CASE STUDY

Swaziland Wastewater Treatment Works
Dam Cleaning & Dewatering

ABOUT ZEBRATUBE®

ZebraTube® has its origins in the manufacturing of large dewatering bags for the deepest mines in the world. We have more than two decades of experience in retaining solids and percolating liquids through our woven geotextile bags. 

ZebraTube® products are designed and manufactured in South Africa. Our geotextiles are produced and woven locally by experienced weavers and thereafter converted to dewatering bags. Our complete control over the manufacturing process allows for design flexibility and speed of delivery. We cater for both large and small projects and our ease of deployment means ZebraTube is ideal for emergency dewatering projects.

Each roll of geotextile goes through a comprehensive set of tests at our factory to ensure absolute quality and traceability. Extruded strands are tested before weaving for tensile strength and once woven, each roll of geotextile is tested for tensile strength and permeability.

 

Background

Development in a specific area at times exceed the rate of critical infrastructure development, such as the expansion of wastewater treatment works infrastructure. The feed load to the wastewater treatment works then exceed the design capacity and would typically lead to drying beds becoming overloaded or waste stabilisation ponds silting up with sludge. Desludging of these wastewater ponds are therefore required in order to regain dam capacity. This project involves the dewatering of a silted up pond at a wastewater treatment works in Swaziland.

 

Figure 1: Dredging barge on the pond

 

DREDGING

ZebraTube was engaged by Integrated Pump Rental for this specific project to supply geotextile dewatering tubes. Integrated Pump Rental is an authorised ZebraTube re-seller and we often partner up on projects such as these. Geotextile dewatering tubes can be used to efficiently and cost-effectively dewater and contain dredged sludge.  

 

Integrated Pump Rental removed the sludge from the dam using a dredging barge capable of extracting high tonnages due to the design and engineering of the dredge head, as shown in Figure 1. Silt is dredged from the pond and fed to the geotextile dewatering tubes.

 

For this specific project, a flocculant was required in order to agglomerate the solids, enabling efficiently settling and dewatering within the tubes. Integrated Pump Rental utilised ZebraFloc® powder flocculant, made up on site using a manual make-up and dosing system (see Figure 4). Flocculant addition is commonly required for the dewatering of bio-solids. Flocculant is injected in-line, downstream from the dredger. The agglomerated sludge particles settle inside the tube, allowing for the formation of a filter cake on the surface of the geotextile. The filter cake enhances filtration efficiency, allowing clear filtrate to percolate through the geotextile.

 

Figure 2: Bags laid out in preparation for pumping

 

DEWATERING PROCESS

It is commonly accepted that in wastewater/bio-solids dewatering one would typically see a volume reduction of around 60 to 75% over time in the pumped tube. This principle is called consolidation. Consolidation time differs based on the particle size distribution of the slurry, as well as the efficiency of flocculant dosing. If flocculant dosing is efficient, you would typically see consolidation and volume reduction occurring over a shorter period, as the solids are properly agglomerated, allowing the in-situ water content of the sludge to percolate at a faster rate through the geotextile.

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.

 

An example from literature documents that a geotextile tube was filled to a height of 1.5m and the contents settled and dewatered to a height of 0.44 m after 65 days of consolidation (Fowler et al, 2000). This equates to a volume reduction of 75% at end of the consolidation period. The literature corresponds with a 60 to 75% volume reduction that ZebraTube® sees as relatively standard based on the other wastewater projects that we have completed and similar consolidation also occurred during this project.

 

For this project, 18 m circumference x 30 m length dewatering tubes were used. The filled dimensions of the tube is shown in Figure 4. The larger circumference tubes allowed for pumping at a higher rate. A total of 800 m3 of silt were removed from the dam during this initial phase of the project.

Figure 3: Filling, drawdown and consolidation process
Figure 4: Dewatering bags during the filling process
Figure 5 Dewatering bag pumped dimensions

ADVANTAGES: GEOTEXTILE TUBES

 

  • Rapid dewatering of large sludge volumes within a short period.
  • The ability to easily scale the dewatering process in line with dredging volumes.
  • Minimal skilled labour required.
  • Stackability of tubes enable 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 is easy and cost-effective.
  • Filtrate can be captured, allowing for containment, recycling and re-use.

 

 

REFERENCES

Fowler, J., Bagby, R.M., & Trainer, E. (2000). Dewatering sewage sludge with geotextile tubes.

 

Figure 6: Flocculant make-up and dosing unit utilized