GEA Decanters as Pioneers for Thermal Hydrolysis of Sewage Sludge

More cost effective disposal

Simple equation: before fermentation, the sewage sludge is broken down by means of high pressure and high temperatures, the structures are destroyed and the water from inside the cells is released. This digestion process makes the naturally occurring cell polymers an easily digestible substrate and the larger surface allows better decomposition by the fermenting bacteria. The result: the sludge contains less bound water and fermentation takes place more efficiently, which modifies the ratio of organic to inorganic components significantly in the direction of the inorganic. The subsequent dewatering by means of decanters can therefore yield considerably higher dry substance values and the aim of more cost effective disposal is therefore achieved.

Both in the batch process and in the continuous inline process

Thermal hydrolysis can be performed both in the batch process and in the continuous inline process. For both system variants, it is critical that the sewage sludge to be treated displays a defined dry substance size. It is at precisely this point that the decanter comes into effect again: in this case, not for dewatering but rather for thickening.

The good news: the sludge management system from GEA is ideally geared towards both processes. The GEA waterMaster is able to thicken into a defined dry substance the sewage sludge from two percent dry substance, for example, extremely precisely and thus achieve the ideal point required for the hydrolysis process for the optimum balance between fermentability and energy demand: 16 - 18% dry substance in the batch process and 12% in the case of continuous inline systems.

The decanter is the most economical alternative to other dewatering technologies such as belt filter presses or screw presses. These work less flexibly and less precisely. They represent an open system and therefore cause dirt, odor nuisance and aerosol formation. There is a trend for these press technologies to gradually replaced with decanters.

New decanter generation significantly reduces energy consumption

Centrifuge - Dewatering Decanter waterMaster

waterMaster 2-phase decanters are available in the right performance class for any size of waste water treatment plant. This new generation raises the efficiency of mechanical separation technology processes to an unprecedented level. By means of conceptual redesigns, a reduction of over 50% has been achieved in energy consumption, which is the critical criterion for operating costs and efficiency. At the same time, investment costs are falling, availability is rising and flexibility is expanding in terms of performance ranges.

One essential feature of the new decanter generation is the summationdrive. With clever kinematics, the performances of the primary drive for the bowl and secondary drive for the scroll have been brought together (“summarized”) then accurately passed on to the bowl and scroll. A further advantage is that a very large differential speed range can be covered with the use of planetary drives. Because of the modular system, all users are able to decide whether they primarily want high clarification performance or alternatively very high torques with generally more difficult output of the dewatered product. This decanter design ensures maximum efficiency in operation and contributes to amortization within the shortest time.

One decanter for thickening and dewatering

The waterMaster is being used in the hydrolysis process for the first time in two waste water treatment plants in England with a total of eight machines of type waterMaster CF 6000 and in the Netherlands with two waterMaster CF 4000 machines.

Decanters have been found in the hydrolysis process for around a decade, for example in the USA, England, Denmark and Lithuania.

The sludge management system holds a further appeal for smaller waste water treatment plants that opt for thermal hydrolysis. Depending on the occurrence of sewage sludge, only one decanter is sufficient, which can be used both for thickening before the hydrolysis and for dewatering after the digestion tower: the swing model. Operation can, for example, alternate the use daily or every two days, i.e. thickening on Mondays and dewatering on Tuesdays, or even hourly and for example thicken in the morning and dewater in the afternoon. This is a cost-saving combination variant.

More dry substance in the dewatering

Ultimately, how great is the benefit of thermal hydrolysis? What is the end result? Without hydrolysis, the decomposed sewage sludge can be dewatered for example to between 20 and 25% dry substance. Thermal hydrolysis allows a change of the ratio of organic to inorganic components in the digestion tower in favor of inorganic material, such that dewatering is possible to between 28 and over 35% dry substance and this massively reduces the disposal costs.