Independent Evaluation of a Mercury-free Commercial Chemical Oxygen Demand Test Kit for the Analysis of Ethanol Distillery Wastewater
DOI:
https://doi.org/10.26534/kimika.v35i1.24-35Keywords:
chemical oxygen demand, commercial kit, titrimetric method, colorimetric methodAbstract
Chemical oxygen demand (COD) is a measure of the oxidizable organic compounds present in a sample and is commonly used as an indicator of water quality. Chloride ions, especially in high concentrations, present a significant interference in COD analysis, and could lead to positively or negatively deviated results. To counteract this interference, mercury-containing reagents, despite being known to be hazardous, are used. In this study, the performance of a commercial Hg-free COD test kit was compared to that of standard protocols from the American Public Health Association (APHA) (colorimetric and titrimetric methods) through the analysis of synthetic samples with known COD values and of actual wastewater samples from an ethanol distillery. Results indicate no significant difference (P < 0.05) between COD percent recoveries when synthetic samples without chloride (Cl-) were analyzed: titrimetric method = (95.02 ± 2.84)%; colorimetric method = (94.05 ± 2.44)%; and commercial test kit = (97.57 ± 3.19)%. However, the COD is severely underestimated (P < 0.05) in the presence of a high concentration of Cl- (1000 mg/L) titrimetric method (91.21 ± 4.80)%; colorimetric method (89.82 ± 4.41)%; and commercial test kit (77.39 ± 0.99)%. In the analysis of the two wastewater samples (Mill Wastewater and Raw Spent Wash), the COD values from the test kit are notably higher compared to the colorimetric and titrimetric methods. Specifically, for the Mill Wastewater sample (with 5535 mg/L Cl-), the COD values obtained from the kit are (18.66 ± 7.23)% and (35.26 ± 8.24)% higher than the values obtained from the titrimetric and colorimetric methods, respectively. Following a similar trend for the Raw Spent Wash samples (with 22140 mg/L Cl-), the COD values obtained from the kit are (16.50 ± 9.02)% and (19.22 ± 15.59)% higher than the values obtained from the titrimetric and colorimetric methods, respectively. Therefore, despite the Hg-free COD kit’s advantage of being environmentally friendly, it is not recommended for the analysis of wastewater samples with very high chloride content.
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