Alum Dosing Theory
Alum 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. Alum (aluminum sulfate) is hydrolzed in water producing aluminum-hydroxy species. At a pH range between 6-7.5, aluminum hydroxide can precipitate in appreciable quantities out of solution and onto the surface of colloidal particles.
At circumneutral pH, aluminum hydroxide has a slightly positive charge on the surface and thus neutralizes the negative surface charge on colloid particles so they are more likely to stick together. Ideally the raw water and coagulant are thoroughly mixed in rapid mix. The particles collide and grow in a flocculator. 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.
Ideal dosing
Ideal dosing is the alum dose at a specific water chemistry, flocculator, and upflow clarifier conditions that gives the best performance. Since it is theoretically impossible to find an ideal dose, we find ranges of alum doses considered to be in the range of an ideal dose to give the best performance.
Underdosing
An underdosed system is lower than what is considered the range of the ideal alum dose. 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. The floc blanket also takes longer to grow.
Overdosing
An overdosed system is higher than what is considered the ideal range of alum 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. The floc blanket takes less time to grow.
In both underdosed and overdosed conditions, the plate settler effluent may still provide an effluent turbidity less than 1 NTU.