~ Pergamon

PIT: S0273-1223(99)00378-9

Wal. Sci. Tech. Vol. 40, No. t, pp. 177-182, 1999e 19991AWQ

Published by Elsevier ScienceLtdPrintedin GreatBritain, Allnghts reserved

0273-1223/99 $20.00 + 0.00

ANAEROBIC TREATMENT OF TEXTILESIZE EFFLUENT

J. Sacks and C. A. Buckley

Pollution Research Group. Department a/Chemical Engineering, University a/Natal.Durban, South Africa

ABSTRACT

This investigation focused on the Kwazulu-Nalal province, where a number of under-utilised and under­performing anaerobic digesters were identified. The aim of the study was to assess the potential for treatmentof high-strength or toxic organic agro-industrial effluents in the available capacity. The anaerobic digestionof a textile size effluent was investigated. Inhibitory components and concentrations of the solution wereidentified. The size solution was degraded anaerobically but could cause overloading of a digester at highconcentrations. The performance efficiency of the anaerobic digesters at the Umbilo Sewage PurificationWorks was evaluated. The digesters were operating efficiently and had available hydraulic and organiccapacities. This investigation confumed the potential for the treatment of high-strength organic effluents inthe available anaerobic digester capacity. <l:i 1999 IAWQ Published by Elsevier Science Ltd. All rightsreserved

KEYWORDS

Anaerobic digestion; textile size; biodegradation; toxicity; acclimation.

INTRODUCTION

In South Africa, the growth of the population and the economy has increased the demand for water, whichhas to be met from limited resources (Department of Water Affairs, 1986). Pressure on these resources iscontinually increased by greater pollution loads and reduced flows in the country's rivers, due to theexpanding demand. Although industry only accounts for approximately 16% of South Africa's direct wateruse, its impact is much higher because the effiuents often contain pollutants (Stander, 1997). In coastalareas, the biggest problem with industrial water users is the amount of fresh water lost via effiuent pipelinesto the sea.

The KwaZulu-Natal region has the potential to attract a significant amount of industry and due to theabundance of water relative to the rest of the country, it is probable that some of these industries will befrom the agro-industrial sector. One of the characteristics of this class of industries is the high concentrationof organic compounds in the effiuents. Industries of this type already exist in the region and, due to thenature of the effiuents that they produce, encounter difficulties with the disposal of the effiuents, with co­disposal into municipal landfill sites or marine discharge being the common solutions.

Tracer tests on a number of anaerobic digesters in the KwaZulu-Natal region have indicated that the mixingvolume can be as low as 50% of the actual volume (Barnett, 1995; Barclay, 1996). Research has also shownthat there are a number of sewage works in the region with under-utilised anaerobic digestion facilities(Barnett, 1995). A digester with available hydraulic capacity can be defined as one which is receiving a

177

178 J. SACKS and C. A. BUCKLEY

smaller volumetric load than its design specifications. Available loading capacity is the ability of a digesterto accept a greater organic load (kg VS/m3.d) without experiencing an overload or shock loading.

The objective of this investigation was to assess the potential for the treatment of high-strength or toxicorganic effluents in available anaerobic digester capacity. A strategy was developed for the simultaneousassessment of available digester capacity and effluent degradability evaluation prior to loading into a full­scale digester. The strategy was applied to determine the feasibility of the anaerobic digestion of a textilesize effluent. The effluent was chosen due to its high organic strength (ca. 100000 mg/I), its close proximityto the Umbilo Sewage Purification Works (which had available anaerobic digestion capacity) and becausethe effluent was being transported 40 km for marine disposal.

TEXTILE SIZE EFFLUENT

Size effluents represent the main component (ca. 60%) of the organic load of the effluents from textilefinishing mills (Schluter, 1991). In the sizing operation, the individual yarns are coated with a protectivefilm of size to resist the abrasive effects of the weaving loom (Water Research Commission, 1983).

Size blends: Starch has been the traditional sizing material used in textile manufacturing, however, in recentyears the trend has been towards synthetic sizes because of the increased demand for synthetic fibres. Toovercome the deficiencies of single component sizes, size blends are commonly used (Water ResearchCommission, 1983).

MATERIALS AND METHODS

For this investigation, a synthetic size solution was used, based on the blend used at the New Germany.textile mill (Table I).

Table 1. Composition of the synthetic size effluent

Component Measured COD Mass added Theoretical(mwl) (!!II) COD/t!

Polyvinyl alcohol (PVA) 53700 30.7 1.79Starch 90526 23.9 1.19Plystran 19200 16.1 1.32Carboxymethyl cellulose 15400 13.6 1.24(CMC) (CMCOxidised modified starch 4900 2.9 0.86(OMS)Acrylic 1900 0.73 0.94Biocide 13 300 0.05 1.94

Biodegradability and toxicity assays: Owen et al. (1979) described techniques for determining the anaerobicbiodegradability of a substrate and its potential toxicity to the methanogenic biomass. Substratebiodegradability was determined by monitoring the cumulative methane production from a sample whichwas anaerobically incubated. The anaerobic toxicity assay measured the adverse effect of a compound on therate of total gas production from a labile methanogenic substrate. The laboratory-scale evaluation of thetextile size effluent was based on these techniques.

The degradability of each component of the textile size solution was investigated. Serum bottles (125 ml)contained the size components (at varying concentrations), 30 ml anaerobic medium (Owen et al., 1979) and30 ml seed inoculum (sampled from an operating primary anaerobic digester). The substrate was dilutedwith distilled water and made up to a working volume of 100 mL. The bottles were incubated at 37 ± 1°Cand shaken manually once a day to facilitate contact between the microorganisms and the substrate. Gasproduction was monitored volumetrically with a glass syringe (Owen et al., 1979). Gas composition wasdetermined by gas chromatography.

Anaerobic treatment of textile size effluent 179

Three concentrations of each size component (Table I) and of the combined size solution were investigatedto give an indication of the degradability and toxicity at a range of concentrations. The concentrationsinvestigated were: (I) the concentration of the normal size solution used at the mill; (2)1.5 times the normalconcentration; and (3) half of the normal concentration. Since a synthetic size solution was investigated,dilution by washwaters etc. was not taken into account. Thus, the laboratory-scale tests could be viewed as aworst case scenario. The assay controls contained only sludge and the anaerobic medium. The function ofthe controls was to quantify the volume of gas produced due to the microbial degradation of residual organicmolecules in the inoculum. Gas production was corrected for by the controls.

Digester evaluation: The regional authorities were approached for information on digesters in their regions.The individual digesters were visited and physical and operating data were obtained (Sacks, 1997). Adetailed assessment was performed on the digesters at the Umbilo Sewage Purification Works. This includedinvestigation and calculation of the following parameters: organic load, volatile solids reduction, gasprodcution, biogas composition, pH and alkalinity, hydraulic retention time (HRT) and mixing efficiency. Atracer test was performed to assess the mixing and flow patterns within the reactors (Sacks, 1997).

RESULTS AND DISCUSSION

Biodegradation of the textile size: A brief description of the results is given; more detailed results arepresented in Sacks (1997). Table 2 summarises the degradation results for the normal concentration of eachsize component.

Table 2. Degradation results of the individual size components

Component Concentration Gas production rate COD reduction Methane content(gil) (mUd) (%) (%)

PYA 30.7 3.24 70 40

Starch 23.9 0.03 85 50

Plystran 16.1 44 78

CMC 13.6 7.04 55 24

OMS 2.9 7.18 64 22

Acrylic 0.73 0.65 45 40

Biocide 0.05 2.88 30 60

Size solution Normal 9.3 32 20

The polyvinyl alcohol (pYA) molecule contains acetate and acrylonitrile groups which prevent decay, thus itwas reasonable to expect PYA to be resistant to biodegradalion. Schluter (199 I) stated that PYA isbiodegradable, however, preliminary acclimation of the biomass is necessary. These tests suggested thatacclimation of the biomass, to the PYA molecule, was not required as gas production was measured after thefirst day of incubation. The chemical oxygen demand (COD) reduction was 70%. The gas production ratedecreased by around 30% with an increase in the PYA concentration which indicated that the PYA becameinhibitory at higher concentrations .

Biodegradation of the starch component was efficient with 85% reduction in COD. No significantconcentration effects were observed. The Plystran formulation (58% starch; 40% PYA; 2% wax) was notbiodegradable and an acclimation period of around 60 d was required before gas production was recorded inthe Plystran assays. The final COD reduction of only 44% was achieved. A concentration effect wasobserved with metabolism of the substrate decreasing with increased concentration .

Degradation of the carboxymethyl ceIlulose (CMC) was observed, however, it was not complete as therewas only a 55% reduction in COD. The methane content of the biogas was also low (24% w/w) suggestingincomplete degradation. Oxidised modified starch (OMS) was degraded in the assays, however, the methane

180 J. SACKSand C. A. BUCKLEY

content of the biogas was only 22% (w/w). The acrylic component became inhibitory atconcentrations> around 4 gil. However, even at concentrations < 4 gil, methanogenic conversion of theacrylic was not very efficient, with only a 45% reduction in COD and a biogas methane content of 40%(w/w). The acrylic component could be treated anaerobically, in low concentrations, with an acclimatedbiomass.

A biocide was added to the size solution to prevent microbial growth which could impair the efficacy of theyarn treatment and could also stain the material. The biocide was severely toxic to the biomass atconcentrations> 5 mg/l. The normal concentration of 0.5 mgll was labile, although gas production was low.This was thought to be due to the limited availability of the biocide due to the low concentration.

o HighA Normalo Low

•A

200I~

o

~------;.

100

600

see

::J'0I00.!"..'il 300::Ie:-200..~

100

0

0

TIme(d)

Figure 1. Plot of the cumulativegas productionfor the syntheticsize solution, in the BMP assay.

•o

100 ISO

•o

o HighA Normalo Low

TIme(d)

Figure2. Plot of the cumulativegas productionfor the syntheticsize solution,in the ATA assay.

From the results for the synthetic size solution (Figure I), it was concluded that the size solution could bedegraded by the microorganisms in the anaerobic digester sludge, however, caution would have to be takenas the efficiency of the process was not great with only around 20% (w/w) methane in the biogas and a 32%reduction in the COD. Caution would have to be taken to prevent overloading of the digester.

Toxicity: The results of the anaerobic toxicity assay (Figure 2) showed that the size solution was not toxic tothe anaerobic biomass. In these assays, a reduction in gas production, relative to the control, is indicative of

Anaerobic treatment oftextile size effluent \8\

inhibition by the added substrate. In the plot, the initial trend of the control gas production is shown to belower than that for the experimental samples, thus verifying the degradability of the size.

Investigation of the individual components identified those with an inherent inhibitory effect on theanaerobic biomass and the concentrations at which each component could be effectively degraded. The PVAwas found to become inhibitory at concentrations greater than 30 gil. Plystran was inhibitory atconcentrations> 10 gil, acrylic at concentrations> 4 gil and the biocide which became inhibitory atconcentrations> 0.5 mgll and toxic at a concentration of 50 mgll.

Acclimation tests: Tests were conducted to assess the biomass acclimation to the Plystran and biocidecomponents of the textile size solution. In these tests the acclimated sludge from the serum bottles,previously containing the Plystran and biocide solutions, was used to seed new serum bottles. The objectiveof the test was to determine whether there was a change in the degradation rate and in the length of the lagperiod with the acclimated sludge (Table 3).

Table 3. Comparison ofbiogas production rates between the unacclimated and acclimated Plystran andbiocide sludges

Unacclimated Acclimated

Size Concentration Rate Ratecomponent (mIld) (mIld)Plystran 16 gil 0.03

0.1

Biocide5mgll

0 0.06

These results showed an increase in the degradation rate with the acclimated sludge. The acclimated sludgecould be used as a seed inoculum to reduce the lag period during subsequent degradation.

Digester evaluation: A detailed evaluation of the anaerobic digesters at the Umbilo Sewage PurificationWorks verified the availability of digestion capacity. The hydraulic load to the entire works was 15% (v/v)below its design capacity and the flowrates to the anaerobic digesters were low which indicated availablehydraulic capacity. The organic load to the anaerobic digesters was low. The digesters were well mixed andheated to 36 ± 1°C, therefore they had the ability to accept an organic load of around 3 kg VS/m3.d (Ross etal., 1992). The digesters were only fed an average of 1.12 kg VS/m3.d; there was available organic capacity.The operation of the digesters was healthy and they could accept a greater load in the form of industrialeffiuents. The average HRT was calculated at around 17.29 d. A residence time distribution test (Sacks,1997) showed that there was no dead volume and that the mixing was efficient as the digester was found tobe comparable to an ideal completely stirredt~ reactor (CSTR).

CONCLUSIONS

There is the potential for treatment of agro-industrial wastewaters in available anaerobic digester capacity.The laboratory-scale tests facilitate the prediction of treatment of a particular wastewater, in a full-scaleanaerobic digester, with indication of the volumes and concentrations that could be treated effectively.

The serum bottle tests identified inhibitory components of the textile size effiuent. PVA became inhibitory atconcentrations greater than 30 gil. Plystran at concentrations> 10 gil, acrylic at concentrations> 4 gil andthe biocide at concentrations> 0.5 mgl!.

Although the synthetic size solution was degraded, and found not to be toxic to the anaerobic biomass, highconcentrations of the size solution could result in an organic overload of the digesters. The New Germanymill produced around 10 m3 size effiuent per day, with a measured COD of around 112,000 mg/l, If all10 m of the size effiuent were loaded into an anaerobic digester at the Umbilo Sewage Purification Works

182 J. SACKS and C. A. BUCKLEY

(1,340 nr'), the additional organic load would be 0.8 kg COD/m3.d. This load is relatively high and couldshock the microorganisms if they were not introduced to the substrate gradually. Treatment of this load ishowever feasible since in the serum bottle tests the equivalent load for the normal size solution was23.7 kg COD/m3 and 35.6 kg COD/m3 for the high size solution. Both of these concentrations weredegraded by the biomass. Thus, the textile size effluent has the potential for treatment by anaerobicdigestion.

REFERENCES

Barclay, S. J. (1996). The Regional Treatment ofTextile and Industrial Effluents. (Project No. 456) Water Research Commission.Barnett, J. (1995). Residence Time Methods for Modelling and Assessing the Performance of Water Treatment Processes.

MScEng. Thesis, University ofNata\.Department of Water Affairs (1986). Management ofthe Water Resources ofthe Republic ofSouth Africa. CTP Book Printers.

CapeTown.Owen, W. F., Stuckey, D. c., Healy Jr, J. B., Young, L. Y. and McCarty, P. L. (1979). Bioassay for monitoring biochemical

methane potential and anaerobic toxicity. Water Research 13,485-492.Ross, W. R., Novella, P. H., Pitt, A. J., Lund, P., Thomson, B. A. and King, P. B. (1992). Anaerobic Digestion of Waste-Water

Sludge: Operating Guide (Project No. 390). Water Research Commission. Pretoria, South Africa.Sacks, J. (1997). Anaerobic Digestion of High-Strength or Toxic Organic Effiuents. MScEng Thesis, University ofNatal.Schluter, K. (1991). Ecological assessment of sizes. Henkel Referate 17,127-132.Stander, G. 1. (1997). Our water supply and water environment pollution problems. Chemical Processing SA 4(4), 13-14.Water Research Commission (1983). A Guide for the Planning, Design and Implementation of Wastewater Treatment Plants in

the Textile Industry. Pretoria, South Africa.