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What is Ultrasound?

Ultrasound is the term used to describe sound energy at frequencies above 20 kHz, above the range normally audible to human beings.

High power ultrasound is usually generated by a transducer and is delivered into a fluid via a horn or probe. The majority of transducers are designed to deliver ultrasonic frequencies of between 20 kHz – 35 kHz. Below 20 kHz excess noise becomes a problem, whereas at higher frequencies the power delivered from the ultrasonic horn is reduced.

Non-water industry applications of ultrasound include amongst others, velocity measurements, medical diagnosis and cleaning baths.

Cavitation - Driving the Reaction

Sound is composed from longitudinal waves comprising rarefactions (negative pressures) and compressions (positive compressions). It is these alternating cycles of compression and rarefaction that, in high power ultrasonic applications, can produce a phenomenon known as “cavitation”.

Cavitation is the formation, growth and collapse through implosion of (usually vapour filled) micro bubbles. Understanding and quantifying the cavitation effect is believed the be the key feature in determining the feasibility and success of ‘high power’ ultrasound in industry.

The minimum amount of energy required to initiate cavitation is referred to as the ‘cavitation threshold’. This threshold will vary for different fluids. In water cavitation will generally occur once the ultrasonic energy rises above 1 W/cm³ (mason, 1991).

Ultrasound to Enhance Sludge Treatment

In conventional anaerobic digestion cell lysis is often the rate-limiting stage for determining the rate of release of intracellular organics for responsible bacterial consumption and conversion to biogas. It has been shown that the extreme conditions produced during ultrasonic cavitation will cause cell disruption/lysis. This action causes the release of intracellular organics that might otherwise be deemed inaccessible to the microbial population in the digester, thus enhancing the digestion process.

Thickened Waste Activated Sludge (TWAS) is mainly composed of dead bacteria cells and it is a particularly difficult sludge to digest anaerobically. Furthermore the physical action produced by the cavitation bubbles reduces the sludge particle size distribution thereby increasing the number of sites available for microbial action. Improvements in downstream dewatering have been observed.

The Beauty of the Sonixs Product

The beauty of the Sonix product lies in the method of sonication. Of particular importance is the design of the sonication 'horn' (i.e. the part that vibrates in the sludges and thus transmits the sound energy into the fluid).

In most conventional systems the sonication horn is a small low power device such as a probe or a block horn. Sound energy emitted from these types of devices attenuates and decreases with increasing distance from the horn. The effect of this is to greatly reduce the effective 'cavitation zone' (i.e. the zone in which ultrasonic cavitation can occur and be controlled).

In the Sonix product however, a novel, patented, radial horn design is used. This design enables very high levels of sonication energy to be achieved.

The Sonix Plant

A number of full scale demonstration trials have been undertaken with the Sonix technology in both the UK and the US. This work has demonstrated conclusively the benefits associated with the sonix product.

In the graph below, data from a recent trial (at Orange County Sanitation District, Los Angeles) is presented. The sonix was installed on the feed line to a full scale digester. The graph clearly shows the increase in gas production due to the sonix (E was the control, H was the test digester). In order to check that the gas readings were real 2 new parametric gas meters (the burgundy and dark blue lines on the graph) were installed. The new gas meters confirmed the improvements in gas production.

An additional benefit of Sonix includes increased dewaterability of the final sludge cake. Typical improvements at Orange County ranged between 1.5 – 2.5 percent. Sludges from the test and the control digester were compared using identical dose rates of polymer. The data is shown in the chart below.

Sonix also improves digester operation and stability particularly at high TWAS loadings. The technology is useful for the control of filament outbursts, as shown under the microscope in figure 5, and to prevent digester foaming.



New Product

VoR Sludge Dehydrator. Dewatering is now made easier. Take the complexity from the process.