10.5 Bench-Scale Jar Testing, Coagulant Optimization & Streaming Current Monitoring

Bench-Scale Jar Testing, Coagulant Optimization & Streaming Current Monitoring

Surface water sources throughout Colorado exhibit dramatic seasonal water quality swings—ranging from pristine, near-freezing winter mountain runoff with low turbidity ($< 1.0\text{ NTU}$) and low alkalinity, to violent spring snowmelts and summer flash storms carrying extreme suspended sediment loads ($> 500\text{ NTU}$) and elevated Total Organic Carbon (TOC). Bench-scale jar testing and online Streaming Current Monitoring are the two primary tools operators utilize to optimize chemical coagulation, maintain finished water clarity, and ensure compliance with the Disinfectants and Disinfection Byproducts Rule (DBPR).

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1. Purpose & Physical Chemistry of Coagulation Optimization

Naturally occurring suspended colloids (clays, silts, organic humic acids, and microorganisms) carry net negative surface electrical charges. These like charges generate mutual electrostatic repulsive forces, preventing particles from aggregating and keeping them in stable suspension indefinitely.

                                [ Negatively Charged Colloids (Zeta Potential: -15 to -30 mV) ]
                                                 | (Electrostatic Repulsion Prevents Settling)
                                                 v
                      +-------------------------------------------------------------------+
                      | Primary Coagulant Injection (Alum Al3+, Ferric Fe3+, or PAC)      |
                      +-------------------------------------------------------------------+
                                                 |
         +---------------------------------------+---------------------------------------+
         |                                                                               |
         v (Charge Neutralization)                                                       v (Sweep-Floc Enmeshment)
[ Trivalent Cations Neutralize Negative Charges ]             [ Insoluble Metal Hydroxide Precipitates Al(OH)3 / Fe(OH)3 ]
[ Zeta Potential Approaches 0 mV                ]             [ Physically Enmesh & Sweep Colloidal Particles Downward   ]
         |                                                                               |
         +---------------------------------------+---------------------------------------+
                                                 v
                                 [ Rapid Floc Growth & Aggregation ]
                                 [ Dense, Rapidly Settling Macrofloc]

Primary Coagulation Mechanisms

1. Charge Neutralization: Positively charged trivalent metal cations ($\text{Al}^{3+}$, $\text{Fe}^{3+}$) or cationic polymers adsorb directly onto the negatively charged colloidal surfaces, compressing the electrical double layer and reducing the zeta potential from $-15\text{ to } -30\text{ mV}$ toward zero (the isoelectric point). Once repulsions are neutralized, van der Waals attractive forces pull particles together into microflocs.
2. Sweep-Floc Enmeshment: At coagulant dosages exceeding the solubility limit of metal hydroxides (typical at $\text{pH } 6.5\text{--}8.0$), amorphous, sticky precipitates of aluminum hydroxide ($\text{Al(OH)}_3$) or ferric hydroxide ($\text{Fe(OH)}_3$) precipitate out of solution, physically entrapping and sweeping suspended colloids downward.
3. Interparticle Polymer Bridging: High-molecular-weight long-chain synthetic polymers bind simultaneously to multiple colloidal particles, building large, tough macroflocs that resist hydraulic shear.

Primary Objectives of Jar Testing

• Establish the optimum primary coagulant dosage (Alum $\text{Al}_2(\text{SO}_4)_3\cdot 14\text{H}_2\text{O}$, Ferric Chloride $\text{FeCl}_3$, Polyaluminum Chloride PAC).
• Determine the necessity, type, and dosage of coagulant aids or filter aids (cationic, anionic, or non-ionic polymers).
• Identify the optimum coagulation pH window (typically pH 5.8 to 6.8 for alum; pH 5.0 to 8.5 for ferric salts).
• Evaluate chemical addition sequencing (e.g., adding lime/caustic before vs. after coagulant; polymer injection delay timing).
• Determine Total Organic Carbon (TOC) / DBP precursor removal efficiency under EPA Stage 1 DBPR Enhanced Coagulation requirements.

2. Standard Jar Test Equipment & Chemical Stock Preparation

Jar Test Apparatus Specifications

• Gang-Stirrer Mechanism: A 6-paddle synchronized drive unit with an electronic digital RPM readout and programmable multi-step timer (such as a Phipps & Bird jar tester).
• 2-Liter Square B-Ker Jars: Square acrylic or glass jars are vastly superior to round laboratory beakers. The square corners act as internal baffles, eliminating bulk fluid vortexing and ensuring uniform energy dissipation ($G$-values) that accurately models full-scale hydraulic rapid mix and flocculation basins.
• Subsurface Sampling Ports: Fixed sampling stopcocks positioned $2.0\text{ cm}$ below the water surface (approx. $10\text{ cm}$ above the container bottom) allow operators to withdraw supernatant without disturbing settled floc.

Preparing Liquid Chemical Working Stocks

To accurately dose milligram-per-liter concentrations into bench-top jars, operators prepare fresh working stock solutions from commercial treatment chemicals:

• 1.0% Working Stock Solution ($10{,}000\text{ mg/L}$): Dissolve $10.0\text{ g}$ of dry chemical (or $10.0\text{ mL}$ of liquid chemical) in deionized water and dilute to $1{,}000\text{ mL}$.
  • $1.0\text{ mL}$ of $1.0\%\text{ stock}$ in a $1.0\text{ L}$ jar = $10.0\text{ mg/L}$ dosage.
  • $1.0\text{ mL}$ of $1.0\%\text{ stock}$ in a $2.0\text{ L}$ jar = $5.0\text{ mg/L}$ dosage.
• 0.1% Working Stock Solution ($1{,}000\text{ mg/L}$): Used for low-dose polymer aids ($1.0\text{ mL}$ in $2.0\text{ L} = 0.50\text{ mg/L}$). Polymer working stocks must be prepared daily to prevent polymer strand degradation.

$$\text{Chemical Dose (mg/L)} = \frac{\text{Stock Volume Added (mL)} \times \text{Stock Concentration (mg/mL)}}{\text{Jar Water Volume (L)}}$$

Worked Example 9.5.1: Dosing Calculation for 2-Liter Jar Test

An operator prepares a $1.0\%$ liquid alum working stock solution ($10.0\text{ mg/mL}$). What volume of stock solution must be pipetted into a $2.0\text{ L}$ square B-Ker jar to achieve an alum dosage of $35.0\text{ mg/L}$?

Step 1: Calculate total mass of alum required:

$$\text{Required Mass} = 35.0\text{ mg/L} \times 2.0\text{ L} = 70.0\text{ mg}$$

Step 2: Calculate volume of 1.0% working stock ($10.0\text{ mg/mL}$):

$$\text{Stock Volume} = \frac{70.0\text{ mg}}{10.0\text{ mg/mL}} = 7.0\text{ mL}$$

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3. Step-by-Step Jar Testing Protocol

1. Raw Water Sampling & Characterization: Collect a fresh $15\text{--}20\text{ L}$ grab sample of raw surface water. Measure and record baseline water quality parameters: temperature ($^\circ\text{C}$), turbidity (NTU), pH, total alkalinity ($\text{mg/L as }\text{CaCO}_3$), and UV-254 absorbance ($\text{cm}^{-1}$). Do not allow the sample to warm up to room temperature, as temperature directly alters water viscosity and coagulation kinetics.
2. Rapid Mix Stage: Fill six $2.0\text{ L}$ square jars with exactly $2.0\text{ L}$ of raw water. Lower paddles and start rapid mix at 100 to 120 RPM (velocity gradient $G \approx 300\text{--}1000\text{ s}^{-1}$). Simultaneously inject the incremental coagulant doses into jars 1 through 6 using calibrated syringes (e.g., Jar 1: Control $0\text{ mg/L}$; Jar 2: $10\text{ mg/L}$; Jar 3: $20\text{ mg/L}$; Jar 4: $30\text{ mg/L}$; Jar 5: $40\text{ mg/L}$; Jar 6: $50\text{ mg/L}$). Maintain rapid mix for exactly 1 minute (60 seconds).
3. Tapered Flocculation Stage: Immediately reduce paddle speeds to simulate the diminishing velocity gradients of plant flocculation stages:
   • Step 1 (High Energy): $50\text{ RPM}$ for 5 minutes (promotes initial microfloc particle collisions).
   • Step 2 (Medium Energy): $30\text{ RPM}$ for 10 minutes (builds macrofloc bridges).
   • Step 3 (Low Energy): $15\text{ RPM}$ for 5 minutes (floc maturation and compaction without hydraulic shear).
   • Total Flocculation Time: $20\text{ minutes}$.
4. Quiescent Settling Stage: Stop the motor, raise the paddles smoothly above the water level without agitating the vessels, and allow quiescent settling for 15 to 30 minutes.
5. Observation & Supernatant Sampling:
   • Visual Observations: Note the time of initial pin-floc formation (typically $< 1\text{--}2\text{ minutes}$ for ideal doses), floc size (fine, pin-point, medium, or large feathery floc), and settling velocity (rate of floc blanket drop in inches per minute; $> 2.0\text{--}4.0\text{ in/min}$ indicates excellent settling).
   • Supernatant Analytical Testing: Open the sampling stopcock ($2.0\text{ cm}$ sub-surface) and discard the first $20\text{ mL}$ line purge. Collect $150\text{ mL}$ of supernatant and measure settled turbidity (NTU), residual pH, residual alkalinity, and UV-254 absorbance.

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4. Interpreting Results & The Risks of Overdosing

Supernatant Turbidity (NTU)
   ^
   |  Underdosing Zone            Optimum Dosing Zone          Overdosing / Restabilization Zone
   |  (Incomplete Destabilization)| (Charge Neutralization)   | (Charge Reversal & Metal Precipitate Rise)
   |
   |  \                                                       /
   |   \                                                     /
   |    \                                                   /
   |     \_________________________________________________/
   +--------------------------------------------------------------------------------------------->
     0     10     20     30     40     50     60     70     80     90    100    Coagulant Dose (mg/L)
                                [ Optimum Dose: 30-40 mg/L ]

Identifying the True Optimum Coagulant Dose

The optimum coagulant dosage is the lowest chemical dose that:

1. Consistently achieves target settled water turbidity (typically $\le 1.0\text{--}2.0\text{ NTU}$, ensuring finished filtered water will meet the $\le 0.30\text{ NTU}$ standard);
2. Produces dense, rapidly settling floc ($> 2\text{ in/min}$) within the allocated basin detention time;
3. Satisfies mandatory TOC removal requirements under EPA Stage 1 DBPR Enhanced Coagulation; and
4. Leaves adequate buffering alkalinity in the finished water (minimum $\ge 20\text{--}30\text{ mg/L as }\text{CaCO}_3$) to prevent corrosive finished water.

Consequences of Coagulant Overdosing

• Charge Reversal & Colloidal Restabilization: Excess positive coagulant cations saturate colloidal surfaces, reversing particle charge from negative to positive. Particles re-disperse, causing settled turbidity to rise sharply.
• Alkalinity Depletion & pH Crash: Every $1.0\text{ mg/L}$ of commercial alum consumes approximately $0.50\text{ mg/L}$ of natural alkalinity (as $\text{CaCO}_3$). Severe overdosing consumes all buffering capacity, dropping process pH below $5.5$.
• Dissolved Metal Breakthrough: At $\text{pH } < 5.8$, aluminum solubility increases exponentially, passing soluble aluminum through media filters into the distribution system (causing post-precipitation turbidity, consumer complaints, and water heater scaling).
• Excessive Sludge Volume: Generates massive volumes of gelatinous, difficult-to-dewater metal hydroxide sludge, overwhelming solids handling facilities.

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5. Streaming Current Detectors (SCD / SCM) for Online Control

While bench jar testing provides vital process optimization data, it is a batch test that cannot respond instantaneously to sudden raw water flash-turbidity events. An online Streaming Current Detector (SCD) (or Streaming Current Monitor, SCM) provides continuous, real-time electrokinetic measurement of colloidal charge immediately downstream of chemical injection.

                       [ Sample Water from Rapid Mix Effluent ]
                                           |
                                           v
                      +-----------------------------------------+
                      |  SCD Annular Cylinder / Sample Chamber  |
                      |                                         |
                      |  +-----------------------------------+  |
                      |  | Reciprocating Piston (Oscillating)|  |
                      |  +-----------------------------------+  |
                      |                                         |
                      |  Electrostatic Counter-Ions Sheared Off | 
                      |  From Cylinder Wall During Fluid Motion | 
                      +-----------------------------------------+
                                           |
                                           v
                      [ Electrodes Capture Displaced Charges ]
                                           |
                                           v (Alternating Current Signal)
                      [ Microprocessor Amplifies & Rectifies ]
                                           |
                                           v
                      [ Digital Display: Streaming Current Units (SCU) ]
                                           |
                                           v (4-20 mA Feedback Loop)
                      [ Coagulant Chemical Metering Pump Adjustment    ]

Operating Principle of the Streaming Current Detector

1. Sample water from the rapid mix basin effluent is continuously pumped through the SCD sensing chamber.
2. Negatively charged colloidal particles temporarily adhere to the stationary walls of an annular Teflon cylinder.
3. A motorized Teflon piston reciprocates up and down inside the cylinder at a high frequency ($4\text{ to } 5\text{ cycles/second}$). As the piston moves, it creates high fluid shear velocities along the cylinder walls, stripping mobile positive counter-ions away from the adsorbed colloids.
4. This continuous displacement of electrical charges generates a minute alternating electrical current (streaming current, microamperes) between two electrodes located at opposite ends of the cylinder.
5. The instrument amplifies and rectifies this signal, displaying a dimensionless output in Streaming Current Units (SCU) (typically spanning $-100\text{ to } +100\text{ SCU}$).

Closed-Loop Chemical Feed Control & Calibration

• Direct Correlation with Zeta Potential: Streaming current is directly proportional to the zeta potential of coagulated water. Negative SCU readings indicate underdosing (incomplete charge neutralization), while positive SCU readings indicate overdosing (charge reversal).
• Automated Feedback Control: The SCD output ($4\text{--}20\text{ mA}$) interfaces directly with the plant supervisory control and data acquisition (SCADA) system and coagulant chemical metering pumps. If raw water turbidity spikes, the streaming current drops negative, prompting the PID controller to automatically increase coagulant pump speed until the process returns to its established setpoint.
• Calibration Against Jar Tests: The SCD is an empirical indicator, not an absolute meter. Operators must establish the target SCD setpoint by conducting a bench-scale jar test, determining the optimal dose, and setting the SCD controller to maintain the streaming current value produced by that optimal dose.