What Size Dewatering Bag Do You Need? 4 Questions That Decide It (With EPA Rules and Real Prices)

The size of dewatering bag you need comes from the pump, not from the hole. Take the pump's rated discharge in gallons per minute, double it, and buy the first bag whose maximum flow rate meets that number. A 2 inch trash pump rated 185 gpm needs a bag rated 370 gpm or more, which on Warehouse Gear Hub is the UltraTech 10 ft x 15 ft dewatering bag at $149.00. The 4 ft x 6 ft bag is $38.00, the 8 ft x 8 ft bag is $100.00, and the 15 ft x 15 ft bag is $213.00. The four questions below take you from the pump to the part number, with the federal rule quoted from 40 CFR 450.21(c), the sizing and replacement rules quoted from state DOT specifications, and every flow rate, capacity and fabric value from UltraTech's spec sheet.

Does the EPA require a dewatering bag?

Not by name. It requires a control, and a dewatering bag is the control most sites use. 40 CFR 450.21 sets the effluent limits for construction sites, and paragraph (c) says "discharges from dewatering activities," trenches and excavations included, are prohibited "unless managed by appropriate controls." The rule stops there. It does not name the control, size it, or say when to replace it. Your permit and your state stormwater manual do that, which is why this guide quotes two of them, the South Carolina DOT's SC-M-815-15 and Indiana's IDEM Section 713.01, alongside the manufacturer's numbers.

Who the rule reaches is set by 40 CFR 122.26. Paragraph (b)(15) defines small construction activity as "construction activities including clearing, grading, and excavating" that "result in land disturbance of equal to or greater than one acre and less than five acres." Paragraph (b)(14)(x) covers five acres and up, and it pulls in any smaller parcel that is "part of a larger common plan of development or sale" when the larger plan will disturb five acres or more. Paragraph (c)(1) says those dischargers "are required to apply for an individual permit or seek coverage under a promulgated storm water general permit." If your site disturbs an acre, you are in, and the water you pump out of a footing or a trench is a discharge the permit expects you to treat.

The bag is the treatment. It is a sewn geotextile sack that the discharge hose feeds into: water passes through the fabric, sediment stays inside. Indiana's manual is blunt about the limit of that: "This measure is not intended to treat or remove contaminates other than sediment." The UltraTech bags we stock are the Oil and Sediment model, which also absorbs hydrocarbons, and Question 2 gives the absorption figure for each size. Contaminated groundwater is a different problem with its own permitting, and a bag does not solve it.

Question 1: How many gallons per minute is your pump moving?

This number decides the size on its own. The sizing method written into SCDOT's dewatering bag specification SC-M-815-15 has three steps: find the pump's peak flow in gallons per minute, find the bag's peak flow from the manufacturer, then select a bag that "passes a minimum of two times the peak flow rate generated from the dewatering pump." The doubling is a stated 50 percent clogging factor, because a bag that passes 350 gpm when it is new passes less every hour it fills with silt.

The bag's number comes from UltraTech's Ultra-Dewatering Bag spec sheet, which lists a maximum flow rate for each size, from 200 gpm for the 3 ft x 4 ft bag to 500 gpm for the 15 ft x 15 ft bag. Use that per-bag figure, not the fabric's lab flow rate of 90 gal/min/ft², because the bag never passes water through its whole surface: SCDOT notes that bags "will not pass flow through their bottom" on low-permeability ground, and the fabric blinds as sediment coats it. UltraTech's own caution on the sheet is that "excessive flow rates or overfilling with sediment can rupture the bag or cause the hose straps to fail."

The pump's number is on its spec page. Honda publishes 185 gal/min for the WT20 2 inch trash pump, 317 gal/min for the WT30 3 inch, and 423 gal/min for the WT40 4 inch. Those are maximum capacities; the real flow at your site is lower once the pump is working against suction and discharge head, so sizing to the rated maximum builds in margin. The table applies the SCDOT rule to every size we stock.

Bag we stock	Part number	Fabric area	Max flow, new bag (UltraTech)	Pump rating it covers under the SCDOT 2x rule	Price as listed
3 ft x 4 ft	9729-OS	24 sq ft	200 gpm	Up to 100 gpm	$29.00
4 ft x 6 ft	9720-OS	50 sq ft	250 gpm	Up to 125 gpm	$38.00
6 ft x 6 ft	9724-OS	74 sq ft	275 gpm	Up to 137 gpm	$48.00
6 ft x 9 ft	9721-OS	110 sq ft	350 gpm	Up to 175 gpm	$71.00
8 ft x 8 ft	9723-OS	130 sq ft	350 gpm	Up to 175 gpm	$100.00
10 ft x 15 ft	9725-OS	302 sq ft	400 gpm	Up to 200 gpm	$149.00
15 ft x 15 ft	9727-OS	452 sq ft	500 gpm	Up to 250 gpm	$213.00

Read the Honda numbers against that last column. The 185 gpm WT20 doubles to 370 gpm, which the 8 ft x 8 ft bag (350 gpm) misses and the 10 ft x 15 ft bag (400 gpm) clears, so the 2 inch pump takes the $149.00 bag. The 317 gpm WT30 doubles to 634 gpm and the 423 gpm WT40 to 846 gpm, and no single standard bag reaches either. For a 3 inch or 4 inch trash pump at full output you have three honest options: run two bags on two separate discharge lines (SCDOT allows one hose per bag, and says nothing against two bags), throttle the pump below its rating, or ask us for a custom size, which UltraTech builds on request with a minimum order. If your spec does not impose the 2x factor, the ceiling is the manufacturer's new-bag number, and bigger is still the safer buy because flow only falls from there.

Question 2: How much sediment will you trap before the bag is half full?

Flow sets the minimum size; sediment decides how many bags you buy, because a bag is finished long before it is full. SCDOT's rule is to "replace Dewatering Bags when trapped sediment has accumulated to 50% of the bag capacity," or sooner if the manufacturer says so, and Greenville County's SC-09 specification carries the same 50 percent line. Both add that a bag is full when it "no longer efficiently filter[s] sediment or pass[es] water at a reasonable rate," and both say not to "clean and reuse a Dewatering Bag after the voids are clogged with trapped sediment."

UltraTech's spec sheet gives the sediment capacity of each bag in cubic feet and pounds, and its product page adds the oil absorption and empty weight for the Oil and Sediment model. The swap column below is half the rated capacity, which is the SCDOT point.

Bag we stock	Sediment capacity (UltraTech)	Swap point at 50 percent	Oil absorption, OS model	Empty weight
3 ft x 4 ft, 9729-OS	12 cu ft (1,440 lb)	6 cu ft	1.2 gal	2 lb
4 ft x 6 ft, 9720-OS	24 cu ft (2,880 lb)	12 cu ft	2.5 gal	4 lb
6 ft x 6 ft, 9724-OS	36 cu ft (4,320 lb)	18 cu ft	3.7 gal	5 lb
6 ft x 9 ft, 9721-OS	54 cu ft (6,480 lb)	27 cu ft	5.5 gal	5 lb
8 ft x 8 ft, 9723-OS	64 cu ft (7,680 lb)	32 cu ft	6.5 gal	7 lb
10 ft x 15 ft, 9725-OS	150 cu ft (18,000 lb)	75 cu ft	15.1 gal	17 lb
15 ft x 15 ft, 9727-OS	225 cu ft (27,000 lb)	112 cu ft	22.6 gal	26 lb

Two things follow from the weight column. First, a half-full 8 ft x 8 ft bag holds about 32 cu ft of wet sediment, more than a cubic yard, so nobody is lifting it. Indiana's IDEM Section 713.01 tells you to let the bag dewater before moving it, disconnecting the hose, or opening it, and it allows the bag to be "left in place, cut open, and its contents spread and stabilized, with all visible fabric removed." Plan the bag's location as the sediment's final resting place, or plan the loader that will move it. Second, keep spares. IDEM says to keep replacement bags on hand for continuous pumping, and SCDOT says to "monitor the Dewatering Bag at all times while the pump is running," which means the swap happens mid-shift, not next week.

Soil type changes the count. IDEM warns that high-clay soils clog geotextile quickly, "so larger bags are likely needed," and it permits flocculants or polymers when they follow the manufacturer's requirements. The practical reading: on sandy or silty excavations, size by flow and buy bags by volume; on clay, step up one size from the flow table and expect to swap on flow loss before you swap on volume.

For a long job with a lot of sediment, UltraTech makes a reusable model: a woven geotextile bag with a 13 lb set of galvanized steel locking rods that hold the opening shut while pumping and come out so the bag can be "emptied and reused repeatedly." We stock the 3 ft x 5 ft bag at $199.00, the 5 ft x 7 ft bag at $240.00, and the locking rods at $503.00. UltraTech notes the reusable fabric does not absorb oil, so it is a sediment-only tool.

Question 3: What hose are you running, and how will you attach it?

Every standard UltraTech bag we stock is listed for discharge hoses up to 4 inches in diameter, and UltraTech's August 2024 specification says the bag is "manufactured to fit discharge hoses up to four (4) inches in diameter." UltraTech's current product page states the neck accepts up to an 8 inch hose. Those two documents disagree, so if you run a 6 inch or 8 inch line, call us and we will confirm the current neck size with UltraTech before you order. For 2, 3 and 4 inch trash pumps, which is most of the market, any size in the table fits.

The connection fails before the fabric does. IDEM lists hose detachment alongside bursting and seam ripping as what happens to an undersized bag, and SCDOT's installation rules are specific: insert the hose "at least 1 foot inside the bag," secure it "with a heavy-duty pipe clamp, rope, or other suitable means," and if you use rope, make "a minimum of 6 wraps around the hose over a 6-inch width of the bag." One hose per bag. Do not tee two pumps into one sack.

UltraTech's own guidance is that the attached straps are enough "at lower flow rates" and that "at higher flow rates, a hose clamp (not included) may be needed." The alternative is the UltraTech 9739 Hose Connector, a PVC fitting that threads onto a 4 inch discharge hose and ships with a quick-release clamp, so swapping a half-full bag for a fresh one is a clamp release rather than a rope job. It weighs 2.5 lb, does not rust, and is reused across every bag on the project.

Question 4: Where will the bag sit, and where does the water go next?

A 15 ft x 15 ft bag needs a 15 ft by 15 ft patch of ground plus a bed under it, and the ground has to do half the filtering. UltraTech's spec sheet says to place the bag "on an aggregate or hay bale bed to maximize water flow through the bag's surface area," and SCDOT repeats it with the reason: on low-permeability or impermeable surfaces the bag will not pass flow through its bottom, so put it on "an aggregate, hay bales, or other highly permeable surface" and move it if flow looks restricted. Half the fabric area in the Question 1 table is the bottom of the bag. Park it on asphalt and you have bought half a bag.

Slope is the next number. IDEM requires "nearly level to slightly sloping surface (less than 5% slope)," allows aggregate pads on slopes from 5 percent to under 10 percent, and says to avoid concave spots where outflow pools. SCDOT wants "a mild slope to ensure incoming water flows downhill through the Dewatering Bag" and warns that on steep slopes "the bag may roll, causing failure." A full 15 ft x 15 ft bag at 27,000 lb of sediment is not a thing you want rolling toward anything.

Then the water has to go somewhere legal. IDEM: "Do not locate filter bags in water resources, wetlands, stream channels, or in concentrated flows." SCDOT: "Do not place Dewatering Bags directly in jurisdictional waters of the state," and when practical pick a location "not in close proximity to receiving surface waters" with "a sufficient permeable buffer to allow natural infiltration." Greenville County adds that bags are for operations that "do not discharge to sediment dams or sediment basins" and routes bag outflow across a vegetated filtering area before it enters any conveyance. Near a stream, wetland, or property line, IDEM calls for a secondary rock berm downslope of the bag. Our storm drain guide covers the drain seals and inlet guards that protect the catch basin the outflow eventually reaches.

Inspection is where the three specs differ, and the strictest one is the one to plan around. SCDOT says monitor at all times while the pump runs. IDEM says inspect at the start of pumping and "at a minimum of once every hour thereafter," and "immediately cease pumping when holes, rips or tears are identified." Wisconsin DOT's dewatering guidance adds the test that matters most: compare the receiving water downstream of the discharge with the water upstream, because discoloration downstream "indicates the dewatering operation needs adjustment." WisDOT also tells you to protect the pump intake with a floating suction hose or clear stone, which cuts the sediment you send to the bag in the first place.

Which dewatering bag should you buy?

Match your pump and your soil to the line that describes them. Prices are as listed on Warehouse Gear Hub as of this writing.

• Any pump rated under 100 gpm, a wet footing or a vault: 3 ft x 4 ft, 9729-OS ($29.00). 200 gpm new, 12 cu ft of sediment, swap at 6.
• Pump rated under 125 gpm, sandy or silty spoil: 4 ft x 6 ft, 9720-OS ($38.00). 250 gpm new, 24 cu ft, swap at 12.
• Pump rated up to 175 gpm, or a 2 inch pump you will run throttled: 8 ft x 8 ft, 9723-OS ($100.00). 350 gpm new, 64 cu ft, swap at 32. The 6 ft x 9 ft 9721-OS ($71.00) carries the same 350 gpm rating with 54 cu ft if the footprint is tight on one side.
• 2 inch trash pump at full rating (185 gpm for the Honda WT20): 10 ft x 15 ft, 9725-OS ($149.00). 400 gpm new clears the 370 gpm the SCDOT rule asks for, with 150 cu ft of capacity.
• Pump rated up to 250 gpm, clay soils, or a job measured in weeks: 15 ft x 15 ft, 9727-OS ($213.00). 500 gpm new, 225 cu ft, swap at 112.
• 3 inch or 4 inch trash pump at full rating: two 10 ft x 15 ft or 15 ft x 15 ft bags on separate discharge lines, or a custom size. Call before you buy and bring the pump's gpm; we will size it with UltraTech and quote it.
• Every job: one 9739 Hose Connector ($56.00) per discharge line, and at least one spare bag on site, because the swap happens while the pump is running.

Every bag above is in the dewatering bag collection. Warehouse Gear Hub is an authorized UltraTech dealer, we match prices, and we quote volume orders for contractors who buy bags by the pallet. Call 1-800-794-1049, 8 AM to 4 PM Central, with your pump's rated gpm, your hose diameter, and whether the spoil is sand, silt or clay, and we will size it in one call.

Frequently Asked Questions

How does a dewatering bag work?

The pump's discharge hose feeds into a sewn sack of nonwoven geotextile. Water passes through the fabric and the surface it sits on; sediment larger than the fabric's openings stays inside. UltraTech's spec sheet lists its fabric at 8 oz per square yard with a No. 80 sieve apparent opening size (0.18 mm), a flow rate of 90 gal/min/ft², and 99.0 percent filter efficiency under ASTM D5141. The Oil and Sediment model also absorbs hydrocarbons, from 1.2 gallons in the 3 ft x 4 ft bag to 22.6 gallons in the 15 ft x 15 ft bag per UltraTech's product page.

What size dewatering bag do I need for a 2 inch pump?

Take the pump's rated gpm and double it, per SCDOT's SC-M-815-15. A Honda WT20 2 inch trash pump is rated 185 gpm, so you need a bag rated at least 370 gpm when new. UltraTech's 8 ft x 8 ft bag is rated 350 gpm and misses; the 10 ft x 15 ft bag (9725-OS, $149.00) is rated 400 gpm and clears. A smaller 2 inch pump with a lower rating, or a WT20 running throttled under 175 gpm, fits the 8 ft x 8 ft bag at $100.00.

How long does a dewatering bag last?

Until trapped sediment reaches 50 percent of the bag's capacity, or until it stops passing water at a reasonable rate, whichever comes first, per SCDOT and Greenville County. On an 8 ft x 8 ft bag that is 32 cubic feet of sediment. How many hours that takes depends on how dirty the water is and how much clay is in it; IDEM notes high-clay soils clog geotextile quickly. UltraTech's spec sheet also rates the fabric at 70 percent strength retained after 500 hours of UV exposure under ASTM D4355, so a bag left in the sun for months is a weaker bag.

Can a dewatering bag be reused?

A standard bag, no. SCDOT and Greenville County both say not to clean and reuse a bag once its voids are clogged with trapped sediment; cut it open, spread or dispose of the contents as the engineer directs, and remove the fabric. UltraTech's reusable model is built for it: a woven geotextile bag with galvanized locking rods that hold the opening shut during pumping and come out for emptying. We stock the 3 ft x 5 ft and 5 ft x 7 ft reusable bags at $199.00 and $240.00 and the rod set at $503.00. The reusable fabric does not absorb oil.

Does a dewatering bag remove oil?

A plain sediment bag does not, and IDEM's manual says bags are "not intended to treat or remove contaminates other than sediment." The UltraTech bags on Warehouse Gear Hub are the Oil and Sediment model, which UltraTech rates for 1.2 to 22.6 gallons of oil absorption depending on size. That absorption figure does not make contaminated groundwater legal to pump; IDEM requires additional controls and appropriate discharge permitting for that.

Can I pump concrete washout water through a dewatering bag?

No. 40 CFR 450.21(e)(1) prohibits "wastewater from washout of concrete, unless managed by an appropriate control," and a sediment bag is not that control; IDEM's manual says bags are not intended to treat anything other than sediment. Concrete washout goes to a washout container. Fuels, oils and other pollutants from equipment maintenance, and soaps or solvents from equipment washing, are prohibited discharges under the same paragraph.

How much does a dewatering bag cost?

On Warehouse Gear Hub as of this writing: 3 ft x 4 ft $29.00, 4 ft x 6 ft $38.00, 6 ft x 6 ft $48.00, 6 ft x 9 ft $71.00, 8 ft x 8 ft $100.00, 10 ft x 15 ft $149.00, and 15 ft x 15 ft $213.00, all UltraTech Oil and Sediment models. The 9739 Hose Connector is $56.00. The reusable system is $199.00 or $240.00 for the bag plus $503.00 for the locking rods.