Two jobs that pull against each other
Retain the fines. Governed by apparent opening size (ASTM D4751, or O90 to EN ISO 12956). Typical requirements sit at a maximum opening equivalent to a No. 80 sieve, about 0.18 mm, with tighter specifications calling for 0.15 mm.
Pass the water. Governed by flow rate and permittivity (ASTM D4491, or velocity index to EN ISO 11058). Highway specifications commonly require in the order of 3,300 litres per minute per square metre, equivalent to a velocity index of roughly 55 mm/s, with permittivity at or above 1.2 s⁻¹.
Fail either way and the bag is useless. Too tight, it blinds off and the pump backs up. Too open, it discharges the fines it was bought to capture.
Three fabrics, three very different outcomes
Slit-film woven tape. The commodity fabric, the same construction used in cheap silt fence. Flat tapes woven tight against each other leave very little open area, so filtration happens only in the plane of the weave. Under sediment load it blinds fast, and flow collapses long before the bag is full. It is cheap because it is easy to weave, not because it is efficient.
Needle-punched nonwoven. The fabric most transport specifications name, and a genuine improvement: a thick, three-dimensional fibre structure that filters through its depth and keeps passing water as a cake builds on the inside face. Its weakness is strength per unit mass. To reach the tensile and puncture numbers a full bag demands, you need a heavy fabric, and heavy nonwoven is expensive.
Monofilament woven. Round, individual filaments woven to leave a controlled, measurable open area between them, rather than the near-zero open area of flat tape. You get the high percent open area and flow of a coarse fabric with the pore control of a fine one, in a woven structure with the tensile and seam strength that a woven gives you. Percent open area is a real specified parameter for woven geotextiles, measured to USACE CW-02215-86, and it is the number slit-film cannot compete on.
Why Flowtex Mono needs no composite layer
The industry's workaround for slit-film's filtration inefficiency has been to laminate a nonwoven layer onto it, producing a composite bag: the woven provides strength, the nonwoven provides filtration.
That composite is a fix for a problem you should not have. It adds mass, adds cost, adds a delamination risk, and a bonded nonwoven layer still blinds under fine sediment.
Flowtex Mono is built from monofilament woven fabric that does both jobs in one layer. The filament geometry gives controlled pore openings for retention and high open area for flow, so the fabric filters and carries load without a secondary layer to compensate. One fabric, one set of test values, no interface to fail.
The practical result on site is throughput that holds up. A bag that keeps flowing at hour six is worth more than a bag with a better opening size on paper that stopped flowing at minute twenty.
The seam is the failure point nobody specifies
A bag full of slurry is heavy, and the seams carry that entire load in tension while the bag is dragged, repositioned and left in the sun.
Specifications recognise this. Some require seam strength of at least 90% of the specified grab strength; others set an absolute minimum of about 0.8 kN, tested to ASTM D4632 or D4884. Good manufacturers publish the result and say where the failure occurred; one published seam test reports failure in the fabric rather than the stitching, which is the answer you want.
A burst seam is not an inconvenience. It releases the full sediment load in one event, usually into the watercourse you were protecting.
Fabric choice depends on what you are pumping
Specifications say this outright: the required structural integrity and flow rate of a dewatering bag depend on the soil type in the dewatering area.
Sandy trench water and clay fines from a mine settling pond are not the same job. Fine clays need tighter pore control and more bag area; silty sands can run higher flow. A supplier who does not ask what you are pumping is selling a sewn envelope, not a filtration product.
Made in Africa, specified for Africa
Flowtex Mono is manufactured locally. That means bag sizes and sleeve configurations built for your pump and your site rather than an imported standard size, short lead times instead of a container wait, and a fabric specification chosen for African conditions, particularly UV exposure and the fine, dispersive soils common across the continent.
It also means someone you can call when a bag underperforms, and who can show you the test data behind what they sold you.
Three questions before you buy
- Can I see the fabric datasheet with minimum average roll values? No datasheet, no specification, nothing to hold anyone to.
- What is the fabric construction, and what is its open area? Slit-film suppliers avoid this question.
- What is the seam strength, and where did the test fail?
The economics
A dewatering bag is a small line item. Behind it sit a pump crew on standby, a programme that cannot move until the water is off the excavation, and an environmental condition in your permit.
A bag that blinds after twenty minutes costs you the crew, the day and the replacement. A burst seam costs you the incident report. Neither appears in the purchase price, which is exactly why the fabric specification is the only part worth arguing about.
Flowtex Mono dewatering bags are manufactured in Africa from monofilament woven geotextile, supplied with full fabric datasheets and seam test data, sized to your pump rate and your soil.