An Investigation into Organic/Inorganic Coagulants and their Effectiveness in Coagulating Colloidal Water
Harrison Lee
Stratford Hall
Floor Location : S 012 V

In numerous parts of the world, clean drinking water is not readily accessible, forcing people to resort to unsafe sources which have a possibility of containing disease. In fact, one out of every ten people rely on unimproved sources of water. For my science fair project, I was interested in exploring chemical purification. My interest for this topic originated from a unit in my geography course that focused on water scarcity and filtration systems. This ultimately led me to the subject of coagulants: my concentration for this investigation.

Water sources may contain particulates, pathogens, and dissolved heavy metals. Each of these contaminants need to be treated differently before the water is safe for drinking. Water sources, such as rivers and streams, can contain large amounts of silt, clogging filters and this interferes with chemicals used for further filtration. This silt makes the water turbid. When water is turbid, it is clouded by colloids: small suspended solids that remain floating due to the molecular movements of water molecules. These colloids can trap microorganisms making disinfection difficult, increasing the likelihood of contracting disease. Colloids are challenging to remove as they are microscopically small and are unable to settle on their own. Through a process known as coagulation, colloids are forced to clump together under the presence of a coagulant, allowing them to sink so they can later be removed. Coagulation is thus a very effective first step in the water treatment process.

There are two main types of coagulants, organic and inorganic, and their use is dependent on the colloids found in the turbid water. Organic coagulants are polymer based and trap colloids between their long carbon chains, creating flocs. Inorganic coagulants act on the charged properties of colloids. Colloids inherently have negatively charged surfaces that attract positive ions creating a double layer and a repulsive force: this prevents colloids from clumping together. When an inorganic coagulant is introduced, it adheres to the colloids, compressing the double layer allowing them to agglomerate.

The purpose of this investigation was to determine the effectiveness of various coagulants that could be used to treat turbid water. The effectiveness of each coagulant was measured by their ability to decrease the turbidity of a controlled turbid suspension.

Calcium bentonite clay was chosen as the material to create the controlled suspension of turbid water. This clay is commonly found in many parts of the world and has excellent colloidal properties; it is, therefore a good representation of non-setting solids. The coagulants chosen are typically used in industrial filtration systems. They were a) organic coagulant: polyacrylamide and b) inorganic coagulants: aluminum sulfate and iron sulfate.

Under the given conditions, it was discovered that the inorganic coagulants were more effective in decreasing the turbidity of the controlled turbid suspension than the organic coagulant. Between the two inorganic coagulants, iron sulfate was most effective. Future experimentation could look into completing tests with other coagulants along with using turbid water from an existing natural source.