Energy Requirements for Sludge Dewatering

Conventional mechanical dewatering equipment compared to geotextile dewatering bags


Mechanical dewatering vs. Geotextile dewatering bags

The dewatering of a slurry is conventionally done using mechanical dewatering equipment such as a filter press or centrifuge. These need to be operated by skilled personnel and their operations are quite intricate, except if the units have been automated. Their operations carry high energy requirements and sometimes limited yields. The procurement of such specialised equipment also usually carry long lead times, and their installations need sufficient space for both operations and maintenance. Since these are fixed structures, there is no flexibility in moving the equipment to another process where slurry dewatering may also be required. Mechanical dewatering equipment does have some advantages such as producing cakes with good moisture content and clear filtrates when filtration is aided by the addition of flocculant.


Figure 1: Filter press


Geotextile dewatering bags offer a cost-effective alternative to mechanical dewatering equipment. These bags are produced in various different geotextiles to accommodate a variety of slurries. They also do not require any technical expertise to operate and no maintenance budget since they do not have any electrically operated or moving parts such as motors. Additionally, if there is a need to dewater a larger volume than initially required, more bags can be easily added and stacked where required to accommodate the increase in feed volume.


Figure 2: Centrifuge


Depending on the type of slurry being dewatered, moisture contents of between 40 – 60% is achievable within a short consolidation period and even lower moisture contents are possible if the bags are allowed to stand for extended periods.


Figure 4 below compares the energy requirements of geotextile dewatering bags with that of mechanical dewatering equipment.


In some cases, the sludge to be dewatered carries no economic value, yet its treatment and disposal remain important. Operating and maintaining a plant is expensive and when it comes to sludge treatment, most clients are reluctant to spend money to ensure that they are treating and disposing of the sludge according to environmental regulations. Energy consumption, therefore, becomes a critical deciding factor when choosing an appropriate dewatering solution.

Figure 3: Geotextile dewatering tube


Advantages: Geotextile Dewatering Bags

  • Rapid dewatering of large sludge volumes within a short period.
  • The ability to easily scale the dewatering process in line with dredging volumes.
  • Minimally skilled labour is required.
  • Stackability of tubes enables efficient usage of the laydown area, reducing the overall footprint required.
  • Permanent containment of the dewatered material is possible, whilst removal and disposal of the dewatered material are easy and cost-effective.
  • The filtrate can be captured, allowing for containment, recycling and re-use.


Figure 4: Energy Consumption Comparison


Figure 4 above illustrates the energy requirements of various dewatering equipment based on kWh per ton of sludge dewatered. It can be seen that a membrane filter press requires about 90 kWh/ton of sludge dewatered, compared to a decanter centrifuge, which only requires about 50 kWh/ton. The technology that boasts the lowers energy consumption for dewatering is Zebratube®’s geotextile dewatering bags, which only requires about 20 kWh/ton. The filling and consolidation process is illustrated in Figure 3. Once the tube is filled and allowed to drain and consolidate completely, the contained material is dry enough to be handled with an excavator for easy disposal or re-use.


Figure 5: Dewatering Process


The energy consumption stated only applies during pumping and filling of the geotextile bag and once that has been completed, no further energy is required because the bag will drain under gravity during the draw-down or consolidation period



Tshepang Dolamo

+27 66 586 6628