Chemical Oxygen Demand- Acid Waste Disposal and Recovery of Mercuric Sulphate to minimise the test impact on our environment

摘要

Johannesburg water with the mandate to provide water and sanitation services to over three million residence of the city of Johannesburg has the responsibility to monitor and ensure compliance of drinking water, environmental water and industrial effluent that is discharged to the system. Over 500 samples are monitored a month at an ISO17025:2005 accredited facilities to ensure compliance with by-laws. Johannesburg water stands by the resolutions for a better tomorrow by preserving our environment and we are committed to the reduction of hazardous material and toxic waste in our tests. Chemical Oxygen Demand (COD) test assesses all chemically oxidizable substances by using Potassium Dichromate as the oxidizing reagent and a strong acid (Sulphuric Acid) each organic compound differs in the amount of oxygen necessary for complete oxidization, the COD test reflects the effect of an effluent on the receiving stream more directly than measurement of carbon content. COD test releases a waste that contains of approximately 25% Sulphuric acid, 360mg/LCr3~+ from reduced potassium dichromate, 1500 mg/L Hg2~+ from Mercuric Sulphate that works as a complex reagent for interferences and 1600 mg/L Ag~+ from the silver sulphate catalyst, this waste is very hazardous and toxic and require very strict and expensive waste management to minimize the effect of this constituents on the environment. This paper aims to discuss and quantify the method to recover chemical oxygen demand (COD) waste and reuse the mercuric sulphate from the waste to save costs and the environmental impacts of the hazardous waste. Experiments were conducted on solutions containing mercury and silver and on actual wastes from chemical oxygen demand to assess mercury and silver recovery on a pilot scale to assess the reduction potential of the material, Reaction order, reaction rate, removal efficiency, and stoichiometry of the reaction were evaluated.