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BOD and COD to characterise wastewater

Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are two of the most important parameters to characterise (measure the degree of pollution) of wastewater.

BOD, the biological demand for oxygen a source of water has, is the amount of oxygen used by microorganisms to break down organic substances. These are usually bacteria (aerobic or anaerobic), yeasts and plankton contained in the water. It is a measure of the degree of contamination and is expressed in mgO2/L. Continue reading


Types of coagulants and flocculants

There is a wide range of commonly used coagulants and flocculants for the clarification of various types of wastewater.

In general, they can be classified into two types: inorganic and organic. The inorganic compounds are usually metal salts, typically aluminium or iron (with the most widely used being aluminium sulfate, iron sulfate and iron chloride); while the organic compounds are polymers (polyelectrolytes), and represent a wide variety of water soluble macromolecular compounds of natural or synthetic origin, which have the property of enhancing the flocculation of suspended particles. Continue reading

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The mechanism of coagulation and flocculation

An essential step in the decontamination of water is clarification, which is the physical-chemical process aimed at removing the particles (suspended solids) that cloud water by precipitating them in the form of sludge.

Flocculation is the chemical process of adding substances, known as flocculants, to cause the colloidal substances in the water to stick together, thus facilitating their subsequent filtration and removal. The flocculation process is preceded by coagulation, so we often speak of coagulation-flocculation processes. Continue reading