How to Use PolyDADMAC as a Primary Coagulant in Water Treatment

Overview

PolyDADMAC (Poly-diallyldimethylammonium chloride, CAS 26062-79-3) is a synthetic, water-soluble cationic polymer used as an organic primary coagulant in water and wastewater treatment. It is a homopolymer of diallyldimethylammonium chloride, with a molecular weight typically in the range of 100,000–300,000 Da and one of the highest positive charge densities available in a commercial water treatment polymer. Unlike polyacrylamide-based flocculants, PolyDADMAC contains no acrylamide monomer, no hydrolyzable aluminum or iron, and is available in NSF/ANSI Standard 60 certified grades for direct use in drinking water at approved dose levels.

The coagulation mechanism of PolyDADMAC is exclusively charge neutralization — it does not bridge particles. Colloidal particles in natural surface water, drinking water source water, and wastewater carry a net negative surface charge (zeta potential typically −15 to −30 mV) that creates electrostatic repulsion and prevents particle aggregation. PolyDADMAC's high cationic charge density neutralizes this surface charge (drives zeta potential toward 0 mV), destabilizing the colloids and allowing van der Waals attraction to cause coagulation. Because the mechanism is charge neutralization rather than polymer bridging, PolyDADMAC works with much shorter polymer chains and much lower dose than bridging flocculants (anionic or nonionic PAM), and overdosing causes charge reversal and restabilization of the colloid — the treated water gets worse, not better, above the optimum dose.

PolyDADMAC's most important commercial application is in drinking water treatment as a partial or complete replacement for inorganic coagulants (alum, PAC, ferric coagulants). It produces no aluminum or iron residual in treated water, generates less sludge volume per unit of turbidity removed, and is particularly effective against low-turbidity, high-color (humic acid) source water where alum doses required for charge neutralization would add more aluminum to the water than is desirable. In industrial wastewater, PolyDADMAC is widely used for decolorization of textile printing/dyeing effluent (where dyes are anionic and strongly attracted to the polymer's cationic charge) and as a primary coagulant before dissolved air flotation (DAF) for oily wastewater or food processing effluent.

Preparation & Dissolution

PolyDADMAC is supplied as a liquid (20–50% active content), so no dissolution is required. However, dilution before dosing is strongly recommended for accurate and uniform application.

Dilute before dosing: Prepare a 0.5–2.0% active solution by diluting the concentrated product with clean water (tap or treated process water). At 20–50% active, the product is too viscous (500–5,000 mPa·s) to disperse rapidly in the treatment process — a diluted 0.5–2% solution disperses within seconds of injection.
Dilution equipment: A simple day tank with a low-speed agitator or static mixer loop is sufficient. No heat is required. Prepare enough dilute solution for 8–24 hours of operation. Diluted PolyDADMAC solution (< 2%) is stable for several days at ambient temperature.
Dosing pumps: Peristaltic or diaphragm metering pumps work well. Unlike lime or PAC, PolyDADMAC solutions are non-abrasive and low-viscosity at working concentration. Clean the dosing pump head weekly to prevent biofilm growth (PolyDADMAC is a nutrient for some bacteria at low concentrations).
Injection point: Inject at the highest-turbulence point in the treatment train — typically just upstream of a static mixer, flash mixer, or rapid mix tank. The injection must achieve complete dispersion within 5–10 seconds of contact with the water. PolyDADMAC is consumed (charge-neutralizes colloids) almost instantaneously upon good mixing — late mixing dramatically reduces efficiency.
Do not pre-mix with anionic polymers or coagulant aids: PolyDADMAC is strongly cationic and will form an insoluble complex with anionic polyacrylamide (APAM) if mixed in concentrated form. Always inject separately into the process stream, not into a combined solution tank.

Dosing Guide

Application	Dose (active PolyDADMAC)	Notes
Drinking water (low-turbidity, high-color source water)	0.5–3 mg/L active	Often replaces 30–80% of alum or PAC dose; confirm NSF/ANSI 60 grade and approved dose level with local regulator
Drinking water (moderate turbidity 10–50 NTU)	1–5 mg/L active, combined with PAC at 5–15 mg/L	PolyDADMAC as primary coagulant, PAC or alum as coagulant aid for sweep floc; jar test to optimize split
Textile dyeing wastewater decolorization	30–200 mg/L active (COD and dye-load dependent)	High dye loads require higher dose; combine with PAM at 1–3 mg/L for improved settling; jar test mandatory
DAF pretreatment (food processing, oily water)	5–20 mg/L active	Often combined with ferric coagulant at 20–50 mg/L; optimize by measuring float layer quality and effluent SS
Paper mill white water / retention aid	0.1–0.5 kg/ton of paper	Contact paper chemical supplier for mill-specific application guidance
Sludge conditioning (co-polymer with CPAM)	20–100 mg/L active alongside CPAM	Improves dewatering of high-organic sludge; pre-test on pilot dewatering unit before full-scale

Application Procedure

Conduct a jar test before any new application or source water change: This is non-negotiable. PolyDADMAC has a narrow optimal dose window due to charge neutralization mechanism — above the optimum, zeta potential goes positive, colloids are restabilized, and performance collapses. The jar test establishes the optimum dose, the sensitivity of performance to dose variation, and the effect of pH on performance.
Jar test procedure: Prepare 1 L jars with representative source water. Dose 0, 0.5, 1, 2, 3, 5, 10 mg/L active PolyDADMAC. Flash mix at 200 RPM for 60 seconds, slow stir at 30 RPM for 10 minutes, settle for 30 minutes. Measure residual turbidity and (if possible) zeta potential. Identify the dose giving minimum turbidity — doses higher than this optimum will show increasing residual turbidity.
Set operating dose at 80–90% of jar test optimum: Full-scale rapid mixing is more intense than jar test, so the effective dose is slightly higher in practice. Starting at 80–90% of jar test optimum allows upward adjustment without risking charge reversal.
Inject at the rapid mix point: Install the PolyDADMAC injection quill immediately upstream of a rapid mix impeller or static mixer. Confirm the rapid mix G-value is 300–1,000 s⁻¹ for sufficient dispersion energy.
Monitor floc formation in the slow-mix (flocculation) stage: PolyDADMAC alone often produces small, dense pin-floc that settles well but may be too small for conventional sedimentation. In many drinking water applications, 0.05–0.1 mg/L nonionic or anionic PAM is added in the slow-mix stage to aid floc growth (bridging). This combination exploits PolyDADMAC for charge neutralization and PAM for floc building.
Adjust dose in real time: Install a continuous streaming current detector (SCD) or zeta potential meter downstream of the flash mixer. Zero streaming current correlates with optimum PolyDADMAC dose and zero-point zeta potential. Use the SCD output to automatically trim the PolyDADMAC dose as source water character changes.

Monitoring & Control

Parameter	Frequency	Target
Jar test (optimum dose determination)	Weekly or on source water change	Establish dose-response curve; re-run whenever raw water turbidity, color, or DOC changes by > 20%
Settled/filtered water turbidity	Continuous	< 0.3 NTU (drinking water); < 5 NTU (industrial clarifier effluent)
Streaming current or zeta potential	Continuous (SCD) or daily grab	Streaming current target: near 0 (−5 to +5 units); zeta potential target: −3 to +3 mV
Raw water pH	Continuous	PolyDADMAC charge density is relatively pH-insensitive from pH 5–9; note if pH excursions occur
PolyDADMAC stock concentration	Each new drum/IBC	Verify active content — supplier specification ± 2%; dilute-solution preparation depends on known active %
Sludge volume and character	Daily	PolyDADMAC-coagulated sludge is typically denser and lower in volume than alum sludge; rising sludge volume may indicate dose is too high (restabilization producing loose floc)

Common Mistakes

Treating PolyDADMAC as a flocculant (bridging agent) and expecting it to work like PAM: PolyDADMAC is a coagulant (charge neutralizer), not a bridging flocculant. Operators who dose it expecting the large, fast-settling floc characteristic of high-MW anionic PAM will be disappointed. PolyDADMAC produces small, dense micro-floc through charge neutralization. To obtain large settling floc, a low dose of anionic or nonionic PAM must be added in the slow-mix stage after PolyDADMAC. Understanding this two-step mechanism — coagulation (PolyDADMAC) followed by flocculation (PAM) — is fundamental to proper application.
Overdosing above the charge neutralization optimum: Unlike inorganic coagulants (alum, PAC) where overdosing simply wastes chemical without dramatic performance loss, PolyDADMAC overdosing causes charge reversal — the colloids acquire a positive surface charge, which is still destabilizing but now generates repulsion between positively charged particles, and the water clarity actually worsens. Jar tests must be used to define the optimum dose, and dose should not be increased beyond that point just because performance is marginal.
Poor injection mixing — injecting into a low-velocity zone: PolyDADMAC must contact every colloidal particle within seconds to achieve charge neutralization before particles recoil into their original stable state. Injecting into a slow-moving pipe section (< 0.5 m/s) or into the inlet of a large, under-mixed basin allows PolyDADMAC to form concentrated threads that neutralize some particles while leaving others untouched. Always inject into a rapid mix zone with G ≥ 300 s⁻¹.
Using non-NSF/ANSI 60 certified product in drinking water: PolyDADMAC contains trace levels of the reactive diallyldimethylammonium chloride monomer and other process impurities. Only grades tested and certified under NSF/ANSI Standard 60 at the doses used should be applied in drinking water. Industrial-grade PolyDADMAC (sold for paper, textiles, oilfield) is not tested for drinking water safety and must not be used in potable water systems.
Neglecting the effect of natural water DOC on dose requirement: Dissolved organic carbon (DOC) from humic and fulvic acids in natural water competes with colloidal particles for PolyDADMAC's cationic charge. High-DOC water (> 10 mg/L) may require 3–5× more PolyDADMAC to achieve charge neutralization relative to low-DOC water of similar turbidity. Operators who calibrate dose on low-DOC water will under-dose when the source water shifts to higher DOC (seasonal algal blooms, storm events), resulting in coagulation failure.

Storage & Handling

Shelf life: 12 months in sealed original drums at ambient temperature; do not freeze — freeze-thaw can cause irreversible polymer precipitation and viscosity loss
Temperature: Store at 5–40 °C; product viscosity increases significantly below 10 °C making transfer difficult — warm to 15–20 °C before pumping from drums
Container: Original 200 kg PE drums or IBC totes; transfer with PE or SS316L pumps and hoses — avoid brass, bronze, or copper wetted parts (heavy metal contamination of drinking water chemical)
Safety: Low acute toxicity; avoid skin and eye contact with concentrated product (mild irritant); no special ventilation required for storage; dispose of empty drums according to local chemical waste regulations — residual polymer is not biodegradable at high concentrations