Auburn University Department of Civil Engineering Introduction to Environmental Engineering CIVL Laboratory #6 Chemical Oxygen Demand (COD)

Transcription

1 Auburn University Department of Civil Engineering Introduction to Environmental Engineering CIVL Laboratory #6 Chemical Oxygen Demand (COD) Experiment Performed by: Anna Breland Sam Broder Jenny Walter November 14, 2013

2 Table of Contents Pg. 1.0 Objective Background Information Results Conclusions References 5 Appendix Appendix A- Procedure Appendix B- Raw Data

3 1.0 Objective The objective of Lab #6 was to become familiar with the biological processes that environmental engineers work with, as well as, learn how to measure the chemical oxygen demand (COD) in a sample using the colorimetric method. To perform the lab, first the COD reactor needs to be preheated to 150 degrees Celsius. For the 2 prepared samples, 2 ml of 275 mg/l glucose (C 6 H 12 O 6 ) diluted to 1 L of deionized water is added to one vial and 2 ml of 185 mg/l sucrose (C 12 H 22 O 11 ) diluted to 1 L of deionized water is added to the second vial. Once the caps are screwed on tightly, the 2 vials should be inverted 10 times while being held with a towel. The blank sample should be prepared similarly, however 2 ml of deionized water should be added to the vial. All 3 vials should be rinsed with water before inverting. After mixing the samples, they should be placed in the reactor for 2 hours and then the reactor should be turned off, and allowed to cool in the reactor for 25 minutes. The vials then should be placed in a dark place at room temperature for 30 minutes for further cooling. After cooling, the photometer can be used to measure and record the COD of each sample. The blank sample should be read first. 2.0 Background Information Biological oxygen demand (BOD) is the amount of dissolved oxygen required by aerobic microorganisms to break down organic material in water. BOD occurs naturally from decaying plant and animal matter and helps regulate dissolved oxygen levels. However, BOD originates in much higher levels from residential and industrial wastewater. Since specific dissolved oxygen levels are an integral part of how an ecosystem functions, releasing wastewater with high BOD levels can have serious negative effects on a natural ecosystem. As a result BOD is regulated as a pollutant and is used in wastewater treatment plants to determine the efficiency of the treatment process. Regulations on wastewater limit effluent BOD to a weekly average of 45 mg/l or a 3

4 monthly average of 30 mg/l (Tchobanoglous, 6). Chemical oxygen demand (COD), similarly, is the dissolved oxygen required to oxidize organic matter by chemical reaction, not biological reaction. The COD includes the BOD, biologically inert organic matter, and inorganics. Measuring COD is much faster than BOD so typically a ratio of BOD/COD is established for a specific process and BOD is estimated from COD (CBOD). Regulations on wastewater treatment limit CBOD to a weakly average of 40 mg/l and a monthly average of 25 mg/l (Tchobanoglous, 6). COD is typically measured using the colorimetric method, which uses potassium dichromate (a strong oxidizer) to oxidize organic matter, which produces potassium chromate. Potassium dichromate and potassium chromate each filter for a different, specific wavelength of light, which can be read in a photometer. The ratio of different wavelengths indicates what percentage of the potassium dichromate was reduced. Theoretically, the BOD and COD can be calculated if you know the chemical reactions that take place and the quantities of reactants and products. The theoretical oxygen demand (ThOD) is found using the stoichiometric ratios of the chemical equations and the number of electrons that need to be oxidized. Although the ThOD can be used to calculate the BOD or COD based on what reactions are required, the actual oxygen demand will never reach the ThOD because no natural process is perfect. 3.0 Results Sample # COD (mg/l) Average COD (mg/l) Sample # ThOD (mg/l)

5 4.0 Conclusions The redox reaction Cr 2 O H + + 6e - 2Cr H 2 O requires high temperatures and acidic conditions. The samples were heated in this lab to match the reaction conditions, hence encouraging the reaction to occur. Sulfuric acid (H 2 SO 4 ) was added to provide the hydrogen ions required. Potassium dichromate (K 2 Cr 2 O 7 ) was added to serve as the electron acceptor. This was added in excess to ensure that the organic compounds would be completely oxidized. The mercuric sulfate (HgSO 4 ) was added so that it would form complexes with the chlorine ions. Chlorine ions tend to interfere with the determination of the COD, being an oxidizable inorganic. Without the mercuric sulfate, the chlorine ions would cause an erroneously high COD result. The ThOD was found to be much smaller than the COD. This is expected since the oxidation process is not perfect and will therefore require more oxygen than calculated. Errors in this lab could have resulted from high concentrations of reduced inorganic compounds being in the sample, or from the organic compounds not being completely oxidized. Human error would have also affected the results. 5.0 References Tchobanoglous, George. "Wastewater Engineering: Treatment and Reuse.". Metcalf & Eddy, inc.. Web. 13 Nov < ddy.pdf>. 5

6 Appendix A Lab #6: Chemical Oxygen Demand (COD) Objective: The objectives of this lab are to familiarize with the biological processes of environmental engineering and to measure chemical oxygen demand (COD) using the colorimetric method. Background: Having knowledge of biological processes is important for environmental engineers to be able to describe and design natural and engineered systems. These processes affect the fate and transport of pollutants and also the stimulation of plant and algal growth. Biological processes are also often used to remove contaminants in water treatment facilities. Biological activity must have a food source to support the organisms. Examples of sources of food include solar radiation, oxidation of chemical species, and organic compounds. Microorganisms use dissolved oxygen to convert the food source into energy for growth and reproduction. Various ways of measuring oxygen required for microbial metabolism include biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Biochemical oxygen demand (BOD) BOD is used to determine the oxygen requirements of wastewaters, effluents, and polluted waters. It provides an indirect measure of the concentration of organic and inorganic material. BOD tests measure the amount of oxygen consumed by microorganisms in decomposing organic matter and measures the chemical oxidation of inorganic matter. There are two main methods used to quantify BOD: ultimate BOD (BOD U ) and BOD-five (BOD 5 ). Theoretically, BOD U takes an infinite amount of time to reach. Therefore, BOD 5, or the oxygen consumed during a 5-day test period, is often used as a surrogate. BOD is important because it directly affects the amount of dissolved oxygen in rivers and streams. Higher BOD means more rapid oxygen depletion which yields less oxygen available for aquatic life. This causes aquatic organisms to become stressed, suffocate, and die. Because high BOD is so critical to aquatic life, every receiving body of water has limits on the amount of BOD it can receive. Each wastewater treatment plant has a permit listing the allowable levels of BOD 5 in its effluent. These permits are called NPDES (National Pollutant Discharge Elimination System) permits and are regulated state by state. Wastewater treatment plants also monitor BOD as an indication of their treatment efficiency. Sources of BOD include leaves and woody debris, dead plants and animals, animal manure, effluents from pulp and paper mills, wastewater treatment plants, feedlots and food processing plants, failing septic systems, and urban stormwater runoff. Chemical oxygen demand (COD) The COD test may be used as an alternative to the BOD test. COD data can often be interpreted in terms of BOD values after sufficient data has been accumulated to determine a correlation between COD and BOD. While both tests quantify the electron acceptor required to oxidize organic material, COD is defined as oxidation by a chemical (instead of a biological) reaction. During COD testing, potassium dichromate (K 2 Cr 2 O 7 ) serves as the electron acceptor. Because strong oxidizing conditions are provided, complete oxidation usually occurs within just few 6

7 hours. This is a strong advantage over BOD testing. However, the COD test is unable to differentiate between biologically oxidizable and biologically inert organic matter. Procedure: 1. Preheat COD reactor to 150 C. 2. Remove the caps from two COD Digestion Reagent vials. 3. Prepared sample: add 2 ml of sample to the vial. 4. Blank preparation: add 2 ml of deionized water to the vial. 5. Replace and carefully tighten cap. Rinse them with water and wipe down with a clean paper towel. 6. Wrap the tube in a towel, hold by cap, and invert gently 10 times to mix. CAREFUL: sample vials become very hot during mixing. 7. Heat the vials for 2 hours, turn off the reactor 8. Allow vials to cool in the reactor to 120 C (~25 minutes). 9. Remove vials from the reactor and place in the dark. Allow to cool to room temperature (~30 minutes). 10. Place blank vial in photometer and zero the instrument. 11. Place sample vial in photometer. Measure and record the COD of each sample. Appendix B Average COD Sample 1 Sample 2 Theoretical OD (ThOD) Using equation C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O Sample 1 Using equation C 12 H 22 O O 2 12CO H 2 O Sample 2 7