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 sensitivity of alum doage and pH changes dramatically if the entering water has a low pH. 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, we can make the 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 6, 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.
PROPERTIES OF LIME
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
Concurrently, the team also continued to search for other information including the cost of distilled water, lime properties (solubility with temperature), and Honduras' water regulation.
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 but not cause any 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.
Figure - 1
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.
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
The problems identified with these trials were:
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 in the resulting water solution.
3) The vertical column alone was not sufficient enough to store and adequate 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, thus, changes were made (as per 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 team could get a more stable pH measure. The figure explaining this description is shown below.
FIGURE - 2 to be added (pH probe arrangement)
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 make changes on the MathCAD calculations to find the changes in the Ph at the plant with respect to changes in this 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 amount go out. Since the velocity in the slanted tube is at an angle, its vertical velocity is less 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 will be 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: It is assumed that 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. In actuality, the lime particles will have a density much larger than the flocs, which means their settling velocities will be much higher than the assumed 10m/day. Also, we do not need ALL the lime particles to settle down - some (a particular amount, not yet determined) will have to eventually fall out of the lime feeder to solve the acidity problem. Consequently, the length of the tube we will need will actually be much less than 1.5m.
The length of the pipe needed to obtain a fully developed laminar flow of water in the pipe, 'Le' was also calculated and determined to be about 10cm as per the given (above) conditions. This was just to verify whether or not there was a fully developed parabolic profile by the end of the pipe or not.
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).
The final target for this duration is 24 hours so that the operator at AguaClara plants in Honduras need only refill the lime feeder once a day. For this trial, distilled water will be used instead of tap water to better simulate Honduran 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