Hazardous Waste Determination Flowchart: TTLC, STLC, and TCLP
Three California toxicity tests decide whether a waste stream can go to a regular landfill, stay on your wastewater discharge permit, or ship out as manifested hazardous waste under your EPA ID. They are TTLC, STLC, and TCLP. Each measures something different. Run in the wrong order, they waste lab money. Run against the wrong analytes, they produce the wrong answer.
Three California toxicity tests decide whether a waste stream can go to a regular landfill, stay on your wastewater discharge permit, or ship out as manifested hazardous waste under your EPA ID. They are TTLC, STLC, and TCLP. Each measures something different. Run in the wrong order, they waste lab money. Run against the wrong analytes, they produce the wrong answer.
This is the California hazardous waste determination flowchart for the toxicity characteristic: what each test measures, when to use it, and what a failure means. For the full process around a determination, see our hazardous waste determination guide. For where this fits in the broader discharge picture, see our industrial wastewater discharge permit overview.
The Three Tests at a Glance
| Test | What it measures | Reported in | Regulatory basis |
|---|---|---|---|
| TTLC (Total Threshold Limit Concentration) | Total amount of a substance in the waste, bound and unbound | mg/kg (solid) or mg/L (liquid) | Title 22, 22 CCR §66261.24 |
| STLC (Soluble Threshold Limit Concentration) | Amount that leaches out under the California Waste Extraction Test (WET) | mg/L | Title 22, 22 CCR §66261.24 |
| TCLP (Toxicity Characteristic Leaching Procedure) | Amount that leaches out under the federal landfill-leachate simulation | mg/L | RCRA, 40 CFR §261.24 |
TTLC asks how much is in the waste. STLC asks how much will leach out under California’s extraction procedure. TCLP asks the same leachate question under the federal method, which simulates municipal landfill conditions. The two leachate tests use different extraction fluids and different ratios, so they produce different numbers for the same waste.
Why California Has Its Own Leachate Test
Federal RCRA (the Resource Conservation and Recovery Act, the federal hazardous waste law) recognizes eight regulated metals (the “RCRA 8”) plus a set of organic compounds and uses TCLP as its toxicity test. California adds more metals to the regulated list (chromium, copper, nickel, zinc, antimony, beryllium, and others) and uses the WET extraction for the soluble test. The STLC thresholds in §66261.24 are often lower than the corresponding TCLP thresholds.
This is why a waste can be non-hazardous under federal RCRA but hazardous under California Title 22. The federal test doesn’t look for the analytes California regulates, and California’s thresholds catch concentrations the federal program does not. We cover the broader split in California vs. Federal Hazardous Waste Rules.
The Testing Hierarchy
A California waste determination runs through the toxicity tests in cost-efficient order. The general flow:
Review SDSs and process knowledge first.
What’s in the waste? What process produced it? Sometimes process knowledge alone supports a defensible determination and no testing is needed. More often, it narrows which analytes apply.
TTLC first.
Total concentration is the cheapest, fastest result. If total concentration equals or exceeds the TTLC value for any analyte, the waste is hazardous for that constituent, with no further testing needed. If TTLC for every regulated analyte sits below the §66261.24 limit, the waste passes that analyte without further testing.
STLC only when it can fail.
The California WET extraction uses a 1:10 ratio of waste to extractant. As a screening rule for many solid samples, if the total concentration is less than 10 times the STLC threshold, the WET result generally cannot exceed the STLC threshold, and STLC testing is not necessary (confirm units, sample basis, and lab method before relying on that shortcut). STLC becomes meaningful only when total concentration is high enough that leaching could push it over the limit.
TCLP when RCRA status matters.
TCLP is required to determine whether a waste is federally RCRA hazardous. Facilities that ship interstate, file biennial RCRA reports, or need to confirm whether a waste is a federally listed waste run TCLP. For California-only classification of non-RCRA metals, TCLP isn’t part of the analysis.
Additional toxicity criteria if the result is still ambiguous.
California recognizes other toxicity endpoints, including aquatic toxicity (96-hour fish bioassay) and oral LD50 thresholds for certain non-metal constituents. Most facilities stop at TTLC and STLC because the additional tests are slower and more expensive. They get run when the lower-cost tests don’t resolve the question.
Done well, this sequence keeps lab costs in check and produces a defensible classification. Done poorly, it produces redundant rounds of sampling. The most common deficiency we see when reviewing a self-prepared determination is STLC ordered for analytes that mathematically couldn’t fail it, or TCLP run when a California-only determination didn’t require it.
What a Failing Result Means
A single analyte over the threshold makes the entire waste stream hazardous. There is no partial pass.
The two leachate tests classify the waste differently:
- STLC failure only: California (non-RCRA) hazardous waste. Manifested and shipped under Title 22.
- TCLP failure: RCRA hazardous waste. Federal rules layer on top of California’s, including land disposal restrictions, RCRA manifesting, and tighter generator-status thresholds.
The distinction matters because RCRA and non-RCRA wastes travel under different paperwork, go to different facilities, and carry different disposal fees. Misclassification in either direction has cost consequences.
A California Scenario
A wastewater stream comes back from the lab as non-hazardous. The lab ran total metals and the values sit below TTLC limits. The CUPA inspector (your local Certified Unified Program Agency, which depending on the jurisdiction may be a fire department, county environmental health agency, or another local agency) reviews the report and asks for further testing: STLC on the metals that were detectable but never extracted. The total numbers pass, but the inspector wants the leachate result on the record before signing off on the discharge.
The STLC runs. It passes. The inspector accepts the result. The facility keeps discharging under its wastewater permit.
The lesson is straightforward: a TTLC pass alone does not always close the loop, especially when an inspector or POTW is in the conversation. The hierarchy exists for a reason, and stopping at the first comfortable answer is not a determination strategy. This is also the heart of the sewer-or-ship decision. The leachate result is often what tells you whether a stream stays on the discharge permit or has to leave the facility as hazardous waste.
Common Mistakes We See in California Toxicity Determinations
When we review a self-prepared determination or a report from a prior consultant, the same patterns come up:
- STLC ordered for analytes where TTLC sits below 10× the STLC limit. The test cannot fail. The lab fee was wasted.
- TCLP run for California-only classifications. For non-RCRA metals, TCLP isn’t part of the California analysis.
- TTLC pass declared “done” when total exceeds 10× the STLC limit. At that concentration, the waste cannot be cleared without an STLC result.
- Federal-only analyte panel. Skipping the California-regulated metals leaves Title 22 questions unanswered, and an inspector will notice.
- Old determinations applied to new processes. Determinations are site-specific and process-specific. A waste characterized two years ago may not represent what comes out of the same drain today after a chemistry change.
Every CDMS determination report goes through a senior technical review before it leaves our office. The cost of an incorrect determination is real in both directions: a wrong “non-hazardous” can become an illegal-discharge or improper-disposal enforcement case, and a wrong “hazardous” can mean a facility pays manifested-waste rates for years on material that didn’t need it.
Where the Toxicity Tests Fit in a Full Determination
Toxicity is one of four hazardous characteristics under California regulations, alongside ignitability, corrosivity, and reactivity. A complete determination addresses all four where they could apply, plus any listed-waste rules. The TTLC/STLC/TCLP hierarchy is the part of the analysis that handles toxicity for metals and certain organics, and it’s where most of the cost and complexity live, especially for metal-bearing wastewaters, sludges, and treatment residuals.
Getting the testing sequence right is what separates a defensible determination from a guess. The flowchart isn’t complicated. Applying it correctly to a specific facility, with the right analytes, in the right order, is where the work is.
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