7.3 Importance of Dissolved Oxygen (D.O) in water The concentration of dissolvedoxygen in water is of vital importance for the support of aquatic life.. Lowerthe concentration of dissolved oxygen, more the water is polluted. Dissolved oxygen of a sample of wateris defined as the mass of oxygendissolved in unit volume of water. It is usually expressed in parts permillion (ppm). The optimum usage value of D.
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O for domestic use is 4-6 ppm whichis able to maintain aquatic life. Oxygen reaches water through two sources. The first is that oxygendissolves at the surface of the water from the atmosphere. Still water takes upoxygen slowly whereas running water takes it up more rapidly. The second sourceof oxygen in water is from photosynthesis. When many aquatic green plants arepresent, the water often becomes supersaturated with oxygen during daylight. Butafter dark, photosynthesis stops but the plants continue to use and actuallyreduce the amount of dissolved oxygen.
Therefore, during a 24 hour period, samewater samples have a considerable range of dissolved oxygen levels. Water bodies receive excess nutrientsfrom biodegradable detergents, nitrogen and phosphate fertilisers applied toagricultural fields and sewage plant discharge etc. This nutrient stimulateexcessive plant growth (algae, periphyton,plant weeds)often called an algalbloom, reduces dissolved oxygen in water.
Water with low concentration ofdissolved oxygen is called hypoxic. This leads to anaerobic conditions thatinhibit the growth of other living organisms in water body. This process in which nutrients enriched water bodies support a densepopulation which kills animal life by depriving it of oxygen and results insubsequent loss of biodiversity is known as Eutrophication Fig 15.4 EutrophicationTomeasure the quality of waste water, two types of methods are used inenvironmental chemistry i) Biochemical oxygen Demand ii) Chemical oxygen Demandi) Biochemical oxygen Demand (BOD): The polluted water may contain largeamounts of organic compounds. Some of these can be oxidised by dissolved oxygenin the presence of microorganism. Organic compounds present in the pollutedwater serve as a food for bacteria.
.Hence the amount of dissolved oxygenutilised by these bacteria is called BOD.Biochemical Oxygen Demand (BOD) of a sample of water is defined as The amount of dissolved oxygen required for the oxidation of organicmatter by aquatic microorganisms under aerobic conditions at 20oCfor a period of 5 days. Its value is expressed in ppm. BOD is used as a measure of degreeof water quality. Clean water would have BOD value less than 5 ppm whereas highlypolluted water has BOD value of 17 ppm or more. To measure BOD, the sample ofwater is first saturated with oxygen. It is then incubated at 293 K for fivedays.
This allows microorganisms in the water sample to oxidise pollutants. Theremaining amount of dissolved oxygen is determined on the basis of difference(DO1 – DO5) between amounts of dissolved oxygen on thefifth day (DO5) and on the first day (DO1).The amount ofdissolved oxygen used by bacteria for degradation of organic material of wastesample can be determined. This is known as BOD (ii) Chemical Oxygen Demand (COD) BOD measurement takes 5 days soanother parameter called the Chemical Oxygen Demand (COD) is measured.
Chemical oxygen demand (COD) isdefined as Theamount of oxygen required by the organic matter in a sample of water for itsoxidation by a strong oxidising agent like K2Cr2O7 inacid medium in a period of 2 hrs. In COD determination, the watersample is treated with a known quantity of an oxidising agent like K2Cr2O7 in acidic medium. This reagentoxidises most of the polluting substances including those which are resistantto microbial oxidation. The remaining K2Cr2O7 isdetermined by back titration with a suitable reducing agent like ferrousammonium sulphate (Mohrs salt). From the concentration of K2Cr2O7consumed, the amount of oxygen used in the oxidation may be calculated usingthe following chemical equationK2Cr2O7(aq) + 4 H2SO4 ? K2SO4 (aq) + Cr2 (SO4)3(aq)+ 4 H2O + 3 (O) aq The results are expressed in terms ofamount of oxygen in ppm that would require to oxidise the contaminants. This iscalled COD.
Thus, generally, the COD value is higher that BOD for the sameliquid waste water. The oxygen equivalent of K2Cr2O7 consumedis taken as a measure of the COD. 1ml of 1N K2Cr2O7 = 0.008 g oxygen.Theresults are expressed in terms of amount of oxygen in ppm that would berequired to oxidise the contaminants.
This is called COD. Thus generally, theCOD value is higher than BOD for the same liquid waste water.