Solid cake: breaking down dewatering stereotypes

After the flocculated sludge goes through the dewatering process, a solid sludge cake is separated. This material at the outlet of the dewatering machine contains a certain percentage of solids, but still around 80% water. Does this mean that the dewatering machine has not done its job 'properly' as many people may think? No! Let's look at the issue of the percentage of water in the sludge cake in the following article.

Why there is "so much" water in the solid cake? 

The task of dewatering technology, whether we are talking about a screw or belt press, or even a centrifuge, is to obtain a final mass with the highest possible dry matter content (and consequently the lowest possible volume). For most people who are not experts in the industry, the way they "read" the information about the 80:20 ratio of water to solids in the resulting cake is that the dewatering technology has failed. But this is not true. We will only get to the truth when we look closely at the composition of the sludge.

Sludge under scrutiny

Before the dewatering process, liquid sludge looks like "dead" mud. Sludge is, in fact, a living organism - mostly made up of living bacteria, microorganisms, and small aquatic animals. Like the human body, their bodies are largely (about 80%) made up of water. These micro-organisms and bacteria pass through the mechanical dewatering machine unchanged: they remain full of water that we can only get rid of by additional drying, and only free water is removed in the process. It is probably becoming clear now why the percentage of water in the dewatered sludge may still appear "high".

Sludge stabilization is for larger WWTPs

The output sludge cake always reflects the origin of the sludge and whether it has been treated, or stabilized, before dewatering. Sludge stabilization aims to reduce the amount of organic matter. Stabilization can be done in various ways, one of the most effective is the use of pure oxygen or an anaerobic chamber with high temperatures of around 40 °C.

The dead bodies of organisms and bacteria then no longer contain water inside, and dewatered are only their shells, which we can imagine as the equivalent of a human skeleton instead of a whole body with organs. This results in a more efficient 'wringing out' of water and a higher percentage of dry matter in the solid cake. However, conditions for effective sludge stabilization have only larger WWTPs, as these are huge investments.

Deceptive percentage of solids

The percentage of dry matter to water ratio is deceptive. And not only in the case of sludge cake. For example, a cucumber or a watermelon contains up to 95% water, yet they are undoubtedly solid objects. Therefore, 15, 18, or even 20% of the dry matter in the resulting solid cake may seem 'little', but in practice, it is a crumbly material that resembles field dirt or compost.

It's all about volume

The more important parameter than the percentage of dry matter? VOLUME! Volume reduction is the basis of the entire dewatering process. It is about making the sludge a compact solid material that allows easier handling and transport. The smaller the volume and the sludge, the lesser the worries and costs for its transportation and disposal. Prices for transport and disposal of dewatered and non-dewatered sludge differ dramatically, so even smaller WWTPs will find the investment in dewatering equipment worthwhile.

Let's take an example: The input sludge, which contains 2% dry matter, is brought to a state where the final output material contains 18% dry matter using a dewatering screw press. The volume is reduced by 88.9 %.

There's no point in trying to dewater the sludge anymore. Here's why: When the input sludge (2% dry matter) is dewatered to the state that contains 25% dry matter, the volume of the sludge is reduced by 92%. The reduction in volume is therefore negligible, but the effort - in particular the electricity consumption - is enormous.

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