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 alum precipitation and affecting alum dosages. 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
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\large
$$
Al_2 (SO_4 )_3 + 6H_2 O = 2Al(OH)_3 + 6H^ + + 3SO_4^
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$$
to release 6 hydrogen ions (H+). These 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 alum is the least soluble in this region and will thus precipitate and stick to clay and organic particles, cleaning the water.
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- Figure 1 - Solubility curve for Alum (Aluminum Hydroxide). Target pH range is shown in orange.
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. To do this, a lime feeding system that will continuously supply a dose of saturated lime water (with a pH of 12) into the entrance tank is 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 treated 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
while the current research is detailed in the link below.
Current research (Fall '09)
The tasks carried out by the present team on a weekly basis are detailed in the ANC Weekly Minutes