November 23, 2016 10:25 pm
Dissolved oxygen (DO) is the amount of oxygen that is present in the water.
It is measured in milligrams per liter (mg/L), or the number of milligrams of oxygen dissolved in a liter of water.
Definition
Dissolved Oxygen test is the amount of oxygen dissolved in waste water which is an important indicator of the health of water body. Presence of organic pollutants in waste water depletes the dissolved oxygen leading to the death of marine organisms.
Range
| CPCB Limits | |
| Drinking Water Source without conventional treatment but after disinfection | Dissolved Oxygen 6mg/l or more |
| Outdoor bathing (Organised) | Dissolved Oxygen 5mg/l or more |
| Drinking water source after conventional treatment and disinfection | Dissolved Oxygen 4mg/l or more |
| Propagation of Wild life and Fisheries | Dissolved Oxygen 4mg/l or more |
Implications
High level of Dissolved Oxygen
Can speed up the corrosion of water pipes.
Excess electricity consumption.
Promotion of unwanted organisms.
Low level of Dissolved Oxygen
Insufficient biological activity.
Can lead to anaerobic zone instead of aerobic zone.
A certain amount of care must be exercised in the collection of samples to be used for dissolved oxygen determinations. In most cases of interest, the dissolved oxygen level will be below saturation, and exposure to the air will lead to erroneous results. For this reason, a special sampling device similar to the one described in “Standard Methods” is needed. All such instruments are designed on the principle that contact with air cannot be avoided during the time the sampling bottles are being filled. Most samples for dissolved oxygen are collected in the “field” where it is not convenient to perform the entire determination. Since oxygen values may change radically with time because of biological activity, it is customary to “fix” the samples immediately after collection. The usual procedure is to treat the samples with the conventional reagents used in the dissolved oxygen test and then perform the titration when the samples are brought to the laboratory. This result will give low results for samples with a high iodine demand, and in this case it is better to preserve the sample by addition of 0.7 ml concentrated sulfuric acid and 0.02 g sodium azide. When this is done, it is necessary to add 3 ml of alkali-iodide reagent rather than the usual 2 ml because of the extra acid the sample contains. Better results are obtained if the fixed samples are stored in the dark and on ice until he analyses can be completed. The chemical treatment employed in “fixing” is radical enough to arrest all biological action, and the final titration may be delayed up to six hours.
Winkler Method
To the sample collected in 300 ml bottle, add 2 ml of manganese sulphate solution followed by 2 ml of alkaline iodide solution. Use separate pipettes of 2 ml capacity for each reagent and take care that tip of the pipette in each case is dipped well below the liquid surfaces carefully. Replace the stopper without the inclusion of any air bubble and thoroughly mix the content by shaking the bottle several times; allow the precipitate formed to settle. After 2-3 minutes of settling, carefully remove the stopper and immediately add 2 ml of concentrated sulphuric acid by running the acid down the neck of the bottle and mix thoroughly to dissolve the liberated iodine. Take 200 ml of the solution and titrate immediately against standard sodium thiosulphate solution, adding 3-4 drops of starch indicator solution. The end point is pale blue to colorless.
Azide Modification
To the sample collected in 300 ml bottle, add 2 ml of manganous sulphate solution followed by 2 ml of alkaline iodide sodium azide solution. Replace the stopper without inclusion of any air bubble and mix the contents thoroughly by shaking the bottle several times; allow the precipitate formed to settle. After 2-3 minutes of settling, carefully remove the stopper and immediately add 2 ml of concentrated sulphuric acid by running the acid down the neck of the bottle; restopper and mix thoroughly to dissolve the liberated iodine. Take 200 ml of the solution and titrate immediately against standard sodium thiosulphate solution adding 3-4 drops of starch indicator solution. The end point is the pale blue to colour less.
Alum Flocculation Modification
Collect the sample in a glass stoppered bottle of 500 ml capacity. Add 10 ml of alum solution followed by 2 ml of ammonium hydroxide, mix the contents gently by inverting the bottle and allowing to settle for 15 minutes. Collect the supernatant liquid into 300 ml dissolved oxygen bottle. Avoid aeration and keep the siphon sufficiently submerged during transfer.
Membrane Electrode Method
Calibration
Follow the manufacturer’s instructions exactly to get specified accuracy and precision. Generally, calibrate membrane electrodes by reading against distilled water sample of known dissolved oxygen concentration as well as with a sample with zero dissolved oxygen (add excess of sodium sulphite and a trace of cobalt chloride to a sample of distilled water to bring dissolved oxygen to zero concentration).
Sample Measurement
Follow the manufacturer’s instructions to get good results. While changing membrane, take care to avoid any contamination of electrodes and trapping of minute air bubbles under the membrane. Provide sufficient stirring of sample or flow of sample across the membrane, till constant reading is obtained.
In liquid wastes, dissolved oxygen is the factor that determines whether the biological changes are brought by aerobic or by anaerobic organisms.
Dissolved oxygen measurements are vital for maintaining aerobic conditions in natural waters that receive pollutional matter and in aerobic treatment processes intended to purify domestic and industrial wastewaters.
Dissolved oxygen determination is one of the most important single tests that the environmental engineer uses.
Determination of DO serve as the basis of the BOD test. Thus they are the most important determination used to evaluate the pollutional strength of domestic and industrial wastes. The rate of biochemical oxidation can be measured by determining residual dissolved oxygen in a system at various intervals of time.
All aerobic treatment processes depend upon the presence of dissolved oxygen and tests for it are indispensable as a means of controlling the rate of aeration to make sure that adequate amounts of air are supplied to maintain aerobic conditions and also to prevent excessive use of air.
Interference
Oxidizing agents give positive interference and reducing agents give negative interference.
Conclusion
Dissolved Oxygen levels impact the living organisms present in the water body. Thus, the data can be used to predict how these DO levels in the water can affect the organisms in different conditions.
Disclaimer
The details given are for general understanding. References should be taken from respective authorized agencies.