Fungal Biodegradation of Dyehouse Effluent and Kinetic Modeling

Abstract
Dyehouse effluent is tough to be degraded because of the presence of dyes, which have complex aromatic molecular structure and synthetic origin. Because the color in dyehouse effluent is highly visible and affects aesthetics, water transparency and gas solubility in water, and especially because many dyes are made from known carcinogens like benzidine, dye effluent treatment becomes inevitable. In this research study, a novel fungus, paecilomyces variotii has been tried degrade the dye house effluent. The effects of environmental factor, namely the concentration of effluent on the % COD removal was determined. A kinetic model was established giving the dependence of COD removal rate on effluent COD (first order) and Biomass concentration (first order). The specific COD removal rate was calculated as

  at 32°C.

Key words: dye house effluent, paecilomyces variotii

Contents

• Introduction
• Materials and Methods
  • Dyehouse Effluent
  • Analytical Methods
  • Influence of Effluent Concentration
• Results and Discussion
  • Influence of Initial Concentration of the Effluent on the COD Removal
  • Kinetic Studies of COD Removal
• Conclusion
• Figures
  • Acknowledgement
  • References

▲ Introduction

Large amounts of chemically different dyes are employed for various industrial applications including textile dyeing. Among the total dyestuff consumption, textile industry accounts for 67% of the total dyestuff market. Azo dyes constitute the largest and most diverse group of dyes used in commercial applications. Azo dyes are a group of compounds characterized by the presence of one or more azo bonds (- N = N -) along with one or more aromatic systems. In spite of their low toxic effect on receiving bodies, these dyes create an aesthetic problem and its colour discourages the downstream use of wastewater.

Colour in the first contaminant to be recognized in the dyeing effluents and has to be removed before discharging into the water stream. Aesthetic merit, gas solubility and water transparencies are affected by the presence of dyes even in small amount. The removal of colour from wastewater is relatively more important than the removal of soluble colourless organic substances, which usually contribute the major fraction of biochemical oxygen demand.

Due to the low biodegradability of dyes, conventional biological treatment processes are inefficient in treating dye wastewaters. In addition, numerous physical & chemical techniques such as flocculation combined with flotation, electro-flotation, flocculation with Fe(II), Ca(OH)₂, membrane filtration, precipitation, ion-exchange, ozonation and katox treatment method involving the usage of activated carbon and air mixtures were also used. Even though some of the above mentioned methods are effective, most of them suffer from the short comings such as excess usage of chemicals, sludge disposal, expensive operating cost, ineffective colour reduction for sulfonated azo dyes and poor sensitivity towards shock load conditions. Biological decolorisation is employed under either aerobic or anaerobic environment. A number of reports discourage the azo dye decolorisation by microorganism under anaerobic conditions as it leads to the formation of corresponding aromatic amines. Even though their reductive cleavage is responsible for colour removal the formation of aromatic amines is highly undesired as they are reported to be carcinogenic. In the presence of oxygen, aromatic amines can be degraded. Degradation of azo dyes has been studied under aerobic conditions using both pure & mixed microbial cultures.

The restrictive environmental legislation, the ecological problem and the high cost of conventional technologies for dye house effluent treatment have resulted in the search of economically viable and technologically suitable wastewater treatment plants. Dye house effluents usually contain azodyes which are highly resistant to biological treatment and these dyes are considered to be recalcitrant xenobiotic compounds because of the presence of N=N bonds and other possible groups like sulphonic group which are tough to be degraded. It was reported that some anaerobic bacteria can biodegrade dyestuffs by azoreductase activity. However the effluent from biodegradation of dyestuffs could be toxic (Kapdan, I.K., Kargi,. F, 2002). Also, reverse colorisation may take place when the degradation products are exposed to oxygen. (Knapp, J.S., Newby, P.S., 1995).

Because of these above-mentioned problems, full-scale application of bacterial degradation is limited. Also, only a few research works have been reported on aerobic degradation of azodyes (Govindaswami, M., et al, 1995). Therefore, a need exists to develop a novel treatment technology for textile dye effluent treatment to ensure environmental protection from these harmful pollutants. Many of the studies with aqueous or synthetic dye solution using bacterial and fungal strains are of very little help in the development of biological Process for treatment of textile dye effluent as there were carried out under conditions which does not resemble an Industrial environment, for example using only a single non commercial synthetic dye. The principal objectives of our study are:

1. To study the feasibility of biodegradation of the dye house effluent using a novel fungus, paecilomyces variotii, under aerobic environment.
2. To study the kinetic pattern of effluent degradation and to evaluate the kinetic parameters.

▲ Materials and Methods

Pure fungal strain Paecilomyces variotii (MTCC No: 1387) was purchased from Microbial type culture collections and gene Bank, Chandigarh, India. The strain was maintained in Potato Dextrose - Agar medium consisting of 20g dextrose, 20g agar and 200g of potatoes in 1 liter of water. The cultures were grown in a controlled environment using orbital shaker, at 250rpm and the working temperature was 30°C. To ascertain the maximum potential for decolorisation, the fungus was grown in Czapek-dox medium consisting of 2g sodium nitrate, 1g dipotassium hydrogen phosphate, 0.5g magnesium sulphate, 0.5g potassium chloride, 0.01g ferrous sulphate, 30g sucrose in 1 lit of distilled water.

▲ Dyehouse Effluent

The dyehouse effluent was collected from a dyeing unit situated in Tirupur region (Tamilnadu, India). The characteristics of the dyehouse effluent were chemical oxygen demand (COD) - 3600mgl^-1, Biochemical Oxygen Demand (BOD) - 1220mgl^-1 and pH - 10.1 ± 0.1 [APHA, 1989]. The colour of the effluent was dark red due to the presence of dyes. The effluent was refrigerated at 4°C and used without any preliminary treatment.

▲ Analytical Methods

Samples were withdrawn from the conical flasks and centrifuged at 13,000rpm for 5minutes to separate mycelia from the media. With the clear supernatant obtained, Chemical Oxygen Demand (COD) was evaluated (APHA, 1989).

Similar procedures were also followed to evaluate the degradation potential of strains incubated for specified period of time. The initial COD removal data were used to find the microbial activity based on per mg/l of COD removed per gram of dry cell mass per unit time. The dry cell mass in suspension was determined after dehydration at 80° C to constant weight. Control experiments were conducted in parallel with uninoculated flasks.

The fungi Paecilomyces variotii was added in predetermined proportion to the effluent along with the growth medium. In order to find the maximum COD removal percentage of the dyehouse effluent by the fungal strain, a test solution containing 0.81g dry cell mass (DCM) per liter and the proportionate quantities of growth medium constituents, was incubated under shaking condition at 32°C .The time courses of percentage COD reduction and effluent COD removal were observed and plotted.

▲ Influence of Effluent Concentration

To find the optimum effluent concentration at which maximum COD removal occur, various dilutions of the effluent were made such that the concentrations were 20%, 40%, 60%, 80% and 100%. In the 125ml Erlenmeyer flasks, 81mg of dry cell mass were added per 100ml of effluent and the growth medium was also added proportionately. The inoculated flasks were incubated in an orbital shaker operating at 100rpm.

The samples were withdrawn at regular intervals of time and centrifuged at 500rpm. The COD removal percentages were calculated from the supernatant solutions obtained.

▲ Results and Discussion

▲ Influence of Initial Concentration of the Effluent on the COD Removal

The influence of initial concentration of the effluent on the COD removal performance of Paecilomyces variotii was clearly shown in Figure 1. The fungi removed 89.7% of the COD in 24h of incubation period when the concentration of the effluent was 20%. Even though, it degraded the other concentrations of effluent (ie. 40%, 60%, 80% and 100% respectively) by percentages greater than 80, the equilibrium time required to remove the COD, varied. As the concentration of effluent increased, the time consumed to reach maximum COD removal increased. This fungus removed only 82.6% of the COD with undiluted effluent at the end of an incubation period of 48h. Increased incubation time for achieving the highest COD removal efficiency with undiluted industrial effluent may be due to the high biodegradability resistance of the dyes and high concentrations of other process chemicals.

From this study, we conclude that the fungi, Paecilomyces variotii can achieve high COD removal efficiency in a shorter duration (i.e. less than 24h) only for diluted effluents like 20% and 40% effluent concentration. It required prolonged incubation period upto 48h to achieve better COD removal with the actual dye house effluent.

▲ Kinetic Studies of COD Removal

To study the kinetic pattern of substrate conversion by enzymes or live biomass under isothermal conditions, a Michaelis- Menten model type equation (equation-1) has been widely applied

The two model parameters (V_m and K_m) in the nonlinear rate expression includes enzyme activity (E₀) and specific rates (k₁, k_-1 and k₂) and are temperature dependent (Yu et al, 2001).

... (1)

The below mentioned equation-2 gives the convenience on analysis of effect of temperature

... (2)

where t is the incubation time(h), M is the biomass concentration (mg dry cellmass l^{- 1}), m is the partial reaction order with respect to biomass, S is the COD value of the effluent (mgl^-1) and n is its partial reaction order. Here, k is the specific COD removal rate with units (mg ^(1-n) l^(m+n-1)/mgDCM^mh). It was reported that equation (2) can be considered as a modified model of Michaelis Menten rate equation (equation-1), when the parameter K_m is not included. The relationship between the COD of the effluent and incubation time depending on the partial reaction order (n) is given by

when (n≠1) ... (3)

and

when (n≡1) ... (4)

When the plot between incubation time and the COD value was made, a high degree of linearity was observed (R²>0.99) and it was shown in Figure 2. Hence the order of the reaction with respect to effluent COD value was found to be 1.0

... (5)

For this set of experiments, used to establish a relationship between biomass concentration and effluent COD removal rate , the incubation time was fixed as 24h. A linear relationship (R² >0.90) was obtained between biomass concentration M and kM^m that indicates m equals to unity. Therefore, the COD removal was a first order reaction with respect to biomass concentration (M)

The kinetic model of COD removal by the fungal strain Paecilomyces variotii was given by

... (6)

S = S₀ exp ( − kMt) ... (7)

The value of specific COD removal rate, k, obtained from the slope of the line in Figure 2 was

  at 32°C.

▲ Conclusion

1. Effective degradation of dye house effluent was achieved by the fungi paecilomyces variotii in an aerobic environment.
2. The kinetic pattern of the dye effluent degradation was studied with respect to the biomass concentration and COD of the effluent.
3. The order of the bio reaction was found to be one with respect to each of the controlling factors, the biomass and effluent COD.
4. The specific COD removal rate was evaluated as
     at 32°C.

▲ Figures

• Figure 1 - Effect of Initial Concentration of the Effluent on % COD Removal
• Figure 2 - Data Fitting and Estimation of Parameters in the Kinetic Model
• Figure 3 - Effect of Biomass Concentration on % COD Removal

Acknowledgement

The authors acknowledge the financial support by the Annamalai University, Annamalainagar. Also the authors acknowledge the technical support offered by the Department of Microbiology, Faculty of Agriculture, Annamalai University.

References

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