New-Oct.

Everything we eat, most of what we wear and many of the ordinary thing that surround us are made of organic material. All living creatures are themselves made of organic material.

Organic material is in itself normally not poisonou. 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. It is used by all living creatures. 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 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.

The proper monitoring and control of all outlets of organic material to our natural waters will help to maintain environmental balance and secure the future for our children.

Methods used for measuring organic pollution in water

The first method for measuring organic material in water was developed almost 100 years ago. By measuring the oxygen consumption in a closed sample, the presence of organic material could be indirectly quantified. Biological Oxygen Demand (BOD) analysis as originally developed is still in use today, with only minor changes. BOD analysis, however, is time consuming and inaccurate. The sample must be monitored for a period of 5~7 days and accuracy is at best ±20%.

In the 1960, a laboratory model of BOD analysis was developed which shortened the analysis period by adding chemical s to the sample. Chemical Oxygen Demand (COD) analysis requires only 3 hours and gives better results than BOD results. 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, are 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 can give accurate results within just a few minutes.

TOC analysis 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 and large particulates and colloidal material of the stream being monitored. For this reason, filtration of the sample should be avoided.

BOD
Biological Oxygen Demand

Indirect measurement based upon oxygen consumption

Analysis period - 5 to 7 days

COD
Chemical Oxygen Demand

Indirect measurement based upon oxygen consumption

Analysis period - 3 hours

TOC
Total Organic Carbon

Direct measurement of organic carbon

Analysis period -few minutes

Oxidation methods used in TOC measurement

Thermal Oxidation
The Thermal Oxidation Method uses high temp. 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 temp. of 600 to 800°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 here 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. Large particulates are not completely oxidized and the oxidation rate varies with the different organic compound present in the sample.

UV light also generates tiny quantities of ozone. However, the quantities are so small as not to effectively contribute to oxidization.

The variable and generally poor oxidation rate for this simplified oxidation method is regarded by a number of experts as unsuitable for either laboratory or on-line applications. 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 oxydizing 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 which can be 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.

TOC Analyzer Informations
Fluorometer for Environmental Science