Ozone success at Tampa Bay
Innovative gas dispersion systems installed at the Tampa Bay Surface Water Treatment Plant in Florida, USA are proving to be efficient and economical. Gary Richards sales director at UK-based Statiflo explains the benefits.
Incoming surface water at the city of Tampa Bay’s water treatment plant has high turbidity, colour, taste and odour values.
To overcome this, the plant design, by consultants Black and Veatch, included solids removal by the Actiflo flocculation and separation process, followed by a lime pH trim.
The water is then subjected to an ozone dose of 4-6mg/L. Ozone is used primarily as a disinfectant for cryptosporidium removal, but the additional benefits of colour, taste and odour removal were attractive to the plant’s designers.
The Statiflo Gas Dispersion System was chosen over a traditional fine-bubble diffusion system or an SVRT J-tube on the basis of capital and operational costs. Capital outlay is 50% less than the other options, but more importantly, the Statiflo GDS leads on performance benefits.
Now successfully commissioned and operated for a couple of years, the dispersal system has been confirmed to consistently dissolve around 98% of dosed ozone across a 5:1 turndown ratio in main water flow. This extremely high transfer efficiency has a tremendous impact on the operating costs of the scheme.
Ozone is an expensive gas to generate and every additional percentage that can be dissolved saves money. Additionally, as any undissolved ozone remaining in the off-gas has to be destroyed by thermal catalysis, the more ozone dissolved, the lower the cost of off-gas destruction.
The project’s original commercial bid analysis estimated that at 95% transfer efficiency, the use of a GDS would return a saving of at least US$616,000 in ozone generation costs over the life of the plant. At 98% efficiency, the savings are considerably higher.
The key to the high efficiency is the ability of a Statiflo mixer to generate fine gas bubbles of a uniform size distribution.
The predisperser mixer within generates a stream of minute gas bubbles all of similar size, irrespective of mainstream flow. These gas bubbles are around one-tenth the diameter of bubbles achieved by traditional diffuser systems, equating to a hundredfold increase in surface area offered for mass transfer by the GDS.
With such a high surface area to volume ratio, the main contactor mixer has the sole function of a residence time reactor, maintaining Ågplug flowÅh at exceptionally low headloss.
The result is high efficiency across a wide flowrate range, perfect for the variable conditions experienced in a drinking water plant.
From a process perspective, the key to efficiency is mixing. The contactor mixer blends the ozone and water completely and efficiently to achieve a minimum mixture quality of CoV=0.05 at the discharge of the GDS. This ensures an ozone “plug flow” condition entering the contact chamber, eliminating ozone concentration gradients. (See www.staticmixers.net for definitions)
The result is excellent residence time distribution within the chamber, ensuring quick and predictable destruction of parasites.
A 3-log inactivation of cryptosporidium is reported about halfway through the contact chamber. This is an impressive performance, as the original design, in accordance with US Environmental Protection Agency guidelines, was a 1-log inactivation at the outlet of the chamber. A significant colour, taste and odour reduction is also obtained.
At peak flow, the two 1100mm diameter GDS units each handle 7200m3/h of water dissolving around 40kg/h of ozone generated at 11% from LOX.
There is one moving part per system, a simple small centrifugal pump, so operating and maintenance costs are minimal. According to the operators, since commissioning, the Statiflo GDS has never needed to be touched, a true ‘fit it and forget it device.’
The success of the Statiflo GDS has led to the publication of a paper defining the selection criteria and critical bid analysis carried out in the design of this equipment. The paper was presented to the International Ozone Association last year.
1. Sidestream Inlet
4. Predipersing Motionless Mixer
5. Isolating Valves
6. Sidestream Injector
7. Contactor Moyionless Mixer with STM (or STL) elements