12.4 Chemical Treatment, Flocculants (PAM/Chitosan) & Jar Testing

12.4 Chemical Treatment, Flocculants (PAM/Chitosan) & Jar Testing

Quick Reference: When construction runoff contains colloidal clays (< 0.002 mm), conventional gravity settling in sediment basins fails because repulsive negative surface electrical charges (zeta potential) keep particles permanently suspended. Meeting stringent water clarity standards (the 2022 EPA CGP dewatering benchmark of 50 NTU, or state benchmarks as low as 10–25 NTU above background) requires chemical treatment. Coagulants (alum, PAC) neutralize negative charges to form micro-flocs, while flocculants (anionic PAM, chitosan) bridge micro-flocs into heavy macro-flocs that settle in seconds. A vital CPESC rule: Anionic PAM has very low aquatic toxicity and is the default choice, whereas cationic polymers bind to fish gill membranes and may not be used under the EPA CGP unless the EPA Regional Office grants written authorization under Part 1.1.9. A site-specific jar test is mandatory to establish optimal dosage and prevent filter-blinding overdoses.

---

The Physics of Colloidal Suspensions: Why Gravity Settling Fails

On construction sites dominated by fine-grained soils—particularly in regions with marine clays, glacial lacustrine deposits, or weathered saprolites (e.g., kaolinite, smectite, illite, montmorillonite)—sediment basins frequently fail to clarify stormwater. Even after 72 hours of quiescent detention, discharged water remains a turbid, milky-brown plume.

The Stokes' Law Stalemate

Under Stokes' Law, the gravitational settling velocity ($v_s$) of a soil particle is directly proportional to the square of its diameter ($d^2$). For a colloidal clay particle with a diameter of $0.001\text{ mm}$ ($1.0\text{ \mu m}$):

$v_s \approx 0.000002\text{ ft/s} \approx 0.17\text{ ft per 24-hour day}$

At this settling rate, a clay particle requires more than 18 to 25 days to settle just 3 feet in completely still water. In active field basins subjected to wind shear, thermal currents, and daily inflows, colloidal clays will never settle gravitationally.

Surface Chemistry, Isomorphic Substitution & Zeta Potential

Colloidal stability is maintained by fundamental electrical forces at the atomic level:

1. Isomorphic Substitution: In the crystalline crystal lattice of aluminosilicate clays, trivalent aluminum ($Al^{3+}$) and tetravalent silicon ($Si^{4+}$) cations are substituted by divalent magnesium ($Mg^{2+}$) or monovalent cations. This lattice mismatch leaves clay platelets with a permanent, structural negative surface charge.
2. The Electrical Double Layer: In water, the negative clay surface attracts a tight boundary layer of positive cations (the Stern layer), surrounded by a looser outer region of mobile counter-ions (the diffuse Gouy-Chapman layer).
3. Zeta Potential: The electrical potential at the slipping boundary of this double layer is defined as the zeta potential (typically $-15\text{ to }-50\text{ mV}$ in construction runoff).
4. Electrostatic Repulsion vs. Van der Waals Attraction: According to DLVO theory, two converging clay particles experience attractive van der Waals forces, but the overlapping negative electrical fields generate powerful double-layer electrostatic repulsion. Because this repulsive barrier exceeds the attractive forces, particles bounce off one another in continuous Brownian motion, maintaining a stable colloidal suspension.

Regulatory Clarity Standards

The 2009 Construction & Development Effluent Limitation Guidelines briefly carried a national numeric turbidity limit of 280 NTU, but EPA stayed it in 2010 and withdrew it in 2014; 40 CFR Part 450 contains no numeric turbidity limit today. The operative federal number is the 2022 CGP standard benchmark of 50 NTU, applied to the weekly average of daily dewatering samples at sites discharging to sensitive waters (Part 3.3.2). Many state agencies and regional permits are more stringent still—commonly 25 NTU above ambient background, or as little as 10 NTU in cold-water trout fisheries. Achieving compliance in clay soils requires chemical destabilization.

Cationic Chemicals Are Conditionally Prohibited, Not Absolutely Banned

Under CGP Part 1.1.9, an operator is not eligible to use cationic treatment chemicals unless it notifies the applicable EPA Regional Office and receives written authorization, using the request format in CGP Appendix J. If authorization is granted, Part 2.2.13f obliges the operator to perform every additional measure attached to that authorization. Part 2.2.13 also requires conventional erosion and sediment controls before and after chemical application, application only where treated stormwater flows to a sediment control before discharge, leak-proof storage under storm-resistant cover with secondary containment, compliance with state and local chemical rules, dosing per the supplier's specifications, and product-specific training for everyone who handles the chemicals. On the exam, the defensible answer is that cationic polymers are barred by default and permissible only with express regulatory authorization — not that they are universally illegal.

---

Coagulation vs. Flocculation: Chemical Mechanisms

Chemical treatment operates as a two-stage thermodynamic and mechanical process: coagulation (charge neutralization) followed by flocculation (polymer bridging).

1. COLLOIDAL DISPERSION          2. COAGULATION (Charge Neutralization)     3. FLOCCULATION (Polymer Bridging)
   Negatively charged clays         Multivalent cations (Al3+, Fe3+)           Long-chain polymers (PAM)
   repel each other                 compress double layer                      bridge micro-flocs into
   (Zeta Potential: -15 to -50 mV)  (Forms Pinpoint Micro-Flocs)               heavy Macro-Flocs

   [-]       [-]                     [+]       [+]                                   /─────────\
        [-]                             [Micro-floc]                           ─────( Macro-Floc )─────
   [-]       [-]                     [+]       [+]                                   \─────────/
   (Indefinite Suspension)          (Flash Mix: 10-30 sec)                    (Gentle Mix: Settles in Seconds)

Stage 1: Coagulation (Electrostatic Charge Neutralization)

• Chemical Agents: Inorganic multivalent metallic salts or specialized inorganic polymers:
  • Aluminum Sulfate (Alum, $Al_2(SO_4)_3 \cdot 14H_2O$)
  • Ferric Chloride ($FeCl_3$)
  • Polyaluminum Chloride (PAC, $Al_n Cl_{(3n-2m)}(OH)_m$)
• Mechanism: When added to turbid water, multivalent cations ($Al^{3+}$ or $Fe^{3+}$) dissociate and dissociate into the solution. These highly charged positive ions compress the electrical double layer and neutralize the negative surface charges (driving zeta potential from $-30\text{ mV}$ toward $0\text{ mV}$).
• Micro-Floc Formation: Once electrostatic repulsion is neutralized, van der Waals forces dominate, allowing colliding clay colloids to adhere and form tiny, pinpoint agglomerations called micro-flocs.
• Mixing Requirements: Requires high-energy, high-shear rapid flash mixing (10 to 30 seconds) to disperse ions throughout the water before hydroxide precipitates form.

Stage 2: Flocculation (Inter-Particle Polymer Bridging)

• Chemical Agents: High-molecular-weight organic polymers (long synthetic or natural carbon chains with molecular weights of 5 to 15 million Daltons):
  • Polyacrylamide (PAM)
  • Chitosan (biopolymer derived from crustacean chitin)
• Mechanism: Flocculant molecules are extended polymer strands thousands of monomer units long. Individual polymer chains attach to multiple micro-flocs simultaneously (inter-particle bridging), tangling thousands of fine particles into large, three-dimensional agglomerations called macro-flocs (often resembling white snowflakes).
• Settling Velocity: Macro-flocs achieve settling velocities of 1.0 to 5.0 inches per second ($150\text{ to }300\text{ ft/hr}$), clearing turbid water from $> 2,000\text{ NTU}$ down to $< 10\text{ NTU}$ in 30 to 90 seconds.
• Mixing Requirements: Requires gentle, low-shear mixing (2 to 5 minutes). High-shear turbulence must be strictly avoided during flocculation because excessive hydraulic shear will physically tear the fragile polymer bridges apart, re-suspending broken flocs.

Polymer Chemistry, Aquatic Toxicity & Environmental Regulations

The application of synthetic polymers to stormwater is subject to strict environmental regulations. The CPESC practitioner must master the critical distinction between polymer charges:

Anionic PAM vs. Cationic PAM: The Toxicity Divide

Core Environmental Rule: Cationic PAM is strictly prohibited in construction stormwater discharges. Fish gill membranes carry a strong negative surface electrical charge. When positively charged cationic polymers enter surface waters, they electrostatically bind to the gill epithelial tissue. This triggers acute gill swelling, hypersecretion of mucus, mechanical clogging of gill lamellae, and rapid respiratory asphyxiation (fish suffocation) at concentrations as low as 0.1 parts per million (ppm). In contrast, Anionic PAM carries a negative charge, repelling biological membranes and posing virtually zero toxicity to fish and aquatic life at field dosages ($1\text{ to }5\text{ ppm}$).

Chitosan Biopolymer Specifications

Chitosan is a natural biopolymer manufactured by the deacetylation of chitin extracted from the exoskeletons of marine crustaceans (crabs and shrimp):

• Mechanism: Unlike synthetic polymers, chitosan is a natural cationic biopolymer. Its positive amine groups bind aggressively to negative clay platelets.
• Environmental Safety & Filtration Mandate: While chitosan is biodegradable, free dissolved chitosan can still exhibit aquatic toxicity to sensitive fish species if discharged unreacted. Therefore, environmental regulators mandate that chitosan must always be followed by pressurized sand media filtration to capture all reacted flocs and ensure zero residual free polymer enters receiving waters.
• pH Operating Window: Chitosan is highly sensitive to water chemistry; it is effective only within a narrow pH range of 6.5 to 8.0.

---

Application Delivery Systems: Passive Dosing vs. Active Treatment Systems (ATS)

Chemical clarifying agents are introduced into construction runoff through two primary engineering methodologies:

1. Passive Treatment Systems (Flow-Activated Dosing)

Passive systems utilize the kinetic energy of flowing water to dissolve and mix solid flocculants without mechanical pumps or electricity:

• Flocculant Blocks ("Floc Logs"): Dense semi-solid blocks composed of site-matched anionic PAM blended with water-soluble matrix waxes. Placed directly in high-velocity open channels, rock check dams, or ditch-line turbulence zones.
• Flocculant Socks & Powder Check Dams: Granular anionic PAM placed inside woven burlap sacks or broadcast over coir/jute check dam faces ($1\text{ to }3\text{ lbs per check dam}$).
• Advantages: Low capital cost, zero power requirements, simple installation in remote highway corridors.
• Operational Limitations: Lacks precise dosage control. During low flows, insufficient polymer dissolves; during torrential floods, excessive water velocity can erode logs, causing chemical waste. Furthermore, passive systems require a downstream settling basin or sediment trap to capture the settled flocs before discharge.

2. Active Treatment Systems (ATS)

An Active Treatment System (ATS) is an automated, containerized industrial water treatment plant deployed on high-risk, highly sensitive construction projects (e.g., California CGP Risk Level 3 sites, projects adjacent to critical salmonid habitats, or contaminated soil excavations):

• Unit Operations:
  1. Raw Water Storage: Inflow enters an untreated retention basin to equalize storm surges.
  2. Automated Metering: Industrial variable-speed pumps inject liquid coagulants (PAC) and anionic polymer into the pipe stream based on real-time flow meter telemetry.
  3. Inline Reaction Train: A static inline mixer provides high-shear flash mixing (coagulation), followed by a baffled reaction tank providing low-shear flocculation.
  4. Lamella Plate Clarifier: Water flows upward through inclined parallel plates ($55^\circ\text{ to }60^\circ$ angle), settling out macro-flocs rapidly within a small footprint.
  5. Pressurized Sand Media Filtration: Dual-cell silica sand filters remove residual pinpoint flocs down to $1\text{ to }5\text{ microns}$.
  6. Real-Time Automated Compliance Feedback: Continuous digital sensors monitor discharge turbidity and pH. If effluent turbidity exceeds permit limits (e.g., $> 20\text{ NTU}$) or pH deviates outside the 6.5 to 8.5 window, an automated three-way valve instantaneously actuates, diverting the effluent back to the raw storage basin (recirculation loop). No non-compliant water is ever discharged to the environment.

---

The Standardized Field Jar Test Procedure

Polymer performance is highly soil-specific. An anionic PAM formulation that clarifies red clay in North Carolina may completely fail to clarify glacial clays in Washington. A standardized field jar test must be performed on site-specific soils prior to chemical deployment.

The Six-Step Jar Test Protocol

Step 1: Collect 5-gal representative site runoff sample
Step 2: Record baseline parameters (Temperature, pH, Initial Turbidity in NTU)
Step 3: Fill four 1,000 mL graduated test jars with turbid sample
Step 4: Dosing Gradient: Jar 1 (0 ppm / Control), Jar 2 (1 ppm), Jar 3 (2 ppm), Jar 4 (5 ppm)
Step 5: Agitate: Rapid mix (100 RPM for 60 s) ──► Slow mix (20-30 RPM for 2-3 min)
Step 6: Measure: Floc size, settling time (seconds), supernatant turbidity at 5, 10, 30 min

1. Collect Representative Runoff: Collect a 5- to 10-gallon representative sample of turbid runoff from the active construction area (or create a slurry by thoroughly mixing site subsoil with local water to match expected runoff conditions).
2. Record Baseline Water Chemistry: Measure and record initial water temperature, pH, and baseline turbidity (using a calibrated nephelometric turbidimeter).
3. Set Up Test Jars: Fill a series of four 1,000 mL ($1.0\text{ liter}$) glass beakers with 1,000 mL of the thoroughly agitated turbid sample.
4. Inoculate Dosage Gradient:
   • Jar 1: Control ($0.0\text{ ppm}$ chemical)
   • Jar 2: Low dose ($1.0\text{ ppm}$ or $1.0\text{ mg/L}$)
   • Jar 3: Optimal target dose ($2.0\text{ to }3.0\text{ ppm}$)
   • Jar 4: High dose ($5.0\text{ ppm}$)
5. Standardized Mixing Protocol:
   • Rapid Flash Mix: Stir vigorously at $\approx 100\text{ RPM}$ for 60 seconds to distribute coagulant/polymer.
   • Gentle Flocculation Mix: Reduce agitation to $\approx 20\text{ to }30\text{ RPM}$ for 2 to 3 minutes to promote inter-particle collisions without breaking polymer chains.
6. Quantify Settling & Clarity:
   • Record the time (in seconds) required for visible macro-flocs to form and settle to the beaker bottom.
   • Observe floc morphology (pinpoint, granular, or large cohesive snowflakes).
   • Extract supernatant samples from the top 2 inches at 5, 10, and 30 minutes; measure final turbidity with a digital turbidimeter.

The Overdosing Hazard: Steric Stabilization

A critical CPESC concept is that more chemical is NOT better:

• Steric Stabilization & Charge Reversal: When polymer is over-applied, excess polymer chains completely coat the surface of every individual clay particle. Because all particles now display identical polymer outer coatings, they repel one another. This phenomenon—known as steric hindrance or restabilization—re-suspends the colloids, rendering the water permanently cloudy.
• Filter Blinding: Excess unreacted polymer forms a sticky, viscous gel that blinds sand media filters and geotextile dewatering bags, causing immediate system clogging and pressurized blowouts.
• Permit Violation: Discharging excess unreacted polymer into public waterways is a severe violation of environmental discharge permits.

Summary: Comparison of Clarification Technologies