How to Commission Flocculant and Coagulant Dosing Systems as One Treatment System

Commissioning should begin at the storage area, not at the injection point.

Verify the recommended storage-temperature range for each product. Protect chemicals from freezing, excessive heat, direct sunlight and contamination. The exact acceptable range must follow the product TDS and SDS.

Temperature changes can affect:

• Product viscosity
• Pump suction performance
• Emulsion stability
• Flowmeter accuracy
• Chemical concentration
• Shelf life

The storage tank should also be inspected for sediment, phase separation, water contamination and incompatible residues from previously stored chemicals.

Before changing products, confirm whether the tank and transfer lines require cleaning. Never assume that two cationic products, two coagulants or two PAM emulsions are automatically compatible when mixed in concentrated form.

The storage tank, pump head, seals, valves, tubing, flowmeter and injection quill must be compatible with the concentrated product and any prepared solution.

Material selection should be confirmed against the chemical supplier’s documentation and the equipment manufacturer’s compatibility data.

Check:

• Tank construction material
• Pump diaphragm or hose material
• Valve and seal materials
• Flowmeter wetted parts
• Flexible tubing
• Static-mixer elements
• Injection fittings
• Flush-water connections

Material compatibility is not only a corrosion issue. Swelling seals, softened tubing or damaged diaphragms can gradually change the delivered flow even before visible leakage occurs.

A dosing pump should operate inside a controllable part of its working range during both minimum and maximum plant loads.

A pump selected mainly for maximum capacity may become inaccurate when operated close to its minimum output. This is especially important for concentrated polymer emulsions, high-activity coagulants and plants with large changes in flow.

Review:

• Minimum required chemical flow
• Normal operating flow
• Maximum required flow
• Pump turndown ratio
• Stroke frequency or motor-speed limits
• Discharge pressure
• Suction conditions
• Expected product viscosity
• Flowmeter measuring range

The normal dosing requirement should not sit at the extreme bottom of the pump or flowmeter range.

If the process requires very low neat-product flow, controlled dilution may improve distribution and measurement. However, dilution does not change the required active-product dose. Operators must clearly distinguish between neat-product flow, diluted-solution flow and active chemical dosage.

Dilution water is a process input, not simply a utility connection.

The system must provide sufficient and stable:

• Flow
• Pressure
• Temperature
• Cleanliness
• Water quality

Rapid pressure changes can alter polymer solution strength even when the neat-product pump appears stable. A partially blocked strainer, fluctuating header pressure or incorrectly sized control valve may cause unstable preparation.

For PAM products, the water chemistry may also influence activation. Excessive hardness, salinity, suspended solids or residual oxidants can reduce preparation consistency in some applications.

During commissioning, record the actual dilution-water pressure and flow under both minimum and maximum plant demand.

The mixer must disperse and activate the chemical without creating unnecessary shear.

For liquid coagulants, the objective is usually rapid distribution into the process stream. For PAM preparation, the objective is controlled chain activation before the solution reaches the treatment process.

Inspect:

• Initial wetting or emulsion inversion point
• Static-mixer size and condition
• Mechanical mixer speed
• Tank geometry
• Dead zones
• Baffles
• Maturation time
• Tank turnover rate
• Prepared-solution transfer system
• High-shear restrictions or valves

Too little mixing can produce uneven solution concentration. Too much mixing can damage activated polymer chains or break formed floc.

Preparation tanks should also operate with a defined turnover cycle. Holding prepared polymer for an uncontrolled period can lead to viscosity loss, contamination or inconsistent performance.

The pump setting shown on a control screen should not be accepted as the actual flow without field verification.

Calibrate the neat-product flow and dilution-water flow independently under normal discharge pressure. Use a suitable calibration column, weighing method, verified flowmeter or another safe site procedure.

For each calibration point, record:

• Pump setting
• Measured volume or mass
• Calibration duration
• Product temperature
• Suction-tank level
• Discharge pressure
• Dilution-water flow
• Final solution concentration

Calibration should cover the expected operating range rather than a single point.

A pump can be accurate at 80 percent output and inaccurate at 10 percent output. Multi-point calibration helps identify this limitation before the plant relies on automatic dose control.

Chemical dosage should be documented using clearly defined units.

Common bases include:

• Milligrams of commercial product per liter of water
• Kilograms of commercial product per cubic meter
• Grams of active ingredient per kilogram of dry solids
• Kilograms of product per ton of dry solids

The site should state whether a dose refers to commercial product, diluted solution or active content.

For flow-paced dosing:

The units and density corrections must be defined in the operating procedure.

For sludge dewatering, mining, paper or other variable-solids applications, water flow alone may be insufficient. Dose control should preferably consider dry-solids loading:

The correct product delivered to the wrong location can still perform poorly.

Coagulants generally require sufficient mixing before the flocculant is added. The polymer injection point should provide enough distribution for particle contact but avoid excessive downstream shear.

Check:

• Distance between coagulant and flocculant injection
• Available reaction time
• Mixer energy at each point
• pH-adjustment location
• Pipe velocity
• Bends, pumps and control valves downstream
• Separation equipment inlet conditions

An injection quill should place the chemical into an active part of the flow. Wall injection into a low-velocity zone can cause localized overdosing, deposits and poor cross-sectional mixing.

The jar-test sequence should be translated into realistic plant contact times and mixing conditions.

Automatic dosing is only effective when it follows the variable that actually drives chemical demand.

Possible control signals include:

• Treatment flow
• Sludge flow
• Dry-solids load
• Turbidity
• Color
• Conductivity
• Streaming current
• Upstream production rate
• Combined feed-forward and feedback control

Flow pacing is often the starting point. When pollutant or solids concentrations vary independently of water flow, additional load information may be necessary.

Online feedback should not replace laboratory verification. Sensors require cleaning, calibration and operating limits to prevent an incorrect reading from automatically driving the system toward severe overfeeding or underfeeding.

Final commissioning should include more than one stable operating condition.

Test the system at:

• Low normal load
• Typical load
• High normal load
• Expected product concentration
• Normal dilution-water pressure range
• Representative seasonal temperature

At each condition, confirm:

• Actual neat-product flow
• Actual dilution-water flow
• Prepared-solution concentration
• Mixing and maturation time
• Process pH
• Floc size and strength
• Clarified-water quality
• Sludge settling or dewatering performance
• Chemical consumption per treated volume or dry-solid load

Short tests can be misleading when chemical residence time is long. Allow enough time for a dosage change to travel from the preparation system to the sampling point.