A mixing breakthrough

Landia details its role in creating a oxygenation system with Air Products and the advantages of using their innovative new process to treat wastewater

Two companies offering different solutions, who may have once worked against each other, have recently combined their problem-solving ideas to bring about an energy-efficient, safer approach to aeration for wastewater treatment.

Oxygenation of biological wastewater treatment systems is widely regarded as one of the most effective processes for treating aqueous waste, but, for many companies the cost and practicality of supplying on-site liquid oxygen has proved prohibitive because of highly detailed risk assessment, site permitting and building regulations.

"A wastewater treatment process that has all the benefits of oxygenation but none of the inconvenience is a significant breakthrough", said Doctor Peter Barratt, commercial technology manager of environmental and clean technology at Air Products. "The vast majority of companies have a limited amount of space within which to bring about a solution, so the advent of a small unit is very advantageous. It removes the burden of bringing gas on-site and only makes oxygen when it is required. Yet compact, modular units are still capable of treating approximately 250-800kg/d BOD."

He added: "We discovered combining our vacuum swing absorption (VSA) oxygen generator with an integrated low-energy mixer/oxygenator from Landia created an easily maintained package capable of delivering dissolved oxygen to any activated sludge basin or oxidation vessel. The Landia mixer had to be highly durable and reliable, and the PODB-I, which is mast-mounted, also allows easy installation and access, without interrupting normal operation of the wastewater basin. The VSA is based upon the robust lobe-type blower technology, typically found in conventional aeration systems.

When oxygen is used, the amount of gas injected into the wastewater basin reduces by around thirty-fold and factors other than gas volume take effect. The other major influence on overall performance is shear force. The amount of shear force the injected oxygen sees determines the gas bubble size produced and, hence, the dissolution efficiency. Another important parameter with oxygenation is dispersion of the injected gas, whether it is as bubbles or as water with high-dissolved oxygen content. The importance of dispersion results in smaller overall volumes of gas being injected.

Aerators will tend to have many more discreet sources of (dilute) oxygen throughout the basin, for example individual diffuser domes, and therefore require shorter dispersion distances to cover the area between one source and the next. For this reason, the process is best served by a modified propeller-type mixer. At the heart of Air Products' oxygenisation system (the Oxy-Dep VSA process) is Landia's mixer-oxygenator, which operates by creating an area of low pressure behind the propeller as a result of water flowing over a closed cup. A gas inlet located in the area of low pressure is connected to the oxygen supply. With VSA supplied oxygen, the gas inlet is simply connected to the VSA outlet via a flexible pipe. As the oxygen, which is sucked into the cup, has to pass through the rotating propeller blades, it is subjected to high mechanical shear. The mixer-oxygenator of the Oxy-Deo VSA unit also creates a high velocity plume of water that carries the oxygen (both dissolved and dissolving) into the main body of the process basin.

Landia's Hugh Vaughan explained: "The mixer-oxygenator is simply mounted on a vertical mast at the side of the wastewater treatment basin. This allows it to be easily winched up and down by hand for maintenance, and positioned with the desired direction of mixing and oxygen distribution. This ease-of-fitting, operation and maintenance fits in perfectly with the VSA. The unit can also be skid-mounted and attached to a special floating frame. As long as power is available on site, the unit can be installed and running within two hours." The advent of new single-bed, single-blower, single-valve VSA technology at the small end of the scale for oxygen production means there are now competitive on-site oxygen generation options in the range <250kg-5,000kg/d. This provides the foundation for a new generation of oxygen-based aeration technology for WwTWs that give plant operators the flexibility, reactivity and process assurance of oxygen-fed processes, with the capital and operating costs of conventional aeration.

Whether replacing worn-out diffusers or surface aerators in an aeration basin, or installing temporary or permanent extra oxygen capacity to a plant during times of peak COD loading, this innovative, mixer/VSA technology offers several cost effective benefits:

  • low emissions - compared to air-based aeration with typical oxygen transfer efficiencies below 20%. A reduction in the volume of gas leaving the surface of the treatment basin by approximately 99%. This gives dramatic reduction in the levels of VOCs and odours originating from in the basin,
  • high shock load resistance - the high rates of biological treatment and rapid dissolution possible with oxygen generally give rise to higher levels of biomass solids (sludges) in a biological treatment basin. This gives the system resistance to shock loads that would typically cause process failure in an air dependent system,
  • high efficiency - specific oxygen transfer rates of approximately 1.5kg/kWh can be achieved in the basin environment with real wastewater. This is comparable with air-based technologies, for which high clean water test figures are usually quoted, but which operate at much reduced efficiency in dirty water and at normal effluent treatment temperatures,
  • high treatment rates - up to 10kgCOD/m3/d can be achieved with oxygen-enhanced systems; the rest of the treatment system is usually the limiting factor. This allows simple and cost-effective 'turbo charging' of existing overloaded plants. New plants can be designed with a much smaller footprint, especially if advantage is taken of complimentary technology such as membrane solids separation (membrane bioreactors),
  • easy installation - all that is required is a flat area near to the treatment basin. This could be concrete, gravel or any firm area of ground. The VSA is easily off-loaded with a forklift truck and only requires an electrical connection to commence operation. The mixer can be installed without draining the activated sludge basin; in fact the whole system can be installed within two hours without any interruption to the normal effluent treatment process.
  • oxygen permits/building regulations may not be required - as oxygen is only made when it is required, a VSA is less demanding than a liquid oxygen storage in terms of site permitting and building regulations.

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