Experiment - Chemical Oxygen Demand
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  Introduction: In recent times, with the increase of pollution by discharging large amount of various chemicals, oxidizable organic substances of different matter enter in the aquatic system. BOD values alone does not give a clear picture of organic matter content of the water sample. In addition, the presence of various toxicants in the sample may severely affect the validity of BOD test. Hence chemical oxygen demand (COD) test is a better estimate of organic matter which needs no sophistication and is time saving. However COD that is the oxygen consumed (OC) does not differentiate the stable organic matter from the unstable form, therefore the COD value are not directly comparable to that of BOD. The amount of organic matter in water is estimated based on their oxidizability by chemical oxidants, such as potassium permanganate or potassium dichromate. For many years, the potassium permanganate was used as oxidizing agent for measuring chemical oxygen demand. But the oxidizing capacity of potassium permanganate varied widely. Nowadays, Potassium dichromate is used instead of potassium permanganate because it is more effective, relatively cheap, easy to purify and is able to oxidize almost all organic compounds. In this method, a fixed volume of oxidant (here potassium dichromate) is added to the water sample. The organic matter present in the water sample is first oxidized with known volume of potassium dichromate and then excess of oxygen is allowed to react with potassium iodide to liberate iodine in amounts equal to the excess oxygen, which is estimated titrimetrically with sodium thiosulphate as an indicator. Objectives: 1. To determine the concentration of chemical oxygen demand in wastewater samples (influent and effluent) taken from Canseleri UiTM Shah Alam Treatment Plant by using EPA Method 410.3 (Dichromate Reflux Method) and EPA Method 410.4 (Reactor Digestion Method). 2. To measure the effectiveness of a wastewater treatment plant based on the % removal of chemical oxygen demand (COD).  Procedures: Name of method: 1. Dichromate Reflux Method 2. Reactor Digestion Method No. of standard method: 1. EPA Method 410.3   2. EPA Method 410.4   Procedures based on reference 1 . Page 15-23. In-situ Data: Influent Effluent Temperature ( ˚ C) 28.87 24.42 DO (mg/L) 2.06 1.59 pH 1.48 1.35 Odour Odourless Odourless Colour Light brown Light brown Conductivity (µs/cm) 5378 4652  Ex-situ Data: 1. Dichromate Reflux Method Group Sample Sample volume, mL Titration of sample, mL Titration sample average, mL Titration of standard, mL Normality of FAS COD for each group, mg/L % removal of COD Initial Final Initial Final 1 I 1  50 0.0 40.0 42.5 0.0   21.2 0.11 475.2 109.4 I 2  50 0.0   45.0 0.0   Blank 50 0.0   69.5 69.5 0.0   2 E 1  50 0.0   54.5 53.0 0.0   22.7 -44.9 E 2  50 0.0   51.6 0.0   Blank 50 0.0   50.5 50.5 0.0   3 I 1 *25 0.0   53.4 49.8 0.0   22.2 200.6 122.8 I 2 *25 0.0   46.2 0.0   Blank *25 0.0   55.5 55.5 0.0   4 E 1  *25 0.0   51.0 51.6 0.0   24.0 -45.8 E 2  *25 0.0   52.2 0.0   Blank *25 0.0   50.3 50.3 0.0   2. Reactor Digestion Method Groups Sample COD digestion reagent vial type and volume, mL Spectrophotometer reading, mg/L COD actual, mg/L  Average of COD value, mg/L % COD removal 5 I 1  HR 0.2 22.0 18.3 17.87 LR = 71.3 HR = ND I 2  HR 0.2 21.0 17.5 Blank HR 0.2 0.0 0.0 0.0 6 I 1  LR 2.0 284.0 236.0 236.0 I 2  LR 2.0 284.0 236.0 Blank LR 2.0 0.0 0.0 0.0 7 E 1 HR 0.2 ND ND ND E 2  HR 0.2 ND ND E 1  LR 2.0 109.0 90.6 67.7 E 2  LR 2.0 102.0 84.8 Standard LR 2.0 121.0 100.5 100.5  Discussions: Dichromate Reflux Method Based on the result obtained, the volume of titration for influent 50 mL are 40.0 mL and 45.0 mL with an average of 42.5 mL, the COD concentration for this influent is 475.2 mg/L. The volume of titration for effluent 50 mL are 54.5 mL and 51.6 mL with an average of 53.0 mL, the COD concentration for this effluent is -44.9 mg/L which can be considered as ND. The volume of titration for influent 25 mL are 53.4 mL and 46.2 mL with an average of 49.8 mL, the COD concentration for this influent is 200.6 mg/L. The volume of titration for effluent 25 mL are 51.0 mL and 52.2 mL with an average of 51.6 mL, the COD concentration for this influent is -45.8 mg/L which also can be considered as ND. Since the reading for effluent obtained is negative, the % removal of COD has become more than 100%. The result of getting negative COD concentration is indicates that the COD is not detectable (ND). This shows that there’s a significant reduction of COD concentration because even from influent the COD reading is not very high which lead to very low concentration of COD in effluent. When concentration of COD too low, it will become negative which proves the COD concentration in the sample is ND. Based on Malaysia Sewage and Industrial Effluent Discharge Standards, the permissible limit for COD is 100 mg/L for standard B which is used for this effluent because it released to Sungai Klang. The result obtained in this experiment for effluent are -44.9 mg/L and -45.8 mg/L which is considered as ND and result is acceptable as well as the effluent is permitted if being discharge into Sungai Klang. Reactor Digestion Method Based on the result obtained, the concentration of COD for influent in high range is 18.3 mg/L and 17.5 mg/L with an average of 17.87 mg/L. For the low range, the concentration of COD for influent is 236.0 mg/L and 236.0 mg/L with having same value for average. For effluent reading, the high range both reading is not detectable (ND) which means the reading of COD concentration of that sample is too low for the detectable limit for spectrophotometer. For effluent reading in low range, COD concentration is 90.6 mg/L and 84.8 mg/L with an average reading 67.7 mg/L.
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