عدد المساهمات : 3513
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: TOXIC ORGANIC COMPOUNDS/PATHOGENIC MICRO-ORGANISM الجمعة مارس 16, 2012 2:05 pm|| |
TECHNOLAB EL-BAHAA GROUP
Toxic Organic Compounds:
An organic compound is any compound which contains carbon, with the exception of carbon monoxide and carbon dioxide, carbonates, or cyanides.
Organic compounds contain chains and/or rings of connected carbon atoms, often with other elements attached.
There are millions of possible compounds, with many useful properties.
Many are biologically active, since all living things are made up of organic molecules.
Industries use and produce thousands of organic compounds in manufacturing such items as plastics, synthetic fibers, rubber, pharmaceuticals, pesticides, and petroleum products.
Some of the compounds are starting materials;
some are solvents;
some are byproducts.
The US EPA lists 116 of them as toxic "priority pollutants";
many states have longer lists.
One of the major groupings is volatile organic compounds (VOC's), many of which are chlorine-containing solvents.
There are also petroleum hydrocarbons and starting materials for plastics, dyes, and pharmaceuticals.
The "semi-volatile" group include solvents, PAH's (polycyclic aromatic hydrocarbons, like naphthalene and anthracene which are coal tar constituents), as well as pesticides (especially chlorinated pesticides) and PCB's (polychlorinated biphenyls, which were formerly used in electrical transformers and other products).
Most of these are analyzed routinely by gas chromatography (GC), often followed by mass spectrometry (MS) for identification.
HPLC is also used for some analytes.
A technique which is becoming available for field measurements for some of these compounds is immunoassay, sometimes called ELISA, for "enzyme-linked immunosorbent assay."
This method, which produces a color reaction related to the concentration of the target compound, or family of compounds, is portable, relatively inexpensive and does not require a great deal of training.
It is in use more for surveying hazardous waste sites, however, than for water analysis.
Sewage contains large numbers of microbes which can cause illness in humans, including viruses, bacteria, fungi, protozoa and worms (and their eggs or ova).
They originate from people who are either infected or are carriers.
While many of these can be measured directly by microscopic techniques (some after concentration), the analyses most commonly performed are for so-called "indicator organisms."
These organisms, while not too harmful themselves, are fairly easy to test for and are chosen because they indicate that more serious pathogens are likely to be present.
For instance, wastewater treatment plants are often required to test their effluents for the group known as "fecal coliforms," which include the species E. coli, indicative of contamination by material from the intestines of warm-blooded animals.
Water supplies test for a more inclusive group called "total coliforms", and in some cases, for general bacterial contamination (heterotrophic plate count, or HTP.)
The two most commonly used methods of analysis for indicator organisms are the multiple tube fermentation technique and the membrane filter procedure.
In the first method, a number of tubes containing specific growth media are innoculated with different amounts of the sample and incubated for a particular time at a prescribed temperature.
The appearance of colors, fluorescence, or gas formation indicates the presence of bacteria belonging to the target group.
The number of organisms per 100 mL in the original sample is estimated from most probable number (MPN) tables, which list the values of MPN for different combinations of positive and negative results in tubes which contained different initial volumes of the sample.
Often, positive results must be confirmed by further innoculation of small amounts of material from the positive tubes into tubes containing a different media, which can extend the test to several days.
The second technique involves filtering a known volume of sample through a membrane filter (made of a material such as cellulose acetate) which has a small enough pore size to retain the bacteria.
The filter is then placed in a dish of sterile nutrient media, either soaked into an absorbent pad or in a gel such as agar, and sealed.
The dish is incubated for the prescribed time and temperature.
The media contain a colored indicator which will identify the target bacteria. Each bacterium in the original sample will result in a colony after incubation, which is large enough to see without a great deal of magnification.
The concentration in the sample can be determined by direct count of the colonies, knowing the volume of sample used.
In some cases, these colonies require further confirmation.
Detection and enumeration of HTP or of specific pathogenic bacteria, such as Salmonella, E. coli, or Enterococcus can be done by similar methods, but utilizing specific growth media for each type.
Viruses are usually measured by concentration, followed by addition to cultures of cells which they infect and counting the number of plaques formed due to cell destruction.
Pathogenic protozoa and ova of multicelled organisms are determined by concentration and direct counting under the microscope, often with the aid of fluorescent staining compounds.
Besides, direct observation, identification of pathogenic microorganisms can be done by standard techniques used in clinical laboratories involving observing reactions in a battery of different indicating media.
Some newer methods use chromatography to identify patterns of compounds which serve as "fingerprints" for certain bacteria; DNA analysis is another recent innovation.
Most wastewater treatment plants, however, confine their testing to simply counting the numbers indicator bacteria.