MONITORING ORGANIC MATTER IN OUR ENVIRONMENT

Human existence on earth is dependent upon the air we breathe, the water in our lakes, rivers and oceans and the quality of our soil. Organic compounds are the chemical substances from which all living organisms are made. They are present in the food we eat, the clothes we wear and the fuels we use. If organic material is not poisonous, why then does it pose a problem for the environment? The answer is found in its decomposition. When organic material decomposes in an environment where oxygen is available, an aerobic destruction process metabolises the organic material into basic fragments, consisting mainly of water and carbon dioxide. Where oxygen is not available, an anaerobic destruction process will release volatile methane gas instead of carbon dioxide. When organic decomposition occurs in water, oxygen is drawn from the water to support the aerobic breakdown. Oxygen is a limited resource in water. If an extra load of organic material is mixed into the natural water environment, some, and sometimes all, of the oxygen in the environment is consumed during the decomposition process of the organic material.

Oxygen depletion of our natural waters leads to an ecological imbalance. In severe cases, the environmental damage can be permanent, resulting in dead water without fish, plankton, algae, or other forms of aquatic life.

Oxygen depletion can occur when sewage outlets or effluents from any modern industry are let out into a river or the sea with surplus organic material. Industries, which present particularly high risk to the environment, would include the food processing, pulp & paper processing, pharmachemical, petrochemical, and textile & dyeing industries. Proper monitoring and control of all outlets of organic matter to our natural waters will help to maintain environmental balance.

METHODS USED FOR MEASURING ORGANIC POLLUTION IN WATER

The first method for measuring organic material in water, Biological Oxygen Demand (BOD), was developed early in the 20th century. BOD measures the rate of uptake of oxygen by microorganisms in a sample of water at a fixed temperature and over a given period of time, usually five days. The test is used to infer the general quality of the water and its degree of pollution. BOD is not an accurate quantitative test and should only be considered as providing an indicator of the quality of a water body.

Chemical Oxygen Demand (COD) is commonly used to indirectly measure the amount of organic compounds in water. By adding chemicals to the sample, the analysis period is shortened to 3 hours. Most applications of COD determine the amount of organic pollutants found in surface water, making COD a useful measure of water quality. In more recent years, a number of on-line methods for measuring correlation to COD have been developed.

Both the BOD and COD methods, however, only provide indirect measurements of organic material. All organic material, no matter how complex, is basically made of organic carbon. When organic matter is oxidized in water, it releases carbon, which then bonds with oxygen to form carbon dioxide. By monitoring the development of carbon dioxide, it is possible to obtain a direct measurement of the organic material in the water.

Total Organic Carbon (TOC) analysis measures both the total carbon (TC) as well as the inorganic carbon (IC) present in the sample. Subtracting the inorganic carbon from the total carbon yields TOC. Another method of TOC analysis involves removing the IC portion first and then measuring the leftover carbon. This method involves purging an acidified sample with carbon-free air or nitrogen prior to measurement and is more accurately called non-purgeable organic carbon (NPOC). TOC analysis can give accurate results within just a few minutes and can be automated for on-line applications. This gives it an inherent advantage over the BOD and COD laboratory measurements. It is important, however, to ensure that any on-line analysis method takes a truly representative sample, including small & large particulates and colloidal material of the stream being monitored. For this reason, filtration of the sample should be avoided.

OXIDATION METHODS USED IN TOC MEASUREMENT

Thermal Oxidation
The Thermal Oxidation Method uses high temperature combustion to convert organic material into carbon dioxide. When applied properly, it gives the best oxidation rate among the three methods. The sample is fed into a small oven, a few drops at a time, where it is oxidized at temperatures of 700 to 1300°C in the presence of a platinum catalyst. For complete oxidation, combustion must be instantaneous. Though thermal oxidation is a good laboratory method, it is less suitable for on-line applications where the filtering requirement prevents the measurement of representative samples. Moreover, this technology leads to a very high requirement for maintenance.

UV / Persulphate Oxidation
In the UV / Persulphate Oxidation Method, the sample is mixed with a solution of persulphate and exposed to UV light. This oxidation system was originally developed for laboratory and has subsequently been adapted for on-line applications. This oxidation system is relatively weak and is inhibited by the presence of chlorides, particulates, or high levels of organics in the sample. Instruments using this oxidation principle usually require a high level of maintenance when used on wastewater or industrial applications. This is because the UV light source can be “blocked out” by scaling, turbidity or particulates. These systems require pre-treatment by filtering of the sample to remove particulates, solid materials etc. The UV Persulphate analyser must be automatically calibrated on a continuous basis and therefore the accuracy depends on the availability and quality of a suitable standard. Despite this, UV/Persulphate oxidation is in widespread use because the instruments are simple to design and inexpensive to construct.

Advanced Oxidation
The Advanced Oxidation Method pioneered by the BioTector uses hydroxyl radicals as its oxidizing agent. By exposing high pH reagents to a heavy concentration of ozone, hydroxyl radicals are created. Hydroxyl radicals are very unstable and highly corrosive. They are one of the most reactive substances found to oxidise organic material. Because this oxidising chemical can be created within the reaction chamber from basic components, it is possible to use larger sample volumes for the analysis and thereby allow the introduction of particles into the sample. Filtering is not required. This method is highly suitable for on-line applications

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