Effect of Ozone Dissolution Systems on Ozone Exposure and Bromate Formation

Abstract

Pilot‐scale data were generated to improve operational guidance for fine bubble diffusion (FBD) and sidestream addition (SSA) systems with respect to unaccounted‐for ozone exposure—i.e., concentration times time (C × T), and bromate formation during ozone dissolution. In FBD systems, results showed significant ozone dissolution C × T (0.31–2.85 mg‐min/L) and bromate formation (0.8–9.1 µg/L) occurring in the bubble column, which is not included in regulatory compliance C × T calculations. Utilities may consider taking contactors out of service to minimize contact time and corresponding bromate formation in the dissolution zone. In SSA systems, ozone dosages (7.2–18.6 mg/L) were transferred to approximately 20% of the overall plant flow for ~5 s, which produced bromate (2.6–5.7 µg/L). As the contact time exceeded the design guidance in the sidestream flow (>30 s), bromate formation became far more significant (40–140 µg/L). When the sidestream flow is operated with short contact times (~5 s), bromate formation can be minimized.

Critical Review on Bromate Formation during Ozonation and Control Options for Its Minimization

“…Because reactor hydraulics should approach plug flow for efficient disinfection and oxidation, there is limited room for changes. However, one factor that can influence bromate formation is the ozone mass-transfer (i.e., dissolution of gaseous ozone in water) method and design. , There are two major methods for ozone mass transfer: (i) fine bubble diffusion (FBD) and (ii) addition of a concentrated ozone solution through a sidestream. Alternative ozone injection methods, such as injection through membranes or as micro/nanobubbles, have been developed, though their ability to minimize bromate formation has not yet been evaluated. In FBD systems, the first chamber of an ozone contactor is used for ozone gas–liquid transfer (Figure a).…”

“…When a treatment plant operates at design flow, the residence time in the FBD chamber can be minimized (often <2 min), which minimizes bromate formation in the ozone-transfer zone. However, during routine operation, flow rates can range from 25% to 60% of the design flow rate, resulting in longer contact times in the mass-transfer zone and thus higher levels of ozone exposure, leading to higher levels of bromate formation (2–7-fold in a pilot study) . Multiple contactor systems can take contactors out of service to minimize the residence time and the corresponding bromate production in the ozone mass-transfer zone .…”

“…Multiple contactor systems can take contactors out of service to minimize the residence time and the corresponding bromate production in the ozone mass-transfer zone . This aspect of FBD should be considered when designing an ozonation process. In sidestream systems, ozone gas is added to a sidestream water flow (10–20%) using venturi or static mixers and subsequently blended with the main water flow rate (80–90%) to achieve a target ozone dose entering the disinfection zone (Figure b), , which can minimize the dissolution zone CT compared to that of FBD systems. However, a nearly 5-fold larger ozone dose than in FBD is required in the sidestream to meet target dosages after blending.…”

“…Hence, the residence time of the sidestream can influence the bromate concentration in the mainstream. Sidestream residence times increased bromate levels from 3–6 μg/L at 5 s to 40–140 μg/L at a residence time of >30 s . Therefore, for the minimization of ozone decomposition and bromate formation, design guidance recommends that the residence time in the sidestream should not exceed 5 s, although time allowance for gas/liquid mass transfer should also be considered …”

Design Considerations for Cost-Effective Ozone Mass Transfer in Sidestream Systems

“…Drinking water treatment plants commonly operate below the design production rate referred to as turndown (i.e., low water flow, low dose, and low ozone production). In FBD systems, lower production rates increase the contact time in the dissolution chamber and could result in considerable ozone exposure (CT) and bromate formation before entering the disinfection zone, where compliance CT is determined (Wert, Lew, and Rakness 2017). Sidestream systems also have turndown considerations.…”