New Design Proposal
Concept
The aim of current design is to create a lime feeder which could provide an effluent pH at around 12 for 24 hours under a certain flow rate, but based on the required lime demand for neutralizing the alum, the current design is hard to scale up in terms of the constraint of its flow rate. The new idea is to construct a lime stock tank beside the entrance tank (see figure 1), operator could pour the lime and water into the stock tank and blend the solution for a while for a better mixing.
In addition, in order to create a good turbine in the tank to prevent lime particle settling down, and the fact that it is not realistic for operator to do it manually, the team's proposal is to set a couple of propeller as shown in figure 2, the idea is to utilize the energy from influent flow to push the propeller in lime stock tank. The way to dose the lime into the entrance tank is also shown in figure 2, with an appropriate orifice size it is expected that the lime doser could provide a constant flow for 24 hours, but it still need further study from the point that a small orifice would possibly cause clogging and the flow rate may change under the decreasing water level in stock tank.
Case study
To evaluate the new design, the team studied the feasibility to implement it in Cuatro Communidades plant. Figure 3 depicts the increase of pH and alkalinity result from an increasing ratio of lime feeder flow over plant influent flow, notice that the initial pH and alkalinity are the measured value from treated water in Cuatro Communidades, so this model has already taken into the negative ANC impact from alum dose( see Flow rate required in Lime-feeder). Knowing the plant flow of Cuatro Communidades is at 6.3 L/s, and the flow ratio we need in order to get a pH of 6.75 for treated water is 0.005 (see figure 3), the required lime per day is 2.5 kg/day. Under the assumption that the concentration in lime stock tank is 5 g/L, the required tank volume is then calculated to be 0.5 cubic meter. Because the required lime in stock tank is calculated on a daily basis, so the flow rate through stock tank into entrance tank could simply use the volume divided by 24 hours, which is 0.093 [gpm]. Figure 1 and 2 below are the demonstration of this possible design in Cuatro Communidades.
There are three major challanges with this new design, the first one is to get enough energy from influent water to turn the turbine in the stock tank, the second one is to avoid clogging in orifice, the last one is to maintain the flow rate through orifice constant, the AguaClara dose controller technology may provide the solution, further study is necessary with this design.
Fig 1. Autocad drawing of new design with dimension description
Fig2. Propeller and Orifice position
Fig 3. ANC and pH as a funtion of flow ratio between lime feeder over raw water