Lecture 3: Storage and preservation of Dissolved Oxygen, BOD and COD

1. The Islamic University of Gaza- Environmental Engineering Department Environmental Measurements (EENV 4244) Lecture 3: Storage and preservation of Dissolved Oxygen, BOD and COD Prepared by Husam Al-Najar

2. 1. Dissolved Oxygen (DO) Dissolved oxygen (DO) levels in natural and wastewaters depend on the physical, chemical, and biochemical activities in the water body. The analysis for DO is a key test in water pollution and waste treatment process control. Collection of Samples Methods of sampling are highly dependent on source to be sampled and, to a certain extent, on method of analysis. Do not let sample remain in contact with air or be agitated, because either condition causes a change in its gaseous content. Samples from any depth in streams, lakes, or reservoirs, and samples of boiler water, need special precautions to eliminate changes in pressure and temperature. Collect surface water samples in narrow-mouth glass-stoppered BOD bottles of 300-mL. Fill bottle to overflowing (overflow for approximately 10 s), and prevent turbulence and formation of bubbles while filling. Record sample temperature to nearest degree Celsius or more precisely.

3. Preservation of Samples Determine DO immediately on all samples containing an appreciable oxygen or iodine demand. Samples with no iodine demand may be stored for a few hours without change after adding manganous sulfate (MnSO4) solution, alkali-iodide solution, and H2SO4, followed by shaking in the usual way. Protect stored samples from strong sunlight and titrate as soon as possible. For samples with an iodine demand, preserve for 4 to 8 h by adding 0.7 mL conc H2SO4 and 1 mL sodium azide solution (2 g NaN3/100 mL distilled water) to the BOD bottle. This will arrest biological activity and maintain DO if the bottle is stored at the temperature of collection or water-sealed and kept at 10 to 20°C. As soon as possible, complete the procedure, using 2 mL MnSO4 solution, 3 mL alkali-iodide solution, and 2 mL conc H2SO4 .

4. Dissolved Oxygen analysis methods The electrometric method using membrane electrodes Based on the rate of diffusion of molecular oxygen across a membrane Iodometric method and its modifications A titrimetric procedure based on the oxidizing property of DO The choice of procedure depends on the interferences present, the accuracy desired, and, in some cases, convenience or expedience.

5. 2. Biological Oxygen Demand (BOD) The biochemical oxygen demand (BOD) determination is an empirical test in which standardized laboratory procedures are used to determine the relative oxygen requirements of wastewaters, effluents, and polluted waters. The test has its widest application in measuring waste loadings to treatment plants and in evaluating the BOD-removal efficiency of such treatment systems. The method consists of filling with sample, to overflowing, an airtight bottle of the specified size and incubating it at the specified temperature for 5 days. Dissolved oxygen is measured initially and after incubation, and the BOD is computed from the difference between initial and final DO. Because the initial DO is determined shortly after the dilution is made, all oxygen uptake occurring after this measurement is included in the BOD measurement. Sampling and storage: Samples for BOD analysis may degrade significantly during storage between collection and analysis, resulting in low BOD values. Minimize reduction of BOD by analyzing sample promptly or by cooling it to near-freezing temperature during storage.

6. 1) Grab samples: If analysis is begun within 2 h of collection, cold storage is unnecessary. If analysis is not started within 2 h of sample collection, keep sample at or below 4°C from the time of collection. Begin analysis within 6 h of collection; when this is not possible because the sampling site is distant from the laboratory, store at or below 4°C and report length and temperature of storage with the results. In no case start analysis more than 24 h after grab sample collection. When samples are to be used for regulatory purposes make every effort to deliver samples for analysis within 6 h of collection. 2) Composite samples: Keep samples at or below 4°C during compositing. Limit compositing period to 24 h. Use the same criteria as for storage of grab samples, starting the measurement of holding time from end of compositing period. State storage time and conditions as part of the results.

7. 3. Chemical Oxygen Demand (COD) Chemical oxygen demand (COD) is defined as the amount of a specified oxidant that reacts with the sample under controlled conditions. The quantity of oxidant consumed is expressed in terms of its oxygen equivalence. Because of its unique chemical properties, the dichromate ion (Cr2O72–) is the specified oxidant it is reduced to the chromic ion (Cr3+) in these tests. 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. COD often is used as a measurement of pollutants in wastewater and natural waters. Other related analytical values are biochemical oxygen demand (BOD), total organic carbon (TOC), and total oxygen demand (TOD). In many cases it is possible to correlate two or more of these values for a given sample. BOD is a measure of oxygen consumed by microorganisms under specific conditions; TOC is a measure of organic carbon in a sample;

8. Sampling and Storage Preferably collect samples in glass bottles. If delay before analysis is unavoidable, preserve sample by acidification to pH ≤ 2 using conc H2SO4. Blend (homogenize) all samples containing suspended solids before analysis. If COD is to be related to BOD, TOC, etc., ensure that all tests receive identical pretreatment. Make preliminary dilutions for wastes containing a high COD to reduce the error inherent in measuring small sample volumes.