The key factor here is not only the digester gas yield, but above all the costs of subsequent sludge dewatering and disposal. In addition, the digestion process should also be stable and digester foaming should be avoided. Countercurrent disintegration (GSD) can be used to exploit the full potential of a digester.
Reforms in Germany
On 3 October 2017, the new ordinance on the reorganisation of sewage sludge utilisation came into force in Germany. It contains obligations concerning the recovery of phosphorus from sewage sludge and sewage sludge incineration ash. Until 1 January 2029, land-based utilisation of sewage sludge from treatment plants with a population equivalent (PE) of over 100,000 may continue in compliance with the criteria of waste and fertiliser legislation – but not thereafter. For plants between 50,000 and 100,000 PE, the cut-off date is 1 January 2032. Land-based utilisation of sewage sludge from smaller plants may continue in the future. The thermal utilisation with phosphorus recovery that is envisaged will have a significant impact on the costs of sludge disposal. Optimum utilisation of the organic portion of the sludge will therefore become all the more important.
Starting gun for the GSD
The development of ultrasonic treatment of sludge took place in 2001. In August 2002, a prototype of a VTA countercurrent disintegrator was put into operation for test purposes at the treatment plant in Attnang-Puchheim, Austria. This was the beginning of a success story in terms of sustainability and sensible environmental protection. Today, GSD systems from the VTA Group are in operation worldwide.
Since 2003, a GSD system has been in operation in Miltenberg am Main in the Lower Franconia district of Bavaria, where it won the first public tender for a disintegration plant in Germany. The treatment plant is operated by the Main-Mud wastewater association, which thanks to the VTA GSD system can look back on 16 years of optimum digestion in which cost savings were achieved.
The Turnhout treatment plant operated by Aquafin has two digesters, one of which is used for excess sludge. In the second digester, filamentous bacteria had taken hold (Microthrix and Nostocoida), causing heavy foaming that resulted in poor dewaterability and low digester gas formation. To get to grips with this, a VTA GSD system was installed in April 2009 after a six-month pilot project. The foaming in the digester soon disappeared. The digester could then be operated without problems. Gas volume and methane content had increased significantly. With annual cost savings of approx. 137,000 euros, the GSD system paid for itself in a very short time.
Large plants in South Korea
Daegu is the fourth biggest city in South Korea and is considered the country’s fashion centre. The city’s four treatment plants treat the wastewater produced by around five million population equivalents per day. All were equipped with ultrasonic disintegration by VTA. A total of 18 reactors with more than 200 ultrasonic transducers were delivered to Daegu. This makes it the biggest ultrasonic disintegration plant in the world.
More success stories
In the municipality of Roth in Germany, the GSD system increased the gas yield by almost a third and reduced the sludge volume by up to 11%. In Domzale, Slovenia, reliable digestion with co-fermentation was achieved by installing a GSD system. In Ajaccio on the island of Corsica, the need for an additional digestion system was no longer necessary after the introduction of ultrasonic sludge pre-treatment. This treatment plant also treats the wastewater from Napoléon Bonaparte Airport.
In view of increased costs for sludge disposal, these examples show the potential of GSD for future applications. Increased sludge disposal costs can be counteracted systematically by reducing the organic content in optimised digestion processes. In addition, the higher volume of digester gas increases the energy efficiency of plants, thereby contributing to climate protection.
Field report from the VTA Group’s scientific journal “Laubfrosch”, issue 81