At the sewage treatment plant Gut Großlappen the surplus sludge was thickened statically. Especially the sludge from biology 2 to be thickened as well as the surplus sludge from the separate process water treatment were considered "difficult" due to their flotation properties. The thickened sludge was subsequently pumped and homogenised over a length of 300 m via eccentric screw pumps with a flow rate of 90 m3/h for further anaerobic treatment. Prior to the project realised with Hiller, the thickened sludge could be pumped to the digestion towers with a maximum of 5 - 6 % DS. A higher degree of thickening led to a pressure build-up in the pipelines that could no longer be handled by the installed pumps.
The plant managers considered thickening the excess sludge from biology 2 and process water treatment by machine in the future. The objective was to increase the degree of thickening to 8 % DS in order to relieve the hydraulic load on the digestion capacities. In March 2019, the Hiller company was therefore commissioned to carry out a large-scale trial with a mobile thickening centrifuge.
The aim of the trial was to provide reliable performance data that can be used for a system comparison of the results obtained with static thickening versus possible results with mechanical sludge thickening using a thickening centrifuge. In addition, the effect of disintegration of the excess sludge by means of the “lysing device" integrated in the thickening centrifuge was to be documented.
Concrete focal points that were to be demonstrated in the course of the experiment were as follows:
- Changed pumpability (flowability) of the thickened sludge with and without active disintegration device
- Result and operational stability of the preselected degree of thickening
- Operating costs (use of flocculant, power consumption)
- Flexibility of the system with regard to different sludge properties
- Supervision intensity of the mechanical thickening process
Documented results:
- The excess sludge can be thickened very stably and optionally from 4 – 10 % DS with or without lysate technology.
- When operating with active disintegration equipment, the viscosity of the thickened sludge can be reduced by approx. 1/3.
- The existing pumping system was able to pump a sludge thickened to 8 % DS over 300 m without any problems. (A pressure reduction of 33 % was documented compared to operation without a disintegration device).
- A stable degree of separation in the centrate > 98.5 %
- The results are reproducible regardless of the individual sludge specifications
The effects achieved from the extended residence time in the digestion tower through volume reduction, an increased digestion gas yield with an improved degree of stabilisation and increased organic degradation were the basis for the decision-makers at Munich's municipal wastewater treatment plant to put a corresponding investment out to public tender in October 2020.
This was followed by a call for projects, which demanded constructive and process engineering experience and concrete references from the applicants in the specific market segment of mechanical excess sludge thickening and disintegration.
Hiller GmbH was able to use its acquired know-how advantage and was awarded the contract for the construction of a complete thickening plant for 100 m3/h, which was to be planned in a double-deck container plant.
The new thickening line was commissioned in April 2022 and confirmed the results from the trial operation right from the start. The process influence of the sewage sludge disintegration up to the improved dewatering properties is to be investigated and documented in detail via an externally commissioned scientific monitoring of the operating results.
We would like to thank ‘Münchner Stadtentwässerung’ for the exceptionally good and constructive cooperation and the trust placed in us by the employees responsible for the project.
