Principles, Procedures, and Heavy Metal Management of Dichromate Reflux Method for COD Determination in Laboratories

Authored by: Mu-Hao Sung Wang , Lawrence K. Wang , Eugene De Michele

Handbook of Advanced Industrial and Hazardous Wastes Management

Print publication date:  October  2017
Online publication date:  October  2017

Print ISBN: 9781466513419
eBook ISBN: 9781315117423
Adobe ISBN:


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Under sponsorship of Water Environment Federation, the standard procedure for chemical oxygen demand (COD) determination as outlined in the APHA/AWWA/WPCF Standard Methods for the Examination of Water and Wastewater was simplified for use by the waste treatment plant operators and researchers. Specifically, the principles, analytical procedures, and waste management processes of dichromate reflux method for rapid COD determination in laboratories are presented and discussed. Proper handling and treatment of spent COD liquid wastes for safe disposal are discussed and emphasized in this chapter. The spent COD testing wastewater contains high concentrations of sulfuric acid, mercuric ions (Hg2+), silver ions (Ag+), and dichromate salts, which are considered to be hazardous wastes and must be handled properly by the operators, chemists, and engineers in the laboratories. Normally a laboratory is a conditionally exempt small quantity generator if it generates no more than 100 kg of hazardous waste in a calender month. Such facilities may be allowed to treat hazardous wastes on-site. This chapter discusses several on-site treatment processes for possible adoption by small laboratories. It is important to note that even if federal regulations allow on-site treatment of COD testing wastewater, the State or local regulations may prohibit it. Hazardous sulfuric acid in the spent COD waste can be neutralized by sodium hydroxide or equivalent alkaline chemicals, such as potassium hydroxide; sodium hydroxide is cheaper. Before neutralization of COD wastewater (18 M hydrogen ion) by concentrated sodium hydroxide solution, the wastewater should be diluted with water by at least sixfold. With the sixfold dilution and adequate mixing, neutralization process increases wastewater temperature to about 40°C when sodium hydroxide is added to acidic COD wastewater carefully over a 5-min period to avoid boiling. The COD wastewater must be diluted by a factor of 6 also to avoid crystallization and precipitation

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