Fundamentals of Bubble Formation during Coagulation and Sedimentation Processes

Abstract

Conventional coagulation and sedimentation processes can be significantly disrupted by gas bubbles, attaching to, and then floating coagulant floc. This study sought to understand the fundamental factors that lead to bubble formation and corresponding floating floc during coagulation and sedimentation. Gas bubbles ͑causing the floating floc͒ can form whenever the total dissolved gas pressure exceeds the local solution pressure, which can occur at localized minimum pressures during rapid mixing at high fluid velocities. Very high rate rapid mixers can cause bubble formation and floating floc even in waters undersaturated with dissolved gas. The formation and stability of floating floc are dependent on the local solution pressure, amount and type of dissolved gas supersaturation, temperature, length of rapid mixing, surface chemistry of the mixing paddle, floc, and attachment forces.

Addressing problems with gas supersaturation at drinking water utilities

“…The mixing paddles had dimensions of 7.5 × 2.5 × 0.1 cm and were positioned in the middle of the jars. All solutions were initially saturated with air and then supersaturated with dissolved nitrogen to 0.2‐atm gauge pressure using a pressure tank as in Scardina and Edwards (2006). The jar‐test water contained 1 × 10 ‐2 M sodium nitrate and 0.52 × 10 ‐3 M as sulfate ion after pressurization.…”

“…The amount of floating floc in these jar tests was estimated by measuring the diameter and thickness of floc floating at the water surface at the end of sedimentation (Scardina & Edwards, 2006). The corresponding volume calculated (mL/L) was reproducible between ±1 mL or ±10%, whichever was greater.…”

“…Design of coagulation mixers and bubble formation . The formation of floating floc is a function of many factors, including the extent of dissolved‐gas supersaturation, type of dissolved‐gas supersaturation, temperature, rate of floc formation, and the speed of various mixing stages (Scardina & Edwards, 2006). As both the jar‐test apparatus and treatment plant experience demonstrated, many floating floc problems originate at the mixing paddle(s).…”

“…In a mixing basin, the hydrostatic pressure at depth is reduced in the localized area of the mixing paddle because of high fluid velocities. Previous work demonstrated that the extent of pressure reduction could be reasonably approximated by Bernoulli's energy equation (Scardina & Edwards, 2006): in which P is the localized fluid pressure, V is the velocity, ƛ is the specific weight of water, g is the earth's gravitational constant, and z is the elevation head. On a given paddle, the maximum velocity ( V ) relative to the water is a function of the mixing rate (rpm) and paddle diameter (i.e., V max = rpm × paddle circumference).…”

“…Gas bubbles forming in a water treatment plant can disrupt treatment processes and reduce plant operating efficiency (Scardina et al, 2006; Scardina & Edwards, 2006, 2004, 2002, 2001). Floating floc, air binding, erratic particle counts, and erroneous turbidity measurements are frequently encountered manifestations of bubble formation.…”