Thickening through Suspended Air Application Aims to Reduce Energy Consumption

Conventional thickening processes use gravitational forces or mechanical methods such as centrifuges or belt filter presses for solids separation. Although these methods have proven to be efficient, gravity based thickening methods require large footprint and mechanical thickening processes can be energy intensive. Recently, several innovative technologies have been introduced that aim to enhance energy efficiency of thickening process while reducing the footprint. Dissolved Air Flotation (DAF) process has been around for over a century and initially used in the early 1900s, primarily in the pulp and paper industry to remove fibers and other solids from water. Interest in DAF resurged in the 1960s for wastewater treatment applications and since then DAF has been used as one of the main thickening processes. DAF introduces air into the wastewater solids, and creates tiny bubbles. Then these bubbles attach to the sludge particles, causing them to float to the surface, where they form a thickened sludge layer. This layer is then skimmed off, leaving clearer water below. However, DAF has shown to be energy intensive since it requires recirculation pumps, compressors. Lately, a new thickening technology called Suspended Air Flotation (SAF) was introduced into the wastewater industry. SAF operates similar to DAF by using air to float sludge. However, there is a key difference: SAF employs charged particles for bubble flotation. SAF also aims to float the particles but without the use of dissolved air. Therefore, thickening performance is not limited by changing conditions such as gas solubility, pressure, and temperature. The flotation occurs by the application of froth made from an anionic surfactant, water, and air instead of air bubbles for flotation. Similar to DAF, SAF mixes feed sludge with a conditioning polymer before it enters the flotation tank. Froth, composed of charged microbubbles called aphrons, is then introduced downstream of the polymer injection. The micron-sized air bubbles, ranging from 7 to 50 µm, in water to attain a volumetric air content of 40 to 50%. These bubbles are coated with a thin layer of soap film derived from an electrically charged anionic or cationic surfactant. The charged bubbles offer a substantial interfacial area for the adsorption of oppositely charged flocculated wastewater solids. In practice, suspensions of charged bubbles are introduced into the flotation tank to interact with wastewater solids. The solids then ascend to the surface and are skimmed off. The clarified effluent is recirculated to the headworks. SAF enhances traditional thickening methods like Dissolved Air Flotation (DAF) by eliminating the need for dissolved air, thus obviating the requirement for pressurization systems, recirculation pumps, compressors, and airlines, leading to considerable energy savings. SAF's capability to manage high solids loads also results in substantial footprint and power savings, high solids recovery (up to 98%), and a high solids loading rate (up to 40 lb/ft2/hr). Moreover, no polymer is needed to thicken sludge to 4% solids. Owing to these benefits, adopting SAF technology can lead to a reduction in greenhouse gas emissions by more than 60-70% and energy savings of up to 90% compared to its conventional counterparts, such as DAF. In a recent project funded by California Energy Commission (CEC), this innovative thickening process was selected to be tested at demonstration scale. The demonstration facility is located at Linda County California, north of Sacramento. The project includes several advanced treatment processes for biosolids. Another innovative technology Supercritical water oxidation (SCWO) is also included after SAF to replace anaerobic digesters. Figure 1 shows the process flow diagram at demonstration facility. SAF process have already shown promising results for their low energy consumption. Recent findings indicated over 90% savings in energy use by switching from DAF to SAF. Table 1 compares SAF process with conventional thickening processes. The cost savings for SAF process was also evaluated with other conventional thickening processes including Rotary Drum Thickener (RDT) and also DAF. Table 2 summarizes the 30 year net present value comparison of SAF process. This presentation will evaluate this novel technology, discuss the critical design criteria for this process and assess its performance focusing on energy savings.