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Choosing a Soil Sterilization Autoclave

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Soil sterilization is notoriously hard on autoclaves. In part, this is simply because the soil sterilization process is inherently challenging. Soil is heavy, dense, and hard to permeate with steam and heat. It really puts an autoclave to the test. On top of that, if you’re regularly sterilizing soil samples, grit invariably infiltrates the autoclave chamber, plumbing, solenoids, fittings, valves, and seals. This grit is abrasive, and it also contains minerals that can be especially hard on autoclave chambers.

Taylor Aris is a Senior Staff Scientist at SiREM, an industry-leading provider of products and services for groundwater, soil and sediment remediation projects. Their services include performing treatability studies on groundwater and soil samples, as well as molecular genetic testing and supplying bioaugmentation cultures for aquifer remediation. Suffice it to say, SiREM autoclaves a lot of soil.

“It varies pretty significantly,” Taylor says. “But on average we run around one cycle per day and on a busy day, we might do two loads in a day.”

The average research lab runs about one cycle per day, so SiREM’s typical daily throughput isn’t particularly high. Nevertheless, SiREM’s soil lab has gone through multiple autoclaves in the last five years.

We can learn a lot from SiREM’s autoclave experiences.

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Autoclaves for soil sterilization? Let the buyer beware!

How Soil Sterilization Autoclaves Fail 

Two of SiREM’s dead autoclaves were “economical” models often recommended as a good compromise between cost and quality. Many labs are happy with these “workhorses,” but their working lifespan varies wildly. In this case, the issue was that this manufacturer reduces costs by relying on clad aluminum chambers, instead of steel.  

“As it turns out,” Taylor explains, “These are really prone to pitting. We literally ate through the pressure chamber in under six months” despite following manufacturer instructions and switching water supplies. “It was definitely concerning.”
It’s tempting to write this off as a case of “you get what you pay for.” That’s not the case. Three of the autoclaves SiREM had trouble with came from one of the most highly regarded autoclave manufacturers in the world. In all three cases, the steel chambers held up. But the autoclaves didn’t last.

The first unit from this manufacturer was refurbished. Although the chamber had been replaced and tested—and performed well for SiREM—the 10-year-old autoclave’s mechanical and electrical systems weren’t up to the demands of soil sterilization. That unit died when a cooling fan failed and damaged the wiring. The next unit—a brand new top-loader—performed inconsistently for unclear reasons. The manufacturer declared it a “lemon” and sent a replacement. But the replacement didn’t last long, either. After just a few months, it was unable to reach sterilizing temperature. Following a string of service calls, Taylor notes, the manufacturer decided that “because of the vertical orientation it was problematic for soil; they were finding soil particles in the solenoids and in the tubing, everywhere.”

SiREM also tried another vertical model from a reputable manufacturer. It worked well for a long time, until the door locking mechanism abruptly failed and, even after repeated repairs, kept failing.

Three Features to Avoid when Choosing a Soil Sterilization Autoclave

“Your autoclave is such a critical piece of equipment,” Taylor points out. “You often don’t realize how important it is until you don’t have one and you need to make some sterile media. And the other problem is that autoclaves take forever to get. They’re not something that you can quickly order, like a centrifuge or a pipette. Right now, it’s an eight month wait, at least.” So it’s best to get the right soil sterilization autoclave from the start, rather than scrambling to repair and replace one later. 

SiREM’s ongoing search for a durable autoclave highlights three key design elements that correlate to early autoclave failures in labs where soil sterilization is a regular part of the work. 

Aluminum Pressure Chambers 

Two of SiREM’s autoclaves failed because of their clad aluminum pressure vessels. Chlorides are hard on aluminum, even clad aluminum, which is the most corrosion resistant. That’s especially the case in environments with high heat and pressure (like an autoclave chamber). Even when SiREM exclusively ran their autoclave with RO (reverse osmosis) purified water at the manufacturer’s urging, the additional chloride load that comes with running regular soil samples seems to have been sufficient to destroy the chamber.  

Instead, look for a lab autoclave with an ASME-Stamped 316L stainless steel chamber. These are manufactured to exceedingly stringent standards and will prove significantly more durable and reliable over the long haul.

Water Recirculation 

Some autoclaves recirculate the water used for steam generation. This is intended as a water-savings measure, although there is some question about how effective it is (in addition to concerns about such schemes creating conditions ideal for breeding some dangerous pathogens). Taylor notes that recirculation almost certainly contributed to the short lifespan of several of their autoclaves.  

“In any lab, you’re going to have a boil-over or a container break or a bag leak,” she says. “When that happens with soil, you get some really great mud in the bottom of the autoclave. That’s annoying on its own. But then when you have an autoclave that recycles the water, it now just pulls all of that mud through its system,” driving grit into every pipe and crevice. 

Even in the absence of a bag break, you are eventually going to get soil particles in the chamber. These are carried up off the load by the air and steam cycling through the chamber as it fills or vents. That will end up in the chamber water that gets recycled, and will likewise begin to grind away at your autoclave’s seals and plumbing.

Top-Loading Vertical Orientation 

This has yet to be fully explored, but we’ve noticed a correlation between vertically oriented top-loading autoclaves and reduced working life in labs that run daily soil sterilization cycles. We suspect that the way air and steam flow through the chamber in this orientation tends to carry grit into the mechanism, accelerating wear.

The “Toughest Autoclaves in the World”

Labs invariably identify durability and reliability among their top concerns when looking for an autoclave. For nearly three decades, Priorclave has dedicated itself to creating the “toughest autoclaves in the world.” Every Priorclave, regardless of size, comes with free lifetime technical support, an ASME-stamped 316L stainless steel pressure vessel, and is built using robust design principles that minimize maintenance headaches and maximize working lifetime.

But no autoclave is one-size-fits-all. Autoclave design choices that are perfectly reliable in a bio-research lab that’s mostly preparing growth media and pipette tips won’t hold up in other labs. This is why every Priorclave is custom built to order, so that you can be sure your machine fits your task. 

Looking for a new autoclave? Give us a call or send us a message whenever you’re ready and we’ll help you find the right autoclave for you.