The anaerobic biodegradation of surfactants is used as an acceptability criterion in some environmental pieces of legislation (eco-label, risk assessment, etc.), without a proper evaluation of the relevance of such a characteristic. Available screening test methods to assess the anaerobic biodegradation do not simulate the real conditions prevailing in these anaerobic compartments but rather reflect more stringent conditions, due to the high test substance/biomass ratio, possibility of inhibitory effects and limited possibility for adaptation. Therefore positive results are indicative of a similar behaviour under environmental conditions, while a negative result cannot be necessarily interpreted as inherent anaerobic recalcitrance. The majority of surfactants entering the environment will be exposed to and degraded under aerobic conditions, and only less than 20 % will potentially reach temporarily anaerobic environmental compartments. In contrast to the well documented adverse effects observed in the absence of aerobic biodegradation, the lack of anaerobic biodegradation does not seem to be correlated with any apparent environmental problem for most compartments after more than 40 years of widely use of such products. The scientific review concluded that anaerobic biodegradability does not have the same environmental relevance as the aerobic one. Anaerobic biodegradability should not, therefore, be used as a pass/fail property for the environmental acceptability of surfactants which are readily biodegradable under aerobic conditions.

Background: Surfactants are widely used across the globe both in industrial and consumer products; their biodegradation characteristics are therefore of high importance. Upon entering a wastewater treatment plant (WWTP), the majority of surfactants are aerobically mineralized to CO 2 and H 2 O. However, a small fraction is inevitably left non-degraded and adheres to the remaining sludge. This sludge is usually further treated in anaerobic digester tanks. Assessment of existing methods for determining anaerobic biodegradability has led to the development of a new test method, which is in principle based on the method DIN 38414 part 8. This new test, named the anaerobic biodegradation under sludge digester conditions test (abbreviated to AnBUSDiC test) allows for a quantification of the degradation of surfactants under conditions encountered in the anaerobic digester tank of municipal WWTPs. The AnBUSDiC test has several advantages over existing methods. The main advantage is that it is particularly suitable for surfactants, because the two-step design minimizes possible unspecific digester gas formation caused by the surface activity of the test substances, therefore avoiding false positive results. Results: In order to further standardize the AnBUSDiC test and gain regulatory acceptance, a ring test was organized involving seven laboratories, and five model surfactants from different surfactant classes (anionic, nonionic (branched and linear) and amphoteric) plus a positive control, glucose. The AnBUSDiC test produced reliable repeatable results between laboratories; however, some additional modifications were suggested. It was identified that the original test method did not identify a clear endpoint from which a biodegradation value should be taken. It was proposed that a new more concise endpoint be defined in combination with the AnBUSDiC test to allow better comparability between test results.