New Developments in Mixing, Flocculation and Flotation for Industrial Wastewater Pretreatment and Municipal Wastewater Treatment

Abstract

Solid/liquid separations are commonly the first step in any wastewater treatment. Such technologies are mature and new developments are rare. However, in the last decade some significant improvements in separation techniques for industrial wastewater pretreatment have been implemented. Advances in the technology include more efficient, faster centrifugal mixing of treatment chemicals and wastewater contaminants, "in situ" continuous flow coagulation and flocculation, implementation of very high molecular weight flocculants and development of more efficient flotation technologies. Recent developments and improvements of commonly used dissolved air flotation units along with development and application of centrifugal flotation units, cavitation air flotation and suspended air flotation will be discussed. Case studies are also described. Hybrid centrifugal -dissolved air flotation technologies provide the best of both systems: efficient continuous flow mixing and in line flocculation with the nucleation and entrainment of fine dissolved air bubbles. This development has resulted in systems with very efficient removal of particulate contaminants, a small footprint, drier sludge, durable long lasting flocs, fast response and treatment of the total wastewater stream (no recycling characteristic for DAFs). The design of on-line turbidity driven sensors for automatic control of coagulant and flocculant dosage is also underway. Computational fluid dynamics (CFD) has been used to design better flotation tanks with a vortical flow pattern that results in the formation of a dense air bed inside the tank. Such fine bubble layers prevent sedimentation of already floated heavier particulates, which results in significantly higher flotation rates.

Dispersed droplet dynamics during produced water treatment in oil industry

“…The range of pH in which optimal performance can be achieved is wider than for aluminium sulphate. This range of pH agrees with the reported practice in separation processes, which is to alter the pH of the mixture to obtain a pH neutral solution [11].…”

“…To enhance the separation methods, bubbles have been used to form agglomerates which transport contaminations at an increased rate. Bubbles can be generated in several ways [11,33] • Injection of air through nozzles.…”

“…Induced air flotation usually leads to relatively large bubbles in the same range as early flotation units (2 − 5 [mm]) [11], which in the oil-industry are deemed too large for efficient flotation purposes. However, devices have been developed in which bubbles remain smaller due to flow phenomena.…”

“…Moreover, small bubbles show much slower settling rates than large bubbles, which also increases the required size of the apparatus. [12,33], ordered with respect to bubble size A novel method to include flocculation and bubble mixing is to use a hydrocyclone for mixing of air and floc generation, subsequently followed by a more conventional settling of the generated flocs [11]. This is claimed to result in good settling, as phases are already pre-separated in the hydrocylone, and a bulk-like separation can take place afterwards.…”

“…Existence of air bubbles during generation of flocs is beneficial for a multitude of flotation methods [11], as generated flocs then already contain bubbles and settle very efficiently. Methods to obtain floc formation in the presence of bubbles often involve centrifugal, or cyclonic devices with an air core.…”