The digestion tower of the Wasserfeld WWTP has a height of 17.5m and a volume of 1,400m³. The sludge is circulated by means of gas injection lances and centrifugal pumps. The last clearing of the digestion tower from deposits was carried out in April 2010 and therefore it seemed important to us to check whether there are deposits in the digestion tower again and in approximately what quantity.
Therefore, in May last year, industrial divers were used at the Wasserfeld WWTP to determine the degree of sedimentation of the digestion tower. This method proved successful for us and with the help of a professional company from Germany the work was carried out. The company has many years of know-how and its own innovative technologies, such as digital measurement of the digestion tower and developed suction techniques to bring the deposits out of the digestion tower.
First, the digestion tower was digitally measured and based on the different measuring points it is possible to determine the deposition form in the digestion tower and to display it in digital form. Here are the results:
It is clearly visible that the center of the digestion tower is almost free of deposits, but already after 2 meters the old deposits rise to about 2.5 meters and end at the periphery of the digestion tower at a height of about 5 meters. In the picture beside you can see the deposits transferred to the cross section of the digestion tower.
After the digital survey, the industrial divers entered the digestion tower to perform a crosscheck. It was found that the quantities of old sludge are overlaid with approximately 1m of soft sludge. The waste sludge volumes are usually very compacted in such shallow digestion bottoms in connection with gas circulation. MAP adherence was also found in palpated and collected sludge samples. It can therefore be concluded that natural degradation of the sedimented old sludge is not to be expected.
Based on the results of the year 2021, the digestion tower was cleared of the detected deposits this year in April. This must be done so that the digestion process is not disturbed. Such a disturbance would have serious consequences for the operation of the plant and would cause immense costs. The digestion tower cannot simply be taken out of operation, since there is no second digestion tower at this plant that could take over the work in the meantime.
When industrial divers are used, however, the digestion tower can remain in operation. Likewise, the temperature does not have to be lowered below 37°C. Accumulating digested sludge can be added and recirculated at any time by arrangement. The old sludge was sucked off and immediately dewatered with a mobile sludge press.
A new start-up of the digestion tower, which would take 14-16 weeks, is therefore not necessary. The cogeneration units can be put back into operation just a few hours after the digestion cover has been closed. This means that electricity and heat can be produced again.
Due to the precise preparation and planning of the work steps as well as the excellent cooperation of the colleagues at the Wasserfeld wastewater treatment plant and the team of industrial divers, the project was a complete success. The digestion tower should now provide its full performance again for about 15 years. The use of professional industrial divers can only be recommended from our side.