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A solid definition of waste forms

Spoiler Alert:  It was dry when I put it in the freezer.

The light was low. The time was college. It seemed to be a good idea at the time. All official records have been scrubbed. The names have been omitted or changed for obvious reasons.

I pulled the jumbo, tie-dyed sponge from the freezer and handed it to my friend.

“Is this wet or dry?” I asked.

He took it from me despite a moment of hesitation. He turned it over in his hands, squeezing it lightly with his fingers. He squeezed it harder with his hands, using fingers and palms together like he was playing an accordion, his eyes locked on the changing patterns as the tie-dyed sponge expanded and contracted.

“Wet or dry?” I asked again.

“It’s definitely wet,” he snapped back, shifting his gaze to me – still squeezing the sponge in and out.  His face changed.  “Well,” he paused and resumed looking at it, ever squeezing… “Maybe not…  I… I don’t know.”

Over the next 5-10 minutes he and three other people passed the sponge around, back and forth, and were unable to come to a consensus (as individuals or as a group) on whether the sponge was wet or dry.

A simple question. Wet or dry is a basic sensation. So, how did it become so difficult for a group of college-educated people to decide? 

Fast forward to the year 2020. My sponge experiment is eerily similar to the situations retailers and waste haulers find themselves in when determining one of the most basic consumer product attributes – Form. Typical forms include solids, liquids, and gases. There are also other forms that are relevant when hauling waste, e.g., aerosols and liquids absorbed in solids (the latter plays a pivotal role in the unfolding story).

Just like my bygone experiment, the determination of liquid or solid…

1. Should be apparent to anyone, in theory

2. Can be more complicated than you think

3. Can be affected by temperature

4. May leave smart people unsure about their choices

5. Is often made without data from the manufacturer

Right off the bat you may say, “You’re cheating! The sponge is a solid. The water it soaked up is the liquid!” Very astute of you. But in the world of waste, it’s not that simple. Form determination at its core is about “free liquid.” So, if I squeeze the sponge, and water hits the floor, is that free liquid? Is it only free after I liberate it? What about gel caps, or just plain ‘ole gel??


The essence of the solid form is that there is no free liquid.  In the world of regulated goods, there are plenty of easily identifiable solids: Chlorine pucks, powdered laundry detergents, rat poison pellets, and fertilizer granules, to name a few. Seems simple enough. What about paste? What about hair gel? It might surprise you to learn that all three can be considered solids. This matters in some very practical ways, especially related to packing, storing, and transporting.

First, containers are only tested and approved to hold and transport certain substances.  Think about a 55-gallon steel drum.  While it may seem logical that it can hold liquids, logic is not always synonymous with regulatory approval. It must be evaluated for the maximum specific gravity of liquids it can contain and the maximum hydrostatic pressure it can withstand. These tests determine if it can safely hold and transport approved free liquids.

With a standard UN-rated steel drum, we could pour used oil or a water/gasoline mixture right inside with no worry about leaks or spills. But, if a container is rated only for solids, then it’s not safe or compliant to pack, store or transport liquids inside. A box is great for collecting and storing batteries. And many waste haulers transport them that way. But you couldn’t pour oil in a box unless you were just itching for a mess and a lawsuit.  

A second consideration is the segregation of liquids and solids.  If you’ve ever done pool maintenance or encountered a bucket of chlorine pucks that got wet, you know that you definitely want to keep them dry.  While most consumer products are not inherently dangerous, there are many products that only “become” dangerous after moisture is added.  

Dry items do not have a pH.  So a bag of Portland cement is not dangerous or regulated. However, adding water produces a substance that is highly alkaline. Some solids react violently with water. The deadly 2015 Tianjin blast may have been triggered by firefighters using water to put out a small fire in a container of calcium carbide. Bottom line, it’s important to keep dry things dry. 

Ok, so there are wrinkles with regard to waste regulations. However, the laws governing solids often apply to non-regulated waste as well. Many large retailers use huge trash compactors to manage their on-site waste. Just like the old cabinet unit your aunt had back in the day, the compactor smashes the crap out of whatever is inside, allowing more waste to be put into the container before final disposal. I think it’s fairly obvious why you wouldn’t want to put something like a propane tank in an industrial strength compactor, but what about liquids? Does it matter?

Yes it does. The waste hauler for regular “landfill” trash requires that no free liquids be included in the compactor. For some retailers, this is defined as “anything that would produce liquid if you put it in the compactor.” But really, this doesn’t hold up. If I smash a vitamin E capsule (a liquid contained in a solid) with my foot, what happens? A little liquid spurts out and leaves a mess on the floor and my shoe.  Same thing happens if I squeeze several wet wipes (a liquid absorbed in a solid).

The cost is very real. The “liquid” material is hauled away by a regulated waste hauler, while the “solid” material is hauled away by the regular trash hauler. Oddly, they both end up in the same place, but incur vastly different costs in the process.


Ok, maybe we’re getting too in the weeds here. Let’s go to the other side of the coin: Liquids. You probably know them well – household cleaners, shampoos and body washes, used oil, hydrogen peroxide solutions – the list could go on and on. Edge cases are bound to come up as we’ve seen, so let’s examine applicable definitions on the subject.

For the U.S. Department of Transportation, a material is defined as liquid if it has a melting point lower than 68℉. Ok great. But, it’s 104℉ in Austin, TX today. So whether or not a substance is solid at 68℉ doesn’t really matter in my world for about 10 months of the year.

Don’t worry. If the information is not available, you can conduct an ASTM D4359 Test. That’s American Society for Testing and Materials, for those of you keeping score. “Ah yes, the ASTM D4359… I can’t believe I forgot about that test.  Let me pull out my ASTM D4359 meter and….” I think you see the problem. This is already frustrating. Let’s put this last part aside for a moment. We’ll come back to the D4359 test in a bit.

The Resource Conservation and Recovery Act (RCRA) is even better. They say that you should put the substance in question in a conical piece of equipment and wait 5 minutes. If you see a drop of liquid, then the substance contains free liquid. And remember, if it contains free liquid, the whole thing is liquid.  I’m not kidding you, THIS is the “science” behind the regulations. “Morning John…  Here’s the cone you use to determine those solid/liquid edge cases.” Really? Next!

The National Fire Protection Association is my personal favorite. They say that a liquid is something that has a fluidity greater than 300 penetration asphalt. What? Those appear to be words, grouped together to convey a thought. But they mean nothing to me. There is some limited explanation. They say to use a test from the American Society for Testing and Materials called the Standard Test Method for Penetration of Bituminous Materials. And yes, you do in fact need a needle for this one.

penetration test

Again, this doesn’t help your average waste technician or retail worker.  They do say another test can be used, however… The old ASTM D4359 test! Now we are getting somewhere. We finally have two regulatory bodies that agree on how to define a liquid!

Don’t get too excited. The ASTM D4359 test boils down to the following – Heating the substance to 100℉ and then inverting its container. If there is no detectable immediate flow of the substance, then the substance is solid. Otherwise it is liquid. Well, at least the temperature consideration seems more grounded in real life. Of course, to have the results of this test, the manufacturer would have to do the test and declare the results. Sounds like something that might be included on a product Safety Data Sheet (SDS)

Safety Data Sheets

Wait, of course! That’s it! Don’t they have to tell us on the SDS? Ahhhh great insight. But it’s a little more challenging than that. The average retail worker or even hazardous waste technician will not have immediate access to an SDS.  

At Smarter Sorting, we have solved this problem by collecting SDSs for over one million consumer products. Our partners have one-touch access to millions of chemical product SDSs, all from a simple product barcode scan. Those SDSs have nearly 10,000 distinct values for product form. Here are some of my favorites:

  • “Cream”

  • “Liquid to cream”

  • “Semi-solid”

  • “Foam”

  • “Aerosol. Liquefied gas.”

  • “Sticks of various colors.”

  • “Paste / Gel, Solid containing liquid”

  • “Polypropylene wiper substrate with a wet solution”

  • “Ivory White with pearls Liquid”

  • “Clear liquid saturating a towelette”

  • “Viscous cream, free of foreign matter”

  • “Solid, Red match head, woody matchstick”

Congratulations! You have the SDS and thousands of distinct values for product form. Problem solved, right? Well, maybe not quite. Even if you have the SDS for reference, the answer may not be as clear as we’d like. Regulations still exist in a strange place where a product can be declared as something that doesn’t necessarily reflect what it is ‘in the livid world.’  The following cosmetic product SDS demonstrates this discrepancy well:


As you can see, this liquid eyeliner product is ironically declared as a solid. However, the Melting Point is described as “Softens at 104°F-158°F (40℃-70℃).” It’s unclear why there is such a large range for the softening, but 104°F is well within the range of temperatures one would experience in the back of a waste hauling truck.

So how do we solve this?

The Smarter Sorting Resolution

“Is this wet or dry?”  Have no fear! Every product registered in the Smarter Sorting Classification Portal is assigned the correct form.  If the manufacturer is unsure of the form type, we determine the truth according to applicable regulations. We resolve that information to retailers and waste haulers with simple instructions on how to handle the item. No more confusion, and no more guessing with those edge cases. A solid definition is here to stay.