How to Audit Your Supplier's Compostability Certifications

Step 1: Start with the Standard – ASTM D6400 vs. EN13432

Before you audit a single document, you need to know which standard applies to your market. If you’re selling in the US, ASTM D6400 is the benchmark for compostable plastics. In Europe, it’s EN13432. These aren’t interchangeable. A bagasse plate that passes ASTM D6400 might not pass EN13432 because the test conditions differ – e.g., EN13432 requires disintegration within 12 weeks, while ASTM D6400 allows up to 180 days. I once had a client import PLA cups from China claiming “compostable” with only a CE mark. That CE mark doesn’t mean compostable; it means conformity to EU directives. We had to run full EN13432 tests after shipment – cost them $3,200 and 8 weeks. Always ask: “Which standard does this certification reference?” If they say “just compostable,” walk away.

Step 2: Verify the Certifying Body – BPI, TÜV, DIN, or OK Compost

Not all logos are equal. In North America, the Biodegradable Products Institute (BPI) is the gold standard. Their certification ensures the product meets ASTM D6400 and has passed independent lab testing. In Europe, look for TÜV Austria’s “OK Compost” or DIN CERTCO’s “Seedling” logo. These bodies audit not just the final product but also the manufacturing process. We audit our suppliers for BPI certification because it’s the most recognized by US retailers. For example, a 500ml PLA cup with BPI certification costs $0.08–0.12/pc at 50k MOQ, while an uncertified cup might be $0.06 but can’t be sold to Whole Foods or Target. Pro tip: Check the certification number on the certifier’s website. Fake BPI logos are common – I’ve seen suppliers Photoshop the logo onto packaging. A quick search on BPI’s site takes 30 seconds.

Step 3: Ask for the Lab Test Report – Not Just the Certificate

A certificate is a summary. The real proof is the test report from an ISO 17025 accredited lab. For compostability, the report should show results for: biodegradation (≥90% in 180 days for ASTM), disintegration (≤10% residue on a 2mm sieve), and ecotoxicity (no adverse effects on plant germination). I request these reports for every new supplier. One supplier sent me a certificate from a lab I’d never heard of – turned out the lab wasn’t accredited. The test report showed 85% biodegradation, which fails ASTM D6400. If they can’t provide a third-party lab report, it’s a red flag. Also, verify the sample tested – it should be the final product, not just the raw material. A PLA resin might be compostable, but when molded into a cup with additives, it might not be.

Step 4: Check for Additives and Coating – The Hidden Pitfall

Many compostable products fail because of coatings. For example, kraft paper bowls are often coated with PE (polyethylene) to prevent leaks – that PE layer is not compostable. Even “compostable” PLA-lined cups may use a PLA coating that’s too thick to disintegrate within the required timeframe. I audit suppliers by asking: “What is the coating material? What is its thickness in microns?” A typical PLA coating for hot cups is 15–20 microns – that’s acceptable. But I’ve seen 40-micron coatings that made the cup non-compostable. Another common issue is additives like colorants or UV stabilizers. A black PLA fork might contain carbon black, which is not allowed under EN13432 because it doesn’t biodegrade. The certification report should list all components. If your supplier can’t provide a full material declaration, insist on it.

Step 5: Audit the Supply Chain – From Resin to Finished Product

Compostability is about the final product, but the supply chain matters. I once worked with a supplier who sourced PLA resin from a BPI-certified manufacturer but then added a proprietary blend in-house that wasn’t certified. The final product had no certification. Now we require suppliers to provide certification for each raw material, plus the final product. For bagasse tableware, the fiber source matters – bagasse from sugarcane is fine, but if it’s mixed with other agricultural waste, the compostability may vary. We also check if the supplier uses any binders or adhesives (e.g., for molded pulp plates). Some binders are starch-based and compostable; others are synthetic. Ask for a list of all inputs and their certifications. A simple table can help:

Step 6: Understand the Disintegration Test – Time and Conditions

Compostability certifications require specific conditions. For example, ASTM D6400 requires industrial composting at 58°C with 50% humidity. A product that composts in 180 days in a lab might take 2 years in a backyard bin. This is a common misconception: “compostable” doesn’t mean “home compostable.” Many B2B buyers assume their customers can throw the packaging in a home compost. That’s not true for most certified products. If you need home compostability, look for TÜV’s “OK Compost Home” or AS 5810 (Australian standard). I advise clients to clearly label: “Industrial Compost Only” unless they have home compost certification. During an audit, ask: “Does your certification cover home composting?” If they say yes, ask for the specific standard and test report. Only about 5% of compostable packaging is home compostable.

Step 7: Conduct an Annual Re-Audit – Certifications Expire

Certifications aren’t permanent. BPI certification requires annual renewal with new testing. I’ve had suppliers whose certificates expired and they didn’t renew – they just kept using the old logo on packaging. We now maintain a spreadsheet with certification expiry dates and request updated certificates 60 days before expiration. For one supplier, we found their BPI certification had lapsed for 3 months. They had to recall a container of 50,000 bowls because the buyer required current certification. Cost: $12,000 in shipping and rework. Avoid this by setting calendar reminders. Also, check if the certification covers the specific product SKU. Some suppliers have certification for one product line but not another. We ask for a list of certified SKUs and cross-check with our purchase orders.

FAQ

What if my supplier only has a certificate from a non-accredited lab?

That certificate is essentially worthless. Accredited labs (like ISO 17025) ensure test methods are valid. If the lab isn’t accredited, the results may be fabricated or flawed. Insist on a certificate from BPI, TÜV, DIN, or a similar body. If they push back, consider it a dealbreaker.

Can a product be certified compostable if it contains a small amount of non-compostable material?

Yes, but only if the non-compostable portion is below a threshold (e.g., <1% for heavy metals under EN13432). However, any coating or additive that doesn’t biodegrade can cause failure. Always check the test report for “heavy metals” and “ecotoxicity” sections. If the report doesn’t include those, ask why.

How often should I audit my supplier's certifications?

At least annually, but I recommend quarterly spot checks for high-volume suppliers. Changes in raw materials or processes can void certification. For example, a supplier might switch to a cheaper PLA resin that isn’t certified. A quick quarterly review of certification status can prevent costly compliance issues.