The Problem Every Fab Shop Knows

If you run a metal fabrication shop or machine shop, you're generating wastewater. It's unavoidable. Cutting fluids break down. Coolant sumps get contaminated. Rinse water picks up metal fines, oils, and emulsions. And all of it has to go somewhere.

Here's what you can't do: dump it down the drain. Municipal wastewater treatment plants (POTWs) have strict limits on what you can discharge—especially when it comes to oils, suspended solids, and heavy metals. Violate those limits and you're looking at fines, permit issues, or worse.

So most shops do one of two things: pay a hauler to take away barrels and totes of liquid waste (expensive), or treat the water on-site to separate the solids and reduce volume before disposal.

That's where aluminum sulfate comes in.

How Aluminum Sulfate Works in Shop Wastewater

Aluminum sulfate (often called "alum" or "liquid alum") is a coagulant. When you add it to wastewater, it does two important things:

1. Charge Neutralization

The tiny particles suspended in your wastewater—metal fines, oil droplets, colloidal solids—carry a negative electrical charge. That charge keeps them repelling each other and floating around instead of settling out. Aluminum sulfate releases positively charged aluminum ions (Al³⁺) that neutralize those charges, allowing particles to come together.

2. Floc Formation

Once neutralized, the particles start clumping. At the same time, the aluminum reacts with water to form aluminum hydroxide—a sticky, gelatinous precipitate that acts like a net, sweeping up particles and binding them into larger clumps called "floc." These flocs are heavy enough to settle.

The result: a layer of concentrated sludge at the bottom and clarified water above. You've just turned hundreds of gallons of liquid waste into a much smaller volume of solids.

Why Fabrication Shops Use Alum

What Kind of Wastewater Can You Treat?

Aluminum sulfate is effective on a range of shop wastewaters, including:

• Spent water-soluble coolants: The milky, emulsified fluids from CNC machines and lathes
• Parts washer solutions: Aqueous cleaners contaminated with oils and metal fines
• Rinse water: From quenching, wet deburring, or interim cleaning steps
• Floor scrubber water: Collected from shop floor cleaning
• General oily wastewater: Mixed streams with emulsified or free-floating oils

Basic Treatment Process

Here's a simplified overview of how shops typically use aluminum sulfate for batch wastewater treatment:

Step 1: Collect Wastewater

Accumulate spent coolant, rinse water, or other wastewater in a treatment tank or tote. Remove any large chips or debris first—a simple screen or settling step helps.

Step 2: Adjust pH (If Needed)

Aluminum sulfate works best in a slightly acidic to neutral pH range (roughly 5.5–7.5 for most applications). The alum itself will lower pH when added. If your wastewater is highly alkaline, the alum addition will help bring it down. If it's already very acidic, you may need to adjust. A pH meter or test strips will guide you.

Step 3: Add Aluminum Sulfate

Dose the alum into the tank. Dosage varies depending on the wastewater—start with manufacturer guidelines or a jar test to determine the right amount. For liquid alum (50% solution), typical starting points range from 100–500 ppm, but heavily contaminated coolant may need more.

Step 4: Rapid Mix

Agitate vigorously for 1–3 minutes to disperse the coagulant throughout the water. This is the "flash mix" stage where charge neutralization happens.

Step 5: Slow Mix / Flocculation

Reduce mixing to a gentle stir for 10–20 minutes. This allows the tiny destabilized particles to collide and form larger flocs. Too much agitation will break them apart.

Step 6: Settle

Stop mixing and let the tank sit. Flocs will settle to the bottom over 30 minutes to several hours depending on the waste. You'll see the water clarify as the sludge layer builds up.

Step 7: Decant or Drain

Carefully remove the clarified water from the top—either by decanting, pumping from above the sludge line, or draining through a standpipe. The concentrated sludge stays behind for disposal.

Handling and Safety

Aluminum sulfate is not as hazardous as strong acids or caustics, but it still requires proper handling:

Aluminum Sulfate from Alliance Chemical

We offer aluminum sulfate in forms suited for shop-scale wastewater treatment:

Liquid Aluminum Sulfate (50%)

Pre-dissolved and ready to dose. Easier to handle and measure than dry product. Ideal for batch treatment tanks where you need precise dosing control.

Dry Aluminum Sulfate (Granular)

Economical for larger volumes. Dissolve in water before use or add directly to well-mixed tanks. Lower shipping cost per pound of active ingredient.

Not sure which form or quantity fits your operation? We can help you estimate dosage and select the right packaging.

Frequently Asked Questions