OVERVIEW
Drinking water needs to have a pH of about 7. 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 below the safety limit. 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 neautralize 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 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.
INTRODUCTION
The former ANC group experimented on different designs for a lime feeder, including a column model, a conical vessel, a funnel - column and lastly an inverted traffic cone model. The most effective design was the inverted cone because it supplied saturated limewater at a pH between 11 and 12 for about 18 hours without having to be unclogged. However, the main problem with this setup is that inverted cones are extremely difficult and expensive to construct, install and maintain. Every other model also had its problems and drawbacks.
For example, the Column model succeeded in keeping the lime suspended for a few hours but the water began to flow in a preferential path after the lime settled on the bottom. For the conical column, the mixing at the bottom of the vessel proved to be inefficient in keeping all the lime in a suspended state. On the other hand, the funnel-column apparatus worked well for 20 hours but only because it was unclogged periodically, which would not possible in a real-time set up. As a result, the inverted cone model that gave a 12 hour success run, without the above difficulties was selected as the best alternative among them. However, due to the difficulty in construction, installation, and maintenance, the ANC team was assigned the task of searching for a simpler solution for the lime feeder design.
ALKALINITY IN HONDURAN WATER
The table below shows actual measures of pH and alkalinity in AguaClara treatment plants in Honduras. To have high accuracy in the lime feeder design, the team will simulate the conditions of raw water in the lab. The results demonstrate a decrease on pH during teh treatment process, it is strongly visible on Cuatro Comunidades and Tamara plants.
Souce: Honduras water reports, 2009
EXPERIMENTAL METHODS AND RESULTS
STAGE ONE - Research
The lime feeder water flow rate required for the experiment was calculated with Mathcad. The analysis was made according with the dosage of lime, the operation time, water velocities, water pH and other variables, most of them were taken from the previous team report. The mathcad file with the results is as below.
<LINK> of mathCAD file
On this stage, the team also continued to search for other information including the cost of distilled water, lime properties (solubility with temperature), and Honduras' water regulation.
PROPERTIES OF LIME
STAGE TWO (Experimental Trials 1, 2 and 3)
The first experimental task was to check the feasibility of a fluidized lime bed suspended in a tall column. This is the simplest model that can be used as a lime feeder. The aim was to find an optimum upflow velocity that would mix the lime uniformly and keep it suspended without causing a great amount of lime to go out with the effluent. Upflow velocity can be calculated by dividing the flow rate by the cross sectional area of the column. Since the area is constant, upflow velocity is directly proportional to the flow rate.
The experimental set up can be seen in the figure below.
Trial 1:
This experiment consisted of measuring the upflow velocity with different flow rates and different quantities of lime, while always maintaining a suspended column of lime. This was an initial experiment and hence most of the quantities were taken through approximation. The details of the exact quantities of lime and flow rates as well as the results obtained can be found in the attached file called "ANC-Initial Experiment using Column".
<LINK> trial 1 - on column
The optimum flow rate which could keep the lime suspended without excessive lime coming out (as observed visually) was found to be about 85ml/min.
The figure above shows the apparatus used for the first trial, it consists in a tube conected with a water source regulated by a pump. The lime was added manually at the top.
Figure - 1 
Trial 2 and 3:
In these trials, Process Controller was utilized to automate the experiment. In addition, the apparatus was equipped with a pH probe whose values could be logged into the computer via the Process Controller.
With those additions, the team measured the pH of the effluent, looking for the optimal flow rate that could maintain 12 pH units for 24 hours.
In trial 2, with a flow rate of 40mL/min, the pH dropped from 12 to 11 after about 3.5 hours. In trial 3, with an increased flow rate of 60mL/min and keeping every other parameter constant, no significant changes were noted.
The results of the experiment are on an attached excel file.
<LINK> Trial 2 and 3 on Column experiment
Figure - 1
Identified Problems:
1) The arrangement of the pH probe was not stable so the pH data was not accurate.
2) The simple column design was not adequate in giving any range in terms of flow rates that could fulfill the objectives of maintaining a good suspension and maintaining a stable pH for 24 hours on the effluent.
3) The vertical column was not enough to store the required amount of lime to run the experiment for 24 hours.
These problems prompted for a modification in the design of the column leading to the next stage of experiments.
By the end of stage two, changes were made (following Professor Monroe's suggestions) in the design of the water outlet in order to ensure an adequate pH measure. It included putting the pH probe in a vertical tube and making a curved outlet for the effluent. With these changes, the apparatus could get a more stable pH measure. A figure explaining this description is shown below.
FIGURE - 2 
On teh second stage, the team also worked on the relationship between the ratio of flow rate between the lime feeder and the plant with the alkalinity provided by the effluent lime. That information was useful to edit MathCAD calculations and to determine the changes in the Ph at the plant with respect to changes in this flow rate ratio (fraction of water flow rate entering the lime feeder from the entrance tank).
STAGE THREE - Redesign
In order to overcome the difficulties faced at the end of stage two, the team considered a new design, which consists of a diagonal column attached at the top of the vertical column. The design would help the saturated lime-water solution stay inside the apparatus, while still having the needed concentration at the exit. Since the velocity in the slanted tube is affected by the angle, its vertical component is lower than the upflow velocity of the primary column. This decreased velocity allows more lime to settle back into the column and thus prevent unnecessary lime loss. Thus the primary column would be used as a storage vessel for the suspended lime bed while the slanted tube above it would allow for a more uniform saturated lime mixture.
The dimensions of apparatus are determined according to MathCad calculations; the relevant criteria being the length of both columns. Below is the link for the mathCAD file:
The team worked on calculations using the following assumptions for simplification:
- Dfrac = 3: When elementary lime particles coagulate, the density of the larger mass stays the same as that of the original particles. This is unlike what happens in flocs, that have a Dfrac of 2.3.
- Density of lime is 2.211 g/m^3: Particles are uniform.
- Shape Factor of lime particles = 1: The lime particles are perfectly spherical.
- Settling velocity = 10 m/day: Given a flow rate of 80 mL/min (as determined by Trial 1). This velocity corresponds to the finer lime particles that have a diameter of about 1 micrometer.
Results of MathCAD calculations: Theoretically, the maximum length of the tube should not exceed 1.5m. Lime particles will have a larger density than the flocs, which means their settling velocities will be higher than the assumed 10m/day. Also, it is not necesary that ALL lime particles settle down - some amount (not determined yet) will have to fall out of the lime feeder to solve the acidity problem. Consequently, the length of the tube needed will be lower than 1.5m.
The length needed for the pipe in order to obtain a developed laminar flow 'Le', was also calculated and determined in 10cm with the given (above) conditions. This is required to verify whether or not there is a parabolic profile at the end of the pipe. In conclussion, the length of the tube must be greater than Le.
With the new apparatus, as shown in figure-3 below, the team will carry out a fourth trial, and will make the required arrangements for more experiments, checking to see if the modification will be successful in maintaining the pH at 12 and if so, for how long.
Figure - 3 : the picture of new apparatus will be taken this week and put up (since we will get the constructed apparatus only by 10/27/09).
For this trial, distilled water will be used instead of tap water. In the picture below (figure 4), the ANC Control team can be seen carrying the distilled water tank on to the platform where the experiment is to be set up.
Figure - 4 
FUTURE TASKS
- Is is required to develop several trials with the new enmseble. The team will determine the amount of lime required to maintain a pH of 12 for 24 hours. In addition the team will evaluate the performance of the new emsemble, if it is necessary, the length will be calculated again.
- Simulate Honuran water conditions. Once the team will have the design working well with distilled water, a new experiment will be required to analyze the performance with a lower pH concentration.
- The team is also working on the calculation of required changes according with the natural cariation of flow rate at the inlet tank in the plant.
