Last week, I received an email query from Kevin H:
Hi, do you know the RLU limit for diesel i see the IATA say

Here’s my response:

Thank you for your query about the existence of an RLU criterion for ATP-bioburdens in non-aviation distillate fuel.

When I suggested the HY-LiTE test low, medium and high levels, I basically translated from the culture test criteria I had developed from the decision matrix I had been using for BCA’s Biodeterioration Risk Assessments. (high risk: CFU bacteria/L fuel >100). That 100 CFU/L equals approximately 1,000 RLU/L.

At the time I was collaborating with Merck to help them have the HY-LiTE method become the basis for an ASTM standard test method. The HY-LiTE works quite nicely in Jet A, where additive usage and water content vary little among samples.

Regarding the relationship between microbial loads and damage risk:
1. I’ve been doing fuel microbiology for about 40 years.
2. I’ve seen systems that have had high levels of contamination, but no evidence of damage, and systems with low levels of detectible microbial contamination and substantial evidence of microbially-caused damage (filter plugging, system corrosion, degraded fuel, etc.). We still don’t have a reliable model for figuring out when a non-damaging population is going to start causing problems (very similar to humans: there are 10x as many microbes as human cells in and on our bodies. Normally they keep us healthy, but sometimes the same microbes cause disease). Key here is that there is no clear link between microbial load and damage risk. Moreover, microbes found in the fuel are generally not the ones to worry about. It’s the microbes growing on the fuel system surfaces. We work with fuel samples, because they are easier to collect. For this reason, both the EI Petroleum Microbiology Committee and ASTM Fuel Microbiology Working Group advise against setting criteria levels for microbial contamination in fuels or fuel-associated waters. The microbiologists on the IATA microbiology task force also resisted resisted putting criteria into the IATA document until the non-microbiologists wore them down.
3. BCA’s Biodeterioration Risk Assessment looks at climate, system design, operations, maintenance practice, fuel chemistry, bottoms-water chemistry, fuel microbiology, bottoms-water microbiology and system component condition. ATP is only one of 10 different microbiological tests that I use to assess biodeterioration risk.
4. All that said, given the patchiness of microbe distribution in fuel systems, I and most of the other fuel microbiologist that I know tend to be conservative. We agree that detecting microbial contamination tells us much more than not-detecting it. Any positive test result indicates a need for further testing. 1000RLU/L equals approximately 100 CFU/mL = 0.1 CFU/mL. In the UK, drinking water is permitted to have 10^6 CFU bacteria/mL (as long as they are not potential pathogens). IATA set a very conservative control level because filter pulling at 50,000 ft can cause an aircraft to fall out of the sky. Filter plugging on the ground can also stalling, but the net impact is not nearly as dramatic.

Long answer to a simple question. Based on their risk tolerance, different companies (particularly marine, rail and trucking fleets) have set in-house criteria. I recommend using ATP as a canary in the cave test. If you get numbers above background, you should run additional tests. I do not recommend relying on any single test when deciding whether you need to take corrective action.