The Effect of Flow-Distance of Industrial Wastewaters on Correlation between Chemical and Biochemical Oxygen Demands

Abstract

Aims: The aim of this study was to investigate the influence of flow-distance on the empirical correlation between biochemical oxygen demand (BOD₅) and chemical oxygen demand (COD) of wastewaters from a cluster of industries. Study Design: Wastewater samples were collected at point-sources and at 25, 50, 75 and 100m distances from the point-sources. Place and Duration of Study: Nasco Household Products Ltd. Laboratory, Jos, Nigeria. Sampling was done twice a week for a full seasonal cycle at peak production period (morning and evening). Methodology: The samples pH were adjusted to 7 using 0.5M acid for basic samples and 1M alkali for acidic samples, de-chlorinated using 0.0125M Na₂S₂O₃ and seeded when necessary and then diluted with de-ionized water. The dissolved oxygen (DO) content of the diluted sample was determined before and after incubation for 5 days by azide modified Winkler’s method. The difference, taking into account the dilution, gave the BOD5 of the sample. For COD the sample to be measured was oxidized under reflux by 0.125M potassium dichromate and 2mg of sulphamic acid with silver sulphate as a catalyst and a suppressor of chloride interferences. Organic matter reduced part of the dichromate and the remainder was determined by titration with iron (II) ammonium sulphate using ferrion as indicator. Results: The empirical correlation between COD and BOD5 was generally maintained despite the distance of flow of the wastewater from point-source and contributions of washings from farmlands and other human activities. COD and BOD₅ of the wastewaters from different sources highly correlated with correlation coefficients ranging 0.93960.9985. Conclusion: The correlation between COD and BOD₅ for wastewaters was not affected by flow distance. The correlation equations for the industries may therefore be used to deduce rapid effluent quality from chemical oxygen demand (COD) of sample from any point along the effluent flow.