عدد المساهمات : 3492
تاريخ التسجيل : 15/09/2009
العمر : 49
الموقع : مصر
|موضوع: SOLIDS/NUTRIENTS/المواد الصلبة/المواد الحيوية الجمعة مارس 16, 2012 1:41 pm|| |
TECHNOLAB EL-BAHAA GROUP
Water, a liquid, can contain quite a bit of solid material, both in dissolved and suspended forms.
The term "dissolved" implies that the individual molecules of a substance are mixed in among the water molecules.
In practice, solids are classified as "dissolved" if they pass through a standard glass-fiber filter with about one micrometer pore size.
Solids captured on the filter are, by definition, "suspended" solids..
Solids which settle out of a water sample on standing for a period of an hour are defined as "settlable." .
Solids are also further classified as "fixed" or "volatile."
Fixed solids are basically the ash left over after burning the dried solids; volatile solids are those that are lost in this procedure.
The sum of the two is referred to as "total". (This can be confusing, as the word "total" is also used in describing the sum of suspended and dissolved solids.) Volatile solids are often used as an estimate of the organic matter present.
Significance: Solids in wastewater contribute to sediment formation; volatile solids may be associated with oxygen demand.
Total solids (TS) are determined by
drying a known amount of a sample at a temperature of 103 to 105 C in a tared (pre-weighed) vessel, such as a porcelain dish,
cooling in a dry atmosphere (in a container known as a desiccator),
weighing on an analytical balance, subtracting the tare weight, and dividing by the original amount of sample.
Results can be expressed in mg/L if the sample was originally measured out by volume;
or percent by weight,
if the sample was originally weighed.
If the sample is then burned in a furnace at about 500 C, cooled, and weighed, the fixed (FS) or volatile solids (VS) can be determined.
If the original sample is filtered through a tared glass-fiber filter, which is then dried, the weight of the material captured on the filter is used to figure the total suspended solids (TSS).
Burning the filter in the furnace allows measurement of volatile suspended solids (VSS) or fixed suspended solids (FSS).
The dissolved solids (DS)
can be estimated from the difference between the total solids and the total suspended solids, but the official method calls for drying the filtrate (the liquid which passes through the filter) in a dish at 180C.
(And, of course, there are TDS, FDS and VDS).
An estimate of total suspended solids can be obtained by an optical/instrumental measurement known as turbidity.
The sample is placed in a glass tube;
a beam of light is shined through it, and the light scattered at right angles to the beam is measured photometrically.
In the same way that COD can be correlated with BOD, turbidity can be correlated with TSS;
but the correlation will hold only for the particular sample from which it was derived.
Similarly, an estimate of dissolved solids is often made by measuring the water's electrical conductivity.
Pure water does not conduct electricity.
If substances which dissociate into electrically charged ions are dissolved in the water, they will conduct a current, roughly proportional to the amount of dissolved substances. Conductivity can be used to track sewage pollution.
Note, however, that many organic materials dissolve in water without producing ions. So, while a salt solution may have a high electrical conductivity, a concentrated solution of sugar would go undetected by this method.
Nutrients are usually thought of as compounds of nitrogen or phosphorus, although certainly other elements, such as iron, magnesium, and potassium are also necessary for bacterial and plant growth.
Nitrogen occurs primarily in the oxidized forms of nitrates (NO3-) and nitrites (NO2-) or the reduced forms of ammonia (NH3) or "organic nitrogen"-- where the nitrogen is part of an organic compound such as an amino acid, a protein, a nucleic acid, or one of many other compounds.
All of these can be used as nutrients, although the organic nitrogen first needs to decompose to a simpler form .
Phosphorus is biologically important in the form of phosphate, the most highly oxidized state of the element.
The most biologically available form is dissolved orthophosphate, (PO4-3). (In solution, there are up to three hydrogens attached to the molecule, each one decreasing the negative charge of the ion by one.
How many hydrogens are attached depends on the pH.)
There are also condensed forms of phosphate, with more than one phosphorus atom per ion, such as pyrophosphate and polyphosphates.
There are also organic phosphates, and all of these forms can be either dissolved or particulate (i.e., insoluble).
The sum of all the forms is known as total phosphorus.
These nutrients are important in natural waters because, in excess, they can cause nuisance growth of algae or aquatic weeds.
In wastewater treatment, a deficiency of nutrients can limit the effectiveness of biological treatment processes.
In some plants treating industrial wastewaters, ammonia or phosphoric acid must be added as a supplement.
Ammonia can be measured colorimetrically, by the Nessler or phenate methods, after distillation from an alkaline solution to separate it from interferences.
It can also be determined by an electrode method, sometimes without distillation, since there are fewer interferences.
Organically-bound, reduced nitrogen can be determined by the same methods after a digestion (the Kjeldahl digestion) which converts the nitrogen in those compounds to ammonia.
The combination of ammonia and organic nitrogen is known as "Total Kjeldahl Nitrogen," or TKN.
(TKN analysis is used for measuring protein content of animal feeds, as well.)
Nitrite is determined colorimetrically. Nitrate can also be determined this way; the most popular way is by first reducing nitrate to nitrite chemically using cadmium, then analyzing the nitrite.
There is an electrode method for nitrate, but it is not considered too accurate.
Finally, ammonia (as the positively charged ammonium ion, NH4+), nitrate, and nitrite can be measured by ion chromatography, as well.
Phosphate can be measured by ion chromatography, also. Greater sensitivity, at lower cost, is obtained by colorimetric methods which measure dissolved orthophosphate.
Some insoluble phosphates and condensed phosphates-- so called "acid-hydrolyzable phosphate"-- can be included by heating the sample with acid to convert these forms to orthophosphate.
If the organic phosphate is to be included, to measure "total phosphate", then the sample must be digested with acid and an oxidizing agent, to convert everything to the orthophosphate form.