Clean water and wastewater contain suspended solids made of particles, as well as colloidal or dissolved substances. Suspended solids consist of particles with diameters greater than 0.001 nm. Colloidal substances have particle sizes ranging between 10-3 mm and 10-6 mm. Smaller particles are classified as dissolved matter. These are substances dispersed as molecules or ions, with diameters ranging from 2×10-8 cm to 10×10-8 cm. Dissolved substances cannot be removed from water by filtration. Suspended solids with diameters larger than 0.01 mm will settle. Colloidal materials, on the other hand, can be removed from water through coagulation. Colloidal materials have two key characteristics:
- The ability to adsorb other substances onto their surfaces.
- The possession of electrical charges on their surfaces, which gives them electrokinetic properties.
Colloidal particles acquire electrical charges depending on the surrounding environment. When two electrodes are placed in a colloidal solution and a direct current is applied, the particles begin to move toward one of the electrodes. Electrically charged particles either attract or repel each other.
The main organic compounds found in wastewater include proteins, carbohydrates, fats, petroleum residues, and urea. In addition, various synthetic organic molecules such as surfactants, phenols, and pesticides are also present. In a moderately polluted wastewater, approximately 75% of suspended solids and about 40% of filterable substances are of organic nature.