International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 3 - October 2015
Optimizing the sludge treatment conditions of municipal
wastewater
S. M. Abdehamid Ahmed*1,2
1
Depatment of Chemistry, Science and Art College, King Abdulaziz University, Rabeg Campus, 21911, Saudi
Arabia
2
Department of Chemical Engineering, Higher Technological Institute, Egypt
Abstract
The objective of this research work was to study
the variables affecting the performance of sludge treatment
in municipal wastewater treatment plants on a laboratory
scales. The effect of chemical addition on the performance
of gravity was studied Jar tests were conducted on the
effluent from the primary and secondary sludge collected
from the main treatment plant and transported to the
thickener of sludge treatment plant, to evaluate all
parameters controlling coagulation process, i.e., coagulant
and flocculants type and dosage, pH, mixing intensity and
settling time. Coagulants studied were ferric chloride (dose
3-230 mg/l, pH 3-9), alum (dose 20-400 mg/l, pH 4-9), and
lime (dose 50-1100 mg/l, pH 5-13) with and without polymer
addition (dose 0.1-0.6 mg/l). Mixing intensity studied varied
between 10 to 50 rpm, and settling range was 15 to 40 min.
for all coagulants. Results obtained from the jar tests on
gravity thickening showed that, the most suitable coagulants
without pH adjustment was 100 mg/l ferric chloride with 0.3
mg/l Magna floc 155. This dose gave 88%TSS removal,
89%turbidity removal and a low sludge volume 100 ml/l
compared to other coagulants (alum, lime with and without
polymer). With pH adjustment the most suitable coagulants
was ferric chloride with a dose of 55 mg/l at pH=4, this
dose gave 99%turbidity removal and 94%TSS removal.
Keywords— Castor oil, PMDI, Polyurethane, Fiberglass,
Gel time, EDS, SEM
I. INTRODUCTION
Current world impacts of pollution problems are
placing emphasis on developing techniques and
economics of wastewater treatment. As population
pressure is continuously increasing, the need for the
usage of municipal water has tremendously developed
[1-3]. As a result, wastewater treatment plants
capacities are growing to fulfill such needs resulting in
large quantities of sludge. The production of sludge is
the essential consequence of treating wastewater and
contains most of pollutants which otherwise would
contaminate water supplies, and thus the introduction
of sewage treatment represents a major development
for protection public health [4-6]. Consequently,
sludge inevitably contain quantities of contaminates
and pathogens, the extent of which depend on the
amounts of industrial and domestic discharges to the
wastewater treatment plant, the effectiveness of
industrial effluent controls. Early the un-stabilized
waste sludge, generated from wastewater treatment
plants was pumped to a remote desert site for disposal
in evaporation lagoons or drying beds [7-9]. This
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method of sludge disposal affected public health and
safety of the people, thus sludge treatment is necessary
and the anaerobic digestion is a superior management
method from an environmental and health point of
view. The main purpose of this study is to develop and
enhance sludge thickening in sludge treatment process
and reduce cost of sludge treatment which represent
40-50% of the total cost of the overall wastewater
treatment plant [10-14]. The methods adopted to
realize this objective is to intensify the rate of settling,
percentage solids removal in the supernatant, reduce
the detention time in the thickener through application
involving chemical addition, flocculation and
sedimentation [15-19]. The influent sludge flow is
regulated using a pinch valve and measured using
ultrasonic flowmeter. The sludge then enters a gravity
thickener. Overflow the thickener flows to drainage
wet well and is pumped back to the wastewater
treatment plant. Settled thickened sludge flows
through grinder and is pumped from the gravity
thickener into two anaerobic digester by two
progressing cavity pumps. The contents of each
digester are maintained at 35oC using sludge
recirculation and heating system [20-22]. The sludge
in the digester is recirculated through two boiler/heat
exchangers by two progressing cavity pumps. The two
boiler / heat exchanger transfer heat from hot water to
the sludge. Digested sludge is pumped from the
digesters to dewatering unit by either a belt press or
centrifuge [23-27].
2. EXPERIMENTAL
2.1. Materials
Sludge sample : is the composite samples of the
influent to the thickener from a sludge treatment plant
Chemicals and polymer used:
a) Primary coagulants
Alum Al2 (SO4)3.16H2O with purity 97% and MW
630.4 (Adwic, El-Nasr pharmaceuticals chemical
co.)
FeCl3 with purity 99% , MW 162.21 (Riedel-de
Haen)
Lime with purity 90% , MW 74.09 (S.d. Fine-chem
ltd Boisar).
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 3 - October 2015
b) Coagulant aid
 Cationic polyelectrolyte polymer (zetac 63), is a
synthetic high molecular weight cationic
polyacrylamide, medium cationic charge,
optimum activity range is 4-9
 Anionic polymer
 Magna floc (1011): is very high molecular
weight anionic polyacrylamide flocculent
 Magna Floc (155): is high molecular eight
anionic polyacrylamide
2.2. Experimental Setup
The conventional jar test procedure was used
to evaluate the parameters controlling coagulation
process, and it is predictive tool of the optimal dosage
of coagulant in which turbidity, particle size
distributions, electro kinetics measurements, and
filterability may be used to enhance interpretation of
test result. The standard jar test apparatus (Phipps and
Bird 400 series six -paddle stirrer) was used in all
experiments. Paddles and shafts were made of
stainless steel. The stirrer features regulated variable
speeds, 0-300 rpm with a digital read out. The jars
used were made of Pyrex and cylindrical in shape with
dimension of approximately 10 cm in diameter and 14
cm height.
2.3. Measurement
All the analytical methods have been
conducted according to “Standard methods for the
examination of water and wastewater ’’. The turbidity
and total suspended solids (TSS) are the key
parameters for the determining the process efficiency.
Complete set of analysis will be carried out for the
optimal condition obtained. Such parameters are
BOD, COD, pH, total phosphorus and heavy metals.
Sludge analysis will be conducted to determine sludge
volume.
Table 1 Tests can be conducted and the equipment used in the
experimental work.
Parameter
Temperature
pH
Turbidity
TSS
Equipment
2.4.Jar testing procedure
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The lowest dosage that provides good
turbidity removal during a jar test is considered as the
first dosage in plant operation. For research the
beakers used in jar testing may be modified to
replicate more closely actual mixing unit constructed
in treatment plant.
2.5. Determination of optimal pH
1- Using 200 ml of sample on a magnetic stirrer, add
a coagulant in small increments at pH of 6.0
2- After each addition, provide a 1 min. rapid mix
followed by 3 min. slow mix. Continue addition
until a visible floc is formed
3- Using this dosage , place 1000 ml of sample in
each of six beakers
4- Adjust the pH to 4,5,6,7,8 and 9 with standard
alkali or acid
5- Rapid mix each sample for 3 min. follow this with
12 min. flocculation at slow speed
6- Measure the effluent concentration of each settled
sample
7- Plot the percentage removal of characterization
versus pH and select the optimum pH
2.6. Determination of optimal coagulant dose
The
Cole
Parmer
economy
pH/mv/OC . Benchtop meter
The
Cole
Parmer
economy
pH/mv/OC . Benchtop meter
DR 2000 spectrophotometer
Suction filtration &glass micro fiber
filter
Imphoff cone
Settleable
solids
2.3. Experimental procedure
Effectiveness of chemical coagulation or
sludge can be experimentally evaluated in the
laboratory by using a stirring device known as a jar
test. In making test, a freshly collected sludge samples
were distributed among the six jars after through
mixing. Coagulant dose was then added in varying
proportions (according to experimental plan) to give a
total volume of one liter in each jar followed
immediately by initiation of flash mixing (100 rpm).
After 1 minute rapid mixing to disperse the chemicals,
the mixing was reduced to 25 rpm and held at this
level for 20 min. for floc formation. Finally, a
quiescent settling period of 30 min. was allowed. At
the end of settling period, a sample of the supernatant
was analyzed for the various parameters, and the
nature of floc are observed and are recorded in
qualitative terms, as poor, fair, good, or excellent. A
hazy sample indicates poor coagulation, while proper
coagulated water contains floc that are well formed
with the liquid clear between particles.
1- Collect 20 to 50 liter of sludge sample. Analyze
the water for pH, turbidity, suspended solids after
filtration
2- Place 1 liter aliquots in 1 liter beakers on the six.
Jar laboratory stirrer and check stirrer operation
3- At a start of a one minute. Rapid mix at 100 rpm,
add the coagulant solution to the five beakers
keeping one beaker as a control
4- Flocculate at 25 rpm for 20 min. m note the size
and appearance of the floc formed
5- After flocculation, remove the paddles and settle
for 30 minutes.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 3 - October 2015
6- Measure the turbidity, suspended solids of the
supernatant in each jar.
7- Plot the percentage removal of characteristics
versus the coagulant dose and select the optimum
coagulant dose
2.7. Effect of mixing
1- Prepare optimum coagulant dosage for all six
beakers
2- Use the same rapid mix as before but vary the
speed of slow mix at 20 min. 10,20,30, 50 rpm
3- Allow 30 min. for settling
4- Measure the characteristics of the supernatant
5- Plot the percentage removal of characterization
versus mixing intensity for each coagulant
2.8. Effect of settling time
1- Prepare optimum coagulant dosage for all six
beakers
2- Use the same rapid and slow mix but vary the time
of settling 15, 20, 25,30,35,40 min.
3- Measure the characteristics of the supernatant
4- Plot the percentage removal of characterization
versus settling time
2.9. Determination of the coagulant aid dosage
The jar test procedure was repeated while adding
coagulant aid toward the end of mix
3. RESULTS AND DISCUSSION
3.1Effect of pH
The effect of pH on the efficiency of the process
has been evaluated for each coagulant as
percentage removal of turbidity and TSS. The pH
varies throughout the experiment and that is
considered one of the negative impacts.
It is noted that the pH has a responsible effect on the
efficiency of the process. Figure 1 represents the
effect of pH on coagulation
process using the
traditional coagulant FeCl3. By increasing pH, the
percentage removal increases until it reaches to a
maximum value at pH 4, above pH 4 the %
removal decreases rapidly but it returns to increase
again at pH 7 to give relatively high percentage
removal . It was noted that percentage removal at pH
7 is less than percentage removal obtained at pH 4 .
The variation of the percentage removal versus pH
using alum as a coagulant is similar to
that
demonstrated in Figure 1 using ferric chloride ,
except
that
percentage removal increases by
increasing pH until it reaches a maximum at pH
5. Above pH 5, the percentage removal decreases,
then increases again at pH 9. Percentage removal at
pH 9 is less than obtained at pH 5 .
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Figure 1 Effect of pH on percentage of removal using ferric
chloride
The double layer theory assumes that the ionized iron
(III) and aluminium (III) will enter the outer layer of
the particle, neutralize it, and make it possible for the
particles to flocculate. It is true, however, that the iron
(III) and aluminium (III) ion do not really exist in an
aqueous solution. They form complexes with water
molecules such as Fe (H2O)6+++and Al (H2O)6+++
and,
in addition, both compounds form
hydroxides with very low solubilities that act
as the bridges between the particles.
These
hydroxides
are long,
sticky molecules with
complex structure and not simple Al (OH)3 and Fe
(OH)3. These hydroxides are soluble at both low
and high pH's within their optimum
pH
range,
defined as the range of least solubility, they
undergo rapid hydrolytic reaction and form the
hydro polymers. At lower pH values, Fe (III) and Al
(III) form free ions, and above these optimum
values,
they are negatively charged hydroxyl
species. The optimum pH range for Fe (III) is
between 4 and 5 and for Al (III), between 4 and 7
But the case of using lime as a coagulant was
completely different than using FeCl3 and alum as a
coagulant. By increasing pH , the % removal curve
started upward until it reaches to a maximum at
pH 9, above pH 9 the% TSS removal decreases
rapidly. The optimal values of pH as observed
from curves are 4, 5, 9 for ferric chloride, alum,
lime respectively. Nearly similar trends were noted
in % turbidity and TSS removal.
3.2 Effect of coagulanttype and dosage
Effect of c o a g u l a n t type and dosage on the
percentage removal of turbidity, TSS with pH
adjustment and without pH adjustment are illustrated
in figure 2.Turbidity removal for FeCl3 as a coagulant
dosages is demonstrated in figures 3, indicated that
FeCl3 is the most effective coagulant (without pH
adjustment). By increasing the dosage of ferric
chloride the percentage removal curve started upward
until it reaches 95% at 120 mg/liter and pH 7 .3 7 .
There is no significant change in removal with
Fe Cl 3 dose above 120 mg/liter. Very Good results
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 3 - October 2015
are attained also with FeCl3, (with pH adjustment).
Turbidity removal increases by increasing ferric
chloride dose until i t reaches 99% at 55 mg/I and
pH 4. Above 55 mg/I FeCl3, at pH 4, the turbidity
removal decreases.
(without pH adjustment) but the percentage removal
reaches 88% with pH adjustment (pH 5). The
suspended solids percentage removal is relatively
identical for both FeCl3 and alum without
pH
adjustment.
Lime
gives nearly 85% TSS
removal without pH adjustment,
but the TSS
percentage
removal reaches
80% with pH
adjustment .
With alum addition the major
fraction of suspended solids settled within the
first 15 minutes of detention time. This detention
was used for comparing settling behavior of chemical
to physical floes.
From the various data, the most suitable doses
of ferric chloride , alum and lime with pH
adjustment
are
55,100,
600
mg/l
respectively .But the most suitable doses without pH
adjustment are 120 mg/l ferric chloride , 200 mg/l
alum and 800 mg/l lime.All lab experiments were
repeated o n e with pH adjustment and one
without pH adjustment.
Comparing the TSS % removal for all coagulant
types if pH adjusted or without pH adjusted. I t i s
noted that FeCl3 gives higher TSS % removal than
the other coagulants.
Figure 2 Effect of ferric Chloride on TSS % removal
without pH adjustment
3.3
Effect
of
slow
mix
intensity
Figure 2 Effect of ferric Chloride on turbidity % removal
without pH adjustment
It is found that good results also are attained with the
other coagulants such as alum, by increasing alum
dose the percentage removal curve rapidly upward
until it reaches about 95% removal with 200 mg/I
alum (without pH adjustment). By using Alum (with
pH adjustment), the turbidity removal reaches about
93% removal at 100 mg/I alum, pH 5.. It was noted
that there was no significant change in the percentage
removal with alum dose above 100 mg/I, pH 5.
Results indicated that by using lime (Without pH
adjustment) the turbidity removal increases by
increasing the lime dose until it reaches 82% at
800 mg/I lime , pH 7.52 . When the lime is used
with pH adjustment, the turbidity removal reaches
76% at lime dose 600 mg/I, pH 9. Using lime did not
produce significant improvement in turbidity
percentage removal, Using lime increase the rate of
settling
Nearly similar trends were noted in the removal of
turbidity and TSS, indicated that high% removal in
total suspend solids (92%) were recorded in the case
of using FeCl3 as a coagulant without pH
adjustment but the percentage removal reaches
94% with pH adjustment (pH 4 ). Also high removal
91% were recorded by using alum as a coagulant
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Studying the effect of mixing intensity on
coagulation dynamics is performed by our
experimental work. The effect of mixing
intensity (expressed as gradient velocity) on
turbidity were investigated using optimum dose of
ferric chloride as a coagulant. The percentage
removal increases continuously as the mixing
intensity increasing up to certain limit (20 rpm)
where strong shear stress forces develop, causing
breakage of aggregates. Above 30 rpm is the limit of
mixing intensity at which the percentage removal
remains constant. The variation of the percentage
removal versus slow mix intensity using alum and
lime as coagulants at their optimum conditions is
similar to that demonstrated using FeCl3. The
optimum slow mix was found to be 20 rpm for all
coagulants.
3.4. Effect of settling time
Studying the effect of settling time using various
coagulant has been studied. Nearly similar trends
were noted in removal of turbidity and TSS, in the
case of using FeCl3 as a coagulant with its
optimum dose (120 mg /1) it was observed that by
increasing the settling time , the % turbidity
removal increases until a certain limit (35 min.),
above this time a percentage removal is relatively
constant whatever a settling time increases. Similar
trend of FeCl3 are also obtained by using alum as
a coagulant with its optimum dose (200 mg /1)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 28 Number 3 - October 2015
except that the optimum settling time is 30 min.,
above this time there is no significant change in%
removal. But in the case of lime addition which is
used as a precipitate , when settling increase the %
removal increase slowly because in the case of
lime the most particles settle within 15 min. ,above
this time the % removal increase slowly until the
settling time reach to 30 min., above this time there
isn't any change in % removal.
3.5. Effect of flocculants addition
The flocculant is tested next using a number of
primary coagulants in conjunction with the
flocculant. Flocculants are usually high molecular
weight anionic polymer of various charge densities.
Two high molecular weight anionic polymer are
tested in our experimental work Magna Floc
1011, Magna Floc 155.
Jar tests is conducted by using primary coagulant as
FeCl3 at a concentration lower than optimal and
varying the flocculant (Magna Floc 1011)
concentration from 0 .1
mg/l to 0. 6 mg/l in
increments 0.1 mg/l. An observation made at all
facilities is that as the flocculant concentration
increased the % removal increased until the
coagulant dose reach to a certain limit 0.3mg/1,
above this concentration there isn't any change in
the % removal whatever the polymer dose increase.
Comparing this results with that obtained using FeCl3
alone, it has been observed that FeCl3 alone gives
better results but the addition of flocculant increase
the settling rate. Increasing in the settling rate means
that the treatment facility can operate at much higher
surface overflow rates when a chemicals (flocculants)
were added to the influent sludge flow. Moreover
using a polymer as a flocculant decreases sludge
volume from 200ml/l by using FeCl3 alone to 100
ml /1 by using FeCl3with Magna Floc 1011,
that
is means that
polymer
increase the
compactness between sludge particles and increase a
floc strengthening. A variation of the percentage
removal versus dosage of the second type of
polymer . (Magna Floc 155) with FeCl3 . Using
magna floc 1011 with FeCl3 except that FeCl3with
Magna Floc155 gives better results than using FeCl 3
with Magna Floc 1011, but in the case of using alum
as a primary coagulant with dose less than the
optimal (150 mg/I) with different doses of Magna
Floc 1011 , it was observed that by increasing the
dose of Magna Floc 1011 there is a little increase in
the percentage removal until the concentration of
Magna Floe 1011 reaches 0.3 mg/l ,above this
concentration there isn't any significant change in
the % removal , it was observed that the results
obtained by using alum with Magna Floc 1011 is
better than the results obtained by using alum alone
with respect to TSS removal and the addition of
Magna Floc 1011 to the alum reduce the sludge
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volume, increase the rate of settling. The variation of
the % removal versus doses of flocculant (Magna
Floc 155) with alum. Using alum with Magna Floc
1011 except that the results obtained by using
Magna Floc 1011 is better than results obtained by
using Magna Floc 155.
4. CONCLUSION
The jar tests using the chemicals were carried out
under two conditions: with pH and without pH
adjustment.
1) The jar test showed that the most appropriate
coagulant and operating condition were 100 mg
/1 ferric chloride with 0.3 mg /1 polymer
(Magna Floc 155) without pH adjustment using
mixing intensity of20 rpm and 30 min. settling
time.
Such conditions gave 88 % TSS
percentage removal and 89 % turbidity
percentage removal.
The sludge volume
produced was 100 ml /1
2) The addition of polymer to the coagulant, resulted
in improving the rate of settling by its action as a
flocculant, also in reducing sludge volume
3) As a result of the reduction in detention time , the
influent sludge flow rate to the thickener can be
increased at least by two folds without affecting
thickener performance.
4) The reduction in detention time , increase of
flow rate and the improvement of TSS removal
which results in reducing the hydraulic loading of
the main plant, should affect both initial and
operating cost of wastewater treatment plant as a
whole
5) The results obtained in the jar test for coagulant
with pH adjustment were 55 mg I 1
ferric
chloride which is the most appropriate coagulant
used at pH 4 , under this condition TSS %
removal reached 94 % and turbidity removal
reached to 99 % .
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