Acid Neutralizing Capacity, Fall 2010

Drew Hart, Roy Guarecuco, Larry Lin

Abstract:

In light of recent successful tests run by the AguaClara Engineers in Honduras with poly-aluminum chloride (PAC), an alternative coagulant to alum which consumes a fraction of the alkalinity, the ANC Control team is preparing to stop research with lime feeders because they will be largely unnecessary when PAC is adopted for all AguaClara designs. In order to bring closure to ANC research the team will write a final paper detailing the insights pertinent to lime feeder technology that AguaClara has gained over five semesters of research. We will also organize the ANC Control wiki page

Plate Settler Spacing, Fall 2010

Tanya Cabrito, Jae Lim, Cosme Somogyi

Abstract:

The goal of the Plate Settler Spacing Team (PSS) is to study the lamellar sedimentation process in plate settlers and the efficiency of the removal of flocculated particles and establish improved guidelines for plate settler spacing. The traditional guidelines for plate settlers state that the spacing between plates cannot be less than 5 cm and little to no justification can be found for this. The team's results show that performance in accordance with the US drinking water standard of 0.3 NTU can be achieved with spacings smaller than 5 cm. Also, all but one of the experiments meet the World Health Organization standard of 5 NTU. For AguaClara plants, having smaller spacings between plate settlers allow the sedimentation tank to be shallower and therefore cheaper.  Smaller spacings also allow for increased head loss across the plate settlers. This would help even out the distribution of flow in AC plants and allow the plate settler system to function at its design capture velocity of 0.12 mm/s throughout. The team had finished velocity gradient experiments with a clay aluminum hydroxide system; however, a recently discovered mistake in documentation caused the team to reassess the data collected this semester and more tube trials must be run. Due to this error, the team changed the capture velocity for the velocity gradient from 0.12 mm/s to 0.10 mm/s.  The major tasks completed by the Fall 2010 PSS team are catching a documentation error that happened at the end of Spring 2010, studying the effects of high velocity gradients and floc rollup on plate settler performance, developing a macro that significantly facilitates data analysis and a plate settler dynamics model that may better shed light into processes governing plate settler performance.

Non-Linear Chemical Dose Controller, Fall 2010

Abstract

Accurate alum dosing is vital for plant operation as it has a great impact on the effectiveness of flocculation and sedimentation. The nonlinear chemical dose controller (CDC) is designed to handle turbulent flow chemical dosing used in conjunction with the newly designed Rapid Mix Tube. In contrast, the linear CDC requires that the chemical flow in the dosing tube is laminar.

The linear CDC uses the linear relationship between laminar flow and major losses in the doser tube to maintain a constant chemical dose with varying plant flow rates. However, when the flow in the dosing tube is turbulent, the linear relationship no longer exists. In this case, a nonlinear CDC, one that uses minor losses to control flow rates, can be used to maintain a constant chemical dose with the varying plant flow rates. By using an orifice to control chemical flow, the CDC will have the same nonlinear response to increasing flow as the plant flow rate, which is controlled by an orifice. A dual scale system on the lever arm will increase accuracy in dosing at smaller doses.

Figure 1 shows a conceptual design of the dosing system. A float in the entrance tank is connected to one end of the lever arm and allows for the arm to move up or down based on varying plant flow rates. As plant flow rates increase, the float rises, and the lever arm connecting the dosing orifice falls increases the elevation difference between the constant head tank and dosing orifice to power chemical flow. As the flow rate decreases the elevation difference decreases and the chemical flow rate slows down.

Figure 1: Nonlinear chemical dose controller schematic

There are two dosing tubes coming out of the constant head tank each with a different size orifice fitting attached to the end. The two different sized orifices allow for the plant operator to dose alum at two different scales, a high and low scale. The operator will choose which orifice will be used based upon the desired alum dose the operator wishes to apply to the water, a parameter that depends on raw water characteristics. The two scales on the lever arm, as seen in Figure 1, correspond to one of the dosing tubes; for instance, the higher scale (20-100 mg/L) will be used when the larger orifice size is needed. The dosing tube which is not in use is merely lifted to a higher elevation than the constant head tank level and is clipped onto a hook to prevent the flow of alum through this unused orifice.

After the alum exits the dosing orifice it flows down through a rigid tube which then injects the alum over a rapid mix conduit which connects the entrance tank and the flocculator. The rapid mix tube has two orifices which create macro and micro eddies to ensure the uniform distribution of coagulant into the raw water.

Laminar Tube Flocculator, Fall 2010

Tami Chung, Alexander O’Connell, Karen Swetland

Abstract

The Fall 2010 Tube Floc Team of aimed to better understand the development of rapid flocculation through a series of experiments and data analysis. In previous semesters, we had focused on the variation of flocculator length and alum dose. With the completion of these studies, we encountered more questions regarding the fundamental mechanisms involved in the process. As a result we chose to focus on characterizing the minimum coagulant dose needed to initiate rapid flocculation for two difference coagulants, alum and PAC. We hope to use this data in the future to develop a theoretical framework describing the onset of rapid flocculation. In order to accomplish this goal and to ensure the accuracy of our data we made a number of improvements and modifications to the FReTA apparatus this semester. We are currently in the process of collecting data and hope to complete our planned experiments by the end of the Fall 2010 semester.

Chemical Dose Controller Development, Fall 2010

Eva Luna, Drew Hart, Larry Lin, Roy Guarecuco

Abstract

The Fall 2010 Chemical Dose Controller team has focused on designing the dose controller to be visually accessible, more aesthetically attractive, and robust. The apparatus will be mounted on a plywood board secured to the plant wall. The team has begun construction on a prototype for the design. In this report we have documented the design process and all of the component parts used in the prototype.

Acid Neutralizing Capacity, Fall 2010

Abstract

The ANC Control team has conducted laboratory research to investigate the feasibility of using a lime feeder to add alkalinity to the influent water of AguaClara plants. This was necessary in order to maintain the pH within the ideal range for flocculation after the addition the chemical coagulant, alum, which has an acidic effect. Low-alkalinity source waters in Honduras do not provide sufficient buffering capacity naturally to resist the drop in pH. The team has found that proposed lime feeder designs consistently fail to produce saturated effluent for a length of time which would make them economically and practically viable. The failure is believed to be caused by precipitation of calcium carbonate on the calcium hydroxide solid surfaces. The recent availability of poly-aluminum chloride, an alternative coagulant to alum with a much smaller acidic effect, in Honduras has reduced the need for lime feeders with AguaClara plants