Anaerobic digestion process comprises of four major steps – hydrolysis, acidogenesis, acetogenesis and methanogenesis. The biological hydrolysis is the rate limiting step and pretreatment of sludge by chemical, mechanical or thermal disintegration can improve the anaerobic digestion process. Ultrasonic disintegration is a method for breakup of microbial cells to extract intracellular material.
Ultrasound activated sludge disintegration could positively affect anaerobic digestion of sewage sludge. Due to sludge disintegration, organic compounds are transferred from the sludge solids into the aqueous phase resulting in an enhanced biodegradability. Therefore disintegration of sewage sludge is a promising method to enhance anaerobic digestion rates and lead to reduce the volume of sludge digesters.
The addition of disintegrated surplus activated sludge and/or foam to the process of sludge anaerobic digestion can lead to markedly better effects of sludge handling at wastewater treatment plants. In the case of disintegrated activated sludge and/or foam addition to the process of anaerobic digestion it is possible to achieve an even twice a higher production of biogas. Here are few examples:
STP Bad Bramstedt, Germany (4.49 MGD)
- First fundamental study on pilot scale by Technical University of Hamburg-Harburg, 3 years, 1997 – 1999
- reduction in digestion time from 20 to 4 days without losses in degradation efficiency
- increase in biogas production by a factor of 4
- reduction of digested sludge mass of 25%
STP Ahrensburg, Germany (2.64 MGD)
- Preliminary test on pilot-scale by Technical University of Hamburg-Harburg, 6 months, 1999
- increase in VS destruction of 20%
- increase in biogas production of 20%
STP Bamberg, Germany (12.15 MGD)
- Preliminary full-scale test, 4 months, 2002 2) Full-scale installation since June 2004
- increase in VS destruction of 30%
- increase in biogas production of 30%
- avoided the construction of a new anaerobic digester
STP Freising, Germany (6.87 MGD)
- Fundamental full-scale study by University of Armed Forces, Munich, 4 months, 2003
- increase in biogas production of 15%
- improved sludge dewatering of 10%
STP Meldorf, Germany (1.06 MGD)
- Preliminary full-scale test, 3 months, 2004 2) Full-scale installation since December 2004
- increase in VS destruction of 25%
- increase in biogas production of 25%
- no foam or filamentous organisms present in the anaerobic sludge digester
STP Ergolz 2, Switzerland (3.43 MGD)
- Full-scale test, 3 months, 2004
- increase in VS destruction of 15%
- increase in biogas production of 25%
STP Beverungen, Germany (2.64 MGD)
- Full-scale test, 3 months, 2004/2005
- increase in VS destruction of 25%
- increase in biogas production of 25%
To sum up, ultrasonication has a positive effect on sludge solubilisation, sludge volume, biogas production, flock size reduction and cells lyses. Ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better degradation of volatile solids and an increased production of biogas.
The use of low power ultrasound in bioreactors may present a significant improvement in cost reduction. Therefore, ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better sludge digestion and efficient recovery of valuables.
