Sewage sludge disposal is expensive: in Bavaria, for example, thermal utilisation already costs well over 120 euros per tonne (up to 200 euros throughout Germany), and the upward trend is continuing. ‘At some treatment plants, this is already the biggest cost factor’, explains graduate engineer Arne Nath, head of the wastewater department at SüdWasser GmbH. Hence, it is important for sludge dewatering to be as effective as possible. In addition to more stable plant operation, there are also more economic reasons for this than ever. This is exactly where the VTA mudinator® comes in, another innovation from the VTA Group. Like the patented VTA GSD, this technology also works with ultrasound, but with one crucial difference: less energy is used, so there is no disintegration in the sludge. The floc structure is modified in such a way that charge balancing with the polymers in use is improved. The floc that is formed is more shear-stable, and intermediate water is removed. This increases the dry substance content of the dewatered sludge, reducing the volume of sludge for disposal. The example of the Piding treatment plant near the spa town of Bad Reichenhall in Bavaria shows what this means in practice. At the plant, the anaerobically stabilised sludge with an input dry residue of approx. 2.3% and a loss on ignition of approx. 60% is dewatered using a screw press with a feed rate of 3.5 m3 per hour. The discharge dry residue used to be between 22 and 24%.
Green light after extensive tests
Before installation of the VTA mudinator® in Piding, extensive laboratory testing and tests involving an external sludge analysis laboratory were performed. In particular, the test determined the optimum dosage of VTA Biocitran®, a high-performance product for dewatering developed especially for its synergy with this technology. In the laboratory tests, ultrasonic treatment achieved an enormous improvement in the dry residue in the discharge, up to 28.6%. The decision was then taken to install a VTA mudinator® with a three-transducer system.
‘The dry residue in the discharge, originally about 23%, was increased to just under 26% during normal operation’, says Arne Nath, summarising the results. With disposal costs of 100 euros per tonne, this increase in dry residue of around 3% is equivalent to an annual cost saving of around 17,000 euros. As disposal costs increase, these savings will increase accordingly; at 200 euros per tonne, they would already amount to 34,000 euros per year. ‘We have now achieved a discharge dry residue rate of 28%, including during normal operation’, adds Arne Nath with reference to the available values, which date from February 2019. The savings are therefore even higher – and the cost benefits of the VTA mudinator® will continue to grow as the disposal costs for sludge rise in the future.
Polymer saving too
In addition, the polymer requirement was also reduced by around 8%. All in all, use of the VTA mudinator® in Piding saves so much in costs that the payback period is only four to five years.
VTA Biocitran® even stronger as part of a team
VTA Biocitran® is a newly developed high-performance product for sludge dewatering based on citric acid, which creates maximum synergies together with the VTA mudinator®. Added before ultrasonic treatment, it further improves the stability of the floc, optimises the effect of the polymer and increases the dry residue discharge after dewatering. The centrate becomes clearer and backcharge at the plant is reduced. A side effect observed in Piding was also less odour development in the sludge.
Field report from the VTA Group’s scientific journal “Laubfrosch”, issue 82