Grantee Research Project Results
2011 Progress Report: Chemical Doser for AguaClara Water Treatment Plants
EPA Grant Number: SU834752Title: Chemical Doser for AguaClara Water Treatment Plants
Investigators: Weber-Shirk, Monroe , Swetland, Karen A.
Current Investigators: Weber-Shirk, Monroe , Swetland, Karen A. , Patel, Akta , Lion, Len , Higgins, Matthew , Guerrero, Christopher , Salwen, Adam , Kendrot, Jordanna , Owusu-Adarkwa, Frank
Institution: Cornell University
EPA Project Officer: Page, Angela
Phase: II
Project Period: August 15, 2010 through August 14, 2012
Project Period Covered by this Report: August 15, 2010 through August 14,2011
Project Amount: $75,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet - Phase 2 (2010) Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , P3 Challenge Area - Safe and Sustainable Water Resources , P3 Awards , Sustainable and Healthy Communities
Progress Summary:
Dose Controller
The linear chemical dose controller (LCDC) is a key technology of the AguaClara suite of technologies that makes is possible to produces safe drinking water without requiring any electricity. Although chemical metering pumps are a relatively small component of the energy budget for water treatment plants, the unreliability and proprietary replacement parts of variable speed pumping systems makes them unsuitable for use in small municipal water treatment plants. The LCDC provides improved reliability and simplicity of operation by simultaneiously measuring the plant flow rate using a Linear Flow Orifice Meter and providing a constant concentration of chemical for flocculation or disinfection. The LCDC design has evolved rapidly as we have characterized the critical design constraints during a combination of laboratory research and full scale deployment at AguaClara facilities in Honduras. Currently four different versions of LCDC are in use at AguaClara facilities. The LCDC installed at the recently completed 22 L=s addition to the Marcala, Honduras AguaClara facility is shown in Figure 1.
Several significant findings during the past year were related to the accuracy of the chemical dosing over a range of target chemical concentrations. We discovered and documented the following constraints
- Minor losses need to be minimized
- The connecting fittings for the dosing tube need to have inner diameters at least as large as the tubing diameter
- The dosing tube needs to be straight: coiling significantly increases minor losses
- The minimum minor loss coefficient that can be obtained is approximately 4
- Major losses need to be increased to reduce the error caused from the nonlinearity of minor losses
Figure 1: Dose Controller installed in a 22 L/s AguaClara plant at Marcala, Honduras.
- Major losses can be increased by lengthening the dosing tube length and reducing the flow rate
- It is quite possible to design a system with dosing errors that are less than 10%
- A significant counterweight is required to keep the float in vertical alignment with the end of the lever. This is particularly an issue in the early versions of the AguaClara entrance tanks where the water inlet and the float are in close proximity.
Minimization of the errors caused by the nonlinearity of minor losses required developing a set of equations to characterize this error. The relationship between major head loss and the chemical flow rate under conditions of laminar flow is given by the Hagen-Poiseuille equation.
The chemical flow rate (QC) is a function of major head loss (hf ), the diameter of the small diameter tube (DTube) that connects the constant head tank (CHT) and the drop tube, the kinematic viscosity of the chemical solution (n) and the length of the small diameter tube (LTube). The Hagen-Poiseuille equation assumes that the chemical flow is laminar, viscous, and incompressible, as well as that the flow passes through a tube with a constant circular cross-section that is significantly longer than its diameter. The total head loss through the system (hl) is the sum of the major (hf ) and minor (he) head losses.
Where Keis the sum of the minor loss coefficients. Equation 3illustrates that the minor losses introduce nonlinearity into the relationship between head loss and flow rate. Equation 3 is approximately linearized by calibrating the LCDC at the maximum flow rate and assuming a linear relationship between head loss and flow with an intercept of zero flow and zero head loss. Calibration is achieved either by adjusting the length of the dosing tube or by adjusting the attachment point of the flow measurement float on the lever arm. The resulting approximate linear equation for the calibrated doser is then
Figure 2: The nonlinearity of minor losses is shown here with exagerated minor losses to illustrate the effect of linearization.
Figure 3: Minimum tube diameter required to prevent excessive minor losses as a function of flow rate. The minor loss coefficient was assumed to be 4.
Equations 1, 3, and 4 are plotted in Figure 2.The calibration at maximum flow results in a slight over dosing of chemical when the head loss through the dosing tube is reduced. This dosing error is maximum as the head loss approaches zero.
This dosing error due to minor losses can be readily reduced to acceptable values by increasing the length of the dosing tube. The minimum length of a dosing tube to achieve a target maximum error is
where IIError is the maximum dosing error that is acceptable. We have tentatively set IIError to 10%. Equation 5 reveals that the minimum tube length increases rapidly with tubing diameter and that it decreases for high viscosity solutions. The diameter dependency results in dosing tubes longer than 3 meters for diameters larger than 3.2mm (1/8 in) assuming the liquid viscosity is close to that of water. Thus we may set the maximum dosing tube diameter to 1/8 in The corresponding maximum flow rate for a particular diameter tube is
The minimum tube diameter required to meet a 10% maximum error target is plotted in Figure 3 by solving equation 6 for DTube. These results are independent of the viscosity of the solution.
Figure 4: Kinematic viscosity of alum and PACl solutions
The error analysis reveals that flow rates less than about 2:3 mL=s can be reliably obtained using dosing tubes that are 3:2mm in diameter. Although this constraint may appear to be severe, it is this flow range that is particularly useful for small water treatment plants. This flow rate corresponds to a daily maximum chemical use of 200L and that works well for the commonly available 210L (55 gal) drums. Lower flow rates are also possible and smaller diameter tubing could be used. Larger diameter tubing is not as easily used because of excessive length unless the viscosity of the solution is signficantly higher than that of water.
Kinematic viscosity testing
The summer 2011 LCDC team investigated the chemical flow limits of the LCDC system to see the range of plant sizes within which the LCDC could be used. The initial hypothesis was that, for plants with higher plant flow rates the coagulant stock concentration could be increased so that the same dose could be delivered into the plant’s water without necessitating a higher chemical flow rate through the LCDC system. With thoughts of using higher coagulant stock concentrations, it was necessary to verify the kinematic viscosity. The team used a Vibro Viscometer to measure the kinematic viscosity of alum and PACl solutions with concentrations ranging from 10 g=L to 600 g=L. The results are presented in Figure 4.
The results indicate that kinematic viscosity must be taken into account when designing the LCDC system. The increasing viscosity with concentration is easily accommodated by shortening the dosing tube as indicated in equation 5. These results suggest that alum stock concentrations above 400 g=L should be avoided because of the rapid changes in viscosity as a function of concentration. The AguaClara facilities currently use stock concentrations less than 150 g=L. We will need to do additional testing at full scale with higher concentration stocks to see if there are any operational difficulties.
The maximum plant flow rate that can easily be accommodated with an LCDC for coagulant dosing is approximately 100 L=s assuming that it is acceptable to use 5 parallel dosing tubes. Chlorination can be handled for even larger plants because the chlorine dose is so much lower than the maximum coagulant dose. As the AguaClara technology scales to flow rate greater than 100 L=s it will be necessary to develop higher flow chemical dosers.
Future Activities:
Next steps:
- Select the best plumbing connectors that meet the criteria of small minor losses, no leaks, no small parts to lose, chlorine resistance, and wide availability.
- Standardize field calibration methods for the LCDC.
- Investigate the maximum chlorine concentration in the chlorine stock tank to avoid losing chlorine as a gas. Finding this upper limit will allow us to use a higher chlorine concentration in the stock tank and therefore have lower chemical flow rates necessary for adequate disinfection.
- Document methods of construction and calibration of the LCDC.
Conclusions
The linear chemical dose controller, LCDC, is a key technology for sustainable, gravity-powered, drinking water treatment plants. The LCDC makes it possible for the plant operator to directly set the chemical dose for the coagulant. The LCDC is a combination of several technologies and the AguaClara team has been steadily improving these technologies since 2004. The next priorities are to verify that all components are resistant to chlorine, to standardize parts and to simplify construction of the LCDC. These dosers are already in use in AguaClara facilities and plant operators routinely rely on these dosers to perform without fail even when the plant operates unattended for hours. Creating a reliable chemical dose controller that can automatically adjusts for variations in plant flow rate has been a major accomplishment of the AguaClara team. Future work will focus on standardizing the technology for widespread deployment.
Journal Articles:
No journal articles submitted with this report: View all 4 publications for this projectProgress and Final Reports:
Original AbstractP3 Phase I:
Dose Controller for AguaClara Water Treatment Plants | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.