By Ayisha I. A. Malik, Cole-Parmer EMEA
A team of scientists at the University of Ottawa, led by Professor Roberto M. Narbaitz, devised and tested a lab-based dissolved air flotation (LB-DAF) system, leveraging instruments from Cole-Parmer. The team investigated the accuracy, and in turn the predictability and feasibility of successful removal of suspended impurities from drinking water using dissolved air flotation (DAF).
What is dissolved air flotation (DAF)?
Drinking water is carefully treated to remove impurities that are detrimental to health. Suspended particles such as oils and naturally occurring solids are removed through a series of processes: coagulation, flocculation, sedimentation and DAF.
In DAF, water is clarified using air that is dissolved under high pressure and released at atmospheric pressure in a flotation tank basin. This forms tiny bubbles that adhere to suspended matter causing the impurities to float to the surface, where they are skimmed off. This technique is more effective in removing low-density particles, such as algae and cyanobacteria, when compared to conventional settling methods. It is not unique to drinking water treatment plants but can be used in other industrial facilities such as wastewater, oil refineries, paper mills and chemical plants.
Gonzalez-Galvis and Narbaitz research project
To test and validate the feasibility and performance of DAF, commercially available bench-scale jars (1 to 2 litres) are routinely applied. However, it has been proven that these kits can over-predict or under-predict turbidity removal results by large margins.
These prediction deviations are attributed to a combination of factors such as the saturated water delivery system, the saturated water application point and the wall effect in the small jars. Particles in a stagnant fluid tend to rotate and equilibrate to a certain orientation in the absence of walls, altering the projected area and drag coefficients. Additionally, the small DAF test jars allow particles to attach to the walls, some of which are resuspended during clarification. Moreover, the saturated water and bubble delivery stem at the base of the jar also creates turbulence that causes particle resuspension during the clarification step.
Juan Pablo Gonzalez-Galvis in Narbaitz’s team investigated the limitations of these commercially available jars and created a lab-scale system that could offer greater accuracy and therefore better predictability of purification parameters by DAF in drinking water treatment.
Tank configurations adjusted to mimic condition in full-scale units
Gonzalez-Galvis’ LB-DAF system was created using a cylindrical tank (21 L) with a large diameter (20 cm) to meet the minimum recommendation for flocculent settling tests and three different flat impellers that meet the requirements for a vertical turbine flocculator, at a radial impeller to tank diameter ratio of 0.5, which is within the values recommended for full-scale units. The impellers were attached to a shaft secured with a bearing to eliminate vibrations and the system also featured four baffles arranged along the vessel wall to meet recommended ratios for full-scale units. The was used to produce motorised mixing at the required operating speed range between 3 and 200 rpm. Other adjustments were also made to ensure appropriate rapid mixing and mimic conditions in a full-scale system.
Bubble size measurement
Gas bubble size is an important parameter in a DAF unit because DAF performance is significantly affected by bubble size and properties in the contact zone. The bubble–particle interactions follow three different steps — collision, attachment and stability; and they are affected by other variable such as floc-particle size and density, number and density of the bubbles, particle–bubble charge and the flotation retention time.
The size of the bubbles generated by the LB-DAF system was quantified using a digital imaging technique. The apparatus consisted of a plexiglass sampling tube connected to the bottom of a viewing chamber. A second tube was connected to the top of the chamber and a Masterflex® L/S® peristaltic pump to create suction. The pump was used at a flow rate of 0.45 mL/min to draw water with bubbles into the viewing chamber through a sampling tube (30 cm above the bottom of the LB-DAF tank). Individual bubbles were then photographed using a high-speed, high-resolution flare camera and quantified using imaging software.
The pH, turbidity, dissolved organic carbon (DOC), and total organic carbon (TOC) concentrations were verified at different points in the experiments to determine comparative values between the samples processed using the commercially available DAF test jars, the LB-DAF system and a full-scale DAF system. Experiments were repeated at multiple temperature points; the Digi-Sense® Type J Thermocouple Thermometer was used to accurately measure water temperature. (This exact product has since been discontinued but we have another option available.)
Gonzalez-Galvis concluded that the LB-DAF apparatus had better prediction capabilities for turbidity removal when compared to commercially available DAF jar test apparatus under research conditions highlighted in his paper.
Research and discovery have always been instrumental in enhancing our lives. Each discovery has its own significance, whether it is revolutionary or merely builds the stepping-stones to something bigger. We must salute scientists for their ongoing dedication, acknowledge the different factors that contribute towards their success, and equip them with tools to pave the way to a brighter future.