Ultrasound improves sewage sludge treatment

By Dr. William P. Barbera

Ultrasound treatment, which involves the introduction of high intensity sound waves into a sludge medium, is one of several technologies that promote hydrolysis during sludge treatment.

The basic principal is based on the destruction of bacterial cells and difficult-to-degrade organics. Bacterial cells release their contents, which are then available for consumption by other species, whilst the organics are broken down into smaller readily biodegradable fractions.

When an ultrasonic field is applied to a liquid medium, minute bubbles form during the cavitation process. These cavitation bubbles implode under extreme forces of temperature and pressure. The force of the bubble implosions breaks up sludge particles when applied in sewage sludge treatment. The effects vary according to the intensity of the ultrasonic field.

At lower intensities, filamentous organisms are shaken off and/or destroyed from the surface of sludge particles, thereby aiding sludge settlement. As intensity increases, the particle size distribution and surface charge of the sludge particles alters. Extracellular polymeric materials are released and, along with altered size distribution and charge, improve dewatering characteristics of the sewage sludge in a process known as "bioflocculation". This process enables sludge to dewater better whilst having lower polyelectrolyte consumption.

If the level of ultrasound is increased, material - including exocellular enzymes - is shaken off the surface of sludge particles. This material catalyses biological hydrolysis reactions and improves biokinetic reaction rates. Consequently, ultrasound technology improves volatile solids destruction, which increases biogas production and lowers solids output during anaerobic digestion, and reduces solids production during activated sludge treatment.

Finally, at the highest level of insonation, sludge particles are completely destroyed and their contents released for consumption by other species. Refractory material is also broken up into smaller biodegradeable fractions. This process improves both biogas production during anaerobic digestion and biological nutrient removal during activated sludge treatment. This has been demonstrated at full-scale with improvements of over 80% with respect to phosphorous, and 90% with respect to ammonia removal.

Ultrasound works well with waste and sludge with large quantities of refractory material and/or cellular matter, such as waste activated sludge streams. These streams can then be readily degraded during anaerobic digestion.

The technology has been used successfully at full-scale in mainland Europe for several years with 14 plants in operation, but recent data from two full-scale treatment plants in Germany documented impressive results.

Ultrasound treatment of secondary sewage sludge improved volatile solids destruction in anaerobic digesters by an average of 50%, which subsequently increased biogas production by 45% at a sewage treatment plant in Süd, Germany. The feed stream of the anaerobic digester at this site is now 100% secondary sludge. In addition, dewatering performance improved by five percentage points whilst using 11% less polymer.

Volatile solids destruction increased from 50% to 62% at a sludge treatment plant using ultrasound technology at Wallerfangen, Germany. Here, the feed stream is a mixture of primary and secondary sludge at a ratio of 35:65. In addition, biogas production increased by an average of approximately 25%. In the dewatering plant, cake solids content increased by up to seven percentage points (an improvement of >40%) from a belt press despite using 33% less polymer. Dewatering performance improved from 17% to 24% dry solids. Together, improved dewatering and volatile solids destruction resulted in 30% less sludge cake leaving the works.