But if that sludge has a moisture content of 80%, only around 200 kg is dry solids. The remaining 800 kg is essentially water.

This raises a simple but important question in sludge management: when a plant transports and disposes of a tonne of sludge, how much of it is actually sludge – and how much is water?

This is where sludge dewatering makes a difference. Dewatering is not simply about making sludge “drier”. It is about removing as much water as practical before the sludge moves on to transportation, storage and final disposal.

Consider a wastewater treatment plant that produces 100 tonnes of sludge per day at 80% moisture content.

That represents approximately 20 tonnes of dry solids and 80 tonnes of water.

If the sludge is dewatered to 60% moisture content, the amount of dry solids remains the same, but the total sludge weight falls to approximately 50 tonnes. The sludge still contains 20 tonnes of dry solids, but the amount of water has been reduced to around 30 tonnes.

In other words:

100 tonnes → 50 tonnes.

The dry solids have not disappeared. What has been removed is the water that would otherwise continue through the downstream handling process with the sludge.

This is why moisture content may look like a simple equipment specification, while its impact can extend across the entire sludge management process.

The first impact is transportation.

If the amount of sludge sent off site falls from 100 tonnes to 50 tonnes per day, the overall transport load can be significantly reduced. This becomes particularly important where sludge has to travel long distances to a treatment or disposal facility.

The second is storage.

The same amount of dry solids can require considerably more storage capacity when they contain a large amount of water. Dewatering reduces the weight and volume of the sludge cake, helping to ease storage pressure and reduce the need for frequent removal.

The most direct impact, however, often comes at the final disposal stage.

Whether sludge is landfilled, incinerated, thermally dried or used in another recovery route, water carried with the sludge can mean additional transportation, treatment or energy consumption.

The more water removed before these downstream processes, the less water there is to transport or process afterwards.

This is why a dewatering machine may sit inside a wastewater treatment plant, while its impact reaches far beyond the dewatering stage itself.

Does That Mean Drier Sludge Is Always Better?

Not necessarily.

This is another point that is easy to overlook.

If reducing the moisture content by a few additional percentage points requires significantly more polymer, electricity, wash water, operating time or maintenance, achieving a lower moisture content may not necessarily be the most economical option.

The right target also depends on what happens to the sludge afterwards.

For sludge destined mainly for landfill or off-site disposal, lower moisture content may primarily mean lower transport weight and disposal volume. Where sludge is sent for thermal drying or incineration, removing water earlier can reduce the amount of water that needs to be evaporated later and, in some cases, reduce additional energy requirements.

So the more useful question is not:

“How dry can the sludge become?”

It is:

“What moisture content makes the most sense for the entire treatment process?”

What Does One Moisture Content Figure Mean in Real Costs?

Let us return to the earlier example.

Suppose a project produces 100 tonnes of sludge per day at 80% moisture content. If the sludge is dewatered to 60%, the theoretical amount of sludge entering the downstream transportation and disposal stages can fall from 100 tonnes to around 50 tonnes per day.

At 300 operating days per year, that represents a theoretical difference of approximately 15,000 tonnes of wet sludge per year.

Of course, real projects are more complex. Sludge characteristics, feed concentration, dewatering technology, polymer conditioning and the final moisture content all affect the actual result.

But even this simple calculation illustrates an important point:

Sludge dewatering does not only change the moisture content of the cake. It changes the amount of material that the rest of the system has to transport, store and handle.

Choosing a Dewatering Machine Means Looking Beyond One Number

For this reason, comparing sludge dewatering equipment based solely on which machine produces the driest cake is rarely enough.

Different applications may call for different technologies, including multi-disc screw presses, belt filter presses, centrifuges and plate and frame filter presses.

The more meaningful comparison is what each solution requires and delivers over its operating life.

That includes treatment capacity, sludge cake moisture content, polymer consumption, power consumption, wash water requirements, labour, maintenance frequency and, importantly, the final destination of the dewatered sludge.

The purchase price of a machine is only the beginning.

What matters over the long term is how many resources the equipment consumes throughout its service life – and how much burden it can remove from the downstream transportation and disposal process.

Dewatering Is, in Simple Terms, About Removing Water Early

Seen from this perspective, sludge dewatering is actually a simple idea: remove, as early as possible, the water that does not need to travel with the sludge any further.

The separated water can continue through treatment or reuse processes, while the solids move on to transportation, storage and disposal with less weight and volume. This is why sludge dewatering increasingly needs to be considered as part of the overall treatment system, rather than judged solely by the performance of an individual machine.

What is really worth calculating is not simply the price of the equipment, but what it can save across the operation once it becomes part of the system.

Haibar has long specialised in solid-liquid separation and sludge treatment equipment. Based on different sludge characteristics, treatment capacities and final disposal requirements, we provide solutions including Multi-Disc Screw Presses, Belt Filter Presses, Dissolved Air Flotation Systems, as well as sludge thickening, conveying and other supporting systems.

If you are evaluating a sludge dewatering solution, we would be happy to discuss your actual operating conditions with you and assess which solution makes the most sense for your long-term operation. >> Please feel free to Contact Us