As a consequence of the Detergents Regulation (Regulation (EC) No 648/2004), all surfactants intended for laundry and home care products in Europe are required to exceed a given biodegradation threshold limit. Hence, they will be more or less quantitatively removed from the wastewater after the second step (secondary treatment) of a wastewater treatment plant [WWTPs]. But while most of the surfactants are biodegraded by microorganisms a considerable fraction will be absorbed by the bacterial biomass (activated sludge) (i.e. microorganisms) present in a WWTP. With the activated sludge they will be transferred to the digester where conditions for anaerobic biodegradation prevail.
Under these anaerobic conditions most surfactants will be further biodegraded by specialised communities of anaerobic bacteria. However, there are molecular structures such as secondary sulphonates that are difficult to be degraded under these conditions. This lack of anaerobic biodegradation does not pose an issue, if the digested sludge is incinerated or disposed of via landfill.
On the other hand, if digested sludge is applied to agricultural areas as a fertiliser, non-degraded surfactants (or biodegradation products thereof) could reach concentrations in soil that are harmful or even toxic to terrestrial organisms despite the fact that additional aerobic biodegradation may take place under these conditions. Therefore, terrestrial toxicity is an issue that requires further research, particularly for anaerobically non-degradable surfactants.