War. Sci Tech. Vol. 32, No. 12, pp. 21-27. 1995. Copyrigbt iC) 1996 lAwo . Published byElsevier Scieoce Ltd Printed in Great Britain.Allrightsreserved. 0273-1223195 $9'50 + 0'00
SULFATE REMOVAL FROM INDIGO DYEING TEXTILE WASTEWATERS Isik Kabdasli, OlcayTiinay and Derin Orhon Istanbul Technical University, CivilEngineering Faculty, Environmental Engineering Department, Ayazaga Kampusu; 80626, Istanbul. Turkey
ABSTRACT Sulfate is an important parameter especially for discharges to sewer systems. The textile industry is a major source of sulfate. Some sulfate sources in the industry have material replacement alternatives. However in some sources, sulfate or species convertible to sulfate are the maio materials. The indigo dyeing process involves sulfur species as main materials. In this study, indigo dyeing wastewaters which contain significant concentrations of oxidized and non-oxidized sulfur components are evaluated in terms of sulfate removal. The approach is a pretreatment at the source before being mixed with other wastewaters. The study is conducted in two steps. In the first step, conversion of species to either sulfide or sulfate is experimentally evaluated. While reduction to sulfide poses problems. oxidation of all species to sulfate is found to be applicable. In the second step sulfate precipitation using calcium. barium and lead is practiced. Calcium precipitation provides up to 30% sulfate removal and these results are supported with existing literature data. Barium sulfate and lead sulfate precipitation provided practically complete removal. Economical evaluation of alternative methods is also given. Copyright IAWQ 1996. Published by Elsevier Science Ltd.
KEYWORDS Chemical oxidation ; indigo dyeing; pretreatment; sulfate precipitation; textile wastewater. INTRODUcnON Textile processing is a significant range of the total industrial activity in Turkey . Pollution resulting from textile plants has been explored in great detail (Germirli et al., 1990). But emphasis is mostly placed on pollution profiles and treatment systems for organic carbon removal (Orhon et al., 1992). However, research data on other characteristic parameters such as sulfur and sulfate are rather limited. Indigo dyeing is an integral part of most textile plants processing cotton 'fabrics, This method is generally observed to impart to the wastewater a relatively high organic content (high COD) of biodegradable nature. In these plants. the problem is often a wide spectrum of compounds and additives incorporating different fonns of sulfur (S2- to S042-). Non-oxidized sulfur forms may be partially converted to during treatment which itself poses a serious corrosion problem in sewers where the wastewater is discharged. around or above a concentration of 1500 mg/l. Therefore. control of sulfur containing compounds is a major task in the wastewater management of such textile plants.
In this study, removal of sulfur containing species in indigo dyeing wastewaters is theoretically and experimentally studied. Oxidation, reduction and precipitation experiments are conducted. Results are 21
I, KABDASU et al;
discussed and conclusions are drawn on the basis of the technical and economical feasibility of the processes. POLLUTION PROFILE Plant operations within indigo dyeing plants can be divided into two groups: the first is stock preparatior processes; the second group is finishing processes. In the first group only dry processes are carried ou therefore no wastewater is originatingfrom the preparation operations. Two main processesin the finishing group are mercerizing and dyeing. Mercerizing consists of caustic baths followed by rinsing baths. Dyeing operation has also a dyeing and rinsing sequence. Sizing and washing after weaving which follows dyeing also producewastewater; however, their amountsare generallynegligible. The plant on which the wastewatercharacterization and treatability studies have been conducted also has stock preparation and finishing processesgroups. Finishing processes are mercerizing, indigo dyeing, sizing and washing after weaving. Sizing and washing steps are determined to be negligible in terms oj wastewaters. Mercerizing is carried out using a caustic bath followed by rinsing. Indigo dyeing is appliec using atmospheric hydrosulfite as reducer and air oxygenas oxidizingagent These operationsare conducted in the continuous operation mode. Mercerizing and dyeing operations are carried out in three lines with different capacities. Wastewater volume for these three lines is given as average flow rates in Table 1. The total amount of wastewater originating from this section is 408 m3d- 1 of which 180 m3d-1 is from dyeing process. Pollution profile based on the characterization study is given in Table 1 (TUnay and Kabdasli
1994). Table 1. Pollution profile Process Mercerizing Dyeing Sample I Dyeing Sample II
238 180 180
1590 4585 2900
150 2690 1750
N.D.: not determined
ASSESSMENT OF TREATMENT ALTERNATIVES Treatment requirementis based on the pretreatment standards given for Istanbul Metropolitan Area by the Istanbul Water and Sewerage Administration, a partial list of which is given in Table 2. If the wastewater characteris calculated by adding two wastewaters togetherthe parameters S2-, soi- and COD are found to exceed the standards. The requiredCOD removal is over 70% and practically all COD is in solubleform and chemical treatment would not be adequate. The quality of wastewater as the sources are considered is biodegradable. Therefore biological treatment will be the main treatment system. In fact, there exists an activated sludge system in the plant although its capacity is limited. In that case all sulfur containingforms will be converted to soi-. Therefore so,> concentration must be evaluated as S04 2- ion plus all oxidizable sulfur forms. Then the basic problem is S04 2- removal. Sulfate removal can be achieved by advanced methodsof treatmentsuch as membrane processor by chemical precipitation. Advancedprocesses are costly and are subject to interferences due to complex natureof the wastewater. Ion exchange which may be one of the suitable techniques for sulfate removal in general would suffer from suspended solids, high organic matter and sulfur content as it is to be implemented as a pretreatment in this specific case study. Chemical precipitation can be applied using calcium. bariumand lead salts. Precipitation application. on the other hand. can be carried out at the source i.e. the dyeing bath effluent or to the total wastewaters. Precipitation at source is advantageous because it applies to only 1/3 of the wastewater and it provides removal of oxidizable sulfide forms which otherwise exert a deleterious effect on the biological treatment
Sulfate removal from indigo dyeing wastewaters
Therefore in this study. sulfate removal from the indigo dyeing bath wastewaters by precipitation is investigated. Table 2. Pretreatmentstandards (lSKI. 1986) Parameter
COD TSS SO 2-
800 350 1500 2 6-10
mgl- I mgl'"
As Table I indicates only 50% or less of the sulfur is in the S042. form. To remove the remaining part by sulfate precipitation there are two alternatives. The first alternative is to oxidize these forms to sulfate and the second alternative is reduction to sulfide. In the first alternative only sulfate precipitation following the oxidation will be needed. In the second alternative sulfide precipitation and sulfate precipitation may be needed because the complete reduction of sulfate to sulfide may not be realized. Experiments are conducted to assess the applicability of the reduction process by using synthetic samples of sulfite. FeCI2 is tried as the reducing agent in a wide pH range to determine the reduction and subsequent precipitation as FeS. Sulfite removal by this application is found to always be below 30%. However. the reduction process is likely to be subjected to interference by some of components in the wastewater. The high pH of the sample needs to be reduced and carbonate should be removed to avoid Fe2+ precipitation. At any rate the fact that sulfate also requires precipitation together with FeS may pose problems in determining optimum conditions. Furtherrnore specific emphasis may be required to deal with the mixed sludge which may at times show a hazardous character. In view of above mentioned factors the reduction method has not been explored in great detail as a candidate process. Oxidation is aimed to convert all oxidizable sulfur forms to sulfate. Catalytic air oxidation. oxidation by chemicalsand combination of these two applicationsare included in the experimental study. FOllowing oxidation the sulfate concentration of the wastewater is between 6000 and 9000 mgll. The wastewatercontains high dissolved solids therefore the ionic strength is expected to be over 0.3 M. On the other hand components originally existing in the wastewater as well as materials added for oxidation are potential inhibition agents for precipitation. A recent study indicated that sodium content is a major parameter determining sulfate solubility (Kabdasli, 1995). Under these conditions the equilibrium sulfate concentrationis expected to be about 4000 mgll with stoichiometric calcium addition. In an experiment with textile wastewater having similar characteristics demonstrated strong inhibition of precipitation in that 9000 mgll sulfate is reduced to 8000 mgll in I day and down to 4500 mgll in I week (Kabdasli, 1995).Therefore the sulfate precipitation with reasonable calcium dosages is not expected to be highly efficient, Barium and lead are considered as alternative precipitationagents. The interference to barium precipitation is assumed to be negligible. The main interference to lead precipitation is the carbonate ion which exists although in small concentrations; oxidation application may give rise to C02 absorptionfrom the air. EXPERIMENTAL STUDY Materials and methods Oxidationexperiments are conducted on composite dyeing samples using I I beakers with magnetic stirring. Air oxidation is provided using pressurized and dried air through diffusers. pH is monitored continuously. Oxidants are added at the beginning. Extent of oxidation is checked by measuring so,> and S2-
I. KABDASU et al:
concentrations. Chemical oxidation experiments are carried out using H 202, KMn04 and NaOCI in the same experimental setup without air feeding. pH and ORP are controlled during experiments. The dosages and conditions of the chemical oxidation processes are adjusted as defined by a study of Cadena and Peters (1988). Chemical precipitation of sulfate following oxidation is tested using calcium, barium and lead salts. CaS04 precipitation is conducted at neutral pH by using CaCI2 or Ca(OH)2 and HCI. Barium and lead are added as BaCI2.2H20 and Pb(N03)2 at the original pH of the sample. All analyses are made in accordance with Standard Methods (APHA, 1989).
Experimental results: air oxidation
Air oxidation is conducted using MnS04 as catalyst with a dosage of 200 mgl- 1 for partial oxidation. 400 and 750 mgl- 1 for complete oxidation. Catalytic air oxidation is carried out at a pH of 12 which is adjusted at the beginning and throughout the process by using a NaOH solution. Complete air oxidation is continued for 96 hours at which complete conversion to sulfate is achieved. Complete conversion is checked by a preexperiment using H 202 as oxidant which yielded a high enough oxidation efficiency and resulted in sulfate concentrations of 8550 and 4150 mgl! for Sample I and II respectively. In tables these values are denoted as theoretical sulfate. In the complete air oxidation experiment, however, a higher sulfate concentration is achieved only in extended oxidation durations. Partial air oxidation with 200 mgl- 1 MnS04 is continued for 7 hours at a pH of 12. Results of complete air oxidation for Sample I are given in Table 3 . Table 3. Results of catalytic air oxidation Reaction Time Sulfate Concentration [mgf'"] [hour] ------------.:....;;:.~---750 mgl"! MnS04 400 mgl"! MnS04
2660 2660 3250
3 6 8 23 30 48
2660 3280 4250 4990
5277 5060 6825 E 9010 9125 A
Table 4. Results of partial air oxidation Sample I Reaction Time [hour]
3 6 7
Result SO/- [mgl- 1]
8550 8550 8550
4070 5100 5630
48 60 68
As shown in the Table 3 air oxidation provided complete conversion. However, long detention times are required to achieve this efficiency. The conversion. however. is not linear with time in that the initial conversion rate is significantly bigger. The effect of catalyst concentration is not significant. Therefore the
Sulfate removal from indigo dyeing wastewaters
partial air oxidation seems to be more efficient as seen from Table 4, where approximately 70% conversion is obtainedin 7 hours. Experimental results' chemical oxidation H202 oxidation is conducted on two different samples at the original pH of the samples (pH> 13) and applying different oxidant dosages. Three different oxidation experiments are conducted on Sample I using H202• The dosages are determinedon the basis of stoichiometricratios with respect to different species and to account for the interferenceof organic matter. A similar experiment with Sample II is conducted on three different sets to check relative oxidationrates of existing species. The reaction time of each experiment is 2 hours. Results of H20 2 oxidationon two differentsamples (Sample I and II) are outlined in Tables 5 and 6. Table 5. Results of H20 2 oxidationfor Sample I H202 dosage [M]
Scope of the process
0.043 0.189 0.424
S032- oxidation SOl- and S2- oxidation Organic matter, SOl- and S2oxidation
8550 8550 8550
4650 80808 8030
SOl- [mgl" sol- [mgl"
55 95 94
Table 6. Results of H202 oxidationfor Sample II H202 dosage [M] 0.062 0.107 0.175
4150 4150 4150
3080 3640 3815
2900 2900 2900
2700 1650 1625
Conversion to Sulfate % 75 88 92
Tables 5 and 6 indicate that organic matter interference is negligible and complete oxidation is readily obtainable. In the Table 6 the first dosage (0.062 M) corresponds to the equivalent of organic matter. the second dosage (0.107 M) is equivalent to non-oxidized sulphur species, the third dosage (0.175 M) is equi valent of the total. Oxidation with KMn04 and NaOCI are tried on Sample I at the original pH of the sample (pH >13). The dosages are selected as 30% excess of the stoichiometric amount corresponding to oxidation of S03 2- and S2-. Oxidation duration is I hour. Results of oxidation experiments with KMn0 4 and NaOCI are given in Table 7. Table 7. Results of oxidation with KMn04 and NaOCI Agent KMn0 4 NaOCI
Result S04 2- [mgl'"]
KMn04 oxidation is effective and interference of organic matter is negligible. However NaOCI oxidation is not complete due to significant organic matter interference as COD is reduced from 4750 to 2900 mgl-1. These COD measurements are carried out after inorganicinterferencesare compensated.
I. KABDASU et al.
Chemical oxidation of partially air oxidized samplesis carried out using H202 and NaOCI both at the dosage of 30 % excess of stoichiometric to remaining non-oxidized forms. assumed to be S032- only. Oxidatior duration is 2 hours for H202 and I hour for NaOCI. pH is over 12 for both experiments. Results of chemica oxidation of partially air oxidized samples are given in Table 8. Table 8. Results of air oxidation plus oxidation with H 202 and NaOCI Agent
Results of this application are similar to those of chemical oxidation with H202 and NaOCI only witl respect to conversion and organic matter interference. Sulfate is precipitated from the oxidized sample. Calcium dosages are selected to present a wide range oj calcium application to assess the solubility limiting species. Barium and lead precipitation is conducted or Sample E (see Table 3). pH is not adjusted but the dosage is selected to compensate for the alkalinity interference due to BaC03 and PbC03 precipitation and carbonate complexation of lead. Results 01 chemical precipitation of sulfate are shown in Table 9. In Table 9 the same notation indicates the type oj oxidation previously applied to the sample. This notation is also indicated in the tables showing the result! of chemical oxidation experiments. Calcium precipitation efficiency is variable and a maximum 30~ efficiency is obtainable. Barium and lead precipitations provided practically complete removal. Table 9. Results of chemical precipitation Sample A B B C 0 E E E E E
Agent Ca(OHh Ca(OHh Ca(OHh Ca(OHh Ca(0H)2 Ca(OHh Ca(OHh CaCI2 BaCl2 Pb(N03h
9.14 6.23 10.54 6.29 5.71 7.94 10.59 23.14 15.63 26.05
9125 8080 8080 8160 7410 6870 6870 6870 6870 6870
7730 7460 6595 5900 5040 6825 4925 5750 480 <10
15 8 18 28 33 28 16 93 100
DISCUSSION AND CONCLUSIONS
In the catalytic air oxidation the concentration of the catalyst does not seem to be of importance which rna) be because of the high pH of oxidation. Different dosages employed in H202 oxidation indicated thai organic matter interference with S032- and S2- oxidation is negligible. Although use of excess KMnO, compensated the organic matter interference. in the NaOCI oxidation conversion to sulfate is not complete because of organic matter interference as measured by the COD reduction.
Sulfateremovalfrom indigo dyeing wastewaters
For the assessment of calcium precipitation two important aspects are the thermodynamic factors affecting solubility and precipitation kinetics. The limit of sulfate solubility was mainly affected by ionic strength and existence of sodium ion as pointed out above. Oxidation especially with NaOCl also has an effect on these parameters. Although theoretical calculations may not provide precise predictions for the complex cases, they at least give an order of magnitude. The magnitude of the sulfate concentration obtained in the experiments agrees with these predictions (Kabdasli, 1995) as discussed in the treatment alternative section. However, experimental results may have a distribution even in similar conditions which can hardly be explained by solubility considerations. These deviations are mostly attributable to kinetic limitations. The above experiments provided technical solutions for the treatment of indigo dyeing wastewaters which contain a complex combination of sulfur compounds. Oxidation by air in the presence of catalyst is not found to be practical since it requires excessively long retention times. KMn04 oxidation is also not effective in reasonable duration which also results in a significant amount of sludge. H 202 and NaOCl oxidations are successful. however they require high amounts of these chemicals. On the other hand. partial oxidation followed by H 202 or NaOCl oxidation is found to reflect an optimum in terms of both duration and chemicals requirement Chemical oxidation by NaOCI and H 202 after partial air oxidation provided the same efficiencies with direct addition with the same chemicals at significantly lower chemical dosages. However as seen from Table 10 the use of H202 is not a viable alternative due to high cost of the material. Table 10. Cost of treatment alternatives Alternative Partial air oxidation + Chemical oxidation with HzO z Partial air oxidation + Chemical oxidation with NaOCI Chemical oxidation with NaOCI
Unit price [$/m 3) 3.13 0.20 0.44
The sulfate removal process studied in this work is a source-based pretreatment which applies to only a fraction of wastewaters originating from a textile factory. Therefore the pretreated wastewater will be diluted by and treated together with other wastewaters. Consequently, although complete conversion to sulfate form is essential. partial precipitation of sulfate may be sufficient depending on the wastewater composition of the textile plant of concern. In such cases precipitation with calcium which is demonstrated to provide up to 30 % sulfate removal may be the first choice if it works properly. Barium and lead salts on the other hand removed practically all sulfate. However. the hazardous nature of the chemicals added and the excessive cost of the treatment operation are important factors to be considered in their rejection and the justified choice of lime treatment with a partial removal potential. REFERENCES APHA (1989). Standard Methods for Examination of Water and Wastewater. 17th edn, American Public Health Association, Washington, D.C. Cadena, F. and Peters, W.R. (1988). Evaluation of chemical oxidizers for hydrogen sulfide control. J. War. Pollut. Control Fed., 60(7), 1259-1263. Gennirli, F., Tiinay, O. and Orbon, D. (1990). An overviewof the textile industry in Turkey - PoUution Profiles and treatability characteristics. Wat. Sci. Tech., ZZ(9). 265-274. ISKI.(1986).EffluentLimitationsto PublicSewerage,IstanbulWater and SewerageAdministration. Kabdasli. N. I. (1995). The Approachesand Applicational Bases for Sulfate Treatment by Chemical Precipitation. PhD. thesis. IstanbulTechnicalUniversity.(in Turkish). Orhon, D., Artan, N., Bilylikmurat, S. and Gorgun, E.• (1992). The effect of residual COD on the biological treatabilityof textile wastewaters. War. Sci. Tech.. Z6(3-4). 815-827. TUnay, O. and Kabdasli, N. I. (1994). Treatabilitystudy of dyeing baths of Akfil San. ve Tic. A. S.. Technical Report, Istanbul TechnicalUniversity. (in Turkish).