13-كتالوج اجهزة معمل تحاليل المياه والهندسة الصحية(الاكسجين المذاب الكيماوى والحيوى)

Chemical parameters of water

Water

biochemical oxygen demand and chemical oxygen demand

Any oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes.

The result is that the oxygen content of the water will be decreased.

Basically, the reaction for biochemical oxidation may be written as:

Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganics such as NO3 or SO4

Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:

S-- + 2 O2 → SO4--
NO2- + ½ O2 → NO3-

Since all natural waterways contain bacteria and nutrient, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above).

Those biochemical reactions create what is measured in the laboratory as the Biochemical Oxygen Demand (BOD).

Oxidizable chemicals (such as reducing chemicals) introduced into a natural water will similarly initiate chemical reactions (such as shown above).

Those chemical reactions create what is measured in the laboratory as the Chemical Oxygen Demand (COD).

Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant.

Both have been widely adopted as a measure of pollution effect.

The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.

The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period.

The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD.

The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.

There are also many different COD tests. Perhaps, the most common is the 4-hour COD.

There is no generalized correlation between the 5-day BOD and the Ultimate BOD.

Likewise, there is no generalized correlation between BOD and COD.

It is possible to develop such correlations for a specific waste contaminant in a specific wastewater stream, but such correlations cannot be generalized for use with any other waste contaminants or wastewater streams.

There is a generally accepted classification of Sewage Strength rated by Oxygen Demand, measured either by the Biochemical method (BOD) or the COD method.

Water Classification BOD COD
Weak < 200 < 400
Medium 350 700
Strong 500 1000
Very Strong 750 1500

Chemical oxygen demand

In environmental chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water.

Most applications of COD determine the amount of organic pollutants found in surface water (e.g. lakes and rivers), making COD a useful measure of water quality.

It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution.

Older references may express the units as parts per million (ppm).

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What is COD?

Chemical Oxygen Demand (COD) is defined as the quantity of a specified oxidant that reacts with a sample under controlled conditions.

The quantity of oxidant consumed is expressed in terms of its oxygen equivalence.

COD is expressed in mg/L 02.

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Why Measure COD?

COD is often measured as a rapid indicator of organic pollutant in water.

It is normally measured in both municipal and industrial wastewater treatment plants and gives an indication of the efficiency of the treatment process.

COD is measured on both influent and effluent water.

The efficiency of the treatment process is normally expressed as COD Removal, measured as a percentage of the organic matter purified during the cycle.
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Methods

COD can be measured by the closed reflux titrimetric method and the closed reflux colorimetric method (Standard Methods 5520 C. and 5520 D.

respectively).

Reactor:

The heater, or reactor, is used to obtain fast organic reactions.

Specifications for this reactor are written in Standard Method 5520 C.

with specific temperature and vessel requirements.

Since it is vital that the reaction take place at 150ºC (±2ºC) for 2 hours it is important to ensure accurate pre-heating.

A light on the Hanna Instruments COD reactor indicates that the temperature has reached the correct level.

The reactor is also equipped with a timer to notify the operator when the reaction is completed.

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Titration:

A sample is refluxed in strongly acidic solution with a known excess of potassium dichromate (K2Cr207).

After digestion the remaining unreduced K2Cr207 is titrated with ferrous ammonium sulphate to determine the amount of K2Cr207 consumed and the oxidizable matter is calculated in terms of oxygen equivalent.

This procedure is applicable to COD values between 40 and 400 mg/L.

Higher COD values can be obtained by careful dilution or by using higher concentrations of dichromate digestion solution (Standard Method 5520 C.).

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Colorimetric:

When a sample is digested, COD material in that sample is oxidized by the dichromate ion.

The result is the change in chromium from the hexavalent (VI) to the trivalent (III) state.

Both chromium species exhibit a color and absorb light in the visible region of the spectrum.

In the 400 nm region the dichromate ion (Cr2072-) absorbs strongly while the chromic ion (Cr3+) absorbs much less. In the 600 nm region it is the chromic ion that absorbs strongly and the dichromate ion has nearly zero absorption.

(Standard Method 5220 D.)

This method covers the ranges from 0 to 15000 mg/L 02:

1) 0- 150 mg/L near 420 nm
2) 0-1000 (1500) mg/L near 600 nm
3) 0-15000 mg/L near 600 nm

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Calculations

The following formula is used to calculate COD:

where b is the volume of FAS used in the blank sample, s is the volume of FAS in the original sample, and n is the normality of FAS.

If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/L.

The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO2 (assume all C goes to CO2), H2O (assume all H goes to H2O), and NH3 (assume all N goes to NH3), using the following formula:

COD = (C/FW)(RMO)(32)

Where C = Concentration of oxidizable compound in the sample,

FW = Formula weight of the oxidizable compound in the sample,

RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO2, water, and ammonia

The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent underacidic conditions.

The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by:

This expression does not include the oxygen demand caused by the oxidation of ammonia into nitrate.

The process of ammonia being converted into nitrate is referred to as nitrification.

The following is the correct equation for the oxidation of ammonia into nitrate.

The second equation should be applied after the first one to include oxidation due to nitrification if the oxygen demand from nitrification must be known.

Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test.

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biological oxygen demand

The amount of oxygen taken up by microorganisms that decompose organic waste matter in water.

It is therefore used as a measure of the amount of certain types of organic pollutant in water.

BOD is calculated by keeping a sample of water containing a known amount of oxygen for five days at 20°C. The oxygen content is measured again after this time.

A high BOD indicates the presence of a large number of microorganisms, which suggests a high level of pollution.

A way of assessing bacterial contamination of water, milk, etc. by micro-organisms which take up oxygen for their metabolism.

Biological Oxygen Demand (BOD) is one of the most common measures of pollutant organic material in water.

BOD indicates the amount of putrescible organic matter present in water.

Therefore, a low BOD is an indicator of good quality water, while a high BOD indicates polluted water.

Dissolved oxygen (DO) is consumed by bacteria when large amounts of organic matter from sewage or other discharges are present in the water.

DO is the actual amount of oxygen available in dissolved form in the water.

When the DO drops below a certain level, the life forms in that water are unable to continue at a normal rate.

The decrease in the oxygen supply in the water has a negative effect on the fish and other aquatic life.

Fish kills and an invasion and growth of certain types of weeds can cause dramatic changes in a stream or other body of water.

Energy is derived from the oxidation process. BOD specifies the strength of sewage.

In sewage treatment, to say that the BOD has been reduced from 500 to 50 indicates that there has been a 90 percent reduction.

The BOD test serves an important function in stream pollution-control activities.

It is a bioassay procedure that measures the amount of oxygen consumed by living organisms while they are utilizing the organic matter present in waste, under conditions similar in nature.

The other traditional tests or indicators for water quality are chemical oxygen demand (COD) and pH.

For results of the BOD test to be accurate, much care must be taken in the actual process.

For example, additional air cannot be introduced.

Temperature must be 20°C, which is the usual temperature of bodies of water in nature.

A five-day BOD test is used in environmental monitoring.

This test is utilized as a means of stating what level of contamination from pollutants is entering a body of water.

In other words, this test measures the oxygen requirements of the bacteria and other organisms as they feed upon and bring about the decomposition of organic matter.

Time and temperature, as well as plant life in the water, will have an effect on the test.

High BOD burdens or loads are added to wastewater by food processing plants, dairy plants, canneries, distilleries and similar operations, and they are discharged into streams and other bodies of water.

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