The question is considered in every single wastewater treatment application – what materials of construction should we use? Regarding DAF tank design, several materials have achieved commercial success: concrete, epoxy-coated carbon steel, stainless steel, and polypropylene; however, the latter two are most commonly used, so they’re the ones we’ll focus on in this discussion. Let’s look at the advantages and disadvantages of each material, and then compare stainless steel vs polypropylene, side by side.
- Stainless Steel is Rust Resistant – a passive film of chromium oxide on the surface of stainless steel blocks oxygen diffusion and rust-forming corrosion. It acts as a shield to preserve the metal’s internal structure, even if it’s scratched.
- Stainless Steel is Strong – the material maintains its strength and rigidity between temperatures of -320°F to 1500°F. It’s well suited for indoor and outdoor use.
- Stainless Steel is Easy to Modify – additions or upgrades can be made to the material on-site, as stainless welders are routinely part of maintenance staff.
- Stainless Steel is Durable – the structure of stainless steel remains sound, even after decades of use. 30 year-old tanks can be polished and hold up as well as the day they came off the fabricator’s floor.
- Stainless Steel is Clean – so long as surfaces are polished to eliminate crevices and pits, bacterial growth on stainless steel is minimal. Cleaning and sanitation is quite easy. This is particularly useful in wastewater environments where the steel is consistently exposed to organic materials.
- Stainless Steel Maintains Resale Value – If at some point processes change and you don’t need a DAF system anymore, units constructed of stainless steel hold their resale value.
- Stainless Steel Can be Expensive – the price of steel fluctuates as nickel supply ebbs and flows. As of the time of this writing, 304 SS in sheets costs $1.85/lb, or roughly $466 USD per 5’x10′, 11-gauge sheet .
- Stainless Steel is Not Suited for High Chlorides – in environments with high chloride concentrations, stainless steel has the potential for pitting and corrosion.
- Polypropylene is Strong and Rigid – like stainless steel, the material holds its strength quite well. However, at temperatures above 260°F and below 32°F, polypropylene may deform and crack.
- Polypropylene is Easily Fabricated – as plastics are molded, fabrication of tanks can be a very quick process. Plastic welders can join the molds together to create larger structures.
- Polypropylene is Suited for Ionized Water Applications – in ultra-pure water applications, like biopharmaceuticals, where ionized water is used, polypropylene resists pitting and can deliver the 100% repeatable cleaning process commonly required in ultra-pure water applications.
- Polypropylene Cannot Withstand UV – when exposed to UV, polypropylene plastics undergo tertiary carbon bond breakdown, forming carbonyl groups in polymer chains that cause cracks and discoloration. In other words, polypropylene tanks are not suited for outdoor applications where they may be exposed to sunlight.
- Polypropylene is Not Easily Modifiable – field modifications or repairs to polypropylene tanks require ultra-clean environments and plastic welders, both of which are not readily available in typical wastewater buildings at industrial facilities.
- Polypropylene Doesn’t Last – manufacturers of polypropylene tanks do not often warranty their structures beyond 10 years.
Now let’s compare the two materials over a variety of characteristics.
|Stainless Steel (304)||Polypropylene|
|Operating Temperature (min.)||-320 °F||32 °F|
|Operating Temperature (max.)||1500 °F||170 °F|
|Heat Distortion Temperature at 20 °C||17.3 x 10 ^-6 / °C||150 x 10 ^-6 / °C|
|Tensile Strength||520 – 720 MPa||33 MPa|
|Compressive Strength||210 MPa||48.2 MPa|
|Density||8.03 g/cm3||0.905 g/cm3|
In the end, understanding the properties of the fluid at hand is the most important factor in considering one material over another. You’ll also need to consider the pros and cons and take a look at how it will fit into the way your plant operates. If you’re still having a hard time deciding which material is a better fit for your facility, contact us and let’s talk about it.
As a bonus, here’s a handy chart with lots of details for materials and chemical resistance.