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Journal of Hazardous Materials 163 (2009) 1076–1083
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Characterization and treatability studies of tannery wastewater using
chemically enhanced primary treatment (CEPT)—A case study of
Saddiq Leather Works
Sajjad Haydar∗, Javed Anwar Aziz
Institute of Environmental Engineering and Research (IEER), University of Engineering and Technology, Lahore, Pakistan
article info
Article history:
Received 19 December 2007
Received in revised form 23 May 2008
Accepted 16 July 2008
Available online 23 July 2008
Keywords:
CEPT
Chromium
COD
Coagulants
Wastewater characteristics
Tannery wastewater
abstract
Chemically enhanced primary treatment (CEPT) is a technology that uses coagulants for enhanced pol-
lutants removal at the primary stage of the wastewater treatment. This paper presents the detailed
characteristics of tannery wastewater. It also explains effectiveness of CEPT in removing pollutants from
tannery wastewater using various metal salts. The results of this study demonstrated that the tannery
effluent had high concentrations of organic matter, solids, sulfates, sulfides and chromium. Alum, ferric
chloride and ferric sulfate were tested as coagulants using jar test apparatus. Alum was found to be the
suitable coagulant for tannery wastewater in a dose range of 200–240 mg/L as Al2(SO4)3. With alum, per-
centage removal efficiency for turbidity, total suspended solids (TSS), chemical oxygen demand (COD)
and chromium was found to be 98.7–99.8, 94.3–97.1, 53.3–60.9, and 98.9–99.7%, respectively. National
effluent quality standards for total suspended solids and chromium were met after CEPT. However, COD
content was high, emphasizing the need of secondary treatment for the tannery effluent.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Tanning industries in Pakistan are causing severe environmen-
tal problems due to the disposal of their untreated effluents on
land and in water bodies. In the past few decades, developing coun-
tries have witnessed a sharp increase in leather production because
such activities have declined in the developed world due to more
stringent environmental pollution control requirements and high
labour costs. Accordingly, Pakistan witnessed a rise in its leather
export from US$ 672million in 20 02 to 1.13 billion in 2007, which
indicates an increase of 68% in a short span of 5 years [1]. The num-
ber of tanneries increased rapidly during this time period, which
currently stands at approximately 650.
Wastewater characterization is an important step in design-
ing effective treatment facilities for industrial wastewaters. This is
especially true for tanneries which exhibit significant differences
in their production processes generating effluents of unique and
complex nature. Characterization is also needed for assessing the
performance of individual unit operations and processes.
Most pollutants in wastewaters appear to exist either in particu-
late form or are associated with particulates [2]. This understanding
∗Corresponding author.
E-mail address: sajj@brain.net.pk (S. Haydar).
led to the wastewater treatment strategy of removing particulate
and colloidal matter in the primary step using suitable coagulants.
The process is commonly termed as chemically enhanced primary
treatment (CEPT). After CEPT, the dissolved matter remaining in
wastewater is dealt within the secondary step. Apart from remov-
ing commonly known pollutants, CEPT is reported to assist removal
of heavy metals, PCBs (polychlorinated biphenyls) and PAHs (poly-
cyclic aromatic hydrocarbons), which are strongly associated with
particles [3]. There are other advantages of CEPT. The application
of CEPT reduces the footprint of primary settling unit as it per-
mits the use of high surface overflow rates. Similarly, reductions in
terms of space and cost of subsequent biological unit are achieved
due to the decreased organic loadings following CEPT. With minor
retrofits, CEPT can also be applied to existing overloaded treatment
plants to improve their efficiency. In addition, CEPT can effectively
be employed for the removal of phosphorus in the effluents to con-
trol eutrophication.
Wastewater treatment in Pakistan is evolving. Application
of secondary biological treatment to industrial and municipal
wastewaters is not widely practiced due to a number of reasons
which include high capital costs, lack of operation and maintenance
skills, and the absence of stringent enforcement of environmen-
tal standards. In this scenario, best management practices must be
used that are commensurate with available financial resources and
skills. CEPT is a technology that appears to havepotential in Pakistan
0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2008.07.074
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S. Haydar, J.A. Aziz / Journal of Hazardous Materials 163 (2009) 1076–1083 1077
to cope with evolutionary demand of environmental protection.
This experimental study was undertaken to investigate the poten-
tial of CEPT for the treatment of tannery wastewater. The objectives
of the study were: (1) to characterize the tannery wastewater and
(2) to examine the treatability of tannery wastewater using dif-
ferent types of commonly used chemical coagulants and jar test
methodology.
2. Materials and methods
2.1. Sampling
This study was carried out on tannery wastewater from Sad-
diq Leather Works (SLW), which is a large size tannery processing
18,000 kg of raw hides per day and producing finished leather for
garments and shoes. The tannery is equipped with a primary treat-
ment plant (PTP) consisting of screens, equalization basin, primary
sedimentation tank, decanter for sludge and sludge drying beds.
At present, addition of coagulants is not practiced during primary
treatment at SLW. Average wastewater flow from the tannery is
1150 m3/day, which discharges into a local river.
Wastewater samples were collected from the equalization tank
of PTP.The equalization tank had an average detention time of 8.5 h.
It was equipped with mechanical mixers and dome type aerators
for the purpose of mixing and homogenization. Samples were col-
lected between 10and 11.30 A.M. During this period the wastewater
from almost all the tannery production processes had reached the
equalization tank. For characterization, sampling was done weekly
over a period of 14 months from May 2006 to July 2007. This period
was considered sufficient to take into account all possible fluctua-
tions in the production processes and all types of working routines
which ultimately affect the effluent quality. For the purpose of jar
tests, 60 L of sample was collected at a time from the equalization
tank and transported within 30 min to the Institute of Environmen-
tal Engineering and Research laboratory. The collected sample was
stored at 4 ◦C for subsequent use.
2.2. Wastewater characteristics
The wastewater samples from SLW were analyzed for tem-
perature, pH, total solids (TS), total suspended solids (TSS), total
dissolved solids (TDS), settleable solids (SS), non-settleable solids
(NSS), volatile suspended solids (VSS), fixed suspended solids (FSS),
total alkalinity,total hardness, calcium hardness, chlorides, sulfates,
sulfides, chromium, five day biochemical oxygen demand (BOD5),
chemical oxygen demand (COD), phosphorus (P) and total kjeldahl
nitrogen (TKN).
Settleable solids (SS) were determined on both volume and
weight basis as described in Section 2540F of the Standard Methods
[4]. Results on weight basis were used for comparison purposes.
Non-settleable solids (NSS) were determined by subtracting the
content of SS (on weight basis) from total suspended solids (TSS).
NSS represent colloidal solids, which do not settle in primary sed-
imentation tanks at normal surface overflow rates. BOD5and COD
tests were conducted on both raw and filtered samples of wastew-
ater to estimate total and soluble contents of these parameters.
Whatman GF/C filter, of pore size 1.2 m, was used for filtration
of wastewater. Wastewater samples were analyzed for the above
mentioned parameters using test procedures outlined in Standard
Methods for the Examination of Water and Wastewaters [4].
2.3. Coagulants tested
Alum Al2(SO4)3·16H 2O, ferric chloride FeCl3·6H2O and ferric
sulfate Fe2(SO4)3·6H2O were tested as coagulants. 2% stock solu-
Table 1
Mixing regime for jar tests
Step rpmaVelocity gradient, G(s−1) Time (min)
Rapid mix 300 380 1
Medium mix 60 54 5
Medium–slow mix 40 32 5
Slow mix 20 14 10
Settling 0 – 30
aRevolution per minute.
tion of each coagulant was prepared and fed into the jars at required
dosages. Doses of above coagulants referred in this paper are with-
out water of hydration. Thus all the alum doses are as Al2(SO4)3,
ferric chloride doses are as FeCl3and ferric sulfate doses are as
Fe2(SO4)3.
2.4. Jar test methodology
Phipps and Birds Programmable Jar Tester with six acrylic square
beakers, each having a capacity of 2 L, was used. Sampling port was
provided with each beaker for drawing sample. Table 1 shows the
mixing regime used for all the jar tests. As given in Table 1,after
adding coagulants to the jars, the contents were rapidly mixed at
300 rpm for a period of 1 min. Rapid mixing was then followed by a
tapered slow mix (flocculation) at three different speeds, i.e. 60 rpm
for 5 min, 40 rpm for 5 min and 20rpm for 10min. Velocity gradi-
ent (G) values for various rpm employed have been shown in the
Table 1.
The mixing speeds and times were selected from the experience
of similar studies published in the literature [5–12].A small portion,
approximately 20mL, was wasted to flush out the sampling tube
when taking sample from each jar after 30 min of settling [13,14].
Jar tests were conducted in the following three series:
2.4.1. Series 1
A series of preliminary jar tests were carried out with the objec-
tives: (1) to examine the comparative suitability of various metal
salts tested; and (2) to determine their optimum dose range. Only
turbidity measurements were made on samples drawn from jars
using Hach 2100AN turbidimeter. Coagulant dose was varied from
0 to 100 mg/L, in increments of 20mg/L. Raw wastewater was given
a pre-settling time of 30 min before conducting jar tests. Jar tests
were performed on wastewater sample having pH of 7.5.
2.4.2. Series 2
Best coagulant from series 1 was selected and used in this series
at a dose range of 0–400 mg/L, in increments of 40 mg/L. Jar tests
were conducted on raw (unsettled) wastewater samples as well as
on samples after subjecting them to 30 min settling. The objective
of this series was to study the effect of pre-settling on the efficiency
of the coagulant. This could provide insight into the evaluation of
using pre-settling tank before the coagulation step. The parameters
tested to evaluate the efficiency of coagulant were turbidity, TSS,
TCOD and chromium.
2.4.3. Series 3
In this series, jar tests were performed at pH of 7.5 and 9.5. These
pH values were the two extremes in which the pH of the tannery
wastewater varied during the period of study (Table 2; serial no. 2).
Pre-settled wastewater was used for this series. Best coagulant from
series 1 was selected and its dose was varied from 0 to 400mg/L,
in increments of 40 mg/L. The objective of this series was to study
the effect of pH on the removal of different pollutants, i.e. turbidity,
TSS, TCOD and chromium.
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Table 2
Characteristics of raw homogenized wastewater of SLW for 14 months
S. no. ParameteraNbRange Meanc
1 Temperature (◦C) 33 20.4–34.2 29.3 ±3.9
2pH 34 7.55–9.51 –
3 Total solids (TS) 34 5330–15,912 9580.6 ±2226.9
4 Total suspended solids (TSS) 34 568–2132 1232.7 ±277.4
5 Total dissolved solids (TDS) 34 4466–14,572 8265.5 ±2086.3
6 Settleable solids (SS) (mL/L) 34 20–88 43.7 ±14.7
7 Settleable solids (SS) 28 448–1690 942.1 ±243.9
8 Non-settleable solids (NSS) 28 102–486 272.1 ±104.4
9 Volatile suspended solids (VSS) 34 226–1344 804.9±203.4
10 Fixed suspended solids (FSS) 33 260–788 420.9 ±117.9
11 Total alkalinity (as CaCO3) 34 520–1720 1227.3 ±311.2
12 Total hardness (as CaCO3) 34 368–1050 770.8 ±181. 6
13 Ca hardness (as CaCO3) 34 208–700 404.4 ±119.4
14 Chlorides 34 1000.2–4548.9 3067.2 ±900
15 Sulfates 34 564.5–121.4 1246.2 ±354.7
16 Sulfides 34 14.8–424.5 156.9 ±98.5
17 Chromium 31 22.9–122.4 68.1 ±24.5
18 Total five day biochemical oxygen demand (TBOD5) 34 390–1320 774.9 ±225.9
19 Soluble BOD (SBOD5) 34 200–765 526.6 ±139.5
20 Total chemical oxygen demand (TCOD) 34 1760–3320 2442.4 ±376.9
21 Soluble COD (SCOD) 34 740–2040 1326.8 ±300.9
22 Phosphorous (P) 3 0.5–1.1 0.8 ±0.3
23 Total kjeldahl nitrogen (TKN) 3 104.4–40.6 118.3±19.3
aAll parameters except pH in mg/L if not specified.
bNumber of samples.
cMean + standard deviation.
Jar number 1, in all the above jar test series, was used as control
jar or “zero chemical” jar. Coagulant was not added to this jar to
simulate conventional primary treatment.
3. Results and discussion
3.1. Wastewater characteristics
The results of the wastewater characterization are presented in
Table 2, which indicate quite a strong character of raw homoge-
nized tannery wastewater. The wastewater contained high organic,
solids, sulfates, sulfides and chromium contents. It was alkaline in
nature and its composition continuously varied due to batch pro-
duction processes with different discharge timings. Total alkalinity
and calcium (Ca2+) hardness were significantly high with a mean
value of 1227 and 404 mg/L, respectively. These high valuescould b e
advantageous as high alkalinity helps in the formation of flocs when
coagulants are used [6] and Ca2+ hardness is needed by anionic
polymers when used as coagulant aid with metal salts [5,8,15].
High sulfate and sulfide concentrations were observed with
a mean value of 1240 and 156 mg/L, respectively. Similarly high
chromium contents with a mean value of 68mg/L were present in
the raw wastewater. The wastewater appears to be high in BOD5and
TKN. However, data in Table 2 suggest that P is the nutrient limiting
parameter for biological treatment of the wastewater. A BOD:N:P
of 100:5:1 is generally considered to be an optimum ratio [16]. This
ratio, based upon average values of these parameters from Table 2,
was 100:15:0.1. Thus phosphorus was deficient in SLW wastewater.
This deficiency has also been reported by Ates et al. [17] in tannery
wastewater.
Experimental data were further examined to obtain useful rela-
tionship between major parameters as shown in Table 3.Itcanbe
seen in Table 3 that sufficient portion of TBOD5and TCOD was in
the particulate form with mean values of 32 and 46%, respectively
(serial no. 1 and 2). Practically, this entire portion can be removed
at primary step using suitable coagulants. The value for particu-
late COD in Table 2 is comparable with the results obtained by Ates
et al. [17] and Orhon et al. [18] for tannery wastewater. The mean
value of TCOD/TBOD5was 3.2 (serial no. 3), which appeared to indi-
cate that a large portion of organic matter in tanneries wastewater
was non-biodegradable or very slowly biodegradable. Out of total
solids, on average, 86% were in the dissolved form (serial no. 5).
Similarly in case of TSS, 80% were settleable (serial no. 6) and 70%
of TSS were volatile in nature (serial no. 7) on the basis of mean
values. Chromium, described as % of TSS, varied from 2.3 to 11.1%
with a mean value of 5.2% (serial no. 8). This is comparable with the
results obtained by Ates et al. [17] and Orhon et al. [18].
3.2. Series 1 jar tests
Comparative performance of alum, ferric chloride and ferric sul-
fate at pH values of 7.5 is shown in Fig. 1.
Fig. 1 indicates that ferric chloride and ferric sulfate performed
in a similar fashion and maximum turbidity removal occurred at a
low dose of 20 mg/L with no further significant improvement. For
alum maximum turbidity removal was observed at the maximum
applied dose of 100 mg/L. However, both ferric salts produced dark
black colour. Black colour was considered to be due to the forma-
tion of FeS by S2−, which is derived from the use of Na2S during
unhairing process. Sulfide reduces Fe3+ (ferric) to Fe2+ to form FeS,
which is soluble and black in colour [19,20]. Further investigation
is required for the removal of this black colour and economical use
Fig. 1. Coagulant dose and residual turbidity at pH 7.5 for various coagulants.
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S. Haydar, J.A. Aziz / Journal of Hazardous Materials 163 (2009) 1076–1083 1079
Table 3
Relationship between major pollution parameters
S. no. Parameter/relationship Range Mean Comparison
Ates et al. [17] Orhon et al. [18]
1 ParticulateaBOD5(% of TBOD5) 18–53 32
2 Particulate COD (% of TCOD) 33–63 46 56 43
3 TCOD/TBOD51.8–4.8 3.2 2.9
4 TCOD/TSS 1.3–4.2 2.0 2.2 2.9
5 TDS/TS 0.7–0.9 0.86
6 SS/TSS 0.6–0.9 0.8
7 VSS/TSS 0.4–0.8 0.7 0.48 0.6
8 Cr/TSS (%) 2.3–10.1 5.53 5.1 5.2
aParticulate BOD5=TBOD
5–SBOD
5.
Fig. 2. Alum dose and residual turbidity with and without pre-settling.
of ferric salts. Alum on the other hand showed no such problem,
rather it exhibited a tendency to completely remove colour already
present in tannery wastewater.
Thus it can be observed in Fig. 1 that good initial turbidity
removal was achieved with ferric salts at low dose. However, the
production of black colour was the major hindrance for ferric salts
to be used as coagulant. Thus on the basis of the preliminary tests,
alum was selected as the metal salt of choice for tannery wastew-
ater.
3.3. Series 2 jar tests
The results of series 2 jar tests are illustrated in Figs. 2–5.
It is evident from Figs. 2–5 that maximum turbidity, TSS, TCOD
and chromium removal occurred at alum doses of 200, 240, 240
and 240 mg/L as Al2(SO4)3, respectively. Removals in pre-settled
samples were slightly better than those without pre-settling. How-
ever, no significant differences existed at optimum dose for both
types of samples in terms of turbidity, TSS, TCOD and chromium
contents. Therefore, on the basis of the experimental results the
Fig. 3. Alum dose and residual TSS with and without pre-settling.
Fig. 4. Alum dose and residual TCOD with and without pre-settling.
optimum dose range for alum was determined to be 200–240 mg/L
as Al2(SO4)3. Additionally, the effect of pre-settling on the removal
of pollutants was not significant. Thus the provisionof a pre-settling
tank may not result in any added benefit. The summary of results
from series 2 jar tests and its comparison with plain sedimentation
are shown in Tables 4 and 5.
Residual values and percentage removals of various parame-
ters resulting from plain sedimentation are compared with CEPT
at optimum alum dose in Tables 4 and 5. This comparison (Table 4)
shows that CEPT enhanced the removal of all the pollutants
when compared with plain sedimentation. Turbidity removal was
enhanced from 86.2 to 99.3% (effluent turbidity = 7 NTU). TSS
removal enhanced from 76.8 to 97.1% (effluent TSS = 30 mg/L).
Similarly TCOD removal enhanced from 18.5 to 60.9% (effluent
TCOD = 720mg/L) and chromium removal from 85.4 to 99.6% (efflu-
ent Cr = 0.4mg/L). Similar observations can be made from Table 5.
It can also be noted from Table 4 that soluble COD for the wastew-
ater sample was 1000 mg/L whereas residual TCOD after CEPT was
720 mg/L. It clearly demonstrates the removal of some soluble COD
Fig. 5. Alum dose and residual chromium with and without pre-settling.
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Table 4
Comparison of plain sedimentation with CEPT for series 2 jar tests (with pre-settling)
Parameter Raw unsettled wastewater Plain sedimentationaAlum dose (mg/L) CEPT
Residual value Removalb(%) Residual value Removalb(%)
Turbidity (NTU) 955 132 86.2 200 7 99.3
TSS (mg/L) 1056 245 76.8 240 30 97.1
TCOD (mg/L) 1840(1000)c1500 18.5 240 720 60.9
Chromium (mg/L) 105 15.3 85.4 240 0.4 99.6
aResults of the pre-settled wastewater used as influent to all the jars.
bFor % removal raw unsettled wastewater was taken as reference.
cValue in parenthesis is SCOD.
Table 5
Comparison of plain sedimentation with CEPT for series 2 jar tests (without pre-settling)
Parameter Raw unsettled wastewater Plain sedimentationaAlum dose (mg/L) CEPT
Residual value Removalb(%) Residual value Removalb(%)
Turbidity (NTU) 955 127 86.7 200 12 98.7
TSS (mg/L) 1056 211 80 240 60 94.3
TCOD (mg/L) 1840(1000)c1350 26.6 240 760 58.7
Chromium (mg/L) 105 20.5 80.5 240 0.6 99.4
aResults of zero chemical jar, which simulate plain sedimentation.
bFor % removal raw unsettled wastewater was taken as reference.
cValue in parenthesis is SCOD.
in CEPT. It might be due to the adsorption of soluble COD on Al(OH)3
gel formed during coagulation process with alum.
The enforced national effluent standards [21] for parameters
tested are given in Table 6. By comparing residual values in
Tables 4 and 5 with those in Table 6, it can be observed that
CEPT generated effluent that met these standards for TSS (effluent
TSS = 30–60 mg/L; standard = 200 mg/L) and chromium (effluent
Cr = 0.4–0.63 mg/L; standard=1mg/L). Additionally, almost com-
plete removal of chromium in CEPT would save any subsequent
biological treatment from chromium toxicity. TCOD in the efflu-
ent from CEPT (720–760mg/L) was still higher than the effluent
standards (150mg/L) thus emphasizing the need for secondary
treatment.
3.4. Series 3 jar tests
The results of the jar tests conducted under this series are shown
in Figs. 6–9.
It can be seen in Figs. 6–9 that maximum removal of turbidity,
TSS, TCOD and chromium occurred at alum doses of 200, 240, 240
and 240 mg/L as Al2(SO4)3, respectively. Moreover, removals were
slightly better at pH 9.5 than at 7.5. However, the differences were
not significant. This clearly demonstrated that pH range normally
associated with homogenized SLW wastewater had no apprecia-
ble effect on the optimum dose of the coagulant. Similar findings
were reported by Ates et al. [17] regarding tannery wastewater. The
present investigations suggest that no pH adjustment is needed for
tannery wastewater before coagulant is applied as long as an ade-
quate equalization basin is provided. It can be seen in Figs. 6–9 that
the initial concentrations of turbidity, TSS, TCOD and chromium
Table 6
Effluent standards in Pakistan [21]
Parameter Effluent standard (mg/L)
Turbidity NAa
TSS 200
COD 150
Chromium 1.0
aNot applicable.
Fig. 6. Alum dose and residual turbidity at different pH.
were different for the two jar tests. This was due to the differ-
ent samples used for these tests. Differences were even observed
when only one wastewater sample was used for multiple jar tests.
These differences arose due to sub-sampling of the bulk sample ini-
tially obtained from the equalization tank. In addition, multiple jar
tests on the same bulk sample were performed on different days.
Therefore, aging of initial sample could be another reason for these
differences. It is clear from Figs. 6–9 that at optimum dose, resid-
Fig. 7. Alum dose and residual TSS at different pH.
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Fig. 8. Alum dose and residual TCOD at different pH.
Fig. 9. Alum dose and residual chromium at different pH.
ual values of various parameters for the two jar tests were in close
agreement thus nullifying the initial concentration differences to a
large extent.
Residual values and percentage removals of various parame-
ters, at different pH values, resulting due to plain sedimentation
are compared with CEPT in Tables 7 and 8. It can be seen in these
tables that reasonable removals of turbidity, TSS and chromium
occurred even with plain sedimentation whereas removal for TCOD
was minimal. With CEPT, at optimum alum dose (Table 7), removals
for turbidity enhanced from 85.4 to 99.8% (effluent turbidity = 2.3
NTU), TSS from 86.5 to 96.4% (effluent TSS = 54 mg/L), TOCD from
16.3 to 53.3% (effluent TOCD= 1120mg/L) and chromium removal
enhanced from 84.2 to 98.9% (effluent Cr =0.8 mg/L). CEPT effluent
met effluent standards for TSS and chromium whereas TCOD values
were still high. Removal of some portion of soluble COD was also
observed. Similar trends are exhibited in Table 8.
The possibility of using CEPT at the existing PTP was also stud-
ied under two options. First option was to augment the existing
PTP with a rapid mix and tapered flocculation unit prior to pri-
mary sedimentation tank (PST). Second option was to skip the
above arrangement and add alum directly into the equalization
Fig. 10. Alum dose and residual turbidity for the addition of alum in equalization
tank.
tank where it would be mixed with wastewater prior to primary
sedimentation. The first option was completely simulated by series
2 and 3 jar tests where wastewater was rapidly mixed at a Gvalue of
380 s−1followed by tapered flocculation at three different Gvalues,
i.e. 54, 34 and 14s−1(Table 1). In order to simulate the second sce-
nario in the laboratory, the velocity gradient (G) in the equalization
tank was determined from the power input of motor for the aera-
tors and volume of the equalization tank. It was found to be 150 s−1.
Thus jar test was run at this Gvalue for a period of 8.5h (average
detention time in the equalization tank) followed by 30min of set-
tlement. Turbidity test was used to evaluate the quality of effluent
as shown in Fig. 10
The optimum dose for second option in Fig. 10 appears to be
400 mg/L as Al2(SO4)3, which is almost double as compared to the
optimum dose (200–240 mg/L as Al2(SO4)3) for first option. This
clearly demonstrates the effect of proper rapid mix and flocculation
on the removal of pollutants. Thus if rapid mix and flocculation
units are avoided, the chemical cost for CEPT would almost become
double.
CEPT results for the treatment of tannery wastewater have been
compared with other treatment methods in Table 9. Three differ-
ent parameters, i.e. TSS, COD and chromium have been selected
for the purpose of comparison. Values of these parameters for raw
and treated samples and percentage removals have been presented.
Five different treatment methods, namely: (1) aerated lagoons; (2)
activated sludge; (3) upflow anaerobic sludge blanket (UASB); (4)
powdered activated carbon (PAC) coupled with membrane biore-
actor (MBR); and (5) vegetated wetlands have been included in this
comparison.
It is evident from Table 9 that CEPT gives good results for per-
centage removal and effluent TSS when compared with aerated
lagoon, activated sludge and UASB. Best results for TSS removal are
obtained with PAC+ MBR. The effluent TSS for vegetated wetlands
is also lower than CEPT. However, it should also be noted that the
value for raw TSS for vegetated wetlands is also quite low. COD
removal with CEPT is comparable to UASB and vegetated wetlands
whereas aerated lagoon, activated sludge and PAC + MBR give better
Table 7
Comparison of plain sedimentation with CEPT at pH 7.5
Parameter Raw unsettled wastewater Plain sedimentationaAlum dose (mg/L) CEPT
Residual value Removalb(%) Residual value Removalb(%)
Turbidity (NTU) 1370 200 85.4 200 2.3 99.8
TSS (mg/L) 1508 203 86.5 240 54 96.4
TCOD (mg/L) 2400(1200)c2010 16.3 240 1120 53.3
Chromium (mg/L) 77 12.2 84.2 240 0.8 98.9
aResults of the pre-settled wastewater used as influent to all the jars.
bFor % removal, raw unsettled wastewater was taken as reference.
cValue in parenthesis is SCOD.
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1082 S. Haydar, J.A. Aziz / Journal of Hazardous Materials 163 (2009) 1076–1083
Table 8
Comparison of plain sedimentation with CEPT at pH 9.5
Parameter Raw unsettled wastewater Plain sedimentationaAlum dose (mg/L) CEPT
Residual value Removalb(%) Residual value Removalb(%)
Turbidity (NTU) 1183 390 67 200 5.4 99.5
TSS (mg/L) 1308 321 75.4 240 54 95.9
TCOD (mg/L) 2480(1220)c2050 17.3 240 1100 55.6
Chromium (mg/L) 61 7.6 87.5 240 0.2 99.7
aResults of the pre-settled wastewater used as influent to all the jars.
bFor % removal, raw unsettled wastewater was taken as reference.
cValue in parenthesis is SCOD.
Table 9
Different treatment methods and their performance for tannery wastewater
S. no. Treatment methods Parameter
TSS COD Chromium
Raw (mg/L) Treated (mg/L) Removal (%) Raw (mg/L) Treated (mg/L) Removal (%) Raw (mg/L) Treated (mg/L) Removal (%)
1 CEPT (Table 7) 1508 54 96 2400 1120 53 77 0.8 98.9
2 Aerated lagoon [22] 1824 258 85.9 4321 1180 72.7 28 0.6 97.8
3 Activated sludge [23] 750 110 85.3 3600 200 94.4 56 0.8 98.6
4 UASB [23] 1398 587 58 1135 566 50.1 76 11 85.5
5 PAC+MBR[24] 976 4 99.5 4051 832 79.5 – – –
6 Vegetated wetlands [25] 79 23 70.8 2093 745 64.4 – – –
removals. Similarly, excellent chromium removals can be obtained
with CEPT as shown in Table 9. These removals are comparable with
those obtained using aerated lagoon and activated sludge process,
and better than those obtained using UASB. It can, therefore, be con-
cluded that with respect to TSS and chromium, CEPT outperforms
other treatment methods. However, it gives moderate removal of
COD and hence further treatment to this effect is required.
4. Conclusions
Following conclusions can be drawn from the above studies:
1. Homogenized wastewater of SLWfrom equalization tank showed
variations in characteristics. It contained high organic, solid, sul-
fate, sulfide and chromium contents. Phosphorus was found to
be deficient for satisfactory biological treatment.
2. CEPT could be a suitable treatment strategy for SLW wastewa-
ter which had high alkalinity and sufficient amount of various
pollutants in the particulate form. Alum was found to be suit-
able coagulant for use in CEPT of tannery wastewater in a dose
range of 200–240 mg/L as Al2(SO4)3, in case rapid mix and floccu-
lation units were provided prior to primary sedimentation tank.
However, without these units, if alum was added to the equaliza-
tion tank, a dose of 400 mg/L as Al2(SO4)3was required to obtain
comparable results.
3. Ferric chloride and ferric sulfate generated black colour when
used as coagulant for tannery wastewater. Nevertheless, an
appreciable removal of turbidity was achieved at lower doses.
Further investigations are needed for the removal of black colour
and economical use of these coagulants.
4. Results indicated that pH range normally associated with
homogenized SLW wastewater had no significant effect on CEPT
efficiency using alum at its optimum dose. Similarly results for
CEPT with and without pre-settling of wastewater for 30 min
showed only slight difference in effluent quality. Hence CEPT
without pre-settling would be more beneficial in terms of the
capital cost.
5. With alum doses of 200–240 mg/L as Al2(SO4)3, removals of tur-
bidity, TSS, TCOD and chromium were found to be 98.7–99.8,
94.3–97.1, 53.3–60.9 and 98.9–99.7%, respectively.
6. At optimum dose, CEPT with alum removed almost all particulate
COD and some portion of soluble COD (in a range from 7 to 28%)
probably due to the adsorption of soluble portion on Al(OH)3gel.
However, organic matter remaining after CEPT was still high and
in the dissolved form and required secondary treatment to meet
effluent standards.
7. At optimum dose, CEPT with alum generated an effluent with TSS
(30–60 mg/L) and chromium (0.2–0.8 mg/L) concentration that
met the enforced effluent standards. However, effluent standard
for COD could not be qualified.
Acknowledgements
The research was funded by the University of Engineering and
Technology, Lahore, Pakistan. The assistance of Mr. Muhammad
Hashmat in laboratory work is acknowledged.
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