Author: Philip McCreanor

The Influence of Metallic Biocide Concentration on the Time of Deactivation of Coliforms in River Water
Erin Brett
Faculty Advisor: Dr. Laura Lackey
Mercer University - School of Engineering, Macon, GA

Copper Concentration Measurement Method

Experimental Results

Single-dwelling biological sand filters can be an effective and appropriate technology for treating drinking water in developing countries. Current literature suggests that the use of metallic biocides in these filters may reduce pathogen levels below those obtained through biosand filtration alone. Design recommendations currently accepted suggest mixing the biocide in the 2 – 3 cm layer of fine sand located closest to the bottom of the filter (Figure 1). It is thought that this placement ensures that the metallic ions have no biocidal impact on the biological layer located in the top 5 to 10 cm of the fine sands of the biofilter.
The overarching goal of this project is to determine the most effective placement of a metallic biocide in a biological sand filter. The work presented here focused on developing a relationship that describes coliform deactivation in river water spiked with a solution containing copper ions. These results can then be used to predict the effect of copper ion concentration on the viability of the biolayer in a point-of-use biofilter. A method for measuring copper concentration in solution was established. Copper was mixed with deionized water until it approached saturation in solution. Water was collected from the Ocmulgee River. The copper solution was diluted with river water and coliform concentration was measured over time. Six different concentrations ranging from 0.028 to 1.65 mg/L of copper in solution were tested and it was determined that coliform deactivation was well characterized using a first-order kinetics model. First order rate constants varied from 0.15 to 19.0 hr-1, respectively.

Abstract

The Total Coliform and E. Coli Membrane Filtration Method was used to determine coliform concentration. The effect of copper concentration on coliform viability in river water was evaluated at six concentrations ranging from 0.028 to 1.645 mg/L Cu2+. The control for each experiment consisted of the same ratio of copper solution to river water, except deionized water was used instead of copper solution. The coliform count for the controls did not change significantly in any of the experiments. The decrease in coliform count over time seen in Figures 3 & 4, as well as in every other experiment justifies that copper in solution does have biocidal effects.

Recommendations

Based on the research conducted copper solutions with concentrations of approximately 0.142 mg/L or less in contact for 30 minutes or less, 0.071 mg/L or less in contact for 60 minutes or less, and 0.028 mg/L in contact for 90 minutes or less with the Schmutzdecke (biofilm layer) all cause minimal damage to the biofilm layer. Anything exceeding these parameters is not recommended.

Figure 2. Copper Concentration Standard for Determination

A method for determining copper concentration was established. Hach CuVer® 1 Copper Reagent and Hach Copper Standard Solution (10 mg/L as Cu) were used to develop a standard curve. A CuVer® packet was mixed with a 1 mL sample of copper solution. The mixture sat for approximately 30 minutes and was then analyzed using an Ocean Optics Inc. USB 2000 spectrophotometer. Spectrophotometer results were displayed graphically through Logger Pro 3.8.3. The maximum absorbance was recorded for eight different copper concentrations as seen in Figure 2 to create a standard curve. For the concentration range tested, the standard curve was modeled linearly as Cu2+ (mg/L) = 7.93xAbs + 0.0619 (R2=0.9998).

Conclusion

The first-order rate constant, k, can be determined if the copper concentration is known by using Figure 7. This knowledge can then be used to determine how long the copper solution can stay in contact with the biofilm layer without causing damage. The rate constants for six copper concentrations ranging from 0.028-1.65 mg/L were 0.0025-0.3169 min-1 respectively.
Further research could be done to determine the diffusion rate of copper through a biological sand filter. If the amount of time it takes copper in solution to reach the biolayer was known, the acceptable height of the metallic biocide layer in the filter could then be calculated based on the diffusion rate of copper through the filter and the frequency of filtration use. Observing the effects of brass and bronze as opposed to copper on coliform concentration over time at different concentrations could prove to be insightful and beneficial.

Figure 3. [Cu]= 0.071 mg/L, t= 0, 30, 60, 90, 120 min, respectively

Figure 4. Control and Copper Solution at 0.404 mg/L

First-Order Kinetics Analysis

Figure 1. Biological Sand Filter

Table 1. First Order Rate Constants
Copper / Concentration (mg/L)	k(min-1)	R2
1.645	0.3169	0.89
0.823	0.1754	0.99
0.404	0.1261	0.90
0.142	0.0296	0.81
0.071	0.0062	0.86
0.028	0.0025	0.62

References

Figure 5. First-Order Kinetics Batch
System Derivation

Dr. Laura Lackey. (2012). [Skematic of biological sand filter April 1, 2013]. Biological Sand Filtration Skematic.
Total Coliforms and E.Coli Membrane Filtration Method. USEPA Approved for Drinking Water. Method No. 10029. Revision 3, June 2003.
Spectrophotometer Powder Pillows or AccuVac Bicinchoninate Method. USEPA Approved. Method No. 8506.

Figure 7. First-Order Rate Constants as Copper Concentration Increases

The first-order rate constants were found based on the equation derived in Figure 5. The slope of the trend line of each normalized coliform concentration data set in Figure 6 is the rate constant, k. The copper concentrations that correspond to the rate constants and R2 values are presented in Table 1. As the copper concentration increases the rate constant also increases. This relationship is displayed in Figure 7.

Acknowledgements

Thank you Dr. Lackey for your guidance, critiques, and willingness to answer questions throughout this process!

Figure 6. Deactivation Trends at Six Different Concentrations

Junior Engineering Honors Project — Mercer University — Spring 2013 Engineering Expo — April 11, 2013 — Macon, GA