OPTIMUM DESIGN OF SEDIMENTATION TANKS BASED ON SETTLING CHARACTERISTICS OF KARACHI TANNERY WASTES WASEEM AKHTAR t* , MUHAMMAD RAUF 2.*, IQBAL ALI 1 and NAYEEMUDDIN 2 l Institute of Environmental Engineering and Research, NED University of Engineering and Technology, Karachi, Pakistan; "PCSIR Karachi Laboratories, off University Road, Karachi, Pakistan (Received 26 June 1995; accepted 7 June 1996) Abstract. A study was carded out in a specially designed settling column to investigate the settling characteristics of effluents arising from Karachi tanneries. The Jar-tests were conducted to determine the optimum dosage of coagulants such as potash alum and ferric chloride to maximize the removal of suspended solids (S.S) from the wastes. For alum, the optimum dosage was found to be 150 mg L- ~, whereas with ferric chloride it was 30 mg L -m. Experiments carried out in the settling column with the optimum coagulant dosages showed that the coagulation with ferric chloride was much better than with alum, as it removes about 84% S.S from the wastes compared to 70% removal obtained in case of alum. Design curves in terms of percent removal of S.S. vs loading rate and detention time were constructed using data obtained from settling column. These curves can be used in designing the settling tanks employed in the tannery wastes treatment plants. Key words: coagulation, settling characteristics, suspended solids, tannery wastes I. Introduction The tanning and leather industry plays a significant role in the economy of Pakistan. Ten years ago, it was the fifth most important export industry in the manufacturing sector and now it has accelerated rapidly to become number two after the textile. At present, there are about 400 tanneries in the organized sector, out of which about 170 tanneries are located in Karachi, a coastal city with a population of 11 million, and the remaining spread all over Pakistan in cites such as Lahore, Kasur, Sialkot and Hyderabad. A complete background information on local tanneries and characteristics of their wastes has been reported elsewhere (Meynell, 1992; Rauf, 1994). Untreated effluent from these tanneries is discharged directly into seweres or open drains, and ultimately into natural coastal waters and in case of inland tanneries into natural streams or stagnant ponds and finally into groundwater (Dreiss, 1986; Shastry, I984). However, with the growing environmental awareness throughout Pakistan, particularly at public and government levels, this practice is under attack and the time for indiscrirninate untreated effluent discharge is growing short. The Pakistan Environmental Protection Council, has recently formulated and approved the National Environmental Quality Standard's for all types of industries * Supervisor of the project, correspondence to be addressed. ** Post Graduate Student, presently employed as research Engineer in PCSIR Laboratories, Karachi. Water, Air, and Soil Pollution 98:199-211, 1997. (~) 1997 Kluwer Academic Publishers. Printed in the Netherlands.
Transcript
OPTIMUM DESIGN OF SEDIMENTATION TANKS BASED ON
SETTLING CHARACTERISTICS OF KARACHI TANNERY WASTES
WASEEM A K H T A R t* , M U H A M M A D RAUF 2.*, IQBAL ALI 1 and NAYEEMUDDIN 2
l Institute of Environmental Engineering and Research, NED University of Engineering and Technology, Karachi, Pakistan; "PCSIR Karachi Laboratories, off University Road, Karachi,
Pakistan
(Received 26 June 1995; accepted 7 June 1996)
Abstract. A study was carded out in a specially designed settling column to investigate the settling characteristics of effluents arising from Karachi tanneries. The Jar-tests were conducted to determine the optimum dosage of coagulants such as potash alum and ferric chloride to maximize the removal of suspended solids (S.S) from the wastes. For alum, the optimum dosage was found to be 150 mg L- ~, whereas with ferric chloride it was 30 mg L -m. Experiments carried out in the settling column with the optimum coagulant dosages showed that the coagulation with ferric chloride was much better than with alum, as it removes about 84% S.S from the wastes compared to 70% removal obtained in case of alum. Design curves in terms of percent removal of S.S. vs loading rate and detention time were constructed using data obtained from settling column. These curves can be used in designing the settling tanks employed in the tannery wastes treatment plants.
The tanning and leather industry plays a significant role in the economy of Pakistan. Ten years ago, it was the fifth most important export industry in the manufacturing sector and now it has accelerated rapidly to become number two after the textile.
At present, there are about 400 tanneries in the organized sector, out of which about 170 tanneries are located in Karachi, a coastal city with a population of 11 million, and the remaining spread all over Pakistan in cites such as Lahore, Kasur, Sialkot and Hyderabad. A complete background information on local tanneries and characteristics of their wastes has been reported elsewhere (Meynell, 1992; Rauf, 1994). Untreated effluent from these tanneries is discharged directly into seweres or open drains, and ultimately into natural coastal waters and in case of inland tanneries into natural streams or stagnant ponds and finally into groundwater (Dreiss, 1986; Shastry, I984). However, with the growing environmental awareness throughout Pakistan, particularly at public and government levels, this practice is under attack and the time for indiscrirninate untreated effluent discharge is growing short. The Pakistan Environmental Protection Council, has recently formulated and approved the National Environmental Quality Standard's for all types of industries
* Supervisor of the project, correspondence to be addressed. ** Post Graduate Student, presently employed as research Engineer in PCSIR Laboratories, Karachi.
Water, Air, and Soil Pollution 98:199-211, 1997. (~) 1997 Kluwer Academic Publishers. Printed in the Netherlands.
2 0 0 w. AKHTAR ET AL.
which will be enforced on July 1, 1996 for all those existing industrial units which are already in production, whereas for new units it has been implemented from July 1, 1994. In order to implement these standards, tanneries which are located closely, are planning to set up a combined treatment plant, whereas the other tanneries which are far enough to have access into the combined treatment plant will have to install their own small treatment units.
One of the important process units in the treatment plant is the settling tank which is designed on the basis of the settling characteristics of the suspended solids present in the effluent to be treated. The suspended particles of the tannery wastes exhibit characteristics of mixed nature. Therefore in order to have appropriate base line data for the design of the settling tanks for the tanneries, it is imperative to investigate the settling characteristics of the tannery effluents being discharged from the local tanneries. There are a number of studies reported elsewhere (Fales, 1929; Sarber, 1941; S utherland, 1947; Clonfew, 1987) dealing with the primary treatment of tannery wastes, particularly of coagulation and flocculation with the help of coagulants such as alum, ferric chloride, ferrous sulphate and polyelectrolytes. These results however, can not be applied directly to optimise the design of the primary treatment settling tanks for local tannery wastes as the choice of coagulant, its dosage, optimum pH and other design parameters depend upon a number of local factors such as the characteristics of the wastes, temperature, and the end use of the treated effluent.
Unfortunately so far no such data is available with respect to the local tanneries and no systematic efforts were made in the past to address this particular aspect. However, only a couple of scattered efforts (Iqbal, 1993; Beg, 1990; Qureshi, 1989) appeared to have been made concerning the data collection and coagulation studies for tannery wastes.
The present work, therefore is a modest effort to initiate the desired type of investigation and fill up the existing gap. This paper presents the results of the settling characteristics of randomly selected composite samples of effluents from typical leather manufacturing units in Karachi. Based on these characteristics, curves have been prepared to provide the optimum design for the settling tanks in the tannery waste treatment process.
2. Materials and Methods
The study was carded out in two phases. In the first phase of experiments, the Jar- tests were conducted to optimize the dosage of the coagulant such as potash alum and ferric chloride for an effective removal of suspended solids from the tannery wastes. These two coagulants were chosen for the study due to their low cost, ease of availability in the local market and their better performance in removing suspended solids of the tannery wastes as reported (Iqbal, 1993; Qureshi, 1989).
OPTIMUM DESIGN OF SEDIMENTATION TANKS 201
The tests were carried out with 1 L sample following the standard techniques generally used in the Jar-test method (Eckenfelder, 1989). This included a 1-min flash mix phase with 80-100 rpm followed by a 20 min slow mix phase with 30 rpm and finally a 15-20 rain settling phase under quiescent conditions.
A series of experiments performed in the jar-test was as follows:
Experiments were carded out without adjusting pH value of the composite samples collected from tannery waste drains at Korangi area in two different days of three weeks apart. Thus the variation in the pH values of the samples was expected. The result of few exploratory Jar-test runs, conducted with alum and ferric chloride, indicated that more than 100 mg L - 1 of alum dosage was required before a visible floc was formed whereas with ferric chloride a small dosage more than 10 mg L -1 was enough. Based on these findings, the above mentioned experiments were performed to optimise the dosage. In each run the supematant samples were drawn from the beaker at the end of the settling period and analysed for turbidity in N.T.U units using a turbidimeter (Model # Hach - 16800).
Once the optimum dosages for the coagulants were determined, the second phase of the experimentation was started in a settling column which was specially designed and fabricated for this purpose (Figure 1). The column was in the cylin- drical shape of 0.146 m diameter, 3.76 m height and 63 L capacity. It consisted of six sections of equal heights of 0.61 meter, which were made using 5 mm thick transparent plastic sheets, each being fitted with PVC pipe and taps to be used as sampling points. Tube lights were fixed along the entire length of the column to observe the settling behaviors during the experiments.
The liquid wastes consisted of the composite samples collected from the differ- ent points of open drains where a number of tanneries, located in sector 7A Korangi Industrial Area, discharged their effluent. Most of tanneries (about 90%), located there, were using chrome tannery process. Each time after collection, a desired amount of formaldehyde was added as a preservative to arrest biodegradation.
The experiments were carried out without and with coagulants such as alum (dosage 150 mg L -1) and ferric chloride (dosage 30 mg L-l) . The required dosage of coagulant was mixed thoroughly with the tannery waste and it was, then, fed from the bottom of the column to maintain the uniform concentration of suspended particles throughout the column. Sampling were drawn off from each samples point after every 20 minutes. The samples were analysed for suspended solids using gravimetric method.
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3. Results and Discussion
In the first group of experiments, the optimum amount for the coagulant dosages required to treat the Karachi tannery wastes were determined. The results from the Jar test are presented in Figures 2 and 3 where percent removal of turbidity is plotted against the coagulant dosage.
OPTIMUM DESIGN OF SEDIMENTATION TANKS 203
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In case of alum (Figure 2), it can be seen that the dosage above 100 mg L -1 was effective in coagulating the turbidity, the optimum dosage being 150 mg L-1 at pH 8.5 which reduce 77 % of the turbidity. However, in case of ferric chloride (Figure 3)
204 w. AKHTAR ET AL.
a lower dosage was needed in flocculating the suspended particles. A dosage of 30 mg L-I ferric chloride was found to be optimum at pH 8.0 with a maximum reduction of 72% in turbidity. In both cases it was observed that on overdosing the suspension with coagulant concentration beyond the optimum dosage, the percent reduction of turbidity which was maximum at the optimum dosage, decreased rapidly. This showed that restabilization of colloidal particles occurred due to overdosing. The restabilization is generally accompanied by a charge reversal ie the net charge on the particles is reversed from negative to positive by adsorption of an excess of counterions.
Second phase of the study was conducted with the settling column. The exper- iments were carded out without coagulant or with coagulants using the optimum dosage of alum or ferric chloride. The results obtained from this phase are shown in Table I. The data are expressed in terms of percent removal of suspended solids (S.S) at each sampling tab and time interval.
Results show that the coagulation of tannery wastes with ferric chloride was sweep-floe type and better than with alum. For example, after 60 min settling time, the percent removal of suspended solids along the entire depth of the column ranges from 63 to 71 with maximum value at 1.22 m depth, whereas in case of alum it ranges 48 to 66 with maximum at 0.61 m depth. In a run in which no coagulant was added, the percent removal varies form 33 to 60. The settling of the floes formed with ferric chloride was found to be faster than with alum. In the ferric chloride run, the settling was almost completed in 100 mins giving rise to an almost uniform concentration of S.S. along the entire depth of the column, while in alum and no coagulant case the settling process was still proceeding. It was also observed that the floes formed with alum was rather somewhat weak, which were breaking down while settling. This was not the case with those formed with ferric chloride. This behavior is believed to be due to the higher solubility of aluminium hydroxide floes than that of ferric hydroxide as expected at the pH range 8-8.5 maintained during runs (Faust, 1983).
In view of the above results, it is concluded that ferric chloride was the suitable coagulant in treating local tannery wastes. Since a small amount of ferric chloride was required for desired results, the economics also favours its application. Table II provides an estimated cost for the two coagulants used for the tannery effluent generated from the processing of 1000 hides per day.
The coagulant cost in the treatment with potash alum is about three times higher as against with ferric chloride.
4. Design Application
Data given in Table I can be used in constructing loading rates (overflow rates) and detention time curves which can be employed in the designing of settling tanks.
Optimum Cost of Total cost for the quantity used coagulant the coagulant per litre (mg) US$/kg in US$
OPTIMUM DESIGN OF SEDIMENTATION TANKS
Table II Estimated cost for coagulants used, bases: 1000 hides processed perday
r
DETENTION TIME-MINUTES
Figure 4. Settling curves (without coagulant).
As it can be seen that some of the S.S. data in Table I are scattered and therefore were streamlined, before plotting them in Figures 4--6 against their respective depth and time. A family of smooth curves representing the isoremoval lines are drawn. These curves are the limiting or maximum settling path for the indicated percent. From these settling curves, utilizing integration techniques given elsewhere (Peavy, 1987; Eckenfelder, 1989), the total percent removal of S.S. was calculated for a particular loading rate and detention time. These data were then plotted in Figures 7
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0 20 40' GO 80 100 120 ~ 160 1~ 200
DETENTION l IME- MINUTES
Figure 5. Settling curves (alum).
and 8 relating the percent removal of S.S. to loading rate and detention time, respectively. The smooth curves drawn in Figures 7-8 are design curves which can be used in the designing of settling tanks for treatment of Karachi tannery wastes. The following steps can be followed to estimate loading rate, detention time, surface area and depth of the settling tank for a desired percent removal of suspended solids from the tannery wastes.
With the help of Figures 7 and 8, the required loading rate and detention time can be estimated for a given removal efficiency desired in the settling tank. It should be noted that the laboratory data presented in Figures 7-8 represent ideal settling conditions; however, the criteria for prototype design must include the effects of turbulence, short circuiting, and inlet and outlet losses. The net effect of these factors is a decrease in the loading rate and an increase in the detention time over that derived from the laboratory analysis. As a general rule, the loading rate and detention times estimated from the laboratory data are divided by a factor 1.25 to 1.75 and multiplied by a factor 1.50 to 2.00 respectively (Eckenfelder, 1989).
OPTIMUM DESIGN OF SEDIMENTATION TANKS 209
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Figure 6. Settling curves (ferric chloride).
After the above computations are completed, the surface area of the settling tank can be calculated by dividing the expected volumetric flow rate with the modified loading rate. Similarly, the effective depth can be estimated by dividing the volumes of tank (modified detention time x volumetric flow rate) with the estimated surface ~ffea.
5. Conclusion
Ferric chloride was found to be more effective than potash alum as a primary coagulants for reducing suspended solids levels in the Karachi tannery wastes. With dosage of 30 mg L -1 as much as 84% of S.S was removed at 180 minutes settling time. However during the same detention period, a 150 mg L -1 optimum dosage of potash alum could only remove about 69% of suspended solids, whereas the settling, under the same operating condition with no coagulant added, could achieve only 61% removal of S.S from the wastes. Since a small amount of ferric chloride was required, the economics also favours the application of ferric chloride
Figure 7. Percent removal of suspended solids vs loading rate.
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Figure 8. Percent removal of suspended solids vs detention time.
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OPTIMUM DESIGN OF SEDIMENTATION TANKS 211
in treating the local tannery wastes. An estimated cost for the ferric chloride application was found to be about one third of that of potash alum.
With the help of data obtained from settling column, the design curves have been constructed in terms of percent removal of S.S vs loading rate and detention time. These curves can effectively, be applied in designing the settling tanks in the tannery waste treatment plants. However these curves are based on the data obtained from experiments using initial concentration of S.S having a limited range of 1455 to 1760 mg L -1, it is therefore recommended that in order to make these design curves useable for a wider application, studies should be carded out using a broad range of initial S.S concentrations expected in the effluents of Karachi tanneries, where a varying degree of processing techniques from vegetable to chrome are being employed.
Acknowledgements
The authors would like to thank N.E.D. University of Engineering and Technology for financial support. They are also indebted to Mr. Idrees Ahmed, Principal Tech- nician of Pakistan council of scientific and Industrial Research laboratories for his help in fabrication of settling column.
References
Beg, M. A. A., Mahmood, N. and Naeem, S.: 1990,J. Sci. Ind. Res. 33(10), 431. Clonfero, G.: 1987, Latin American Seminar on Treatment of Tannery Wastes, UNIDO, Port Alegre,
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Qureshi, A, W., Rizvi, N. and Iqbal, M.: 1989, Pak. J. Sci. Ind. Res. 32(12), 795. Peavy, H. S., Rowe, D. R. and Techobanglous, G.: 1987, Environmental Engineering (International
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Sarber, R. W.: 1941, J. Am. Leather Chem. Assos 36, 463. Shastry, C. A.: 1984, Paper presented in the 19th Tanners Get-Together, Madras, India. Sutherland, R.: 1947, Ind. Eng. Chem. 39(5), 630.
