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Demo Plant

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Project Objectives

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Since many of the modules in the AguaClara plant are being updated and created anew, the previous Demo Plant was not reliable as a reference for the full-scale plant. New parts needed to be machined and tested based on the current AguaClara technologies. This semester, the demo plant team focused on creating a Linear Flow Orifice Meter (LFOM), a chemical dose controller (CDC), an updated sedimentation tank with an operable floc blanket, and a Stacked Rapid Sand Filter  (SRSF).

Flow Control and Chemical Dosing 

The purpose of the Flow Control and Chemical Dosing is to set the Demo Plant flow rate and to create a linear relationship between the plant flow rate and the chemical dosing flow rate. Initially we intended on using a LFOM to set the flow rate of water leaving the entrance tank. However, after much consideration, it was realized that at the Demo Plant Scale the size of the holes that comprise the area that creates the Sutro Weir effect are too small and thereby inapplicable. Now, Laminar flow tubes are to be used to set the flow rate throughout the plant. Not only is this easier, but it is also more reliable on the small scale. Further developments have even informed setting the flow rate by adjusting the height difference between the entrance tank and the constant head tank for the raw water influent.

In order to make sure there is a linear relationship between the flow rate and the dose rate, a lever arm will be put in place so that the dosing method in real AguaClara plants can still be observed at the Demo Plant scale. Through the use of a bob and surface tension, the lever is able to increase the dose rate as the flow rate increases. A picture is shown below. Notice that while the flow rate is set simply by changing where the entry point is for the raw water, the lever allows the alum dose rate to change automatically based on the height of the water in the entrance tank. In order to reduce the chance of having clay settle in the bottom of the entrance tank, it is now being constructed as a pipe. This will reduce the area at the bottom of the sedimentation tank and increase the velocity, thereby reducing the chance for the clay to settle at the bottom.

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The purpose of the sedimentation tank is to remove large particles from the water. The !2012-04-27 15.37.48.jpg|align=right,border=1,height=300!demonstration plant achieves this by using two functional sections: the floc blanket and tube settler, which are corollaries of the large-scale sedimentation tanks with plate settlers used in the real AguaClara plants. Although a full floc blanket has yet to be achieved in a full-scale AguaClara plant, floc blanket theory is the driving factor behind sedimentation tank design, and thus the illustration of a floc blanket and its functionality in the demonstration plant is a useful way to educate the public about AguaClara technologies. At the scale of the demonstration plant, tube settlers are more practical, and fully analagous to the full-scale plate settlers from the point of view of solids in the water.

The purpose of the floc blanket is to grow flocs, making them easier to capture. The tube settler acts to capture flocs grown in the floc blanket, cleaning the effluent water and feeding flocs back into the floc blanket. The floc blanket works by preventing flocs from setttling out: water enters at the bottom of a vertical column, and flows upward, acting to counter the gravitational force on the flocs and fluidize them. The balance of the upward flow rate with the downward gravitational pull on the flocs means that flocs must circulate throughout the column, rather than simply passing through. This forced circulation causes increased particle collision, meaning that the flocs grow. Flocs that grow to the point where the gravitational force is strong enough to cause them to settle out are resuspended by the influent jet. The floc blanket thus consolidates particles in the water into large, capturable flocs.

The majority of those flocs are drained out by a floc weir at the top of the floc blanket column. The resultant, cleaner water then travels through the tube settler, an angled pipe of the same diameter as the floc blanket column. The angle of the tube settler's walls causes flocs traveling in the water to settle out on the sides of the tube. Gravity then rolls them back down into the floc blanket, where they will grow and be drained out. The combination of the floc blanket and tube settler effectively cleans water of a large portion of particles before it is processed by the stacked rapid sand filter.

 Stacked Rapid Sand Filter

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The purpose of the SRSF is to remove small particles that did not settle out in the sedimentation tank. The advantage of a stacked rapid sand filter over a traditional rapid sand filter is that the backwash process is more efficient. Since backwash of the layers occurs in series, the SRSF uses significantly less water to clean the filter. During filtration, water flows into the inlet tubes, out slotted pipes and through the sand layers. The purpose of the slotted pipes is to allow water to flow out but prevent sand from clogging the pipes. Therefore, the slots have to be smaller than a grain of sand for this to work. As water flows through the sand layers, any remaining particles stick to the sand effectively filtering the water. The water then flows back into a slotted pipe and through outlet tubes. After a while, the filter performance decreases due to particle buildup in the sand. At this point, it is necessary to backwash the filter.

During backwash, water only flows through the bottom inlet tube, creating a backwash velocity equal to the filtration velocity times the number of layers with the same flow rate used for filtration. This high water velocity fluidizes the sand bed, washing flocs out of the sand and through the backwash outlet.

In order to reproduce the SRSF on such a small scale, we based our design calculations on the MathCAD worksheet used to design the full scale SRSF. Due to the size, several modifications were made, such as reducing the number of sand layers to four and using valves instead of a siphon system for backwash. Additionally, we had to find some alternative way to replicate the slotted pipes which are impractical to fabricate on this scale.

80/20 Aluminum Frame

A new frame was constructed out of modular pieces of aluminum from the company 80/20 to house the new processes. This frame is easily deconstructed, is simple to pack for transportation, and is easily modified to accommodate changes in the required relative positions of the unit processes. The new frame is shorter, lighter, and more professional looking than the previous frame, while continuing to be simple to operate.