Acid Neutralizing Capacity (ANC) Control Team

Overview

Abstract

The ANC Control team has conducted laboratory research to investigate the feasibility of using a lime feeder to add alkalinity to the influent water of AguaClara plants. This was necessary in order to maintain the pH within the ideal range for flocculation after the addition the chemical coagulant, alum, which has an acidic effect. Low-alkalinity source waters in Honduras provide little buffering capacity themselves. The team has found that proposed lime feeder designs consistently fail to produce saturated effluent for a length of time which makes them economically and practically viable. The failure is believed to be caused by precipitation of calcium carbonate on the calcium hydroxide solid surfaces. The recent availability of poly-aluminum chloride, an alternative coagulant to alum with a much smaller acidic effect, in Honduras has reduced the need for lime feeders with AguaClara plants.

Introduction and Objectives

Flocculation in AguaClara plants relies on sweep coagulation of particles, which works optimally within a pH range of 6.5 to 7.5. The addition of alum, the chemical coagulant generally used with AguaClara plants, causes aluminum hydroxide to precipitate, lowering the pH of the influent water. Low alkalinity source waters often found in Honduras provide little buffering capacity to counter the acidity of alum, so the pH often falls below the ideal range for flocculation performance. The team would like to add a base to the plant flow along with alum to provide acid neutralizing capacity and maintain a more desirable pH.

Calcium hydroxide, or slaked lime, is an inexpensive and readily available chemical used in making tortillas in Honduras. Because it is relatively insoluble, chemical dosing using a pre-mixed stock solution saturated with calcium hydroxide would require impractically large volumes of water. Instead, the team has investigated the possibility of using a lime feeder to continuously deliver calcium hydroxide-saturated effluent to the influent water. A lime feeder is a reactor in which lime is continuously dissolving in the water flowing through a fluidized bed of solids which are kept from being carried out with the effluent.

During laboratory experiments with lime feeders the concentration of the effluent consistently drops prematurely. The team now believes that the failure to maintain saturated effluent for the anticipated duration of a run, calculated based on the amount of lime initially added, is due to calcium carbonate precipitation within the reactor which inhibits dissolution of calcium hydroxide.

An alternative coagulant, poly-aluminum chloride (PAC), which is now economically competitive with alum in Honduras due to the recent availability of a transport-friendly crystalline form, does not consume alkalinity to the same degree as alum when dissolved in the influent. AguaClara Engineers in Honduras have tested the chemical with AguaClara plants very successfully with respect to both maintaining optimal pH and reducing raw water turbidity. We anticipate that all AguaClara plants will soon adopt PAC as their coagulant, reducing the demand for lime feeders because it will no longer be necessary to buffer against high acidity. The Fall 2010 ANC Control team has decided to discontinue research with lime feeders because the declining need no longer warrants the difficulty of the research. The team will now focus on writing a final report which organizes all of the knowledge produced by ANC Control teams to facilitate any future lime feeder investigations.

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 objective of the ANC Control team has been to design a lime feeder system to deliver saturated calcium hydroxide effluent to be mixed in with the influent surface water to increase its alkalinity. The lime feeder design must be inexpensive and simple to construct, install, and operate. As with all AguaClara technology, it cannot include 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.

Research Previous research (Fall 2005- Spring 2006) can be found here
The results of Fall '09 experiments can be found here
The results of Spring '10 research can be found here
The results of Summer '10 research can be found here
Current research (Spring '10)

The tasks carried out by the present team on a weekly basis are detailed in the ANC Weekly Minutes. Keep up to date with our detailed task list and future challenges.