Acid Neutralizing Capacity (ANC) Control Team

OVERVIEW 

Research on designing a lime feeder (for the plant at Ojojona) had been carried out until the Spring of 2006 by the former ANC control team. It was discontinued because the plant at Ojojona was working successfully without the need of lime feeders. However, presently, it has been reported that reduced alkalinity in Honduran source water is interfering with aluminum hydroxide precipitation, which facilitates particles sticking together and is important for floc formation. Consequently, plant performance is suffering because insufficient quantities of Aluminum Hydroxide are precipitating. The lime feeder model needs to be re-designed so that it can increase the pH of entering water to at least 6.5, without the use of electricity.

When alum is introduced into the entrance tank, it reacts with influent water according to the following equation

to release 6 hydrogen ions (H+). Hydrogen ions increase the acidity of the water, making the precipitation of alum less effective. As seen in the solubility graph in figure 1, the pH range for effective coagulation is 6.5 to 7.5. This is the target zone since the aluminum ion is the least soluble in this region and will thus precipitate and coat the surface of clay and organic particles. The coating increases the efficiency of subsequent flocculation and sedimentation processes with the goal of removing the majority of these particles from water.

 
Acid Neutralizing Capacity (ANC) is the measure for the overall buffering capacity against acidification for a solution. ANC is achieved by raising the alkalinity of the water. Alkalinity, is the ability of a water to neutralize acid; the higher the alkalinity, the higher the capacity to neutralize water. To increase the alkalinity of raw water and thus buffering capacity of raw water, a lime feeding system that will continuously supply a dose of saturated lime water (with a pH of 12) into the entrance tank as needed.

OBJECTIVE

The main objective of the team is to design a lime feeder system to deliver effluent with a pH of 12 (saturated pH of limewater) to be mixed in with the influent surface water to increase its pH. For efficient plant operation, the lime feeder design must be simple, easy to construct and install, and cost-effective. It must also be robust, avoiding the use of electricity and be easy to maintain, such that the operator will only have to add a specific amount of lime every 24 hours.

The results of the previous experiments (Fall 2005- Spring 2006) can be found here
The results of Fall '09 experiments can be found here
while the current research is detailed in the link below.
Current research (Spring '10)

The tasks carried out by the present team on a weekly basis are detailed in the ANC Weekly Minutes