ANC Control

Acid Neutralizing Capacity (ANC) Control Team

OVERVIEW
Drinking water needs to have a pH of about 7 for effective treatment. During the process of water treatment, alum is added to remove organic particles and clay in the process of flocculation. Alum decreases the pH of water making it acidic for low alkaline waters. Moreover, the neccessary alum dosage is very sensitive to pH changes in the entering water and vice versa. This is often the case in many of the Honduran plants. For this reason it is imperative to ensure that the pH of the water entering the flocculator is maintained near 7. Lime ( Calcium Hydroxide) is an alkaline substance popularly used to neutralize low pH water. By devising a system (lime feeder) which will continuously supply a dose of saturated lime into the entrance tank, the target pH of the water as desired.

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 causes the pH of treated water to go as low as 4.5, interfering with alum precipitation and affecting the alum dosages. Consequently, plant performance is suffering. The ANC team has hence been presented with the opportunity to re-design a model for a lime feeder system that could increase the pH of entering water to at least above 6.5, without the use of electricity.

OBJECTIVE

The main objective of the team is to design a lime feeder system to deliver effluent with a pH of around 12 +(saturated pH of water) to be mixed in with treated water to increase its pH. For efficient plant operation, the lime feeder design must be simple, easy to construct and install, and be cost-effective while also being robust enough as to avoid using electricity and to not require maintenance more than once a day.

Previous Research by the AguaClara team in Fall of 2005 to Spring of 2006

The lime feeder research team investigated the possibility of building a lime feeder for the AguaClara plant at Ojojona. In the end flocculation at Ojojona was successful without lime addition and the lime feeder project was dropped. Daniel Smith, Fulbright Scholar, is [evaluating the performance of AguaClara water treatment plants in Honduras]. He is routinely measuring pH and alkalinity of the raw and treated waters. From Dan's research we know that low alkalinity waters are common in Honduras and that setting an appropriate alum dose is extremely difficult because addition of a little bit too much alum consumes all of the alkalinity and results in low pH water. At low pH the solubility of

increases
[ANC control with Lime|^ANC control with Lime.docx] by Amy Yeh and Melina Diaconis
The following analysis needs to be reviewed and converted to analysis in MathCAD. The analysis includes cost comparisons for ANC control using lime,

, and sodium carbonate,

. The mass of the chemical stocks per day is also calculated.
[Preliminary chemistry analysis spreadsheet|^ANC control.xlsm]