Efficient Sludge Dewatering with Decanters

Green Power

Most of the green power for Münster is produced by the combined heat and power plant at the site in Coerde, where GEA decanters have also been used since the beginning of last year.

The so-called “Bio-Stab” method is used for converting organic waste into energy. The organic waste is collected and cleaned carefully to remove foreign substances such as plastics, metal or stones/sand; it is then broken down into small particles, mixed with process water and fed into the digestion tank for fermentation. At 55 ° centigrade, the sludge-like mass is allowed to ferment for 15 days under strictly anaerobic conditions, i.e. with the complete exclusion of oxygen. During this time, the installation produces biogas with a methane content of 50 to 60 percent and a carbon dioxide content of 40 to 50 percent; it is fed to the neighboring combined heat and power plant, where it is converted into energy. In this way, 3000 MWh electricity and 3500 MWh heat are generated in Münster on average every year (net).

Two decanters dewater 300 to 400 m³ biomass every day

City of Muenster_UCA 501

Since April 2009, the digested sludge has been dewatered by two type UCA 501 decanters. With low operating costs and low power consumption, the decanters dewater 300 to 400 m3 biomass every day. The two environmental decanters have replaced two previous older machines. Invitations to tender were sent to six manufacturers, and three companies initially submitted quotations in the Stadtwerke Münster selection process. This process filtered out the other companies, and only GEA remained at the end of the process. “The joint intensive project planning, the visit to decanter installations in reference installations and, not least, the local trial with a mobile installation convinced Stadtwerke Münster to replace the existing centrifuges using technology supplied by GEA.

We were particularly impressed by the decanters from GEA as a result of the good references in comparison with other biogas fermentation installations”, stated Udo Hibbeler, Operations Manager at Stadtwerke Münster for organic waste fermentation.

Maintenance agreement provides budget reliability

“A further crucial factor was to ensure that it would be possible to calculate repair costs in the medium term. GEA was the only decanter manufacturer able to offer us a reasonable maintenance agreement. We consider that a very positive aspect. A manufacturer must trust its own machines. The company must be so certain of its operations that it is able to calculate a maintenance agreement, and this is what GEA did“, added Udo Hibbeler. Together with GEA we developed a formula including such variables as the number of machines, speed, throughput per hour and operating time per day, and these data have been used to calculate an annual operating time and a defined maintenance rate. “The maintenance costs are therefore not fixed; on the contrary, we are able to influence the annual costs by the way in which we operate the installation.”

The maintenance agreement defines values for throughput capacity, degree of dewatering and centrate quality. “This maintenance agreement, which runs for a period of ten years, focuses on the key aspects of budget reliability, and the extent to which repair costs are plannable. However, we are also assuming that, viewed over the entire period of ten years, we will be able to operate much less expensively than would be the case with unplanned “ad hoc repairs” which cannot be calculated”, said Udo Hibbeler.

GEA satisfied a further criterion with regard to the placement of the order. “We wanted to see positive results in the pre-trials, and in the final analysis this was achieved only by two providers”, stated the Operations Manager. GEA carried out the pre-trials in a particularly elegant manner by means of a mobile installation which operated on site with original material. One of the two existing machines was disconnected for a period of one week, and the mobile installation was used instead.

Standard machines with standard spare parts

Moreover, Stadtwerke Münster took a deliberate decision not to use customized products, as had previously been the case, and instead decided to use standard machines. “In this way, we are certain that, if the need arises, an original rotor would be held in stock as a spare part at GEA”, said Udo Hibbeler. With the UCA 501, GEA has succeeded in constructing a high performance dewatering decanter of modern design and a fundamentally improved performance combined with a simultaneous reduction in service and operating costs. The environmental decanter has a deep pond design. The special rotor geometry combined with the high rotor speed guarantee the high throughput performance. The robust multiple stage planetary gear itself generates high scroll torque for optimum dewatering, which also remains constant even at high differential speeds. Despite the high throughput capacity and the significantly higher torque, the specific energy requirement is less than 1 kW/m³.

High installation availability

Stadtwerke Münster also decided to remain on the safe side with regard to installation availability by doubling the dewatering capacity per decanter compared with the old machine from 15 to 30 cubic meters per hour. In this way, if a machine has to be serviced, the process can be adjusted by increasing the bowl speed and throughput capacity so that only one machine can handle the dewatering of the residues. “Because we normally dewater approximately 15 to 18 cubic meters per hour per machine during a period of eight to twelve hours every day, we are in a position to handle the total quantity with only one centrifuge running at 30 cubic meters per hour if one machine has to be serviced or shut down. This means that we have much higher installation availability”, said Udo Hibbeler.

“The new decanters operate at 2600 revolutions per minute; this speed is lower than the previous speed of 2950 revolutions per minute, resulting in a further reduction in energy consumption. We attain centrate values which are mostly lower than the figures which had previously been attained. The dewatering efficiency has remained roughly unchanged, at approx. 35 percent dry matter.

The solids discharged by the decanters with approx. 35 percent dry matter, with its valuable organic components, is fresh compost which has to be rotted down further in accordance with the Organic Waste Ordinance. For this purpose, the Bio-Stab soil which is obtained from this process is mixed with fresh garden waste; it is composed in the adjacent garden waste composting facility of the city of Münster, producing very high quality compost which the citizens of Münster can acquire for a small charge.

Changeover without interrupting operations

The city of Münster took double advantage of the fact that the new environmental decanter is also available as a mobile installation for test purposes. Following the successful trials with the mobile installation, the semi-trailer was used again during the process of assembling and commissioning the decanters. “This is because the new machines had to be installed at the location of the existing machines”, explained Udo Hibbeler. In order to ensure that it was not necessary for the ongoing operation of the fermentation installation to be interrupted despite the need to disassemble the old machines, the mobile decanter was used for sludge dewatering for a transitional period of two weeks, thus ensuring trouble-free operation. “Otherwise, we would have had to cope with the challenge posed by a significant interruption of operations. The delivery times and disassembly/assembly times were significantly lower than the times specified by the competition. And finally, GEA accepted the used machines as part payment. Very convenient from our point of view.” Stadtwerke Münster has been satisfied with the way in which the decanters have been operating for more than one year without any problem. And GEA met the requirement of Stadtwerke Münster for financial planning reliability with the 10-year maintenance agreement. ”This gave us a very good feeling about the whole thing.“, said Udo Hibbeler.