Ultrasound facility

Ultrasound treatment of sludge increases the biogas production by 20% and significantly reduces the sludge volume

Sludge treatment can be a heavy financial entry at many purification plants but with a payback time of just a few years, the installation of Ultrawaves for ultrasound treatment of sludge is a very attractive investment.

Ultrasound is sound with vibrations above “the audible” area. It is successfully put to use in a variety of scenarios – now including sludge treatment.

You can read more about Ultrawaves and ultrasound treatment here:

Ultrawaves high-power ultrasound reactor for disintegration of waste water sludge

Sound is made up of vibrations in various materials such as air. Ultrasound is sound that has a frequency (number of vibrations/second) above “the audible” area. Sound with a frequency above 18,000 Hz is defined as ultrasound. Ultrasound can spread in solid matters, liquids and gasses.

It is used for the following purposes:

  • cleaning of items in immersion baths
  • welding of plastic and metal
  • medical use
  • alarm units
  • industrial measuring instruments
  • opening of bacteria cells

The picture shows the Stjernholm agency on ultrasound treatment of biological waste water sludge. Ultrawaves Wasser & Umwelttechnologien GmbH is a German company that cooperates closely with Technische Universität Hamburg-Harburg. Ultrawaves is well-known for its research into and vast knowledge of ultrasound treatment of sludge.

Information about the ultrasound reactor

5 kW ultrasound reactor in a stainless steel box
· 5 x 1 kW sonotrodes (surface material: titanium)
· 5 air-cooled ultrasound converters
· 5 x 2 kW air-cooled automatic generators (230 V, 50 Hz, 3 phases) 
in separate 48 cm box.

Reactor size: 130 cm x 110 cm x 130 cm, weight 130 kg.

Optional: Sound casing for isolation of the sound from the ultrasound reactor. The sound goes down to 90 dB. Sound casing size: 150 cm x 120 cm x 150 cm.

The picture shows the inside of the ultrasound reactor. The Ultrawaves reactor has a volume of 29 litre and it is equipped with five 20 kHz sonotrodes (sounding bodies). Each sonotrode is equipped with a 2 kW generator. Generator intensity is adjustable in an area from 25 to 50 W/cm2.



Biological sludge is pumped upstream through the reactor and ejected at the top of the reactor in order to prevent accumulation of gas bubbles produced during degasification to the sludge phase.

The sonotrodes are designed as 1-2 kW sonotrodes and they have a converter (transducer made of piezo ceramics) at the end of the flow passage. Ultrasound is created by sending electrical energy to the converter which then turns the electrical energy into mechanical energy. This mechanical energy continues to a booster and then to the sonotrode itself which, in the case of Ultrawaves, is designed as a horn. This horn transmits the ultrasound energy to the medium/organic sludge. When electrical energy is transmitted through the ceramic unit, it starts to move. The ceramics unit cannot cope with very high temperatures. In order to solve this problem, Ultrawaves has designed the reactor so that the sonotrodes are surrounded by air-cooling. The surface material on the sonotrodes is made of titanium.

The purpose of the generators is to keep a relatively constant amplitude. Amplitude means vibrations in the air (variation from zero position/0 point). The generator must continuously send in the “power” needed to keep the amplitude at the given level. The amplitude is 15 to 20 µm.

The phenomenon – cavitation with ultrasound energy

When ultrasound vibrations spread into the sludge, small, unstable air bubbles are created (cavitation).  Those air bubbles collapse during release of very high local amounts of energy, temperature and pressure increase (see figure above). In this way, the organic sludge is exposed to strong mechanical forces that destroy the bacteria cells and degrade large molecules. During this process, the amount of accessible and convertible CO2 is released and increased. This leads to an acceleration in the speed-limiting hydrolysis step in the following digestion process which, according to experience, leads to around 20% more organic material (ignition loss) being converted in the digestion tank.

Ultrasound is characterised by a wide range of frequency and intensity levels; however, only a small number of ultrasound parameters are suitable for destroying microbiological cells so that the organic material inside the cells becomes accessible.

Limitations of the ultrasound technology: the effectiveness limitations of this technology strongly depend on various factors. It is very important to follow below mentioned guidelines in order to achieve an optimal effect of the ultrasound sludge treatment.

  • Please note that the pressure through the ultrasound reactor must not exceed 2 bar. If the pressure in the reactor exceeds 2 bar, no cavitation bubbles are created. In addition, the sonotrodes will be damaged if the pressure is too high.
  • The medium for the ultrasound reactor must be able to flow freely.
  • Viscosity must not exceed 0.5 Pa/s.
  • The TS concentration of the medium must not exceed 9%. Optimal amplitude, and thus effect, is achieved for pre-dewatered, biological sludge with a TS of 4-5%.
  • The medium must be neutral with a pH value between 6 and 8.
  • It is important that coarse fibres, corn, grain etc. have been removed from the medium prior to the ultrasound treatment in order to avoid blockage in the reactor.
  • The flow must be strong enough to avoid sedimentation inside the reactor. The flow must not be less than 0.5 m3/h to avoid too much heat generation in the reactor which will damage the sonotrodes.

If above-mentioned guidelines are not followed, the ultrasound reactor might be damaged.

Important service hints for everyday use:
After each 24 hours of service, the ultrasound reactor must be cleaned in order to avoid deposits and blockage by solid matter. Use water with a strong flow (equal to 5 m3/h for 10 min.) for this procedure. The ultrasound must be on during the purging process.

The purging interval must be increased if more ultrasound power is required to keep the amplitude. If more frequent purging does not help, check above-mentioned guidelines.

It is recommended to use the unit continuously (24 hours/day) in order to achieve a high degree of cell disintegration.

Parameters for Ultrawaves 5 kW ultrasound facility

From no. 1 to 7: Based on systematic research by Technische Universität Hamburg-Harburg, showing that low-frequency, high-intensity sonication is the optimal area for disintegration of waste water sludge. Ultrawaves has developed a special sonotrode that works with 20 kHz and an ultrasound intensity of 25-50 W/cm2 which is the most suitable area for breaking the biological sludge cell wall.

No. 9: The optimal amplitude of the sonotrode is in the area of 15 to 20 µm for pre-dewatered sludge with a TS of 4-5%. The optimal amplitude depends on the type of sludge and must be determined through an ultrasound test at the Ultrawaves laboratory.

No. 11: The average retention time must be at least 1 minute to allow the cavitation bubbles to be divided evenly within the reactor. Retention time and reactor design are based on a mathematical cavitation model that ensures the best result with regards to disintegration of waste water sludge.

No. Parameter description Value
1 Ultrasound frequency 20 kHz
2 Design generator effect for 2.0 kW each sonotrode
3 Current power consumption per sonotrode 1.0 kW for normal operation
4 Power efficiency % ~85%
5 Number of sonotrodes in an ultrasound reactor 5
6 Maximum treatment capacity 30 m3/d = 1.25 m3/h for a ultrasound reactor m3/h
7 Power intensity ( W/cm2 ) Variable: 25 – 50 W/cm2
8 Sonotrode amplitude~area ( µm ) 15-20µm
9 Optimal, required amplitude ~18µm for pre-dewatered sludge with 4-5% TS.
10 Power density ( kW/L ) Density in W/I reactor: Vreactor = 5×1 Kw/29l => 170 W/I
11 Needed average retention time (min) 1 – 2 min
12 Designed reactor volume 29 L
13 Module dimension ??? 25 cm x 119 cm x 102 cm
14 Cooling system Air cooling
15 A number of sensors are delivered together Indication of amplitude and output in a
module, power, and control system in the automatically cooled ultrasound
Laboratory test

Laboratory test:
In order to examine whether a purification plant might benefit from exposing their sludge to ultrasound treatment, a sludge sample is forwarded for testing to Ultrawaves at the laboratory of the Hamburg/Harburg university. The sample must be sent to the university along with a mass balance of the purification plant. Such an examination will show whether the sludge in question can be degraded even more through ultrasound treatment in order to increase gas production and minimise sludge volumes. The results of the examination are presented in a report.

Advantages of ultrasound:
Sludge treatment at purification plants can be very costly. Sludge treatment with ultrasound can be an attractive way of reducing the final amount of sludge. Ultrasound treatment of sludge offers the following potential advantages:

  • Increased conversion of organic matter (ignition loss) of 20-25% in the digestion tank
  • Increased biogas production => around 20-25%
  • Reduction of polymer consumption for final dewatering
  • Improved final dewatering=> the sludge can be used for a higher TS equal to 3-5% higher TS
  • Control of filamentous bacteria and thereby reduction of foam formation issues in the digestion tank

Please note, however, that a reduced ignition loss will also cause an increased release of ammonium in the reject.
Disintegration can also take place in other ways as mechanical, thermal and chemical disintegration. Ultrasound application does, however, show significant advantages with regards to operational conditions: the Ultrasonic/Ultrawaves reactor is very compact, easy to install and easy to operate.

When you cooperate with us, you know that we are focusing on delivering a solution to you that adds value to your facility.



  • Typically 3-5% higher TS for final dewatering
  • Biogas production increases by 20%
  • Residual sludge volume is reduced by 10-15%
  • 15-20% less polymer consumption


  • Better utilisation of resources through increased gas production
  • Lower sludge volume, less deposits and thereby less transport contribute to lower CO2 emissions from the facility.
  • No waste production such as when using sludge heat treatment
  • Higher efficiency level, lower loss


  • Service-friendly and operationally reliable
  • Closed system, hygienic conditions for the staff
  • Utilisation of resources that would otherwise be wasted

Ultrasound facility


 Contact us


Ulrik Folkmann

Mobile phone: +45 92 15 22 15

© Stjernholm A/S 2020