Water_e - UNIVERSITY OF HONG KONG OF DEPARTMENT OF CIVIL...

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U U NIVERSITY NIVERSITY OF OF H H ONG ONG K K ONG ONG D D EPARTMENT EPARTMENT OF OF C C IVIL IVIL E E NGINEERING NGINEERING CIME 2001 CIME 2001 W ATER ATER Q Q UALITY UALITY : C : C ONCEPTS ONCEPTS AND AND M M EASUREMENTS EASUREMENTS N AME : C HAN L OK F AI E MILY (2) U NIVERSITY N O .: 1999087170 G ROUP : C IV 1 S UBMISSION D ATE : 4 TH M AY 2001
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C HEMICAL HEMICAL O O XYGEN XYGEN D D EMAND EMAND (COD) T (COD) T EST EST Date of experiment: 29 th March 2001 Objective To determine the COD of given samples by Closed Reflux Method. Reagents 1. COD reagent (potassium dichromate: 0.005M) 2. Standard ferrous ammonium sulfate, FAS (0.10M) Procedures 1. 5ml water sample and 10ml COD reagent were pipetted into culture tubes. 2. The tube cap was tightened and mixed thoroughly. 3. The tube was placed in preheated oven at 150 ° C for 2 hours. 4. After cooling to room temperature, COD was determined. COD determination by titration 1. About 2 drops of ferroin indicator was added. 2. The mixture was then titrated with standard FAS solution. 3. The end point would be a color change from blue-green to reddish brown. Molarity of FAS solution 0.03 titration in used FAS ml reagent COD ml × = COD as mg O 2 /L = (A-B) × M × 8000 / ml sample where A = ml FAS used for blank B = ml FAS for sample M = molarity of FAS Results and Calculations Samples Sample source: Stanley wastewater treatment plant Sample 1: Blank Sample 2: Treated wastewater Sample 3: Raw wastewater Titration results Sample Volume of FAS (ml) COD Reagent 3.5 Deionized Water 3.3 Treated Wastewater 3.17 Raw Wastewater 2.13 2
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Calculation of the molarity of FAS solution Molarity of FAS solution = 03 . 0 titration in used FAS ml reagent COD ml × = 03 . 0 5 . 3 10 × =0.086M Calculation of COD COD as mg O 2 /L = sample ml 8000 B) - (A × where A = ml FAS used for blank B = ml FAS for sample = 5 ml M = Molarity of FAS Sample COD as mg O 2 /L Treated Wastewater 17.83 Raw Wastewater 160.46 Discussion The chemical oxygen demand (COD) test is widely used as a means of measuring the pollution strength of domestic and industrial wastes. This test allows measurement of waste in terms of the total quantity of oxygen required for oxidation to carbon dioxide and water in accordance with the following equation: 3 2 2 2 ) 2 3 2 ( ) 4 3 2 4 ( cNH O H c a nCO O c b a n N O H C c b a n + - + - - + + It is based upon the fact that all organic compounds, with a few exceptions, can be oxidized by the action of strong oxidizing agents under acid conditions. Among all the oxidation of compounds in wastewater, three main types are involved. Organic compounds During oxidation, organic compounds change into carbon dioxide and water. Organics + H + CO 2 + H 2 O K 2 Cr 2 O 7 oxidize Volatile organics like low-molecular weight fatty acid are present in the vapour space and do not come into contact with the oxidizing liquid. To oxidize them, a catalyst Ag + must be added to decompose them into CO 2 and H 2 O.
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