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Study on Flocculation Efficiency of Okra Gum inSewage Waste Water

Monika Agarwal, Rajani Srinivasan, Anuradha Mishra*

Department of Chemistry, Institute of Engineering and Technology, CSJM University, Kanpur-208 024, India

IntroductionThe process of flocculation involves gathering together ofsmaller masses from a destabilized colloidal suspensionto larger masses called flocs. Flocculation is the primaryand essential phenomenon in domestic/industrial wastewater treatment, before the waste is discharged into therivers, to prevent water pollution. In recent years naturalpolymer based flocculants have started gaining impor-tance due to their eco-friendly nature.

Grafted and ungrafted natural water soluble polysac-charides have the capability of flocculating small parti-cles.[1] Natural polymers such as starch,[1, 2] sodium algi-nate,[1, 3] amylopectin,[1] guar gum,[1] xanthan gum,[1]

kendu gum,[4] and chitosan[5] find extensive application asflocculants. Many starch based products have been usedfor removal of toxic wastes like hexavalent chromium,[6]

cadmium,[7] and gallium[8] which are usually present inmany industrial waste water such as those from textiles,leather tanning, electroplating and metal finishing indus-tries.[9]

Okra gum is botanically known as Hibiscus esculentus.It is soluble in cold water and is used in the food industryas a good emulsifying and foam stabilizing agent. Okragum is a natural polysaccharide composed of d-galactose,

l-rhamnose and l-galacturonic acid. In the present study,the flocculation efficiency of this polymer as flocculantwas tested in sewage effluent for the very first time. Theflocculation efficiency was studied by varying the poly-saccharide dose, contact time and pH with sewage wastewater. X-ray diffractograms of pure gum, solid wastefrom sewage and of flocs after treatment of the effluentwith polysaccharide were obtained to detect the incor-poration of waste material in flocs formed by the polysac-charide.

Experimental PartThe raw material, seedpods of Hibiscus esculentus wasbought during summer. The okra gum was obtained by aqu-eous extraction of the seedpods of okra plant followed byprecipitation with alcohol. It is a white amorphous polysac-charide consisting largely of d-galactose, l-rhamnose and l-galacturonic acid.[10] The precipitated polysaccharide wasthen washed with acetone 2–3 times to remove impuritiesand finally dried. The FTIR spectrum of purified okra gumwas recorded on a Bruker Vector-22 spectrophotometer. Theviscosity of the polymer solution was measured by an Ost-wald viscometer. The intrinsic viscosity was obtained (fromthe point of intersection) after extrapolation of two plots,

Full Paper: Okra gum, a polysaccharide consisting of d-galactose, l-rhamnose and l-galacturonic acid is used asfood additive. In the present investigation, the flocculationefficiency of okra gum for the treatment of sewage wastewater is tested. The flocculation study has been done bystandard jar test method. The effect of polymer dose, con-tact time and pH on the per cent removal of solid waste isreported. Okra gum is found to be very effective and to becomparable with commercial flocculants. X-ray analysisof the flocs is also reported to support the interaction ofsewage waste with the gum.

Macromol. Mater. Eng. 2001, 286, No. 9 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1438-7492/2001/0909–0560$17.50+.50/0

Macromol. Mater. Eng. 2001, 286, 560–563

(a) Plot of per cent removal of suspended solid vs. polymerdose of sewage waste (0), temperature = 32 8C, contact time =1 h. (b) Plot of per cent removal of total dissolved solids vs.polymer dose of sewage waste (9), temperature = 32 8C, con-tact time = 1 h.

Study on Flocculation Efficiency of Okra Gum in Sewage Waste Water 561

i.e., gsp /C vs. C and lngrel /C vs. C to zero concentration.Here C is the concentration of polymer in g/dL and gsp /C =grel – 1/C, where grel = g/g0 = t/t0 , t being the time of flow ofthe polymer solution (of viscosity g) and t0 the time of flowof the solvent (of viscosity g0) at the time of measurement.

The sewage waste water (mixed, domestic and industrial)was collected from municipal manhole. The pH values forthe sewage waste water and gum solution in water weremeasured by Microprocessor pH meter CP 931. The conduc-tivity of the waste water was measured by the CenturyMicroprocessor conductivity meter CC 631 and COD wasmeasured by usual standard method. The buffer solutionsprepared by using ready-made buffer tablets (E-Merck che-micals) were used for maintaining the pH of the waste water.Flocculation studies of okra gum were conducted by standardjar test described by Huck et al.[11] Beakers of 1000 mL capa-city each equipped with variable speed (0–100 rpm) agitatorwere used. 500 mL of waste water was taken in each beakerand the polymer solution of desired concentration was addedinto it by means of a syringe (capacity 1–2 mL). The agitatorwas first adjusted to 100 rpm for 1 min and then continuedfor the total of 10 min at 50 rpm. The agitator was subse-quently stopped and the waste water was allowed to settlefor 1 h.

A measured volume of 20 mL of the samples was taken todetermine the solid content of the sewage waste before andafter treatment with the polysaccharide. The suspended solidcontents were calculated by the Equation[12]

Solids ðmg=LÞ ¼ ðAÿ BÞ61000volume of sample ðmLÞ

where A = weight of the dried residue + crucible, B = weightof the crucible.

To determine the total dissolved solids, known volumes ofthe samples were filtered and the solids obtained were driedand weighed; for total solids unfiltered samples were taken.Suspended solids in waste water were determined by sub-tracting total dissolved solids from total solids.

Flocculation efficiency of the solution was tested at threepH values: 4.0, 7.0 and 9.2. The pH of the solution wasmaintained by addition of 450 mL of a buffer solution of therequired pH to 50 mL of waste water.

X-ray powder diffraction patterns of the polysaccharideused, sewage solid waste and flocs after treatment of thewaste with polysaccharide were obtained on an X-ray dif-fractometer model Iso-Debyflux-2002 (Rich and Scifert)using Cu Ka radiation.

Results and Discussion

Characterization

The IR spectrum of the purified sample of okra gumshows characteristic peaks of 1OH between 3609–3288cm–1, of 1COOH between 1574–1557 cm–1, of lactoneat 1734 cm–1, and of 1CH3 at 2923 cm–1. The intrinsicviscosity of okra gum was found to be 4.45 dL/g.

The pH values of 100 mL of aqueous solutions havingdifferent concentrations of okra gum were found between6.57 and 8.00. Sewage waste water has a pH of 7.63, con-ductivity 2.80 mS, turbidity 225 NTU, total solids1865 mg/L, total dissolved solids (TDS) 1700 mg/L, sus-pended solids (SS) 165 mg/L and COD 425 mg/L. ThepH of the waste water after addition of the okra gum wasbetween 6.90 and 7.04.

Flocculation Studies

Effect of Polymer Dose

The flocculation efficiency of okra gum with sewagewaste is shown in Figure 1. Figure 1(a) shows the plot ofper cent removal of suspended solids vs. polymer dose insewage waste water and Figure 1(b) shows the plot of percent removal of total dissolved solids (TDS) vs. polymerdose in sewage waste. From the plots in Figure 1, it isapparent that with increasing polymer dose, the per centremoval of solid waste increases; after a certain dose ofpolymer, a decreasing trend in solid removal is seen. Fig-ure 1(b) also shows the same trend as Figure 1(a). It isalso apparent from Figure 1 that the most effective doseis 1.2610–4 g/L, at which a maximum solid removal ofsolid waste is seen. The above behavior could beexplained by the fact that the optimal dose of flocculantin suspension causes a larger amount of suspended solidto aggregate and settle, thus decreasing the TDS in theeffluent. However an over-optimal amount of flocculantin suspension would cause the aggregated particle toredisperse in the suspension and would also disturb parti-cle settling, thus increasing TDS in the effluent.

Figure 1. (a) Plot of per cent removal of suspended solid vs.polymer dose of sewage waste (0), temperature = 32 8C, contacttime = 1 h. (b) Plot of per cent removal of total dissolved solidsvs. polymer dose of sewage waste (9), temperature = 32 8C, con-tact time = 1 h.

562 M. Agarwal, R. Srinivasan, A. Mishra

Effect of Contact Time

The flocculation efficiency of the gum is shown withvarying contact time in Figure 2. Figure 2 shows the percent removal of suspended solids and per cent removal ofthe total dissolved solids from sewage waste water at dif-ferent polymer doses. The maximum solid removal wasseen after the fifth hour of contact time at 1.2610–4 g/Lof polymer dose. Maximum solid removal from the efflu-ent was seen after this particular time duration. After thisduration the reverse trend was seen. The most plausiblereason for this trend may be the restabilization of the col-loidal particles after optimal time duration.

Effect of pH

The flocculation efficiency of the gum at its best dose of1.2610–4 g/L is shown with varying contact time at dif-ferent pH in Figure 3. It is apparent from the graph thatan acidic to neutral pH is suitable for the maximum solidremoval from the effluent. At neutral pH, 84.63% and atacidic pH, 86.86% of solid removal are seen after oneand five hours, respectively.

Usually a change in pH does not effect the efficiencyof natural polymers. Here changes in percent solidremoval with varying pH are due to the effect of pH onthe constituents of the sewage waste water. At neutralpH due to the formation of hydrogen bonding betweenneighboring hydroxyls and surface adsorbed water resultsin disruption of surface hydroxyls, thus increasing the percent solid removal.[13] In acidic pH (4.0) the metallic ionspresent in the effluent get oxidized, resulting in an aggre-gation of solid wastes.[14]

Mechanism

Figure 4(a) shows the crystalline nature of the wastematerial in sewage waste water whereas the pattern for(b) shows a complete amorphous nature of the okra gum.In Figure 4(c), the XRD patterns of flocs obtained after

Figure 2. Plots of per cent removal of suspended solids vs.contact time of sewage waste with varying polymer dose, (0)0.04 mg/L, (9) 0.08 mg/L, (J) 0.12 mg/L, (j) 0.16 mg/L. Plotsof per cent removal of total dissolved solids vs. contact time ofsewage waste with varying polymer dose, (f) 0.04 mg/L, (F)0.08 mg/L, (g) 0.12 mg/L, (G) 0.16 mg/L.

Figure 3. Plots of per cent removal of suspended solids vs.contact time of sewage waste at polymer dose 1.2610–4 g/L,with varying pH (0) 4.0, (9) 7.0, (J) 9.2.

Figure 4. X-ray diffraction patterns of solid waste (a), polymer(b) and flocs of sewage waste (c).

Study on Flocculation Efficiency of Okra Gum in Sewage Waste Water 563

the treatment of the effluent with the polymer are quitedifferent from the diffractograms of Figure (a) and Fig-ure (b). The 2h and d values observed in (a) are changedaltogether in pattern (c), and this constitutes primary evi-dence that a different crystal type was found.[15–17] Thischange in angle and d values may be due to strong inter-actions between the free hydroxyl groups and carboxylicgroups and contents of the sewage waste. Though theXRD patterns do not give any specific evidence for themechanism of flocculation, and anionic polymers areknown to cause larger flocs by a bridging mechanism, inthis case the extent of change observed in the patternssuggests that, apart from secondary bonding betweenflocculant and solid waste, there may also be an involve-ment of primary bonding like chelation between the crys-talline matter of the waste and the polymer.

Comparison with Commercial AnionicPolyacrylamide

The flocculation efficiency of okra gum was compared tothat of anionic polyacrylamide (available at local level).The per cent removal of solids from the effluent is nearly88% when synthetic flocculant was used at almost thesame conditions as maintained for the flocculation usingokra gum. The results show that okra gum is almost aseffective as a commercial flocculant.

ConclusionOkra gum is used as a flocculant for sewage effluent forthe very first time. It is found to be a very effective floc-culant with almost 86% removal of suspended solids. It isas effective as a commercial one and, in addition, it iseco-friendly by virtue of being biodegradable and of foodgrade. The interaction between the sewage waste andokra gum is shown by means of X-ray diffractograms.

Acknowledgement: The authors are grateful to Dr. TapanRaoth, Incharge, Zonal Laboratory (Kanpur), National Environ-

mental Engineering Research Institute, Dr. Padma S. Vankar,Facility for Ecological and Analytical testing, Indian Institute ofTechnology, Kanpur, and Dr. R. P. Mishra, Scientist, CentralPollution Control Board, Kanpur, for their valuable help in thiswork by providing some of the research facilities.

Received: February 20, 2001Revised: June 26, 2001

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