Background: Surfactants are chemicals with a high production volume and a wide dispersive use, i.e. surfactants have a high environmental impact. Most commercial surfactants are aerobically biodegradable. Only a small fraction of the surfactants is not aerobically broken down during the usual hydraulic retention times of modern WWTPs.
This fraction, due to the predominantly hydrophobic nature of surfactants, adheres to the sludge. The sludge is usually collected and further treated under anaerobic conditions in digester tanks. Therefore, the knowledge about anaerobic biodegradability under digester tank conditions is important to gain an understanding about the environmental fate of surfactants.
Results: A new test method suited for the assessment of the anaerobic biodegradability of surfactant under sewage plant simulation conditions is proposed. The test method foresees that an accurately known amount of the test substance is added to the sludge inoculum, and that the test substance is added in two sequential steps to overcome possible interferences from unspecific digester gas formation caused by the surface-activity of the surfactant test substance. By measuring the difference in the gas volumes produced in the sludge inoculum plus test substance and the corresponding control (sludge inoculum only) and converting the gas volumes to the percentage degree of biodegradation, this test allows the quantification of the anaerobic biodegradability of the test substance.
Conclusions: Tests with commercial surfactants indicate that the newly developed test method allows for a quantification of the degradation of surfactants under conditions encountered in the anaerobic digester tank of municipal waste water treatment plants. The described test is particularly suitable for the testing of surfactants, because the two-step design overcomes any problems related to unspecific digester gas formation caused by the surface-activity of the test substances, therefore avoiding false positive results.