ENCM Environmental Impact Assessment and Environmental Monitoring. Determination of Chemical Oxygen Demand in water/waste water

Transcription

1 ENCM Environmental Impact Assessment and Environmental Monitoring itoenvironmental Monitoring Practical Number 3 Determination of Chemical Oxygen Demand in water/waste water Learning outcomes At the end of the practical the learners will be able to gain familiarity with the standard procedures of determining Chemical Oxygen Demand(COD) in surface water/waste water determine the chemical oxygen demand of the waste water samples provided following the standard procedures present COD analysis data in a scientific report and comment on the pollution status of the analyzed samples Introduction The Chemical Oxygen Demand (COD) of a surface water/waste water sample is a measure of the chemically oxidisable organic material present in that sample. It differs from Biochemical Oxygen Demand (BOD) determinations in that COD is determined chemically by using a strong oxidizing agent. COD is a rapidly measured and important variable for characterizing water bodies, sewage, industrial wastes and treatment plant effluents. In this practical, the dichromate method has been selected as a reference method for the COD determination because it has advantages over other oxidants owing to its oxidizing power, its applicability to a wide variety of samples and its ease of manipulation. COD is defined as the amount of a specified oxidant (potassium dichromate) that reacts with the sample under controlled conditions; it is the amount of oxygen consumed by organic matter from boiling acidic potassium dichromate solution. It provides a measure of the oxygen equivalent of that portion (mainly organic matter) of the water sample susceptible to oxidation under the conditions of the test. Both organic and inorganic components of a sample are subject to oxidation, but in most cases the organic component predominates and is of the greater interest. Where the sample contains only readily available organic bacterial nutrients and no toxic matter, the COD results can be used to obtain an approximate estimate of the ultimate carbonaceous BOD values. Two standard methods with potassium dichromate as the oxidant are available for COD analysis; Open Reflux method and Closed Reflux method. The open reflux method is suitable for a wide range of wastes where a large sample size is preferred. The closed reflux method is more economical in the use of metallic salt reagents and generates smaller quantities of hazardous waste. Interferences: Oxidation of most organic compounds is 95 to 100% of the theoretical value. Pyridine and related compounds resist oxidation and volatile organic compounds will react in proportion to their contact with the oxidant. Straight-chain aliphatic compounds are oxidized more effectively in the presence of 1

2 a silver sulphate catalyst. But if chloride ions are present, chloride reacts with silver ion to precipitate silver chloride, and thus inhibits the catalytic activity of silver. Bromide, iodide, and any other reagent that inactivates the silver ion can interfere similarly. Such interferences are negative in that they tend to restrict the oxidizing action of the dichromate ion itself. However, under the rigorous digestion procedures for COD analyses, chloride, bromide, or iodide can react with dichromate to produce the elemental form of the halogen and the chromic ion. Results then are in error on the high side. The difficulties caused by the presence of the chloride can be overcome largely, though not completely, by complexing with mercuric sulphate (HgSO4) before the refluxing procedure. Nitrite (NO2 ) exerts a COD of 1.1 mg O2/mg NO2 N. Because concentrations of NO2 in waters rarely exceed 1 or 2 mg NO2 N/L, the interference is considered insignificant and usually is ignored. To eliminate a significant interference due to NO2 in sulfamic acid is added (10 mg for each mg NO2 N present in the sample volume used; same amount of sulfamic acid is added to the reflux vessel containing the distilled water blank). Sample handling Samples should be collected to glass-stoppard glass bottles. Unstable samples should be tested without delay, especially wastewater and polluted water samples. Natural, not heavily polluted, water should be analysed on the same day or at least within 24 hours and the sample should be kept cold before analysis. If delay before analysis is unavoidable, the sample may be preserved by adding conc. sulphuric acid, about 2 ml H2SO4 to each 1L of sample (ph <2) and refrigerate at 4 C until 28 days. Safety Precautions: Avoid skin and eye contact with caustic and acidic solutions. If contact occurs, rinse your hands and/or flush your eyes for several minutes. Seek immediate medical advice for eye contact. Some apparatus used in this procedure may be hazardous to the safety of the user if inappropriately or accidently misused. 1. COD determination using Open Reflux method 1.1 Principle The sample is boiled under reflux with a known excess of potassium dichromate and silver sulphate catalyst in strong sulphuric acid. Part of the dichromate (Cr2O7 2- ) is reduced to the chromic ion (Cr 3+ ) by the organic matter in the sample and the remaining unreduced dichromate is titrated with ferrous ammonium sulphate to determine the amount of K2Cr2O7 consumed. The quantity of oxidant (dichromate) consumed is expressed in terms of its oxygen equivalence. 1.2 Chemicals/Reagents (Note to the technical officers: reagents should be prepared only the required amounts for the practical based on the following proportions to minimize wastage of chemicals: Consult the lecturer in charge). Sulphuric acid reagent: Add Ag 2 (reagent or technical grade), crystals or powder, to conc H 2 (d =1.84) at the rate of 5g Ag 2 /L H 2. Let stand 2 days to dissolve. Mix. Standard potassium dichromate solution, ( M): Dissolve g of K2Cr2O7 primary standard grade, previously dried at 150 C for 2 hours, in distilled water and dilute to 1000 ml. 2

3 Dilute standard potassium dichromate solution, M. Dilute 100 ml of the standard potassium dichromate solution to 1000 ml.(use only for low COD samples) Standard ferrous ammonium sulphate solution, M. Dissolve 98 g of Fe(NH4)2(SO4)2.6H2O analytical grade crystals in distilled water. Add 20 ml of conc. H2SO4 (d=1.84), cool and dilute to 1000 ml. This solution may be standardised against the standard potassium dichromate solution as follows: Dilute 10.0 ml of standard potassium dichromate solution, M, to about 100 ml. Add 30 ml conc. H2SO4 (d=1.84) and allow to cool. Titrate with the ferrous ammonium titrant, using 2 or 3 drops of ferroin indicator. Molarity of the ferrous ammonium sulphate solution Dilute standard ferrous ammonium sulphate solution, M. Dilute 100 ml of the standard ferrous ammonium sulphate solution to 1000 ml. Standardise daily against the dilute standard potassium dichromate, M..( use only for low COD samples) Mercuric sulphate, analytical grade crystals. Ferroin indicator solution. Dissolve g of ferrous sulphate, FeSO4.7H2O, in distilled water. Add g of 1,10-phenanthroline monohydrate, shaking until dissolved. Dilute to 100 ml. This solution is also commercially available. Sulphamic acid, analytical grade (required only if the interference of nitrites is to be eliminated).-this will not be used in this practical assuming no nitrite interference) Anti-bumping granules that have been previously heated to 600 C for 1 hour. 1.3 Apparatus A reflux apparatus consisting of a 250-ml Erlenmeyer flask with ground-glass neck, and a 300- mm double surface condenser (Liebig, Friedrichs, West or equivalent) with a ground-glass joint. Since absolute cleanliness is essential, flasks and condensers should be protected from dust by inverted cups when not in use. The glassware must be cleaned well and used exclusively for COD determinations. A heating mantle or hotplate. - A hotplate producing at least 1.5 W cm -2 of heating surface to ensure adequate boiling of the liquid in the flask. Heating mantles are preferred because they prevent the problem of overheating. Electric Blender Pipetes (Wide bore) 1.4 Procedure for the Open Reflux Method Blend (homogenize) all samples containing suspended solids before analysis to obtain reproducible results. If COD is to be related to BOD, ensure that all tests receive identical pretreatment. Make preliminary dilutions for wastes containing a high COD to reduce the error in measuring small sample volumes (note the dilution factor). 3

4 Samples with low chloride concentrations If the sample contains less than 100 mg L -1 chloride, proceed as follows: 1. Place in an 250 ml refluxing flask 20.0 ml of the blended sample or an aliquot diluted to 20.0 ml with distilled water (if high COD value is expected). 2. Add 10.0 ml of standard potassium dichromate solution, M, and a few anti-bumping granules. Mix well. 3. Add very slowly, with caution, 30 ml of concentrated H2SO4 containing silver sulphate, mixing thoroughly by swirling while adding the acid. If H2SO4 containing silver sulphate is not used, add 0.15 g of dry silver sulphate and then, slowly, 30 ml of concentrated H2SO4. Caution: If the liquid has not been well mixed local heating may occur on the bottom of the flask and the mixture may be blown out of the flask. 4. Attach the condenser to the flask and turn on cooling water and reflux the mixture for 2 hours. Cover open end of condenser with a small beaker to prevent foreign material from entering refluxing mixture. 5. Allow to cool and then wash the condenser with distilled water. Disconnect the reflux condenser. Dilute the mixture to about 150 ml with distilled water, 6. Cool to room temperature and titrate excess K 2 Cr 2 O 7 with standard ammonium ferrous sulphate, using 0.10 to 0.15 ml (2 to 3 drops) ferroin indicator. Although the quantity of ferroin indicator is not critical, use the same volume for all titrations. 7. Take the first sharp color change from blue-green to reddish brown that persists for 1 min or longer as the end point of the titration. The blue-green may reappear. 8. Reflux a blank consisting of 20 ml of distilled water together with the reagents and titrate as in steps 2-7 above in the same manner Samples with high chloride concentration (If the sample contains more than 100 mg L -1 chloride) after appropriate dilution, proceed as follows: 1. To 20.0 ml of sample (or diluted sample to 20 ml with deionized water if COD >900 mg O2/L) in the 250 ml refluxing flask add 0.5 g of mercuric sulphate, several glass beads and shake thoroughly. (if a slight precipitate develops, ignore. It will not affect the determination). 2. Very slowly add 5 ml sulfuric acid reagent, with mixing to dissolve HgSO4. Cool while mixing to avoid possible loss of volatile materials. 3. Add 10 ml of standard K 2 Cr 2 O 7 solution M and add a few antibumping granules and mix. 4

5 4. Attach flask to condenser and turn on cooling water. Add remaining sulfuric acid reagent (25 ml) through open end of condenser. Continue swirling and mixing while adding sulfuric acid reagent. Caution: Mix reflux mixture thoroughly before applying heat to prevent local heating of flask bottom and a possible blowout of flask contents. 5. Cover open end of condenser with a small beaker to prevent foreign material from entering refluxing mixture and reflux for 2 h. Cool and wash down condenser with distilled water. 6. Disconnect reflux condenser and dilute mixture to about twice its volume with distilled water. 7. Cool to room temperature and titrate excess K 2 Cr 2 O 7 with standard ammonium ferrous sulphate, using 0.10 to 0.15 ml (2 to 3 drops) ferroin indicator. Although the quantity of ferroin indicator is not critical, use the same volume for all titrations. 8. Take as the end point of the titration the first sharp color change from blue-green to reddish brown that persists for 1 min or longer. Samples with suspended solids or components that are slow to oxidize may require additional determinations. The blue-green may reappear. 9. In the same manner, reflux and titrate a blank containing the reagents and a volume of distilled water equal to that of sample Adjustments for other sample sizes Table 1 : Quantities of reagents for different sample sizes (source UNEP/WHO 1996) If a water is expected to have a higher or lower than normal COD, a sample ranging in size from 10.0 ml to 50.0 ml may be used with the volumes, weights and concentrations adjusted accordingly. Table 1 gives the appropriate reagent quantities for different sample sizes. When using large samples, increase the size of the Erlenmeyer flask to 500 ml to permit titration within the refluxing flask. 5

6 Samples with low COD (eg. Less polluted River water) Follow one of the procedures given above for high and low chloride concentrations with the following differences: Use dilute standard potassium dichromate, M (instead of M). Perform the back titration with either M or 0.01 M ferrous ammonium sulphate (instead of 0.25 M). Use redistilled water for the preparation of all reagents and blanks. Exercise extreme care with this procedure because a trace of organic matter in the glassware or the atmosphere may cause a gross error Calculations: Where A = ml ferrous ammonium sulphate used for blank B = ml ferrous ammonium sulphate used for sample M = molarity of ferrous ammonium sulphate 8000 = milliequivalent weight of oxygen X 1000 ml/l 2. COD determinations using Closed Reflux method Chemical reactions are usually similar to the open reflux method. Volatile organic compounds are more completely oxidized in the closed system because of longer contact with the oxidant. Instead of reflux flasks, digestion vessels (borosilicate culture tubes with TFE-lined screw caps) are needed for sample digestion. In addition a block heater is needed to operate at 150 ± 2 C, with holes to accommodate digestion vessels. Digestion vessels with premixed reagents and other accessories are available from commercial suppliers. Premixed reagents contain all the reagents/chemicals used in the open reflux method including silver as a catalyst and mercury to complex the chloride interferences. Instead of the titration, colour development can be measured spectrophotometricaly to determine COD values of the digested samples. Note: The waste generated from the COD analysis should not be thrown to the normal drainage system as it may contain hazardous waste. Pour the waste to the specific waste containers provided. 6

7 Lab Exercise You are provided with surface water/waste water samples from two sources for COD analysis. 1. Determine the COD contents of the two water/waste water samples using the open reflux method following the standard procedures given in this handout. 2. Determine the COD content of the two water/waste water samples using the closed reflux method (reactor digestion method) and the colorimetric procedure following instructions given by the commercial supplier (Hach Company). 3. Compare the COD values of the same water/waste water samples obtained from the two methods. 4. Comment on the pollution status of the two samples analyzed. Note: You are expected to write a Lab Report based on the COD and BOD analysis of the water/waste water samples provided (Practical 2 and 3). The Lab report should contain the following sections. Title Introduction (Do not repeat the whole description given in the practical handouts) Objectives Methodology (Write sentences, use impersonal form ) Results: short description of results in sentences. include raw data tables, calculations, final data tables under results. Discussion and Conclusions: Briefly discuss your main results. Discuss uncertainties (if any) associated with the final results and propose corrective measures. Compare your COD data obtained for the waste water samples with the Sri Lankan tolerance limits for industrial effluent discharge into surface waters. Any References you may have read and used in writing the report Due date for the lab report submission 22nd May 2015 References used for preparation of the practical: APHA (1999). Standard Methods for the Examination of Water and Wastewater. 20 th ed. American Public Health Association, Washington DC, USA UNEP/WHO (1996).Water Quality Monitoring- A practical guide to the design and implementation of freshwater quality studies and monitoring programmes, Chapman and Hall, Prof A Pathiratne/Department of ZEM/UOK,