~ Pergamon War. S CI. Tech. Vo/. 39, No. 10-11 , pp. 347-3SI, 1999e 1999

Publishedby ElsevierScienceLtdon behalfof the IAWQPrintedin GreatBritain. All rightsreserved

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AN ASSESSMENT OF THE FEASIBILITYOF ANAEROBIC DIGESTION AS ATREATMENT METHOD FOR HIGHSTRENGTH OR TOXIC ORGANICEFFLUENTS

J. Sacks*, C. A. Buckley**, E. Senior** and H. Kasan***

• Pol/utionResearch Group, DepartmentofChemicalEngineering, University ofNatal,Durban, 4041. SouthAfrica•• DepartmentofMicrobiology and Plant Pathology, University ofNatal,Pietermaritzburg, SouthAfrica••• DepartmentofBiotechnology, Technikon Natal, SouthAfrica

ABSTRACT

The anaerobic digestion process converts organic materials into ,a methane-rich biogas. The KwaZulu-Natalregion has the potential to attract a significant amount of industry. The objective of this research was toassess the feasibility of using anaerobic digestion as a treatment method for high-strength or toxic organiceffluents, A strategy was developed to evaluate the degradabil ity and toxicity of effluents and, ultimately,predict the efficiency of treatment in a full-scale digester. This paper details the strategy and investigates thedegradation potential of a textile size effluent (COD ca. 140,000 mgll). The ultimate degradability of theeffluent was determined as well as the concentrations and volumes, which could be treated effectively. Theinhibitory components of the size effluent were found to be Plystran (10 mgll) and the biocide (5 mgll).Anaerobic digestion was found to be feasible, on a laboratory-scale . Thcse results are being applied for scale­up. to full-scale implcmentation in an existing anaerobic digester. 10 1999 Published by Elsevier Science Ltdon behalfof the lAWQ. All rights reserved

KEYWORDS

Anaerobic digestion; biogas; degradability; Kwazulu-Natal; textile size; toxicity; tracer tests.

INTRODUCTION

Anaerobic digestion is a process by which a wide variety of organic materials can be converted into a gasrich in methane, In view of the current problems, both in the protection of the environment and the searchfor sources of renewable energy, anaerobic digestion appears to be a favourable biotechnological process todispose ofan organic waste through bioconversion into energy.

The Kwazulu-Natal region has the potent ial to attract a significant amount of industry and, due to itsabundance ofwater, relative to the rest of the country, it is probable that a number of these industries will befrom the agro-industrial sector. These industries usually discharge high-strength organic effluents, Tracertests on a number ofanaerobic digesters in the Kwazulu-Natal region have indicated that the average mixing

347

348 J. SACKS et al.

volume is 50% of the actual volume. During these investigations it was also found that there are a number ofsewage works, in the region, with under-utilised anaerobic digestion facilities.

The basic objective of this research was to identify available capacity in existing anaerobic digesters, as wellas high-strength or toxic organic effluents which could be effectively treated by anaerobic digestion. in thespare capacity. In order to achieve this, a strategy was developed. The overall strategy follows twoconcurrent pathways. The one investigates the effluent and, the other, the digester.

EfJIuent: the first step was to identify an appropriate effluent and examine options for waste minimisation,with the aim to concentrate and segregate the high-strength stream. A laboratory-scale test protocol wasdeveloped to assess the potential degradability of the effluent and any inherent toxicity that it may cause tothe anaerobic biomass. Enrichments should be made for the potentially inhibitory compounds; the biomassis exposed to small but increasing concentrations of the molecule, over a period of time. Themicroorganisms tend to acclimate to the compound and there is normally selection for the organisms thatcan metabolise the compound. This culture can then be used to seed digesters, thus preventing digesterfailure and reducing the lag period. It is important to identify intermediates and degradation products, asthese can be inhibitory to the biomass. Finally, it is important to determine the degradation kinetics,including the degradation rate and the maximum biodegradability that can be attained.

Digester: a locally available anaerobic digester was identified and a tracer test performed to assess itsmixing efficiency. Other important considerations regarding the digester include mass balances and kineticparameters. The pathways then merge; knowledge of the operational parameters of the anaerobic digesterand the biodegradation kinetics of the effluent should enable the prediction of whether the effiuent could bedegraded anaerobically, on a large scale. The reduction in chemical oxygen demand (COD) is an importantparameter as it indicates the extent of organic degradation. The laboratory-scale tests facilitate knowledge ofvolumes and concentrations of an effluent that could be effectively treated. This could then be applied toscale-up the digesters to a pilot-size plant, and ultimately full-scale treatment in an existing digester.

The strategy was applied to determine the anaerobic degradability of a concentrated, segregated textile sizeeffluent, from a textile mill located within 10 km of the Umbilo Sewage Purification Works.

METHODS

EfJIuent: A laboratory-scale test protocol was developed to determine the anaerobic biodegradability of aneffluent, as well as the potential toxicity to the anaerobic biomass. The assays were based on the method ofOwen et al. (1979).

The laboratory-scale batch tests were made with anaerobic sludge collected from the Umbilo SewagePurification Works (Carliell et al., 1996). Serum bottles (125 ml) were overgassed with oxygen-freenitrogen. Biomass (30 ml) and nutrient medium (30 ml) were added to each bottle.

Table 1. Composition of the synthetic size effluent

Component

Polyvinyl alcohol (PVA)Acrylic (as 25 % solids)Carboxymethyl cellulose (CMC)StarchPlystranOxidised modified starch (OMS)Biocide

Measured COD(mgll)537001900

1540013 100192004900

13 300

Amount in synthetic effluent(gil)

30.70.7313.623.916.12.9

0.05

Anaerobic digestion as a treatment for organic effluents 349

The wastewater investigated was the size effluent produced by the textile mill. The biodegradability of eachsize component was determined . The effluent was synthesised in the laboratory, according to the mass ofeach component utilised by the mill (Table I).

Substrate (40 ml) was added to each bottle. Three concentrations were investigated for each constituent,namely, the concentration of the typical factory effluent (Table I), one half of this concentration, and oneand a half times this concentration. Duplicates of each concentration were made, for reproducibility. Thebottles were incubated in a constant temperature room (37°C) for 120 d. The assay bottles were shakenmanually once a day.

Gas volume measurement and removal, during incubation, was performed with a graduated glass syringe(20 ml) fitted with a 22-gauge disposable needle. Readings were taken at the incubation temperature and thesyringe was held horizontal for measurement. Volume determinations were made by allowing the syringeplunger to move and equilibrate between the bottle and atmospheric pressures (Owen et aI., 1979). Gas waswasted as necessary to prevent gas leakage due to excessive pressure. Whenever gas was wasted, themethane, carbon dioxide and nitrogen contents were determined, by gas chromatography (ChrompakCP9000). A stainless steel column (poropak N, 2 m by 3 rom, 80 to 100 mesh) was used for the separation,with a thermal conductivity detector (TCD).

Digester: The physical and operating data were obtained for the digesters at the Umbilo Sewage PurificationWorks . From these, performance criteria and operating efficiencies were calculated. Basic digesterperformance efficiency was evaluated by doing chemical oxygen demand (COD) balances over thedigesters.

The mixing efficiency was assessed by a tracer test from which the residence time distribution (RTD) wasdetermined. In an RTD test, an inert salt is added to the inlet of the reactor and it is assumed that the tracer isrepresentative of the fluid. The outlet is monitored and the resulting concentration versus time profileprovides a representation of the flow characteristics of the reactor (Barnett, 1995; Grobicki, 1989).Regression was used to determine the configuration of ideal flow components that would result in a similarresidence time distribution.

RESULTS AND DISCUSSION

Since several anaerobic digesters, in KwaZulu-Natal, are not in operation, or being under-utilised, theycould be used to treat high-strength organic effluents. A longer retention time than the conventional 30 d, formunicipal waste, may be required, to reduce the COD sufficiently.

Sizes represent the main component (ca. 60 %) of the COD load of textile effluents (Schluter, 1991). TheCOD of the size effluent sampled at the mill was analysed (APHA, 1989) and found to be 137662 mgll. TheCOD of the synthetic size effluent was 138 500 mgll. The mill produced a total of 110m3 of effluent perday; 100 m3 constituted dye effluent and 10m3 size effluent. The two effluent types were segregated at themill, thus it was feasible to investigate anaerobic digestion of only the size effluent. Based on the quantitiesofeach size component utilised by the mill, it was calculated that the size effluent contained 88 g of size perlitre.

The purpose of the biocide was to prevent bacterial or fungal growth in the size or on the woven cloth. Itwas expected that the biocide would prove problematic in the microbial treatment of the effluent. Thedisinfectant that was included in the size was a water-soluble compound, Dodigen 2451 (Hoechst), whichwas composed ofalkyl dimethylbenzyl ammonium chloride.

Gas production is a measure of anaerobic metabolism and, ultimately, organic degradation. The sizecomponents that showed the greatest degrees ofmicrobial activity are illustrated in Figure 1.

120

Starch

1008060

Time (d)

~ ---_--~Slze

J. SACKS et al.

4020

~c::~~.........................--+----+---CMC

350

400

:::i' 300

£~"'0 200e0.

~c100

0

Figure I. Cumulative gas volume s for the starch,carboxymethyl cellulose and the syntheti c size solution.

There was a ca. 3 d lag period before gas was produced in the synthetic size solution. This was due to theinhibitory components. Gas production stabilised after ca. 20 d, suggesting that all of the degradablesubstrate was utilised within the first 20 d of digestion.

120

Acrylic

Plystran

10080

~----,o--{OMS

60

_ _ .....-&--....----"---PVA

4020

100

80

:::i'£'0!l"'0e"-~c

-200

Time (d)

Figure 2. Cumulat ive gas volumes for the individual sizecomponents.

These results show that Plystran was the most inhibitory compound to the anaerobic biomass . Plystran is acommercial blend consisting of ca. 58 % (w/v) modified potato starch, 40 % (w/v) PVA and 2 % (w/v) wax.Wax degradation was not investigated. Figure 2 shows the PVA to be slightly inhibitory. It is believed thatthe Plystran formulation also contained a biocide resulting in the inhibitory effect on the anaerobic biomass.The biocide was found to be toxic to the biomass at a concentration of ca. 5 mg/l, and Plystran at 10 gil.

The composition of the biogas was analysed to determine the CO2 : CH4 ratio; which is indicative of theefficiency of the anaerob ic process . The ratio varied for the different components, but for the synthet ic sizesolution , it stabilised with a methane content or ca 40%. The COD reduction of the synthetic size was 89%.

The results showed that the anaerobic microorgan isms were active, suggesting degradation of the organiccompounds . Thus, the laboratory-scale test protocol was successful. It is necessary that these tests be scaledup in order to determine the effect of the effluent on the biomass, on a larger scale.

Anaerobic digestion as a treatment for organic effluents

CONCLUSIONS

351

The textile size solution was found to be degradable on a laboratory-scale. The test must be scaled up to alarger reactor, ca. 20 I, and finally to full-scale. The preliminary tests are critical to prevent digester failureand to assess the efficiency of the process for the treatment of a particular effluent. Laboratory-scale testsalso provide the opportunity to identify areas of inefficiency in the digester, thus facilitating better digesterdesign to optimise treatment.

REFERENCES

American Public Health Association (1989). Standardmethodsfor the examination ofwaterand wastewater. Washington.Barnett, J. L. (1995). Residence time methodsfor modelling and assessingthe performance of water treatmentprocesses. MSc

Eng Thesis. University ofNatal, Durban.Carliell, C., Barclay, S. J. and Buckley, C. A. (1996). Treatment of exhausted reactive dyebath effluent using anaerobic digestion:

Laboratory and full-scale trials. Water SA 22(3), 225-235.Grobicki, A. (1989). Hydrodynamic characteristics and performance of the anaerobic baffled reactor. Ph.D. Thesis, Imperial

College, University of London.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.Schluter, K. (1991). Ecological assessment ofsizes. HenkelReferate27,127-132.