Fighting against POLLUTION to Save Environment
Fighting against POLLUTION to Save Environment
J. Microb. World: 2 (1) pp.49-58, 2000.
D.B. Boralkar1 and Vinita M. Dhupkar2
1. Central Pollution Control Board, Parivesh Bhawan, East Arjun Nagar, Delhi - 110 032.
2. Central Laboratory, Maharashtra Pollution Control Board CIDCO Bhawan, Belapur CBD, New Mumbai-400614
Biodegradation is "the destruction of chemical compounds by the biological action of living organisms". It can be divided into primary and ultimate biodegradation. Primary degradation is the maximum extent of degradation needed to change the identity of the compound. Ultimate biodegradation or mineralization involves the complete conversion of a compound to CO2, H2O, and other inorganic compounds. OECD-301 B (modified sturm test) is based on the ultimate biodegradation of surfactant.
Biodegradability can be affected in the presence of other compounds. It was found that there is retardation of biodegradation of LAS by sublethal concentrations of mercuric chloride. Microbial organisms adapted in activated sludge can degrade phenol and can utilize phenol as a sole source of carbon for their metabolism. However, degradation is affected by the presence of phenol above 360 mg/l. This is substrate inhibition.
A variety of tests are used to assess the biodegradability of surfactants. They include screening tests to indicate ready biodegradability, tests of inherent biodegradation, and simulation tests to assess removal by waste treatment processes.
Tests can be conducted using radiolabelled surfactants as well. There are other tests, like BOD measurement, CO2 evolution, shake culture tests, and enrichment cultures.
OECD divides biodegradation tests into three types: screening, inherent, and simulation.
Screening tests generally employ a simple aqueous medium containing mineral salts and a small number of unacclimatised microorganisms to which the test compound is added. The purpose of screening tests is to provide unequivocal evidence that the test compound will biodegrade in the environment.
Inherent tests employ a higher concentration of microorganisms and may last for several months. Compounds passing the test for inherent biodegradability are also tested using simulation methods, such as continuous activated sludge, trickling filter systems, etc., to assess their behaviour during wastewater treatment.
Degree of biodegradation for a specific compound varies with the test method, and the chemical, physical, and biological characteristics of the compound must be considered when choosing the test.
Can be employed to estimate the extent of biodegradation in an isolated system of known composition. Interpretation of the results is rather difficult, due to the complexity of the metabolic processes involved, the variety of metabolic by-products formed, and the multiple pathways of possible oxygen utilisation by the microorganisms.
Used to assess ultimate biodegradation. This method is not quantitative because the CO2 evolved usually falls short of 100% of the theoretical value. Some of the carbon is converted to new cells, i.e., used for biomass production. Some carbon is converted to CO2 and a third possible short-term fate of substrate carbon is the formation of intermediate metabolites, which may accumulate in the test medium.
Therefore, OECD (1981) concluded that results of at least 60% of the theoretical oxygen demand or of theoretical CO2 production indicates that the chemical should be classified as readily biodegradable and is hence readily removed in the environment.
Aerobic microorganisms, utilizing an organic substrate as carbon and energy source, convert the molecules into new cells, CO2, and wastes. By measuring the amount of CO2 provided and comparing the quantity with the theoretical yield, calculated from the substrate carbon content, a measure of ultimate biodegradability can be made. The test duration is limited to 28 days, and it has been found that materials degrading extensively in this period are readily degraded. Hence, a result regarded as positive by this test procedure indicates that the substance tested is readily and ultimately biodegradable.
Assembly is prepared as shown in Fig. (2). First four flasks are to get the CO2-free air passed through four 4-litre flasks. Flasks A and B are for test material. Flask C is standard sodium acetate (220 ppm) for checking the performance of test as "control". Sodium acetate should give 100% biodegradation at the end of 28 days. Flask D is inoculum blank - Inoculum used is the activated sludge (1%). The inoculum should normally contain 10^6 to 20 x 10^6 colony forming units per ml.
Bubbles of CO2-free air are passed through the solution at the rate of 50-100 ml/min per flask, i.e., approximately 1-2 bubbles per second.
Carbon dioxide produced in the flask is then absorbed in 0.025 N Barium hydroxide, and precipitated as barium carbonate. The amount of CO2 produced is determined by titrating the unreacted barium hydroxide with standard 0.05 N HCl. Periodically, every 2-3 days, the two absorbers (4-1 flasks) are removed and filled with 100 ml fresh barium hydroxide solution placed at the far end of series.
The test is run at 25 ± 2°C temperature.