7.7 Mixing, Loading & Handling Operations

7.7 Mixing, Loading & Handling Operations

Pesticide mixing and loading operations constitute the most hazardous phase of pesticide handling. During mixing, handlers work directly with undiluted, highly concentrated chemical formulations. A spill or splash at the mixing site exposes the handler to toxic concentrations far exceeding field application spray mixtures and presents severe potential for localized environmental contamination.

Personal Protective Equipment (PPE) for Mixing & Loading

Pesticide container labels specify mandatory PPE for mixing and loading, which is typically more stringent than PPE required for application:

┌─────────────────────────────────────────────────────────────────────────┐
│                     MANDATORY MIXING & LOADING PPE                      │
└────────────────────────────────────┬────────────────────────────────────┘
                                     │
     ┌────────────────┬──────────────┼──────────────┬────────────────┐
     ▼                ▼              ▼              ▼                ▼
┌──────────┐    ┌──────────┐    ┌──────────┐   ┌──────────┐    ┌───────────┐
│ Chemical │    │ Heavy    │    │ Chemical │   │Protective│    │Respirator │
│ Apron    │    │ Nitrile  │    │ Boots    │   │ Eyewear  │    │(If labeled│
└──────────┘    └──────────┘    └──────────┘   └──────────┘    └───────────┘

• Heavy-Duty Chemical Apron: Made of rubber, neoprene, or PVC covering the chest down to the boots to block spills when opening containers or pouring concentrates.
• Unlined Chemical-Resistant Gloves: Heavy-duty nitrile, neoprene, or butyl rubber gloves. Gloves must never have cloth linings, which trap liquid chemicals against the skin.
• Eye Protection: Chemical splash-proof goggles or a full-face shield to protect against accidental splashing during container opening and pouring.
• Chemical-Resistant Footwear: Unlined rubber or neoprene boots worn with pant legs outside the boots to prevent chemical splashes from running down into the footwear.

Tank Mixing Sequence: The W-A-L-E-S Rule

When tank-mixing multiple pesticide products or adding liquid fertilizers and adjuvants, adding formulations in the wrong order can cause severe physical incompatibility—resulting in chemical curdling, heavy sludge formation, phase separation, or tank-bottom precipitation that ruins expensive chemicals and clogs spray equipment.

Applicators must strictly follow the industry-standard W-A-L-E-S sequence:

┌─────────────────────────────────────────────────────────────────────────┐
│                      THE W-A-L-E-S MIXING SEQUENCE                      │
└────────────────────────────────────┬────────────────────────────────────┘
                                     │
     ┌────────────────┬──────────────┼──────────────┬────────────────┐
     ▼                ▼              ▼              ▼                ▼
┌──────────┐    ┌──────────┐    ┌──────────┐   ┌──────────┐    ┌───────────┐
│    W     │    │    A     │    │    L     │   │    E     │    │     S     │
│  WATER   │    │ AGITATE  │    │ LIQUID   │   │EMULSIFI- │    │SURFACTANTS│
│ CARRIER  │    │ DRY PROD │    │FLOWABLES │   │   ABLE   │    │ ADJUVANTS │
└──────────┘    └──────────┘    └──────────┘   └──────────┘    └───────────┘

1. W — Water Carrier: Fill the spray tank 1/4 to 1/2 full with clean water (or liquid fertilizer carrier) and start the tank agitation system.
2. A — Agitate Dry Products: Add dry formulations—such as Wettable Powders (WP), Water-Dispersible Granules (WDG), and Dry Flowables (DF). Allow full agitation until dry products are completely suspended before adding other formulations.
3. L — Liquid Flowables: Add liquid flowable suspensions—including Flowables (F), Liquid Suspensions (L), and Suspension Concentrates (SC).
4. E — Emulsifiable Concentrates: Add petroleum-based liquid formulations—specifically Emulsifiable Concentrates (EC), Solutions (S), and Soluble Powders (SP).
5. S — Surfactants & Adjuvants: Add non-ionic surfactants, crop oil concentrates (COC), drift retardants, and defoamers. Finally, top off the spray tank with the remaining water volume to achieve the full target dilution.

Physical Compatibility & The Jar Test Procedure

Before mixing unknown product combinations in a 500-gallon sprayer tank, perform a small-scale Jar Compatibility Test:

Jar Test Protocol

1. Wear eye protection and chemical-resistant gloves.
2. Fill a clean 1-quart glass jar half-full with water from the actual spray carrier source.
3. Add chemical components one by one in proportional amounts following the W-A-L-E-S sequence (e.g., 1 teaspoon of dry product per pound per acre label rate; 1 teaspoon of liquid product per pint per acre rate).
4. Cap the jar tightly, invert 10 times, and let stand for 15 to 30 minutes.

Evaluating Compatibility

• Compatible: The mixture remains a uniform liquid suspension or emulsion without clumping or separation.
• Incompatible: The mixture exhibits phase separation (oil layer floating top), heavy sludge or paste precipitating at the bottom, gel formation, clumping ("cottage cheese" texture), or unexpected heat generation. Incompatible mixtures must never be loaded into spray tanks!

Protecting Water Sources: Back-Siphonage Prevention

Filling spray tanks directly from surface water bodies (ponds, streams) or municipal water supplies carries severe risk of back-siphonage. If supply line pressure drops suddenly while the filling hose is submerged below the tank liquid level, liquid chemical can be siphoned backward through the supply line into the water source.

                  ┌───────────────────────────────────────────────┐
                  │    BACK-SIPHONAGE PREVENTION MECHANISMS       │
                  └───────────────────────┬───────────────────────┘
                                          │
                  ┌───────────────────────┴───────────────────────┐
                  ▼                                               ▼
      ┌──────────────────────┐                        ┌──────────────────────┐
      │   PHYSICAL AIR GAP   │                        │ MECHANICAL PREVENTER │
      └───────────┬──────────┘                        └───────────┬──────────┘
                  │                                               │
      • Unobstructed vertical space                   • Reduced Pressure Zone (RPZ)
      • Distance >= 2x Pipe Diameter                  • Double Check Valve Assembly
      • Absolute minimum 1-inch gap                   • Installed on fill supply line

1. Physical Air Gap: Maintain an unobstructed vertical physical space between the lowest end of the water supply pipe/hose and the top flood-level rim of the spray tank. The air gap distance must be at least twice the inside diameter of the supply pipe, and never less than 1 inch.
2. Mechanical Backflow Preventers: Install an approved Reduced Pressure Zone (RPZ) backflow preventer or double check valve assembly on water supply lines where an air gap cannot be maintained.

Container Rinsing & Equipment Clean-Up Protocols

Empty Container Rinsing Standards

Empty pesticide containers must be cleaned immediately upon emptying. Unrinsed containers sent to landfills or recyclers constitute hazardous waste violations.

• Triple-Rinsing Procedure:
  1. Empty remaining product concentrate into the spray tank; allow container to drain for 30 seconds.
  2. Fill container 1/4 full with clean water.
  3. Replace cap tightly; shake and rotate container vigorously for 30 seconds.
  4. Pour rinsate into the spray tank; drain for 30 seconds.
  5. Repeat steps 2–4 two additional times (total 3 rinses).
  6. Puncture container bottom or sidewall to prevent reuse; send to an approved agricultural plastics recycling program.
• Pressure-Rinsing: Use a specialized high-pressure nozzle probe to spray the inner container surfaces for at least 30 seconds, draining rinsate into the spray tank.

Sprayer Tank Decontamination & Washdown

To avoid cross-crop injury (such as phenoxy or sulfonylurea herbicide residues injuring sensitive broadleaf crops), sprayers must undergo thorough decontamination:

1. Drain spray tank and flush system with clean water in an approved wash pad area.
2. Fill tank half-full with water, add label-recommended tank cleaning agents (e.g., commercial tank cleaner, household ammonia, or detergent), and circulate through all boom lines and hoses for 15 minutes.
3. Flush solution through nozzles and drain.
4. Remove and clean all nozzle tips, strainers, and inline filters with a soft brush (never use metal pins or mouth blowing).
5. Perform a final rinse with clean water.