Report FR/INV 0001

The Foundation for Water Research, through discussion with water companies and research organisations, identified a need for a review of ultrasonic technology as used in existing applications and for an investigation into potential further uses in new application areas and the resulting potential benefits to water-industry operations.

FWR commissioned FTD Infinite Ltd, East Kilbride, to conduct an independent technology review to establish the feasibility and potential value of using ultrasonic technology in water- industry operations.

The objectives of this technology review were to study existing technology, document typical applications, highlight key design issues, identify future development trends and potential application areas along with their benefits to water-industry operations.There are a number of applications in general industry which currently use high and low intensity ultrasound. In general industry high intensity applications include cleaning, surface decontamination and the welding, machining or assembly of plastics. The use of ultrasound is well suited to industrial applications since the medium itself can transmit the sound, the main limitation to its use with large volumes and flow rates is the cost of equipment and the running costs. Low intensity applications in general industry include flow and level measurement and non-destructive evaluation of materials. These applications are now well established, having proved their feasibility and potential value in industrial applications.

In the water industry the use of low intensity ultrasound is also well established in level measurement and relatively well established in the area of flow measurement, although in this latter area electromagnetic flow measurement is still seen by some users as being more accurate.

In the area of high intensity (power) ultrasound in the water industry it has been proven at laboratory scale that power ultrasound can have a considerable effect in enhancing various processes and therefore has good potential on a range of applications. The effect can either be to enhance an existing process, thereby improving efficiency and/or productivity, or to cause a mechanical or chemical reaction which is not feasible with other technologies. The commercial feasibility of using power ultrasound in water and wastewater processes is still unclear since there are as yet very few pilot plants, and even fewer full-scale uses of power ultrasound.

The power ultrasound process applications which are showing the highest potential are currently in the areas of disinfection, decontamination and dewatering. Other processes which are less well researched but are showing good potential are particle agglomeration, degassing, gasification (enhanced oxidation) and dispersion processes. Work carried out at laboratory scale has demonstrated that the use of ultrasound at specific frequencies and intensities can improve or enhance process reactions and efficiency, and can achieve high bacteria kill rates and destruction of pollutants and specific organisms.

Although the successful scaling-up of power ultrasound or sonochemical reactors for water applications has not yet been fully proven, ultrasonic technology has been demonstrated to be effective in large scale, general industrial, continuous processes such as surface decontamination, cleaning, tube drawing and defoaming.

It is important in the scaling-up process that process technologists understand the methods by which power ultrasound operates. Changes in ultrasonic power input may not only influence the overall rate of the reaction but also the ultrasonic intensity distribution within the medium.

A current problem in the development of larger scale applications of power ultrasound is the lack of proprietary large scale equipment.

Industrial and water companies who are currently attempting to use larger scale equipment for novel applications in specific areas have generally funded the development of the equipment themselves. Due to confidentiality agreements only a small amount of results are published from such applications.

There are two main transducer types generally used in power ultrasound applications. These are piezoelectric and magnetostrictive.

The amount of power required will be one of the main problems for large-scale power ultrasound applications. The efficiency for converting electrical energy into sound energy for systems using piezoelectric transducers is approximately 85%. The efficiency for systems using magnetostrictive transducers is between 50 and 55%. Magnetostrictive transducers do however have a much longer life.

There are additional inefficiencies connected with transferring the ultrasound energy from the transducer into the liquid. The overall efficiency of the Harwell sonochemical reactor in transferring input electrical energy into delivered, ultrasound energy within the liquid, is claimed to be approximately 50%.

Copies of the Report are available from FWR, price 25 less 20% to FWR Members.

July 1995