Dewatering screw press vs. centrifuge: energy consumption, durability and noise level comparison

Various devices can be used in the process of sludge dewatering at wastewater treatment plants. In this article, we will focus on two in particular: the dewatering screw press and the centrifuge. While percentages of dry matter in output material are comparable, the technologies used in these two machines are quite different. Deciding which machine to invest in? Which one is most suitable for your purposes? If this is the case, you will be interested in factors that have the greatest impact on the profitability of your investment. In the paragraphs below, we will talk about operating costs of these machines, energy consumption, service life and repairs, and costs associated with noise and vibration during the dewatering process.

Similar results, different technology

The aim of all dewatering machines is the same: to get rid of as much excess water as possible and to obtain a final material with the highest dry matter ratio and the smallest volume possible. Both the screw press and the centrifuge achieve the best results in these areas compared to any other machines.

Remember: higher dryness during dewatering gives you a higher volume reduction compared to the original liquid state, so you save more money on material disposal and handling. And that's what it's all about.

Comparison #1: Energy consumption

In terms of energy consumption, centrifuges have the highest energy consumption of all dewatering machines. This is also one of its main disadvantages and something you should take into account when looking for a suitable dewatering solution. This is particularly true if you consider the current costs in the energy market.

Compared to a centrifuge, the energy consumption of a screw press is up to ten times lower. In the case of a screw press, we are talking about approximately 20 Wh per kilogram of processed material (dry matter), whereas a conventional centrifuge consumes more than 200 Wh. Modern centrifuges are also available on the market, who's energy consumption is slightly lower. These are still around 60 to 80 Wh which is still several times more when compared to a screw press.

Comparison #2: Percentages of dry matter

There are two opposing camps: screw press supporters and centrifuge supporters. These two camps argue about which of these devices achieves better dewatering results. The truth is that the proportion of dry matter in a final output material is comparable. Differences are negligible, around 1 to 2%. What needs to be taken into account is the significantly higher energy consumption in the case of centrifuges.

The correlation between energy expended and volume reduction is not linear. For example, the original sludge contained 2% dry matter. After passing through a screw press, an output material contained 18% dry matter. In doing so, its volume was reduced by 88%.

The same original sludge containing 2% dry matter was dewatered in a centrifuge. The output material contained 20% dry matter, which means 2% more than in the screw press. Its volume was reduced by 90%. But the energy we used for this insignificant volume reduction was disproportionately large.

In reality, results of sludge dewatering do not depend on the principle on which equipment works better, but mainly on the stability and quality of the sludge itself. For example, dewatering sludge from dairy farms, which contains a high proportion of fat, or sludge saturated with chemicals, is always more challenging, regardless of the machine used. It also depends on the polymer used, which is mixed into sludge in polymer units, and also on the mineralization of the sludge.

Comparison #3: Maintenance and service life

The fundamental difference between a screw press and a centrifuge is in service life and maintenance requirements. If you have a well-trained operator who follows prescribed work procedures and takes care of routine daily maintenance, a screw press could be described as essentially maintenance-free. This machine can easily handle up to 10 years of operation (about 12 to 15 thousand operating hours) without major intervention. With preventive maintenance, it is not difficult to replace individual components on-site, at the customer's premises. Even a wastewater treatment plant operator or anyone with a technical mindset can handle simple repairs and replacement of operating parts according to instructions.

The situation with centrifuges is completely different. Their lifetime is limited, and compared to a screw press, it's very short. The average lifetime of a centrifuge until major servicing is normally one year, after which worn-out components, which suffer from high speeds and strong vibrations during operation, have to be completely replaced. Repairing this machine on site is not possible in most cases. The centrifuge must therefore be dismantled and taken to the manufacturer for repair. This requires heavy equipment and trucks, as it is a device weighing hundreds of kilograms. The financial cost of disassembly and repair is 7 to 15% of the price of new equipment and you will also incur additional costs to rent a backup device for the duration of the repair (which is typically 2 to 3 weeks).

Comparison #4: Noise level, vibrations, and location

A centrifuge is a device that operates at a very high speed. When it's running, you can't be near it without protective headphones, or you could suffer serious hearing damage. Its noise level can go up to 100 to 120 dB, which is comparable to a jet aircraft taking off. This must be taken into account when installing them in wastewater treatment plants. For example, centrifuges cannot run at night because they disturb the surroundings. They are not suitable for urban areas and they should be located between 100 and 200 meters away from human habitation.

In contrast, a screw press can be installed wherever environmental conditions allow. Noise and vibrations are almost negligible, ranging from around 60 to 70 dB. When the press is running, at no stage of its operation does it make more noise than the normal noise in a shopping mall, for example, and it is possible to talk to people in its immediate vicinity.

The Czech company MIVALT s.r.o. has been manufacturing screw presses since 2014 and supplies them to smaller and medium-sized wastewater treatment plants in the Czech Republic and all over the world. For larger treatment plants, the screw press has not yet fully established itself, mainly due to the conservative approach of treatment plant operators who have been used to using centrifuges for several decades. However, the advantages of screw presses described above are also valid for large water treatment plants (200,000 - 400,000 equivalent) and we hope to see them become established here in the future.

Are you convinced by the advantages of dewatering screw presses?

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