Page 315 -

10.56 CHAPTER TEN

sured. Several guidelines have been developed to assist the designer in ensuring adequate
contact between the ozone gas and the liquid:
• Maintain a gas-to-liquid ratio between 0.05 and 0.20. This guide comes from the chem-
ical engineering field for effective transfer and contacting in a two-phase (liquid and
gas) system. However, this may be difficult to maintain under a wide range of flow and
dose conditions.
• Maintain a minimum gas floor loading rate of 0.12 ft3/min/ft 2 (0.037 m3/min/m2). This
guide comes from studies on activated sludge systems and represents the minimum gas
flow rate for proper mixing of a liquid mass.
• Maintain uniform floor coverage with diffusers spaced at no more than 3 ft (0.9 m).
This guideline was developed in recognition of opportunistic flow of the liquid through
areas low in or devoid of gas bubbles.
If followed, these guidelines will yield a design that effectively promotes uniform con-
tacting and distribution of ozone throughout the liquid. In disinfection applications, this
means the designer can be assured that all the liquid has been equally exposed to the dis-
infectant. These guidelines work well with conventional air feed systems operating at typ-
ical water treatment doses.
However, recent developments in ozone generation, resulting in ozone concentrations
in excess of 10% when using high-purity oxygen as the feed gas, have made it almost im-
possible to meet these guidelines when operating at typical disinfection doses of 1 to 3
mg/L. Solutions to these applications have included supplemental mixing with air or wa-
ter injectors. One manufacturer recommends using more, small [4-in.-diameter (10-cm)]
diffusers with smaller bubbles (1 to 2 mm) to improve the floor coverage. Ultimately, the
higher ozone concentration available from the latest generation of generators is forcing
the industry to consider modifications, and alternatives, to the conventional multistage
fine bubble diffuser contactor.
The search to optimize the hydraulic efficiency Tlo/T in fine bubble contactors has
been investigated by using tracer dye studies of existing basins, on computer modeling
based on finite element analysis, and using computational fluid dynamic techniques. While
complex in development, this work provides practical results for the designer.
By using D, L, and W to represent the depth, length in the direction of flow, and the
width perpendicular to the direction of the flow, respectively, it has been shown that hy-
draulic efficiency is very closely related to D/L. While previous sources have recom-
mended basin configurations of 1.5 to 1.0 to 1.0 (D, W, L), the TIo/T can be improved
by 50% or better if D/L is increased to 4.0 or higher. Hydraulic efficiency appears to be
independent of width. Consequently, to optimize Tlo for contactors with a depth of 20 ft
(6.1 m), the cell length should be less than 5 ft (1.5 m).
Appendix O of the SWTR Guidance Manual provides extensive discussion in evalu-
ating CT for existing basins. For most applications, the designer may use the T10 method
for estimating basin performance. However, the determination of CT for ozone contact
basins is complicated by the time required to establish a residual concentration and the
relatively rapid decay of ozone residual in water. Consequently, a cell-by-cell evaluation
is necessary, and the CT for an ozone contact basin is the sum of the CT values for all
cells.
The designer may conservatively estimate the Tlo/T for the entire contactor. The D/L
value may be estimated by

D
L
D × number of cells
total contactor length (excluding baffle thickness and counting chimneys as cells)