Geo Tube Dewatering Containers

Ocean Geosynthetics manufactures geo tube dewatering containers from engineered woven geotextile made of polypropylene multifilament and monofilament yarns. Geo tubes are used to dewater sludge and dredged sediment, and, when filled with sand slurry, to build marine structures such as revetments, groynes and containment dykes. Tubes are made in different sizes to suit the volume and space available, and smaller units can be mounted in mobile roll-off containers and moved around a site. This page explains how dewatering tubes work, how they are sized and tested, and how a dewatering site is set up.

What are geo tubes?

A geo tube is a large tube sewn from high-strength woven geotextile and filled hydraulically: a slurry is pumped in through filling ports along the top. The fabric holds back the solids and lets water drain out through its pores. In dewatering work, the tube is filled repeatedly until it holds as much consolidated solid material as it safely can. In marine work, the tube is filled with sand slurry, the sand settles inside by gravity, and the filled tube becomes a heavy, continuous unit for shoreline and coastal structures.

The system has no belts, gears or moving parts. Compared with mechanical dewatering (belt presses, centrifuges, filter presses), a tube installation needs little equipment and few operators, and capacity can be increased by laying more tubes or stacking them where the design allows.

Geo tubes are different from geo bags. Geo bags are small sand-filled bags placed by hand or by barge; geo tubes are much larger, filled by pumping, and used for dewatering and continuous marine structures.

How geo tube dewatering works

The dewatering process has three stages.

Step 1: Filling

The sludge or dredged slurry is pumped into the tube. In most dewatering jobs, a polymer flocculant (and sometimes a coagulant) is dosed into the slurry line on the way to the tube. The polymer binds fine particles into larger flocs, so the solids separate from the water quickly and the fabric pores do not blind.

Step 2: Dewatering

Clear filtrate drains out through the pores of the geotextile, and the solids stay inside. As the water leaves, the volume inside the tube drops, so the tube can be filled again. Filling and draining are repeated over several cycles. The filtrate is collected on the lay-down area and can be returned to the treatment plant or discharged if it meets the required quality.

Step 3: Consolidation

After the final fill cycle, the retained solids keep losing water by drainage and then by evaporation through the fabric, so they continue to densify. The solids content and volume reduction achieved depend on the sludge type and the polymer conditioning. When the tube is full and consolidated, the tube and its contents can be taken to a landfill or left on site, or the tube can be cut open and the solids removed for disposal or land application where permitted.

Testing before you order

Dewatering performance depends on the slurry, so it should be tested before tubes are sized:

• Sludge characterisation: solids content, particle size distribution, organic content and contaminants. A simple soil study tells you how fine the solids are.
• Polymer selection (jar test): different flocculants and dose rates are mixed with samples to find the one that gives fast separation and clear water.
• Hanging bag or pillow test: conditioned slurry is poured into a small bag of the same geotextile to measure drainage rate, filtrate clarity and retained solids. ASTM D7880 is the standard method for measuring flow rate and suspended solids retention from a closed geosynthetic bag.

These tests let you select the fabric and pore size, and confirm that the filtrate will meet discharge requirements.

Key properties of dewatering geotextile

When comparing geo tubes, ask for these properties of the fabric and the finished tube:

• Tensile strength in both directions (wide-width test, ASTM D4595 or ISO 10319). Fill pumping creates hoop tension in the fabric, so strength sets how high the tube can be filled.
• Seam strength: the factory seams must withstand pumping pressure. Seams are usually the weakest point, so ask for tested seam strength, not only fabric strength.
• Apparent opening size (ASTM D4751) and permittivity (ASTM D4491): pore size and flow rate. They must be matched to the conditioned solids, which the hanging bag test confirms.
• UV resistance (ASTM D4355): tubes often sit in the sun for months during dewatering and consolidation.
• Chemical and biological resistance: polypropylene is inert to biological degradation and to most chemicals found in sludge.

Sizing a geo tube

Tubes are specified by their circumference (or theoretical diameter) and length, and are made in a range of sizes to suit the volume to be dewatered and the space available. Larger tubes hold more solids per square metre of lay-down area but need more space and a firmer base. Each tube has a maximum safe fill height set by the strength of the fabric and seams; the manufacturer’s fill-height limit must be followed during pumping, because overfilling can split a tube. Tube geotextiles can be custom-made to suit the particle size of the solids, so that the tube walls retain as much of the solids as possible.

Setting up a dewatering site

• Level and compact the lay-down area, with a slight fall towards a collection point.
• Line the area with an impermeable HDPE geomembrane where filtrate must not enter the ground, and add a drainage layer of gravel or geonet so filtrate flows freely from under the tubes.
• Lay the tube flat along its marked centreline, connect the fill pipe to a port and keep other ports closed.
• Monitor fill height during pumping with a height gauge or measuring stick, and stop at the specified limit.
• Allow drain-down time between fill cycles; stack tubes only where the design and manufacturer allow it.

Applications

Dewatering

• Municipal sewage and wastewater treatment plants: digested and waste-activated sludge.
• Pulp and paper mills and other industrial plants, wherever solids and liquids must be separated.
• Mining and mineral processing: tailings and fines, including recovery of valuable material.
• Oil extraction, agriculture, and food and beverage processing.
• Construction and pipeline projects, including seepage dewatering and drilling mud.
• Environmental and marine remediation: contaminated sediment from lakes, ponds, harbours and lagoons.
• Dredging: sediment pumped from shipping lanes, cooling-water intakes and berths (dredged so that deeper-keeled vessels can dock) can be contained and dewatered in tubes on shore.

More on sludge applications is in our article on geotubes for dewatering sludge.

Marine and hydraulic structures

Filled with sand slurry, geo tubes form shoreline protection, revetments, containment dykes, groynes, jetties, breakwaters, wetland creation, construction platforms and island and land reclamation works. Tubes can be filled on site with native sand or dredged material, which saves the cost of importing rock or fill. A ring of tubes can also enclose an area that is then filled with dredged material to create new land. Placed as offshore breakwaters or groynes, tubes reduce wave energy reaching the beach and help limit loss of beach sand.

Related products

• Geo bags: smaller sand-filled containers for bank protection and flood fighting.
• Geomembranes: impermeable lining for dewatering lay-down areas and sludge lagoons.
• Geotextiles: woven and nonwoven fabrics for filtration and separation.

Frequently asked questions

What is a geo tube used for?

Geo tubes dewater sludge and dredged sediment, and when filled with sand they form marine structures such as revetments, groynes and breakwaters.

Is a polymer always needed for geo tube dewatering?

Not always, but most fine sludges need a flocculant so solids separate quickly and the fabric does not blind. A jar test and hanging bag test show whether polymer is needed and at what dose.

How do I choose the right geo tube size?

Size depends on the volume and solids content of the slurry, the space available and the lay-down area’s bearing capacity. Share these with us and we can suggest tube circumference and length.

What is a hanging bag test?

A small bag of the same geotextile is filled with conditioned sludge to check drainage rate, filtrate clarity and solids retention before full-size tubes are ordered.

What happens to the tube after dewatering?

The full tube can be disposed of in a landfill with its contents, left on site, or cut open so the dewatered solids can be removed and disposed of or reused where regulations allow.