Sludge dewatering in the 21st century

Sludge, a waste liquid that contains solids of impurities, accumulates in WWTPs. It is the management of sludge, specifically its dewatering, that is one of the key processes in its treatment. However, several different paths lead to a single objective, i.e. to obtain a solid mass with the smallest possible volume and the highest dry matter content. Let's take a look at the different types of dewatering machines that find their application in municipal and industrial wastewater treatment plants of different sizes.

Problem? The volume

The biggest problem with sludge is its huge volume. There is only one solution: dewatering. In this process, the excess water is extracted from the sludge by various mechanisms, thereby significantly reducing its volume. The sludge becomes a compact solid, which makes it easier to handle and transport. 

For the purpose of sludge dewatering, various dewatering devices were developed. They all have the same objective: to remove as much water as possible from the sludge and to obtain a final solid “cake” with a reduced volume. These devices or machines differ significantly from each other in the principle on which they operate, energy consumption, durability, and cost of servicing.

Let's introduce the basic types of dewatering equipment. We'll start with the simplest ones and work our way up to the more complex devices, outlining their advantages and disadvantages.

Dewatering bags

Historically, the oldest dewatering system: bags of woven fabric into which sludge is pumped. The water then drains away through the perforated canvas of the bag, while the solids remain inside. This happens until the bags are completely full, losing their ability to let water out. The clogged bag is then disposed of and replaced with a new one. This outdated method of dewatering is now mainly used in developing countries. In our conditions today, we rely on more efficient devices. 

PROS:

  • Simple drainage method that is easy to install.

  • Low energy consumption and total cost.

CONS:

  • The sludge volume reduction is insufficient compared to more advanced technologies.

  • Difficult handling with full bags.

  • Limited disposal options and possibilities of bags reusing. 

Belt press

Belt presses are similar to belt conveyors. The sludge passes over a perforated belt, where the excess water is removed first by filtration and then by a system of pulleys and presses. The dewatered sludge then leaves the belt and the perforated belt is cleaned by rinsing with pressurised water.

PROS:

  • Continuous process that operates with minimal intervention.

  • The length of the filter belts and the high pressure ensure a relatively high level of sludge dewatering.

CONS:

  • Huge footprint of the plant.

  • High consumption of process water required for continuous flushing of contaminated belts. At the same time, additional wastewater is produced due to the belt cleaning.

  • It is an open facility which requires extraction of odour and moisture.

  • Difficult and expensive maintenance. Replacing a belt of huge size and weight is complicated.

Centrifuge

Centrifuges work on the principle of high operating pressures and maximum water squeezing out of the material. The sludge is fed into the rotating drum of the centrifuge, where a huge force mechanically spins the sludge and the centrifugal force "squeezes" the water out of it. The solids are continuously conveyed out by a conveyor and the separated sludge water is also continuously removed.

PROS:

  • A continuous process that does not require human operator intervention.

  • Dry matter with a relatively high solids content.

  • Can handle high loads. Its use pays off in huge applications such as WWTPs for hundreds of thousands or millions of people. 

CONS:

  • Highest energy consumption of all equipment. Compared to a screw press, for example, it can be up to ten times higher.

  • Noise. Centrifuges make a sound comparable to a jet airplane taking off when running. This has to be taken into account in the installation in the WWTP, for example, they cannot run at night, they disturb the surroundings, they have to stand in soundproof rooms, people cannot be near them because of the risk of hearing damage.

  • Low durability. As it is a high-speed device, rapid wear and tear is natural. The average life of a centrifuge is one year, after which the bearings and other wear parts must be completely replaced.

  • High cost of repairs, spare parts and reconditioning. If an object (e.g. a stone) enters the equipment, it can damage the equipment significantly as everything happens at high speeds and high operating pressures. Repairs should only be carried out by a certified professional company, ideally by the manufacturer itself.

Chamber filter press

It is a rectangular container (chamber) with perforated bottom and sides. The sludge is pumped into the chamber under high pressure. When the chamber is full, it is further compressed to filter out the excess water. The attendant then has to remove the filtered sludge in the form of a solid cake, and clean the chamber manually.

PROS:

  • A simple solution that can be used, for example, in the treatment of industrial wastewater where smaller quantities of sludge are processed, such as in electroplating and galvanizing plants.

  • Compact solid cake that is easier to handle.

CONS:

  • There is no possibility of automation. 

  • Manually removing the sludge and cleaning the chamber is time-consuming and physically demanding. 

  • This system operates under high pressure, making it energy intensive.

Dewatering screw press

This sophisticated solution works on a relatively simple principle. At the center of this press is a shaft that presses the sludge and squeezes out the excess water as it rotates. The clean water separates and drains away, while the solid cake moves towards the outlet, falling out either into a container or onto a conveyor.

PROS:

  • The high dry solids content of the sludge and low energy consumption, many times lower than that of a centrifuge.

  • It is a continuous solution, and complete automation is possible.

  • Thanks to the moving rings, this equipment has a self-cleaning capability. 

  • Low noise and vibration. The device can be placed anywhere, no need to build soundproof walls.

  • Rinsing water consumption is negligible, unlike the belt filter.

  • Long service life. Up to ten years of trouble-free operation is no exception.

  • Low-maintenance device. Easy repair and replacement of operating parts.

The bottom line is…

We stand by the fact that screw presses are qualitatively superior to all other mentioned equipment. Our screw press MP-DW, which we have developed over the years, can be tailored to smaller and larger wastewater treatment plants, industrial and food processing plants and other sectors of human activity. We also offer complete accessories for screw presses: belt conveyors, polymerisation stations, and equipment for mechanical pre-treatment of raw wastewater.

 

Are you looking for suitable equipment for sludge treatment and dewatering?

Contact us. We will be happy to find the ideal solution for you.


 

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