Alum Dosing Theory
Alum, or aluminum hydroxide, is added to raw water in order to coagulate the particles suspended in the water. Before alum is added, the particles have a slight negative surface charge and therefore repel one another. Dosing the water with alum neutralizes the charge on these particles so they are more likely to stick together. The particles are allowed to grow in a flocculator, where the dosed water is thoroughly mixed in order to disperse the alum and promote the flocculation of the particles. These conglomerates are referred to as flocs.
Following the flocculation, the water enters a sedimentation tank, where the flocs begin to settle. This settling creates a "blanket" of flocs, which serves to trap more flocs and provide effective filtration as it grows. The floc blanket is an integral part of the AguaClara technology and enables the system to achieve much lower effluent turbidity.
Underdosing
An underdosed system occurs when the alum dosing is lower relative to the dosing conditions that are considered "ideal." In this situation, the flocs that are formed will be made up of a larger portion of suspended particles, which in our system are clay particles, and a smaller portion of the flocs will be composed of the alum. This results in a smaller floc and, consequently, a more dense floc blanket.
Overdosing
An overdosed system is accomplished by providing an alum dose that is at a higher concentration than what is considered to be the "ideal" dose. In this case, the flocs that are formed will have a higher alum-to-suspended-particle ratio. The flocs are therefore larger and more "fluffy" and the resulting floc blanket is less dense.