12.1 Dry Chemical Feeders, Gravimetric/Volumetric Systems & Slakers

Bulk Dry Chemical Handling in Water Treatment

Municipal water treatment facilities rely on bulk dry chemicals for coagulation, pH adjustment, softening, taste and odor control, and sludge conditioning. Common dry chemicals include aluminum sulfate (alum), hydrated lime ($Ca(OH)_2$), soda ash ($Na_2CO_3$), potassium permanganate ($KMnO_4$), powdered activated carbon (PAC), and dry coagulant aid polymers.

Storing and conveying dry chemicals presents distinct physical challenges. Unlike liquids, dry powders and granules vary widely in particle size, hygroscopic tendencies (moisture absorption from ambient air), and flowability:

• Hygroscopic Chemicals: Hydrated lime, soda ash, and dry polymers absorb ambient atmospheric humidity, leading to caking, crusting, and clumping that obstruct feeder hoppers and feed throats.
• Dust and Safety Hazards: PAC produces fine, highly combustible carbon dust that presents an explosion hazard if suspended in air near ignition sources; explosion-proof motors and electrical fixtures (Class II, Division 1) are required. Potassium permanganate is a powerful National Fire Protection Association (NFPA) Class 2 oxidizer that accelerates combustion of organic materials. Hydrated lime is a severe respiratory irritant and caustic dust hazard requiring dedicated personal protective equipment (PPE).
• Arching (Bridging) and Rat-Holing: In storage bins and hoppers, fine powders interlock into a structural bridge over the discharge opening (arching), completely halting chemical flow, or form a narrow vertical pipe where only the material directly above the orifice discharges (rat-holing), leaving the surrounding bulk inventory stagnant.

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Volumetric Dry Chemical Feeders

Operating Principle and Mechanical Configurations

A volumetric dry feeder discharges a constant physical volume of dry chemical per unit of operating time. The mechanism delivers chemical at a controlled rate governed by drive speed or displacement volume:

1. Screw (Auger) Feeders: The most widely utilized volumetric configuration. A motorized rotating helical screw (single or twin auger; open-helix or solid-shaft flighting) draws dry material from the base of a conditioning hopper and propels it horizontally into a dissolution tank. Feed rate is adjusted by varying the screw drive motor RPM.
2. Rotating Disk Feeders: A horizontal rotating grooved plate positioned beneath the hopper discharge throat. An adjustable plow or scraper blade shears off a fixed depth of chemical as the disk turns, directing it into the solution tank. Rate adjustment occurs via disk rotation speed or scraper blade position.
3. Oscillating Hopper (Reciprocating Plate) Feeders: A grooved tray or feed shoe reciprocates back and forth beneath the hopper throat. Chemical drops off the edge on each forward and reverse stroke. Adjustments are made by altering stroke length or reciprocating frequency.
4. Roll Feeders: Utilize a rotating cylinder with a precision doctor blade or gate opening to meter uniform layers of granular material.

[ Volumetric Feeder Principle ]
Hopper -> [ Rotating Screw / Auger ] -> Measured Volume -> Dissolution Tank
* Feed rate governed strictly by Motor RPM or Stroke Opening
* Does NOT measure actual mass or weight

Inherent Limitations and Accuracy Errors

The fundamental limitation of volumetric feeding is that water treatment chemical stoichiometry depends strictly on mass (weight), whereas volumetric feeders deliver only volume. The actual weight delivered per unit time depends directly on the chemical's bulk density (pounds per cubic foot, $lb/ft^3$):

$\text{Mass Delivery Rate } (lb/hr) = \text{Volumetric Feed Rate } (ft^3/hr) \times \text{Bulk Density } (lb/ft^3)$

Bulk density is not constant. It fluctuates substantially due to:

• Fluidization and Aeration: When a bulk delivery tanker pneumatically blows dry powder into a storage silo, entrained air fluidizes the powder. The bulk density of hydrated lime can drop from $35–40\ lb/ft^3$ down to $20–25\ lb/ft^3$. A volumetric screw turning at a fixed RPM delivers the same volume but up to 40% less chemical mass, causing severe underdosing.
• Compaction: Material standing in a tall storage bin settles and consolidates under the weight of the column above it. Bottom layers become compacted, increasing bulk density and causing chemical overdosing.
• Moisture Absorption: High ambient humidity adds water weight while causing swelling and agglomeration.

Class II Exam Diagnostic: Standard volumetric dry feeders exhibit delivery errors of ±3% to ±5% under stable conditions, with errors climbing to ±10% or more following pneumatic filling or humidity swings. They cannot detect or correct for bulk density changes.

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Gravimetric Dry Chemical Feeders

Operating Principle and Precision Engineering

A gravimetric dry feeder measures and controls the chemical delivery based on actual weight (mass) rather than physical volume. By continuously monitoring the weight of chemical delivered and dynamically modulating drive speed, gravimetric feeders provide precision accuracy of ±0.5% to ±1.0%, automatically compensating for aeration, compaction, and moisture fluctuations.

[ Gravimetric Loss-in-Weight Feeder ]
+------------------------------------------------+
| Storage Hopper + Variable Speed Feeder         |
| Mounted entirely on Precision Load Cells       |
+------------------------------------------------+
                       |
                       v [Continuous Mass Loss Signal: lb/min]
             [ Microprocessor Controller ]
                       |
                       v [Modulates Screw Motor RPM to maintain setpoint]

Primary Gravimetric Configurations

1. Loss-in-Weight Feeders: The entire assembly—including storage hopper, variable-speed metering screw, and discharge nozzle—is suspended from or supported by precision strain-gauge load cells. The microprocessor controller reads the declining gross weight dozens of times per second. By taking the derivative of weight with respect to time ($dW/dt$), the controller calculates the instantaneous gravimetric delivery rate ($lb/min$ or $lb/hr$). If the rate drops below the setpoint (due to powder aeration), the controller instantly accelerates the feed screw to maintain exact mass flow.
   • Refill Cycle: When the hopper reaches a low-level threshold, an automated refill valve opens to replenish chemical from an overhead silo. During the 10-to-30-second refill, the feeder temporarily switches to "volumetric lock" mode, locking the screw speed at its pre-refill average until gross weight stabilizes.
2. Weigh-Belt Feeders: Chemical discharges from an agitated conditioning bin onto a moving endless conveyor belt. A section of the belt passes over a suspended weighing deck connected to a load cell. The controller continuously calculates mass flow by multiplying belt speed by the weight of chemical per foot of belt:

$\text{Mass Flow Rate } (lb/min) = \text{Belt Speed } (ft/min) \times \text{Belt Loading } (lb/ft)$

The controller modulates either belt travel speed or the height of an adjustable shear gate to maintain the target mass feed rate.

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Dry Feeder System Appurtenances

Reliable dry chemical feeding requires auxiliary equipment to maintain continuous material flow, manage fugitive dust, and dissolve solids before application.

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Quicklime Slaking Equipment & Thermodynamics

The Exothermic Hydration Reaction

Quicklime (calcium oxide, $CaO$) is manufactured by calcining limestone ($CaCO_3$) in high-temperature kilns, driving off carbon dioxide ($CaCO_3 + \text{heat} \rightarrow CaO + CO_2$). Before quicklime can be fed for pH adjustment, softening, or coagulation, it must undergo slaking—reacting pebble quicklime with water to produce slaked (hydrated) lime slurry ($Ca(OH)_2$), commonly called milk of lime:

$CaO + H_2O \rightarrow Ca(OH)_2 + \text{Heat} \uparrow$

$\Delta H = -65.3\text{ kJ/mol} \quad (\approx 490\text{ BTU per pound of pure } CaO)$

This reaction is violently exothermic. The heat released expands the solid pebble structure, shattering the lime particles into submicron hydrated crystals, producing a vast reactive surface area. The reaction requires a precise water-to-lime weight ratio (~3:1 to 4:1) to maintain the reaction temperature without boiling or drowning.

Slaker Equipment Types: Paste vs. Detention

Water utilities utilize two primary mechanical slaker designs, differentiated by water-to-lime stoichiometry, reaction viscosity, and detention kinetics:

[ Paste Slaker: ~2:1 Ratio ]           [ Detention Slaker: ~3.5:1 - 4:1 Ratio ]
Water:Lime = 2:1 by weight            Water:Lime = 3.5:1 to 4:1 by weight
Consistency: Thick viscous paste       Consistency: Fluid milk of lime slurry
Temperature: 170°F - 190°F             Temperature: 160°F - 180°F
Detention: ~5 minutes                  Detention: 10 - 15 minutes
Agitation: High-torque pug mill        Agitation: Turbine impellers with baffles

1. Paste Slakers: Operates at a tight water-to-lime ratio of ~2:1 to 2.5:1 by weight. The slaker produces a dense, high-viscosity paste. Two counter-rotating, high-torque pug-mill shafts thoroughly knead the paste. Because less water is heated, the reaction achieves elevated operating temperatures of 170°F to 190°F (77°C to 88°C) with a brief detention time of ~5 minutes. After slaking, dilution water is injected at the discharge weir to reduce the paste to a 10% to 15% milk of lime slurry.
2. Detention Slakers: Operates at a more dilute water-to-lime ratio of ~3.5:1 to 4:1 by weight, producing a fluid slurry directly in the reaction chamber. Slakers utilize vertical turbine impellers with tank baffles to keep solids in suspension. The operating temperature is maintained between 160°F and 180°F (71°C to 82°C) with a detention time of 10 to 15 minutes.

Critical Temperature Boundaries: Drowning vs. Boiling

Precise temperature regulation inside the slaker dictates the quality, reactivity, and safety of the slaked lime:

• Drowning (<140°F / 60°C): If excessive cold slaking water is admitted, or if incoming water is below 50°F (10°C), the water chills the lime faster than the exothermic reaction can generate heat. Below 140°F, hydration slows dramatically, producing a coarse, gritty, unreactive hydrate with poor settling and neutralization characteristics. Large quantities of unhydrated $CaO$ pass through unreacted and are discarded as waste.
• Optimal Reaction Window (160°F to 180°F / 71°C to 82°C): Yields the highest chemical reactivity, submicron crystal formation, and maximum solubility. Modern systems utilize tempered water systems or heat exchangers that capture heat from the discharging slurry to preheat incoming slaking water.
• Boiling and Steam Binding (>200°F / 93°C): If the water-to-lime ratio drops too low, or if slaking water is excessively hot, the slurry temperature approaches the boiling point (212°F). Water flashes to steam, causing eruptive boiling, foaming out of the slaker housing, vapor binding of drives, and hazardous steam expulsion. Steam binding can bake dry lime onto internal paddles and walls, causing drive motor overload.

Grit Removal and Classification

Commercial pebble quicklime contains 5% to 15% inert impurities, including uncalcined limestone core ($CaCO_3$), silica sand, and iron oxides. This non-reactive material is termed grit. If pumped into the plant piping, grit causes abrasive destruction of pump impellers, cuts valve seats, and clogs chemical feed lines.

Slakers incorporate an automated grit classifier: an inclined trough containing a slowly rotating helical screw or reciprocating rake mechanism. As milk of lime slurry overflows a discharge weir, heavy grit particles settle to the bottom of the classifier trough. The helical screw conveys the grit up the inclined dewatering ramp, where fresh spray water washes residual lime slurry back into the slaker. Dewatered grit discharges into a collection dumpster for landfill disposal.

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Comparative Technical References

Table 12.1.1: Comparison of Volumetric vs. Gravimetric Dry Chemical Feeders

Table 12.1.2: Lime Slaker Operating Parameters