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 below the safety limitfor 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 neautralize 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.
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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.
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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 insufficient 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 the treatment process, it is strongly visible on Cuatro Comunidades and Tamara plants.
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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. (I would add the equation for upflow velocity)
The experimental set up can be seen in the figure below.
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