/m2
Conversion of pressure to heads
h
P
g

.................................................................... Equation 11
51285
1000 9.8
= 5.23 m
Total head required from the pump is the sum of dynamic and static head
Dynamic: (a) velocity head
(b) Friction head
Static: (a) pressure head
(b) Head due to height
Dynamic head= 5.23m
Static head = 5.3m
Total head = 5.3 + 5.23
= 10.53 m
27
Due to high concentration of total suspended solids 10% of the total head has to be added which is
(1.05m). This is to compensate slight density and viscosity change that is caused by solids in the
sludge. The density of the sludge and viscosity will increase as the concentration of solids increases.
Total head after adding 10% of it = 10.53+1.05
= 11.58 m
Bernoulli’s equation is used to calculate how much work is required for pumping the sludge in above
mentioned piping system. Work required is the function of pressure difference (ΔP) between pump
intake and the discharge, as well as pressure loss due to friction (ΔPf) caused by bends, fittings and
valves.
P
gz 


Pf

W  0
...................................................... Equation 12
(9.8)(-5.3)+0-(51285)/(1000)-W=0
-51.94-51.285-W=0
W = -103.225 J/kg
A negative sign implies that work is done by the system (pump).
5.3.3 Power requirements
Volumetric flow rate (v) in hourly basis
V = 0.0314 m2 × 1m/ s
= 0.0314 m3/ s
= 31.4 l/s
.
Mass flow rate (m)
.
m = 0.0314 m3/s × 1000 kg/m3
= 31.4 kg/s
.
W m
Power 

............................................................................
Equation 13
= (103.225)(31.4)/(0.73)
=4.4 kW 20% of power required will be added for safety factor
Total power to be supplied 5.28 kW
28
The pumps evaluated consist of variable speed drive that drives the pump impeller in different
speeds. The speed at which the impeller is running has direct effect in head produced by the pump
at the same flow rate, which will also affect the efficiency of the pump as well. The following table
shows different speeds that possibly this pump can run on and the head produced.
Table 3: Variable speeds of the impeller
Q (l/s)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
VSD
1780
30.5
28.2
26
23.7
21.5
19.5
17.5
15.5
13.3
11.2
9.2
VSD
1460
22.6
20.9
19
17.2
15.5
14
12.2
10.5
8.7
7
5.4
VSD
1180
14.6
13
11.5
10
8.5
7.1
5.8
4.2
2.8
VSD
970
NPSH
10.3
9.2
7.9
6.8
5.8
4.5
3.2
2
operating
curve
5.2
5.5
5.8
6.2
6.7
7.2
8
8.8
9.7
10.8
11.8
13.2
14.6
Pump Performance Curve - Macassar pump
station
35
30
VSD 1780
25
VSD 1460
20
VSD 1180
15
VSD 970
10
NPSH
oparating curve
5
0
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
Figure 14: Pump curves of different speeds
29
6. Heat transfer operation
In Macassar Waste Water Treatment Plant there was no units that transfers heat in order to
evaluate heat transfer operation, hence I decided to theoretically design a heat exchanger for
cooling of a hot fluid to an adequate temperature.
a. Introduction
The transfer of heat to and from process fluids is an essential part of most chemical processes. The
most commonly used type of heat-transfer equipment is the shell and tube heat exchanger, as this
conceptual design will be based on it. As its name implies, this type of heat exchanger consists of a
shell with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over
the tubes to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may
be composed by several types of tubes: plain, longitudinally finned, etc. the following diagram is a
three dimensional view for illustration of tubes arrangement.
Figure 15: Shell and tube heat exchanger
The fluid flowing on a shell side is directed by baffles for an adequate time of cooling or warming.
See the following diagram for fluid flow paten.
30
Figure 16: Shell and tube heat exchanger
b. Design procedure
Let us assume that we are designing a shell and tube heat exchanger for cooling a process stream of
methanol from temperature of 95 oC to 40 oC using final effluent of Macassar WWTW that is at
ambient temperature. The flow rate of methanol is 100,000 kg/h. water temperature raised from 25
o
C to 40 oC. Now I need to work out how much area of heat transfer is needed to accomplish the
duty in terms of sizing the heat exchanger.
Properties of the fluids used in a heat exchanger such as corrosiveness of fluid should be
investigated and known for decisions to be made in terms of materials of construction and to decide
which fluid must flow inside the tubes or outside. According to the above mentioned problem
statement water is more corrosive than methanol, there for water should flow inside the tubes and
methanol in a shell side outside the tubes simple because it will less costly to repair corroded tubes
after long time operation of an heat exchanger than to repair corroded shell.
c. Design calculation
Heat capacity methanol (Cp) = 2.84 kJ/kg oC (amount of energy required to increase temperature for
1 oC)
Heat load (Qh)= mCpΔT

......................................... Equation 14
100,000
 2,84(95  40)
3600
=4340 kW (rate at which heat is transferred)
31
Heat capacity water = 4,2 kJ/kgoC oC (amount of energy required to increase temperature for 1 oC)
Cooling water flow 
4340
4,2(40  25)
= 68.9 kg/s
Log mean temperature difference ( ΔTlm)
T1  t 2   T2  t1 
T  t 
ln 1 2
T2  t1 
ΔTlm 

........................................... Equation 15
95  40  40  25
95  40
ln
40  25
= 31oC
Log mean temperature difference is a function of heat exchanger design, it varies with the number
of tube passes and shell passes. Correction factor has to be taken in to consideration in order to
obtain correct temperature difference. Correction factor is taken from a chart that resembles the
specific design of a heat exchanger in terms of number of tubes that passes and a shell passes.
Use one shell pass and two tubes passes
R
T1  T2 
t 2  t1 

................................................. Equation 16
95  40
40  25
= 3,67
P

t 2  t1 
T1  t1 
............................................... Equation 17
40  25
95  25
= 0.21
32
Point of intersection
Figure 17: Correction factor chart
Correction factor (Ft) from the chart
Ft = 0,85
Tm = Ft x Tlm
= 0.85 x 31
= 26 oC
Overall heat transfer co-efficient (U)
U = 600 W/m2 oC
Heat load is also expressed as
Qh  UATm
A
Q
UTm

4340 103
600  26
.............................................. Equation 18
= 278 m2
33
7. Evaluation of mass transfer operation
Filtration and the drying unit were chosen for the evaluation of mass transfer. This is the small scale
unit used in the laboratory for sludge concentration analysis.
a. Background
Sludge of variable concentration which is mixed liquor suspended solids and the return activated
sludge were analysed for the determination of their concentration for process optimization in a
biological reactor. As the concentration of sludge in a reactor was to be maintained roughly around
3500 mg/l, due to food per micro-organism ratio and optimum performance of a reactor. A sample
of sludge that was to be analysed was grabbed directly from the reactor, with muddy appearance
that was caused by mass of solids that were suspended in the solution. The main objective of the
experiment was to completely separate the solids from known amount of clear water. The solids has
to be dried and weighed in order to determine its total mass that was in suspension per unit volume
of clear water.
The known volume of grabbed sample will be charged in a vacuum filtering funnel to be filtered,
using a thin filter paper that allows clear water to pass through but it blocks all other solids from
penetrating, this forms a layer of solids packed upon that known mass filter paper. Water will be
stored in the flask receiver at the bottom of the filter as a filtrate of known volume.
Figure 18: Vacuum Flask
34
Although the cake of solids that are upon a filter paper are still wet since some of the moisture is
bound inside the solids structure, hence drying has to take part in order to remove all the moisture
that was left inside the solids structure, in order to obtain the mass of bone dry solids. The operation
is carried out such that the mass of wet sludge solids is weighed before drying takes place in a unit
called moisture analyser. (Citation) has discovered from scientific research that some of the solids in
the sludge are more volatile than others
.
Figure 19: Moisture Analyser
The experiment was carried out to determine the total suspended solids both volatile and nonvolatile solids that forms the sludge. The temperature of a drying unit was maintained at 1050C in
order to vaporise only water and leave out all the solids dry.
Drying time was determined graphically. Since the mass of wet sample was known and the moisture
analyser has built-in weighing balance for continuous mass weighing during drying process. Mass
data captured during drying process at consecutive intervals when drying at constant temperature,
produced a hyperbolic graph. After all the moisture has been removed the graph becomes linear.
35
Table 4: Drying Experimental Results
time (s)
0
60
120
180
240
300
360
420
480
mass (g)
0.906
0.7
0.552
0.401
0.332
0.307
0.281
0.279
0.279
1
0,9
0,8
0,7
0,6
Mass (g)
0,5
No moisture
present
0,4
0,3
0,2
0,1
0
0
100
200
300
400
500
600
Time (s)
Figure 20: Drying Curve
The total amount of suspended solids was obtained by the use of empirical formula obtained from
the conversions done on the experimental findings.
S .S 
C  A  106
 B  C  A  D
.................................................................... Equation 19
36
A = weight of filter paper (g)
B =weight of wet sludge and filter paper (g)
C = weight of dried sludge and filter paper (g)
D = filtrate volume (ml)
S.S = total suspended solids concentration (mg/l)
Derivation of formula
Since the main objective of the experiment was to determine the amount of solids per unit volume
of sample, the total amount of water in the sample was worked out.
Total water = (weight of wet sludge +filter paper) – (weight of dried sludge +filter paper) – (weight of
filter paper) + (filtrate volume)
Total water (ml) = B – C – A + D
Conversion of water volume from millilitres to litres was necessary as the concentration of sludge
was expressed in terms of (mg/l).
Total water (l) = ml÷1000 = litres
Total amount of dried sludge (g) = (weight of dried sludge + filter paper) – (weight of a filter paper)
=C–A
Conversion of weight of dried sludge from grams to milligrams by multiplying with 1000
Amount of dried sludge (mg) = (C-A) × 1000
S .S 
C  A  1000
 B  C  A  D
1000
S .S 
C  A  106
 B  C  A  D
b. Sample calculation
From the experiment that was carried out the following data was recorded.
a.
b.
c.
d.
Filter paper was weighed in a weighing balance and the mass was = 0.211 g
20 ml was filtered in a vacuum funnel and gives a filtrate of 19 ml
The amount of wet sludge in a filter paper was weighed = 0.965 g
Wet sludge was dried in a moisture analyser, dried sludge mass = 0.287 g
37
Equation 19 was used to calculate the number of total suspended solids from above data.
S .S 
0.287  0.211  106
0.965  0.287  0.211  19
= 3904 mg/l
The sample calculation shown above was for calculation of mixed liquor suspended solids
concentration; the same formula can be used for calculation of return activated sludge
concentration which is roughly twice as much that of mixed liquor suspended solids under good
conditions of process operation. The results obtained will be used to work out the sludge age or
wasting rate for a day in the plant using the following formula.
38
Table 5: Suspended solids result
MIXED LIQUOR SUSPENDED SOLIDS (MLSS)
Filter paper
+ Wet
sample
Total
mass of
dried
sludge
Filtrate
Suspended
Solids
(A)
g
0.212
0.211
(B)
g
1.059
0.965
(C)
g
0.295
0.287
(D)
ml
18
19
(SS)
mg/l
4474
3904
0.211
0.213
0.213
0.208
0.205
0.971
0.956
1.042
1.055
0.943
0.281
0.281
0.284
0.278
0.288
18.5
18.5
18.5
17
18
3635
3744
4045
3984
4499
0.208
0.208
0.197
0.209
0.205
1.143
1.146
1.441
0.842
0.885
0.307
0.306
0.32
0.262
0.256
18.5
18.5
18.5
18
18.7
5175
5122
6332
2884
2667
0.205
0.209
0.208
0.21
0.208
0.879
1.03
0.815
0.758
1.055
0.267
0.271
0.275
0.245
0.278
17.5
17
17
18
17
3464
3506
3704
3655
3984
0.206
0.211
0.212
0.209
0.979
0.965
0.998
1.03
0.267
0.287
0.256
0.271
18
19
18
17
3561
3904
4002
3506
Filter paper
Date
weight
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
39
8. Conclusion
From this report I concluded that there is less accurate information and data available about physical
and chemical properties of the biological process. In water purification processes for completion of
material balance calculations in terms of their reaction kinetics accurate data is required. For process
optimization process controllers should be receiving test data from lab technicians inside treatment
plant frequently to enable decisions to be made in terms of process operation. The data will be
modelled by mass balance equations and also to fore see problems that may take place, to minimize
process risk that can lead to failure of the plant or equipment in the plant.
The energy balance performed in a centrifugal pump of Macassar sludge pump station showed
power requirement of 5.28 kW. Comparing power requirement of the centrifugal pump with the
actual installed motor of 5.5 kW, I can come to conclusion that this centrifugal pump still operates
under its specification. Determination of suspended solids in the sludge was of vital importance as
the sludge age was worked from it. Much deviations were observed in the results because of
inaccurate operation of the plant, which was caused by lack of realisation about the importance
modelling material balance equations.
My in-service training was a good experience to me, I learned to understand how biological process
work, problem solving skills and lot more in terms of process operation. It is quite interesting to see
different departments co-operate to achieve one objective, which is to improve the quality of waste
water discharged to the environment. For the improvement of the quality of training, structural plan
should be made such that the students can be rotated in various departments to obtain as much
knowledge as they can.
40
References
1.
2.
3.
4.
5.
Coulson and Richardson’s (chemical engineering design 5th edition 1963)
Robert. L (chemical engineering journal of mass and heat transfer 1992)
www.google.co.za/Wikipidia/heat exchanger/valves
Core notes of N3 waste water treatment course
T.W. Fraser Russell, Anne Skaja Robinson, Norman J. Wagner( Heat Transfer Series 1934)
41