How to Size a Chemical Dosing System — Pumps, Tanks, and Mixing Design

A properly designed chemical dosing system is essential for reliable and efficient water and wastewater treatment. Whether you’re dosing polyaluminum chloride (PAC) for coagulation, Polyacrylamide (PAM) for flocculation, or any other treatment chemical, the design of the dosing system directly impacts process performance, chemical consumption, and operational costs. An undersized system limits treatment capacity, while an oversized system wastes capital and may have operational issues from operating too far from design conditions.

This article provides a comprehensive guide to sizing chemical dosing systems, covering pump selection, tank design, mixing requirements, and practical considerations for PAC and PAM systems.

Components of a Chemical Dosing System

A typical chemical dosing system consists of several key components, each serving a specific function in the process of storing, preparing, and delivering treatment chemicals to the water stream.

Component	Function	Key Design Considerations
Chemical storage tank	Bulk storage of raw chemical	Material compatibility, size, secondary containment
Day tank / makeup tank	Working volume of prepared chemical solution	Sizing based on daily usage, mixing
Dosing pump	Metered delivery of chemical to process	Flow rate, pressure, accuracy, type
Mixer / agitator	Dissolving and keeping chemicals in solution	Mixing intensity, type, placement
Piping and valves	Transporting chemical between components	Material, sizing, isolation points
Control system	Regulating dosing rate	Manual, timer-based, flow-proportional, feedback
Safety features	Protecting personnel and equipment	Leak detection, overflow, pressure relief

Step 1: Define Design Parameters

Before sizing any component, you need to define the key design parameters for your specific application.

Flow Rate and Chemical Dosage

Start by determining:

• Design flow rate: The maximum water flow to be treated (m³/h, MGD, or gpm). Include a safety factor of 10-20% for peak conditions.
• Chemical dosage: The required dosage of active chemical in mg/L or ppm. This should be determined through jar testing or pilot studies.
• Chemical solution strength: The concentration of the working solution to be dosed. This varies by chemical type and application.

Basic Dosing Calculation

The fundamental equation for calculating chemical feed rate is:

Feed Rate (L/h) = (Flow Rate (m³/h) × Dosage (mg/L)) / (Solution Concentration (mg/L) × 1000)

Or in US units:

Feed Rate (gpd) = (Flow (MGD) × Dosage (mg/L) × 8.34) / (Solution % × 8.34 × 100)

For example, if you need to dose PAC at 50 mg/L into a flow of 500 m³/h using a 10% PAC solution (approximately 100,000 mg/L):

Feed Rate = (500 × 50) / (100,000 × 1000) × 1,000,000 = 250 L/h

Understanding MOQ and pricing tiers for the chemicals you’ll be using can help optimize tank sizing for cost-effective procurement.

Step 2: Dosing Pump Sizing and Selection

The dosing pump is the heart of the chemical feed system. Selecting the right pump type and size is critical for accurate, reliable chemical delivery.

Pump Types

• Diaphragm metering pumps: The most common type for water treatment chemical dosing. Available in mechanical and hydraulic configurations. Excellent accuracy (±1-2% of setpoint), good for low to medium flow rates, and compatible with many chemicals.
• Peristaltic (hose) pumps: Good for viscous fluids, slurries, and shear-sensitive chemicals. The chemical only contacts the hose, making them ideal for aggressive or high-purity applications. Lower pressure capability and periodic hose replacement required.
• Progressive cavity pumps: Used for high-viscosity fluids and high flow rates. Good for emulsion polymers and slurry applications.
• Centrifugal pumps: Not typically used for metering but may be used for bulk transfer. Not suitable for precise dosing applications.

Pump Sizing Guidelines

When sizing a dosing pump, follow these principles:

• Design point at 50-70% of pump capacity: Pumps operate most accurately and reliably in the middle of their range. Sizing so the normal operating point is at 50-70% of the pump’s maximum capacity provides turndown flexibility for both higher and lower dosing requirements.
• Maximum flow capacity: The pump should be able to deliver at least 150% of the calculated maximum dosing requirement to handle peak conditions, water quality variations, and future capacity increases.
• Pressure rating: The pump must be rated for at least 125% of the maximum discharge pressure at the injection point. Account for line losses and elevation changes.
• Turndown ratio: Consider the range of dosing rates you’ll need. Diaphragm pumps typically offer 10:1 to 100:1 turndown depending on the control method.

Multiple Pump Configurations

For critical applications or systems with wide flow variations, consider installing multiple pumps in parallel:

• Duty + standby: One pump operating, one pump on standby for reliability
• Lead + lag: Multiple pumps that stage on/off to handle wide flow ranges
• Trim pump: A small pump for low-flow conditions combined with larger pumps for high flow

Step 3: Storage and Day Tank Design

Proper tank sizing ensures you have enough chemical on hand while minimizing capital cost and the risk of degradation or contamination.

Bulk Storage Tanks

Bulk storage tanks hold the raw chemical as delivered. Sizing depends on:

• Consumption rate: Daily chemical usage at average and peak conditions
• Delivery frequency: How often deliveries can be made (weekly, bi-weekly, monthly)
• Safety stock: Extra capacity for supply disruptions, typically 7-14 days
• Shipping economics: Order quantities that optimize shipping costs and unit pricing

A common guideline is to size bulk storage for 30 days of average consumption plus 7-10 days safety stock. For liquid PAC, typical storage volumes range from a few hundred gallons for small systems to tens of thousands of gallons for large municipal plants.

Day Tanks / Makeup Tanks

Day tanks hold the working solution that is fed by the dosing pumps. For liquid products like liquid PAC, the day tank may simply be a smaller tank fed from bulk storage. For dry products like powdered PAC or PAM, the day tank is where the chemical is dissolved and mixed to the working concentration.

Day tank sizing guidelines:

• Liquid chemicals (PAC, etc.): 4-24 hours of maximum dosing. Enough to provide surge capacity and allow bulk delivery without interruption.
• Dry PAM solutions: 8-24 hours of maximum dosing. PAM solutions have limited shelf life once mixed (typically 24-72 hours for anionic, less for cationic).
• Multiple tanks: Consider two or more day tanks to allow one to be mixed while the other is in service, especially for PAM systems.

For PAM systems in particular, proper aging time in the day tank is critical. PAM molecules need time to fully hydrate before dosing — typically 30-60 minutes for emulsions and 1-2 hours for powders. The PAM molecular weight and charge density affect hydration time and required mixing intensity.

Tank Materials

Select tank materials that are compatible with the chemical being stored:

• Liquid PAC: HDPE, fiberglass (FRP), rubber-lined steel. PAC is acidic and corrosive to many metals.
• PAM solutions: HDPE, fiberglass, stainless steel 316L. PAM is generally non-corrosive.
• Ferric chloride: Rubber-lined steel, HDPE. Highly corrosive.

Always check chemical compatibility charts and consult with the chemical supplier before specifying tank materials. Our article on PAC shelf life and storage provides additional guidance on proper storage conditions.

Step 4: Mixing System Design

Proper mixing is essential for dissolving dry chemicals, maintaining uniform solutions, and preventing stratification or settling in tanks.

Mixers for Dissolving Dry Chemicals

When dissolving powdered PAC or PAM, the mixing system must:

• Create a vortex or eductor to draw powder into the water without clumping (fish eyes)
• Provide sufficient energy to wet all particles thoroughly
• Avoid excessive shear that could degrade polymer molecules (for PAM)

PAM is particularly sensitive to mixing conditions. Too much shear can break polymer chains, reducing molecular weight and effectiveness. Too little mixing results in incomplete dissolution and “grease balls” of undissolved polymer.

Tank Mixer Sizing

Mixer sizing depends on the tank volume, the chemical being mixed, and the purpose (dissolving vs. maintaining uniformity):

• For maintaining uniformity (liquid chemicals): 0.1-0.3 kW per 1000 liters (0.0004-0.001 hp per gallon)
• For dissolving dry chemicals: 0.5-2.0 kW per 1000 liters (0.002-0.008 hp per gallon)
• For polymer solutions: Moderate intensity — enough to dissolve without degrading. Typically 0.3-1.0 kW per 1000 liters.

Impeller selection is also important. Pitched-blade turbines are commonly used for general mixing, while high-shear impellers may be used for initial powder wetting (with caution for PAM). For more information on mixing theory and design parameters, see our article on rapid mix vs slow mix design.

Step 5: PAC and PAM Dosing System Considerations

PAC Dosing Systems

PAC dosing systems are relatively straightforward because PAC is typically supplied as a liquid solution (10-18% Al2O3) that is ready to use. Key design considerations include:

• Dilution: PAC may be dosed neat or diluted with water. Dilution can improve distribution and mixing at the injection point, but is not always necessary.
• Materials: Use corrosion-resistant materials — PVC, HDPE, or PVDF piping; EPDM or Viton seals; plastic or rubber-lined tanks.
• Injection point: Inject before the rapid mix basin or into a pipeline with adequate turbulence for mixing.

Learn more about polyaluminum chloride products and available grades on our product page.

PAM Dosing Systems

PAM dosing systems are more complex because PAM must be properly dissolved and aged before use. Key considerations include:

• Makedown system: PAM must be carefully mixed to avoid lumps. Use a wetting eductor or vortex mixer.
• Aging time: Allow 30-120 minutes for polymer hydration before use. Exact time depends on polymer type (emulsion vs powder), molecular weight, and water temperature.
• Solution concentration: Typical working concentrations are 0.1-0.5% for anionic PAM and 0.5-1.0% for cationic PAM.
• Pump selection: Progressive cavity or peristaltic pumps are often preferred for PAM solutions, especially higher viscosity solutions. Diaphragm pumps work well with dilute solutions.

When deciding between powder vs emulsion PAM, consider the impact on the dosing system design. Emulsion PAM dissolves faster but requires different mixing equipment and has shorter solution shelf life.

Step 6: Control Strategy

The control strategy determines how the dosing rate is adjusted to maintain treatment performance as flow and water quality change.

Control Options

• Manual control: Operator adjusts pump stroke or speed based on lab results. Simple but slow to respond to changes. Suitable for stable systems with consistent water quality.
• Flow-proportional control: Dosing rate automatically adjusts based on incoming flow rate. Chemical dosage (mg/L) stays constant as flow changes. Common and effective for flow variations.
• Feedback control: Uses a process parameter (turbidity, streaming current, pH) to adjust dosage automatically. Can compensate for both flow and water quality changes.
• Feedforward + feedback: Combines flow-proportional control with trim based on effluent quality. The most sophisticated and effective approach for variable conditions.

Frequently Asked Questions

How do I calculate the required chemical feed rate?

To calculate chemical feed rate, use the formula: Feed Rate (L/h) = (Flow Rate (m³/h) × Dosage (mg/L)) / (Solution Concentration (mg/L)). First determine your design flow rate, the required chemical dosage (from jar testing), and the concentration of your working solution. Always include a safety factor of 20-50% above the calculated maximum to handle variations in water quality and peak flow conditions.

What size dosing pump do I need?

Size your dosing pump so that your normal operating point falls at 50-70% of the pump’s maximum capacity. This provides good accuracy and flexibility for both higher and lower dosing requirements. The pump should be able to deliver at least 150% of your calculated maximum dosing rate to handle peak conditions. Also ensure the pump’s pressure rating exceeds the maximum discharge pressure by at least 25%. Diaphragm metering pumps are the most common choice for water treatment chemical dosing.

How large should my chemical storage tank be?

Size bulk chemical storage based on your consumption rate, delivery frequency, and desired safety stock. A common guideline is 30 days of average consumption plus 7-10 days of safety stock as a buffer against supply disruptions. For day tanks (working solution tanks), size for 4-24 hours of maximum dosing for liquid chemicals, and 8-24 hours for PAM solutions (considering solution shelf life). Always consider the economics of larger order quantities vs. tank capital cost.

What type of mixer do I need for PAM makeup?

PAM makeup requires careful mixing — enough to fully dissolve the polymer without degrading it through excessive shear. For initial powder wetting, use an eductor or create a gentle vortex in the makeup tank. For mixing during aging, use moderate intensity mixing (approximately 0.3-1.0 kW per 1000 liters). Avoid high-shear mixers that can break polymer chains. The mixer should be sized to keep the solution uniform without creating excessive turbulence at the surface. Two-speed mixers — high for makeup, low for holding — are ideal.

What is the best control strategy for chemical dosing?

The best control strategy depends on your specific application. For systems with relatively stable water quality, flow-proportional control (dosing rate adjusts automatically with flow) is usually sufficient and cost-effective. For systems with variable water quality, feedback control using streaming current or effluent turbidity provides better performance. The most robust approach is feedforward + feedback control, which adjusts for flow changes immediately and trims based on actual treatment results. Consider the complexity, cost, and expected variability when choosing a control strategy.

What materials are compatible with PAC?

Polyaluminum chloride (PAC) is acidic and corrosive to most metals. Compatible materials for tanks and piping include HDPE, fiberglass-reinforced plastic (FRP), PVC, CPVC, and PVDF. For seals and gaskets, EPDM and Viton are generally compatible. Stainless steel should be used with caution — 316L may be acceptable for some grades but check with the chemical supplier. Always verify material compatibility with your specific PAC product and concentration before specifying equipment.