An extensive experimental investigation has been carried out i) to isolate and identify a potential microbial strain from soil which can biodegrade pesticides, typically present in wastewater from the sources of industrial and agricultural related activities, ii) to study the growth potential of microbial isolate in the presence of Malathion, Methamadiophos, Cartap and Cypermethrin iii) to assess the performance of microbial isolate for Cypermethrin degradation in wastewater using pilot 8.ctivated sludge process (biosimulator) iii) to study the effects of different parameters (pH, temperature, dissolved oxygen) on the biodegradability of the potential strain for Cypermethrin iv) further statistical assessment of the performance efficiency of the system with he help of principal component analysis and regression models.
During the investigation, two sets of experiments were conducted. In the first set of experiments, attempts were made to isolate soil borne strains, potential candidates; capable of biodegrading pesticides. In the second set of experiments, the effects of parameters on the biodegradability of the isolated strains for the Cypermethrin, were investigated. Experiments were designed and performed in such a way as to study the effects of parameters like Cypermethrin concentrations, retention time, temperature, pH, mechanism of aeration, and t!1e amount of Dissolved Oxygen (DO), on the degree of biodegradation of Cypermethrin (in terms of the removal of COD and Cypermethrin ).
Following were the ranges of the above mentioned parameters investigated during the experiments. Concentration of Cypermethrin: 40-125 mg/l Retention Time: 24,48 and 72 hours Temperatures: ambient temperature (l8-25°C), 28-30°C, and 38-40°C pH: 7.3-8.8 Mechanism of aeration: diffused and mechanical aeration at 180 al1d 250 rpm Amount of dissolved oxygen: 5-12 mg/L
For growth kinetic studies, a number of batch experiments were carried out in conical flasks (without shaking) in the presence of different concentration of pesticides such as: Malathion, Methamadiophos, Cartap and Cypermethrin. In order to perfom1 the parametric; sensitivity study, as mentioned above, biodegradation experiments were perfon:1ed on small scale in 250 ml flask, using shaking water bath, and on large scale in “Biosimulator”(borosilicate glass of 20 L capacity). In the Biosimulator, the operating conditions of a typical “Activated Sludge” process was simulated.
The data, obtained from parametric sensitivity experiments, are employed to develop "Regression Models" in order to study the effects of parameters, statistically, on the removal of COD and Cypermethrin.
Cypermethrin pesticide was selected for the detailed investigation because its low aqueous solubility, such toxic compounds are "cry difficult to be removed from the environmental systems by conventional means. Information obtained from this study may be helpful in designing and operating an efficient pollution control treatment system. A particular problem, which is being faced by the environmental engineers, is the difficulty in predicting the performance of these systems with respect to high load of individual organic compounds. This has led to difficulty in predicting effluent concentrations by traditional models. An effective pesticide waste treatment technology is therefore needed 10 prevent water pollution and to comply with increasing regulatory pressures.
Following are the main findings from the present extensive investigation:-
1. Using the enrichment technique, a soil borne strain Fseudomollas designated as IES.Ps-l was isolated some time back at the Institute of Environmental Studies, University of Karachi. In the present study IES-Ps-l was allowed TO adap1 in the presence of Cypermethrin. Adapted IES-Ps-l strain was identified by microscopic examination and confirmed by biochemical tests.
2. During growth studies, IES-Ps-1 strain was found to grow in a wide range of concentrations of Malathion (35-220 mg/L), Methamidophos (80-320 mg/L), Cartap (60- 120 mg/L) and Cypermethrin (40-180 mg/L). However, the optimum concentration of each pesticide which support the growth of IES-Ps-1 during 24 hours incubation period (experiments without shaking) was 120 mg/L of Malathion, 160 mg/L of Methamidophos, 80 mg/L of Cartap and 60 mg/L of Cypermethrin.
3. When compared with the control test (without pesticide), a significant increase in bacterial population was observed at low concentration of each pesticide, however at high concentration lag phase markedly increased but no zone of inhibition ob served.
4. Tolerance to much higher concentration of Cypermethrin was obtained through enrichment. Even at 180 mg/L of Cypermethrin dose, the toxic effect did not appear, but the growth rate significantly decreased.
5. Experiments, carried out on shaking water both, showed about 22% biodegradation of Cypermethrin at 40 mg/L initial concentration with 24 hour retention time. In contrast negligible degradation (about 5%) was observed in uninnoculated sample of wastewater. However, in the Biosimulator, the performance efficiency of. IES-Ps-l significantly improved and the disappearance of Cypermethrin at 24 and 48 hours was noted to be 48 % and 80 % respectively.
6. Under ambient temperature (18-25°C), biodegradation of Cypermethrin was studied for approximately 48 hours using d0sage of 20 , 40, 80 and 125 mg/L. Results showed a complete biodegradation of Cypermethrin at 20 mg/l dosage. However, at other dosages it was 82%, 50% and 17% respectively. These findings indicate that increased concentration of Cypermethrin has a marked effect on the rate of degradation.
7. The optimum temperature for Cypermethrin degradation by IES-Ps-1 was found in the range of 28-30°C. Even at higi1 concentration of Cypermethrin (80 mg/L) the removal efficiency was between 78-88% after 48 hours of treatment. This is considered to be satisfactory especially with respect to high organic load and retention time. In contrast, at ambient temperature (l8-25°C) and at 38 + I DC, using similar concentration of Cypermethrin, the removal rate significantly decreased to 48-51 %.
8. During biodegradation, the COD removal was found to be proportional to the disappearance of Cypermethrin. The corresponding decreased in COD values provided an evidence of Cypermethrin removal from the system.
9. In biosimulator experiments, it was observed that the IES-Ps-I retained their biodegradation capability at a wide range of pH (pH 7.3 - pH 8.8). Therefore, the alkaline pH which was achieved during treatment need no further adjustment of pH.
10. Dissolved oxygen have been found to be an important rate limiting factor for Cypermethrin degradation. At Optimum temperature using 8-9mg/L DO concentration, 78% degradation of Cypermethrin (80 mg/L) was achieved, where as at 5-6 mg/L DO only 38% degradation occur. When the concentration of DO further increased to 12 mg/l no significant difference observed in the rate of biodegradation when compared with 9 mg/L DO. It is therefore recommended that 9 mg/L DO would be sufficient for effective biodegradation of Cypermethrin. Increasing DO concentration would mean wastage of resource and material. thus making the system uneconomical.
11. The multiple regression models proposed for COD and Cypermethrin removal was statistically significant. Statistical model based on COD (INOUT) and Cypem1etl1rin (IN-OUT) removal demonstrates that enhancement of biodegradation is primarily a function of DO concentration and time of exposure of organisms (IES-Ps-I) in the reactor. Moreover, the model further demonstrated that ,he time for treatment of Cypermethrin even at high concentration (80 mg/L) could effectively be reduced by maintaining optimum operating temperature (28-30°C) and DO concentration (8-9 mg/L) in biosimulator.
12. In addition to the above mentioned factors, required for the treatment of least water soluble compounds like that of Cypermethrin, a well-mixed aeration system is required to support growth of IES-Ps-1 and to ;promote maximum contact of bacteria with the compound they used as , carbon and energy source. Therefore the mechanical aeration system at 250 rpm was found to be effective for maintaining high dissolved oxygen level as well as to keep Cypermethrin in suspension.
From the results of the research study, it can be concluded that biodegradation rates :significantly depend on Cypermethrin concentration as well as operating condition like temperature, DO, rate of agitation /aeration. Moreover, the models obtained from the results would be a valuable addition in the improvement of the designing and operation of biological treatment system used for the: biodegradation of agricultural pesticide wastes. This finding may be the first instance in which high concentration of Cypermethrin detoxification has been achieved in short retention time of 48 hours using aerobic biological treatment system. Such approach of using potential organisms in the treatment of pesticide wastes would be more preferable to current methods, such as those of incineration and disposal to land.