Several experiments have been done at the University of New Orleans since January 2004 to evaluate the kinetics of the electro-disinfection process. In the beginning, a pilot scale treatment plant located adjacent to the Engineering Building at the University of New Orleans was used to reveal the influence of important parameters such as contact time and power consumption on the process performance.
This first experimental set up consisted of three reaction chambers in parallel, formed by 2 concentric, vertical, seamless aluminum pipes connected to a power supply. The outer pipe on each chamber was 89 mm (3.5 in) in diameter, 1.82 m (6 ft) long and the internal concentric pipe was 63.5 mm (2.5 in) in diameter, 2.44 m (8 ft) long. A plastic cone supported the inner pipe and sealed the center opening, therefore, a 13 mm (0.51 in) annular space was provided between the pipes for water to flow from bottom to top. The total
anode/cathode/water contact area on each chamber was 0.78 m2 (8.4 ft2) and it could handle
continuous flow rate up to 4.5 L/min (1.2 gal/min). Figure 4 shows a diagram of the unit.
The electrodes were connected to the positive and negative outputs of a 40-volt, 20- amp direct current rectifier that takes alternate current (AC) and transforms it into direct
DC output was adjustable by varying either the current or the voltage. Reversing polarity minimized the formation of polarized layers and distributed the aluminum electrode consumption. It also allowed the required voltage and the power consumed to be maintained at a lower level.
Figure 4. Diagram of the Pilot-Scale Electro-Disinfection Unit
Few experiments were performed with this tentative system because many difficulties were experienced and some parameters were extremely complex to control, particularly the flow rate and energy consumption. In addition, a significant amount of sludge was produced and it was difficult to remove it from the effluent with this particular system configuration. Conductivity, pH, ORP and temperature of influent and effluent samples were monitored. Total
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coliform count, total COD, total suspended solids and total aluminum concentrations in each sample were determined.
Nevertheless, coliform removal efficiencies between 60% (0.4 log removal) and 96.5% (1.5 log removal) were observed. It was also determined that for better coliform removals the power consumption must be increased, and that as the power is raised more aluminum
suspends into the water from the electrodes, thus generating larger quantities of sludge.
These preliminary tests demonstrated the usefulness of this technology for wastewater disinfection. However, further examination of the technology was required to find out if chlorine disinfecting species are produced or if electrocution and/or free radical and oxygen species are the major coliform death causes. The successful preliminary results warranted the development of carefully controlled experiments to optimize the electro-disinfection technology, as presented below.
To further confirm the disinfective effectiveness of the system, four bench scale batch electrochemical cells were assembled and operated in the UNO Schlieder Urban Environmental Systems Center Analytical Laboratory. Based on previous researches and available resources, the electrode materials selected where: aluminum, stainless steel, and titanium. The first electro-disinfection reactor was set with aluminum electrodes, the second with standard 316 stainless steel electrodes, the third one with titanium electrodes, and the fourth one was assembled with a standard 316 stainless steel cathode and a titanium anode.
Each cell is cylindrical in shape, consisting of a tube-shaped column as the outer
electrode, and a similar thinner pipe placed in the center surrounded concentrically.A plastic
section supports both pipes and seals the center opening, providing an annular space between the pipes to place the water sample. Dimensions of the pipes for each cell, as well as the total
anode/cathode/water contact area utilized on each case, are presented in Table 10. Figures 3
and 4 illustrate, respectively, a basic diagram of the electrochemical reactor and a photo of the laboratory unit (aluminum electrodes).
Table 10. Dimensions of the Electrochemical Cells
Aluminum Titanium Stainless Steel
cm in cm in cm in Inner Pipe Inside Diameter 5.26 2.07 5.54 2.18 5.26 2.07 Outside Diameter 6.03 2.37 6.07 2.39 6.05 2.38 Thickness 0.40 0.16 0.28 0.11 0.40 0.16 Length 71.00 27.95 71.00 27.95 71.48 28.14 Outer Pipe Inside Diameter 7.74 8.31 3.27 7.81 3.07 Outside Diameter 8.89 3.50 8.95 3.52 8.94 3.52 Thickness 0.55 0.21 0.31 0.12 0.55 0.21 Length 66.00 25.98 62.90 24.76 62.40 24.57
Anode/Cathode/Water Contact Area 1828 cm2 1044 cm2 1742 cm2
Again, the rectifier had automatic polarity reversal capabilities. The reversal of polarity among the system facilitates electrochemical reactions at the average electrochemical potentials set by the equilibrium between surface groups and the electrolytic solution. In addition, a self-cleaning effect occurs due to the periodic reversal of current, which changes the nature of the substances produced at each electrode, thus preventing deposits and other undesired cumulative effects.
Figure 5. Diagram of the batch Electro-Chemical Cell
The DC output was adjustable by varying either the current in the range of 0 - 40 amp
or the voltage in the range of 0 - 20 v. A multimeter (AMPROBE®, model ACDC-400) was
used to read the current values.
Figure 6. Electro-Chemical Cell connected to the Power Supply.
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Electro-Disinfection Reactor
The chemical composition of the standard 316 stainless steel electrodes is presented in Table 11. Aluminum and titanium electrodes were 100% pure.
Table 11. Typical chemical composition for 316 stainless steel alloys Element % Iron 62.00 - 69.00 Carbon <0.08 Manganese 2.00 Phosphorus 0.045 Sulfur 0.03 Silicon 1.00 Chrome 16.00 - 18.00 Nickel 10.00 - 14.00 Molybdenum 2.00 - 3.00