Experimental theory
Chemical Oxygen Demand (COD) is one of the significant concerns of parametric indicators of water quality in wastewaters. It usually represents the quantity of oxygen that is consumable through redox reactions. The Korean ministry of the environment set the minimum COD of municipal wastewater to be 40mg/l in 2005 (Choi, 2007). This parameter is majorly concerned with how much oxygen would react with organic compounds in water. Thus, it is an essential aspect of water quality that determines the extent of contamination in regards to organic composition.
The measurement of chemical oxygen demand is important as it prevents adverse effects on aquatic life. Organic compounds in water utilize the oxygen present in water for oxidation (Ibanez et al. 2010). These organic compounds are converted to carbon dioxide. The general equation is represented as CnHaObNc + (n+a/4-b/2-3c/4)O2——-ànCO2 +(a/2-3c/2)H2O +cNH3. The oxygen in water is depleted. This depletion is disadvantageous to aquatic life as they entirely rely on dissolved oxygen for survival. Hence, the consumption of greater amounts of oxygen by organic compounds poses a significant risk of rendering water bodies inhabitable by aquatic animals such as fish.
The measurement of COD in a laboratory is expressed in mg/L or ppm. Usually, a sample of interest obtained from a bulk wastewater matrix is subjected to tests of COD, and its concentration can be determined either through titration or by use of spectrophotometer (Ibanez et al. 2010). Titration is more labor-intensive as it requires more laboratory equipment. Besides, the likelihood of having more significant erroneous measurements is high because it involves making observations, which sometimes might be wrong. Also, while calculating the COD manually, errors may arise and result in false findings. However, a more precise method of the method for the determination of COD utilizes spectroscopic techniques that provide results within the shortest time. Spectrophotometers measure the concentration of organic compounds in wastewater as a function of wavelength (Ibanez et al. 2010). This method is the fastest as it gives results instantly, thus reducing the time for COD analysis.
Furthermore, the spectroscopic equipment calculated the values of the concentrations automatically and presented the results in terms of mg/L or ppm. This implies that no erroneous measurements are undertaken, and thus the report provided is precise and more reliable. Usually, the process takes the least time. Therefore, this method is the most reliable.
The COD analysis uses organic oxidizers such as potassium permanganate and potassium dichromate. However, acidified potassium dichromate is the most preferred choice as the compound is a better oxidizer than potassium permanganate (Ibanez et al. 2010). Potassium dichromate contains the chromate ions that primarily oxidize organic compounds. Consequently, chromate ions are reduced to form reduced species, hence a transition in color. The completeness of the process is determined by a change of color from orange to pale green. Thus, its profound use is attributed to its ability to oxidize an array of organic compounds, ranging over 95%.
The general remix reaction is represented as CnHaObNc +dCr2O72- + (8d +c)H+——————ànCO2+{(a+8d-3c)/2}H2O + CNH4++ 2dCr3+.
Reagent and experimental apparatus
Diluted sulphuric acid
Potassium dichromate
Distilled water
Pipette
10mm-sized Vials
Digestor
Spectrophotometer
Experimental Methods
A standard solution of the COD sample was prepared and kept for use in the calibration of the UV-VIS spectrophotometer. The laboratory reactor was switched on to pier it early enough to allow the temperature to build up to 150°. A blank sample was prepared by sucking 2.0ml of deionized water and putting in one of the sample cell vials of size 10mm. Similarly, for the sample to be tested, 2.0ml was also sucked using a pipette and put into the second sample. Dilutions of x5, x10, and x20 were carried out to obtain three different concentrated samples for analysis. An oxidizer reagent of 0.2ml was sucked and put into the blank sample; the same volume was put into the samples to be tested. The contents were mixed thoroughly by shaking. The samples were introduced to the preheated reactor, where they were heated for two hours. The samples were removed and placed in a rack for cooling. After 10 minutes, the UV-VIS spectrophotometer was switched on, and a wavelength of 420nm was set. The cold blank sample was put into the spectrophotometer for calibration of the machine. The three diluted samples were then introduced one after another, during calibration after every single test. The readings of the spectrophotometer for each example were recorded for analysis.
Figure 1. A table of dilution factor and spectroscopic readings of concentrations.
| Dilution factor | Reading(mg/L) |
| X5 | 632 |
| X10 | 404 |
| X20 | 149 |
Figure 2. A graph of concentration of COD vs. Dilution factor of samples
Discussion of results
The readings for the diluted wastewater samples showed lower values at higher dilutions. Thus, from the curve of concentration against UV-VIS readings, concentration of COD of the sample was identified to be approximately 700mg/l when the graph was extrapolated. This value is far much higher than the recommended amount of 40mg/l approved by the MOE, 2005.
References
Choi, E. (2007). Piggery waste management: Towards a sustainable future. London: IWA Publishing.
Singh, R. L., & Singh, R. P. (2019). Advances in Biological Treatment of Industrial Waste Water and their Recycling for a Sustainable Future. Singapore: Springer Singapore.
Ibanez, J. G., Carmen, D. S., Infante, A. F., Singh, M. M., & Esparza, M. H. (2010). Environmental chemistry: Fundamentals. New York, NY: Springer.