Calculating bubble rise velocity and bubble size in thickened waste activated sludge

wastewater treatment plant

A recently released paper by RMIT University has for the first time developed equations for calculating bubble rise velocity and size in a concentrated sludge with Herschel Bulkley flow behaviour  to help improve efficiency in the design and operation of aerators.

Associate Professor Nicky Eshtiaghi, School of Engineering, RMIT University, said these equations are useful for engineers and operators to understand the hydrodynamics of highly-concentrated, complex, opaque fluids such as sludge and to optimise process parameters for efficient operation using an online rheometer.

“Since these equations were based on direct measurements of the sludge physical properties and gas velocity, they are easy to apply to the actual processes,” Nicky said.

“Accurate and in-situ estimation of sludge physico-chemical properties and gas phase characteristics is important for the efficient operation and process design optimisation.

“These equations will facilitate engineers and plant operators to appropriately understand the change in physiochemical properties without having to perform these measurements offline, which are time consuming and tedious. And it’s not even possible to measure gas phase characteristics offline.”

The researchers measured stress imposed by gas injection in-situ by measuring the viscoelastic properties of sludge, which has not previously been shown, and can therefore provide a more accurate representation.

“An accurate measurement of stress imposed (corresponding shear rate) is important in the design and operation of aerators in waste activated sludge processes,” Nicky said.

“Measuring the physicochemical and gas phase characteristics can be used as a method for monitoring efficiency of biological treatment process.

“Since a correlation has been shown between stress imposed and sludge physicochemical properties as well as gas phase characteristics, it may be possible to monitor the thickened waste activated sludge process performance by only measuring the sludge viscoelastic properties. Additionally, rheological measurements can be done online in real time.”

Developing the equation

The calculation of effective shear rate for the Herschel Bulkley fluid presented in the paper is based on the HB model which eliminates a lot of errors caused by using the power law model and therefore increases the accuracy in process design and optimisation.

The simple equations were developed for the quick estimation of gas phase characteristics (such as gas hold up, bubble size and bubble rise velocity) in waste activated sludge.

There were three key findings from the research:

  • Gas injection imposes extra shear on sludge and makes significant changes in sludge viscoelastic properties. The knowledge of stress (or corresponding shear rate) imposed by the gas injection is important as the shear rate impacts on power consumption, mixing characteristics and mass transfer phenomena in aerators
  • The imposed stress by gas injection has a direct relationship with the sludge physicochemical properties, which will help to optimise the aeration rate and coagulant dosage, reduce the excess energy consumption and increase the oxygen transfer efficiency
  • There is a relationship between stress imposed and gas phase characteristics, which directly affects the mass transfer and fluid flow pattern within the aeration tank

“The change in viscoelastic properties of sludge can be used as a tool to monitor the changes in physicochemical and gas phase characteristics during gas injection. This will pave the way to use an online rheometer for monitoring process performance,” Nicky said. 

“We proposed a technique for finding the stress imposed by the gas injection in complex opaque fluid. Also, for the first time, an effective shear rate for the Herschel Bulkley fluid was calculated, which was a big challenge due to the massive discrepancy among the available equations for shear rate calculation.”

The equations are useful for a number of professions and sectors including engineers, designers and plant operators who are in the field of wastewater sludge treatment; researchers who study the rheological behaviour as well as gas phase behaviour in opaque non-Newtonian complex fluid; and can also potentially be utilised in different industrial applications such as aerated gels, aerated pharmaceutical and cosmetic products to increase the efficiency of the process and quality of product.

For more information, the full report can be found in Chemical Engineering Research and Design I48 (2): 119-128 (2019).

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