『Original by HaiChuan Translation Team』Gas flotation for wastewater treatment

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English title: Gas Flotation For Waste Water Treatment
Original link: Click here
Original author: ankur2061
Translator ID: @hbmetals
Those who wish to subscribe to the articles from the “Haichuan Translation Group” are invited to leave a message below the post saying “I want to subscribe”, or send me a short message; I will notify everyone as soon as new articles are available. Gas flotation is the oldest method used for removing solids, oils, and fibrous substances from wastewater. It is an important stage in wastewater treatment, with an efficiency of up to 99% in removing suspended solid particles and oils. Air flotation is a simple process in which tiny bubbles are generated within a container or tank containing wastewater, causing suspended solid particles and oils to rise to the surface of the container or tank for further removal by mechanical means. Please note that the term used here is “air flotation” rather than “air floating”. Air flotation is frequently mentioned in various technical documents, as air, as a gaseous medium, is most widely used in urban wastewater treatment and other applications where there is no risk of flammability or explosiveness associated with gases dissolved in water. In the upstream oil and gas industry, other gas media such as natural gas or nitrogen are used due to the inherent risk of flammability and explosiveness of air mixtures. The air flotation process can be further classified into “dissolved air flotation (DGF)” and “air-sparged flotation (IGF)”. Essentially, both processes promote the flotation process by generating bubbles. But their main differences lie in the method of generating bubbles and their effectiveness. Let’s first learn about dissolved air flotation. In a dissolved air flotation system, water flows into a pressure pump, where air is mixed in and held under high pressure for a sufficient length of time to allow the air to dissolve fully in the water and reach saturation. The typical pressure is 1.5 to 5 barg. When air is dissolved in water under high pressure until saturation is reached, the mixed gas-water fluid and straight water flow out through a pressure-reduction nozzle at low pressure into the air flotation chamber, where the air emerges in the form of tiny bubbles. The bubble diameter produced by the dissolved air flotation process is between 50 and 100 micrometers. These tiny bubbles are key to causing solid particles and grease to rise to the top of the flotation chamber and thus be removed. The total surface area of bubbles obtained by dividing a certain volume of air into bubbles with a diameter of 50 micrometers is 10 times that of bubbles with a diameter of 500 micrometers. In other words, if bubbles with a diameter of 500 micrometers are generated, compared to bubbles with a diameter of 50 micrometers, you need 10 times more air to achieve the same gas-liquid interface area. Air-sparged flotation is a variant of the dissolved air flotation process. It involves introducing gas into the fluid by using a sensing device (hydraulic type) or a mechanical rotor (mechanical type). In the hydraulic type, the wastewater is pumped into a recirculation tank and then fed into a series of Venturi nozzles. The water flowing through the nozzle absorbs gas in the vapor zone and then ejects small bubbles through a nozzle. The rise of these bubbles causes flocs to form a layer of foam in the air flotation chamber, which can be scraped off by a mechanical skimmer. In the mechanical type, swirls and negative pressure are generated by using a rotating shaft and impeller to draw in gas to the bottom of the vertical tube, allowing it to mix thoroughly with water within the chamber. When the gas in the chamber is released in the form of tiny bubbles, it carries the grease and suspended solid particles to the top. The foam can later be scraped off using a mechanical skimmer. Dissolved air flotation (DGF) has some clear advantages over air flotation (IGF); some of these advantages are as follows: 1. Compared to IGF, DGF can produce finer bubbles, thereby improving the efficiency of removing suspended particles. 2. Compared to IGF, DGF requires less power. 3. The operating and maintenance costs of DGF are lower. 4. The overall efficiency and performance of DGF are higher than those of IGF. 5. Since the addition of flocculants can enhance the flotation process, DGF units require fewer chemical additives than IGF units. The IGF system is becoming obsolete. Most new air flotation devices are of the DGF type. For the dimension table of the dissolved air flotation process, please refer to the image below, which shows two different types of design sketches for this process, using air and gas as the medium respectively.