PAM 5 acres silt loam NPDES

PAM (polyacrylamide) in a sediment basin: from 65% to 92% trap efficiency

PAM is the only practical way to capture the fine-silt and clay tails of a typical influent PSD. The 7-bin Stokes/Camp arithmetic on the same 5-acre basin shows what a 5 ppm anionic PAM dose does: trap efficiency 65.5% → ~92%, no change to basin geometry. Worked example, application options, and the regulatory framing.

Without PAM: 65.5% trap efficiency on silt-loam influent (limited by bins 5–7). With 5 ppm anionic PAM: bins 5–7 flocculate into ~0.05 mm aggregates, which settle like bin 3. New trap efficiency ≈ 92%. Cost: ~$50 per design storm event. Application: floc logs in the inlet ditch, no power, no operator. The catch: anionic PAM only — cationic is acutely toxic to aquatic life and is prohibited on most NPDES permits.

Baseline (no PAM)

Same site as the 5-acre subdivision: 22,500 ft² basin, 22.2 cfs design Q, silt-loam soil. The 7-bin trap efficiency without chemical addition was 65.5%. Bottleneck: bins 5, 6, 7 (fine silt and clay) — together 40% of the influent mass — are essentially uncatchable in a gravity basin.

What PAM does to the PSD

Anionic polyacrylamide bridges fine particles by adsorbing onto multiple surfaces simultaneously. The mechanism is physical (charge interaction + bridging), not chemical reaction. The result: clay and fine-silt particles aggregate into flocs with characteristic diameters in the 50–200 μm range — solidly in the coarse-silt to very-fine-sand bins.

From the basin's perspective, the PSD changes:

Bin	d (mm)	% — no PAM	% — with 5 ppm PAM	Note
1 (sand)	0.500	8%	8%	Unchanged — sand doesn't flocculate
2 (very fine sand)	0.100	12%	12%	Unchanged
3 (coarse silt + flocs)	0.050	18%	58%	Original 18% + ~40% of bins 5–7 mass migrated as flocs
4 (medium silt)	0.020	22%	22%	Some flocculation, conservatively unchanged
5 (fine silt)	0.010	15%	≈ 0%	Flocculated into bin 3
6 (very fine silt)	0.005	12%	≈ 0%	Flocculated into bin 3
7 (clay)	0.002	13%	≈ 0%	Flocculated into bin 3

The bin 3 settling velocity is 7.38 × 10⁻³ ft/s — fast enough that, in a 22,500 ft² basin at 22.2 cfs, η = 7.48 → 100% capture. So everything that flocculates into bin 3 gets caught.

Trap efficiency with PAM

Re-running the 7-bin Stokes/Camp on the post-PAM PSD:

Bin	% (post-PAM)	η_i	Captured (% of total)
1	8%	100%	8.0%
2	12%	100%	12.0%
3 (incl. flocs)	58%	100%	58.0%
4	22%	100%	22.0%
5–7 (residual)	0% (flocculated up)	—	0%
System trap efficiency (theoretical)			100%

Theoretical 100%; field-measured 88–95% in EPA studies. Use 92% as the design value. The 8% gap covers under-flocculated fines, dose timing across the storm, and short-circuiting effects.

Net effect: trap efficiency 65.5% → 92% in the same basin with no civil-engineering changes. NCDEQ's 80% target is comfortably met.

Dosing rate and application

For typical construction-site stormwater (1,000–3,000 NTU influent), anionic PAM at 5 ppm is the workhorse rate. EPA's recommended range is 1–10 ppm depending on turbidity and clay fraction.

For the 5-acre site, 10-yr design storm produces total runoff:

$V_{runoff} = Q_{eff} \cdot A = 3.84 \text{ in} \cdot 5 \text{ ac} \cdot \frac{43{,}560 \text{ ft}^2}{\text{ac}} \cdot \frac{1 \text{ ft}}{12 \text{ in}} = 69{,}700 \text{ ft}^3$

$V_{runoff} = 521{,}000 \text{ gal} \approx 1{,}975 \text{ m}^3$

PAM mass for 5 ppm:

$m_{PAM} = 5 \text{ mg/L} \cdot 1{,}975{,}000 \text{ L} = 9.9 \text{ kg} \approx 22 \text{ lb}$

At ~$3-5/lb for granular anionic PAM, that's $66 to $110 per design storm event. Floc logs (compressed PAM blocks for in-channel passive dosing) cost $20–30 each and last for one or two storms.

Application options

Floc logs in the inlet ditch. Passive, no operator. Best for sites with continuous overland flow into the basin. Fails when flow is erratic.
Tablet feeders. Slow-dissolve cartridges in the inflow line. Flow-actuated, no power needed. Good for diversion inlets.
In-line liquid dosing. Pump + reservoir + flow meter. Best mixing, highest cost, requires power and an operator. Typically only used on linear projects (utility crossings, road widenings) with continuous discharge.
Sock + jute matting. PAM-impregnated geotextile in the inlet weir. Cheapest at small scale.

The two ways PAM goes wrong on a project

Wrong product. Cationic PAM is widely available (it's used in pulp/paper, water treatment) and is acutely toxic to fish at low concentrations. Almost all NPDES permits prohibit it explicitly. Specify anionic PAM, water-soluble, with an MSDS confirming acute toxicity testing per ASTM E724. NCDEQ §6.83 lists pre-approved products.
No mixing. A floc log in still water does nothing. PAM needs turbulent contact with the sediment to bridge. The inflow channel is the right place — never the basin itself, where flow is by design quiescent. If your application point is the basin pond, redesign.

What changes if you tweak the inputs

If you change…	The result moves…
Dose rate 5 → 2 ppm	Under-flocculated fines pass through; design η drops to ~85%
Influent PSD sandier (less clay)	PAM less needed; baseline η was already higher; ROI on PAM falls
Influent PSD finer (more clay)	PAM more needed; design dose may rise to 8–10 ppm
Storm duration shorter (flashy)	Mixing-time becomes critical; tablet feeders or in-line dosing favored over passive socks
Cold water (winter, < 5°C)	PAM hydration slows; pre-mix on warmer water or increase dose 25–50%

Toggle PAM on the basin in HydroComplete

Same scenario, PAM mode on/off — see the per-bin η jump and the NPDES turbidity prediction. Engine includes the floc-log application calculator.

Open in app 14-day free trial

Sources and further reading

EPA. Best Management Practices for the Use of Polyacrylamide for Erosion Control. EPA-841-B-12-007.
NCDEQ. NC Erosion and Sediment Control Planning and Design Manual. §6.83 (Polyacrylamide).
Sojka, R. E. & Lentz, R. D. (1997). Reducing Furrow Irrigation Erosion with Polyacrylamide. J. of Production Agriculture 10(1).
Caltrans. Construction Site Best Management Practices Manual. Section CASQA SE-12 (Polyacrylamide).

— Michael Flynn, PE
PAM works. The two failure modes seen in the field are (1) wrong PAM type — get the anionic, water-soluble, NCDEQ-approved product, and (2) inadequate mixing — passive floc logs in still water do nothing.