Fiber-based super-bridging agents improve flotation and settling during water treatment

Abstract

Increasing demand for water poses a major challenge to the water treatment industry. To maintain their floc removal efficiency, water treatment plants are expected to require larger separation units and use more chemicals, namely, coagulants and flocculants. Conventional physicochemical treatments produce flocs that are limited in size, which limits floc removal efficiency via gravitation-based processes such as settling and flotation. Introducing fiber-based super-bridging agents has improved the floc size, which is 10–100 times larger than conventional flocs. Such improvements could lead to important gains in floc separation and ultimately increase the capacity of water treatment plants. This study analyzed the behavior and interaction of fibers under various coagulation/flocculation conditions to improve flotation and settling. Residual turbidity < 1.5 NTU was systematically achieved when the fibers were combined with conventional physicochemical treatments (alum and polyacrylamide). The results also showed that fiber-based super-bridging agents can allow a ~50 % reduction in coagulant. Three types of renewable fibers originating from the residues were selected for jar tests: softwood cellulose, brown fibers extracted from recycled cardboard boxes, and hemp fibers. The floc settling rate increased considerably when fibers were previously incorporated into the floc structure during aggregation, thus acting as a super-bridging agent. The benefits of fibers on floc settling velocity were particularly pronounced during suboptimal coagulation, as the injection of fibers compensated for poor coagulation conditions. Air bubbles during air flotation were also better incorporated into the larger and more porous floc structure obtained via the fibers, which drastically improved floc removal during flotation. Ultimately, such fiber-based super-bridging agents can be introduced in existing water treatment plants for wastewater and drinking water applications to increase plant capacity, reduce the demand for coagulants/flocculants, and improve contaminant removal.