This is again shown by pilot project lasting almost two years at the treatment plant in the town of Traunstein, which produced up to 30,000 kWh more electricity per month and a 14% reduction in the volume of sewage sludge for disposal. The approach pays for itself within 3.5 years. The disposal costs for sewage sludge now account for up to 15% of the total annual costs of treatment plants. This innovative approach to the disintegration of sewage sludge using ultrasound therefore continues to gain in importance. As a sustainable material flow management process, it yields benefits in terms of waste management and saves costs.
The sewage sludge produced is treated with ultrasound immediately before digestion. This causes the floc to break upon the one hand, cells to be destroyed on the other, releasing cell fluid. In the subsequent digestion process, these released substances are available more quickly and are also generally available for further biological transformation processes. This disintegration intensifies the degradation of the organic substances in the sewage sludge in the digestion system and their conversion into biogas. The increased production of biogas ultimately generates more electricity and heat in a combined heat and power cycle. In addition, the dewaterability of anaerobically stabilised sludge can also be improved. The disintegration of sewage sludge significantly reduces the amount of sludge produced, leading to considerable savings. In the VTA Group’s patented countercurrent disintegration system (GSD), part of the thickened excess sludge produced flows through the disintegration reactor from bottom to top. There are up to twelve ultrasonic transducers in the reactor, and an agitator is used to continuously push the sludge suspension past them for treatment. The degree of digestion (ACSB, enzyme exposure) is determined by the sludge retention time in the reactor, flow rate, agitator speed and energy input for the installed transducer elements. The treated medium is then returned to the digester’s feed pipe/circulation pipe.
Traunstein test setup The Traunstein treatment plant has a design capacity of 92,000 PE, and the load on the plant is currently around 80,000 PE. It features a conventional preliminary clarifier, activated sludge process and secondary clarifier for further wastewater treatment. A VTA GSD system was used for the disintegration of excess sludge at the Traunstein treatment plant over a period of five months from May 2014 until the end of September 2014 and over 15 months in a second trial from September 2015 until the end of 2016. The goal of the long-term trial was to prove that use of the VTA ultrasound system is cost effective. For the assessment, the results from 2014 – 2016 and the previous years were used, divided into periods with and without the use of VTA GSD. This approach provided valid data for the result.
Significantly higher degradation rate Through pre-treatment of the sludge with ultrasound, the potential for organic degradation was activated, resulting in a higher degradation rate of the organic content and thus a higher gas yield. With VTA GSD, the organic degradation rate was increased by approx. 15% to 54%, and in a second trial period to as much as 58%.
Increase in digester gas yield
The comparative value for the specific gas yield (litres of gas per kg of organic substance added) was used as a basis for calculation, because seasonal fluctuations are also taken into account here. Compared to the years 2012/2013, with 350 litres of gas per kg of added organic mass, 436 l/kg OTR added was produced with disintegration in the first trial period. This is equivalent to an increase of 24.5% In the further course of the long-term project, the specific gas yield was ultimately increased to 582 l/kg OTR added.
On the basis of the electricity consumption of the entire treatment plant per month, it can be shown that with the VTA GSD system, in addition to its own energy requirement (approx. 155 kWh/day), there is an additional energy gain of approx. 500 kWh/day through the use of ultrasound. ‘This was a long-term trial in which a standardised VTA GSD system was used. Even without being specially adapted to the individual conditions, significantly more energy was gained than was put into it’, emphasises graduate engineer Dr Bernhard Eder, Head of R&D at VTA Technologie GmbH.
Reduction in the volume of sewage sludge for disposal
Prior to the disintegration trials, the digestion system at the Traunstein treatment plant achieved up to 47% organic degradation. Use of the VTA GSD system increased the degree of degradation in the digestion system to 54%. ‘In addition, the use of GSD also saves on polymer for dewatering’, says Dr Bernhard Eder. A further consequence of the significantly better degradation behaviour of the sludge as a result of the use of the VTA GSD system is that there is less sludge requiring subsequently disposal. Assuming consistent dewaterability, the calculated savings in sewage sludge disposal are about 14%. If dewatering is also improved as a result of the lower organic load in the sludge, additional savings can be achieved. A welcome benefit in times of high disposal costs.
Energy surplus of 130,000 kWh per year
If the electricity requirement of around 50,000 kWh per year is compared to the electricity yield of approx. 180,000 kWh per year, the balance of accounts is clearly positive. A comparison of revenue and consumption in addition to investment and depreciation as well as interest gives a payback period of 3.5 years.
Degradation rate with VTA
- Improvement of the degree of organic degradation in the digestion system by up to 15%
- Increase in specific electricity generation by up to 24%
- Over the trial period, up to 30,000 kWh more electricity was generated per month (on average approx. 15,000 kWh/month)
- Calculated reduction of sewage sludge disposal approx. 14% or 300 t/a (1,759 t compared to 2,045 t)
- Polymer savings of up to 15% possible
- Amortisation up to three years
Field report from the VTA Group’s scientific journal “Laubfrosch”, issue 80