Microbial Degradation of Steroid Hormones in the Environment and Technical Systems

Steroid hormones are naturally produced by human and animals. Most important steroids are 17β-estradiol (E2), estrone (E1) (both estrogens), testosterone as well as the synthetic ethinylestradiol (EE2). All substances consist of four carbon rings what makes them stable in the environment. Steroid hormones are secreted in urine and mainly enter the environment by waste water treatment plant (WWTP) effluents. The problem of steroid hormones is that they are endocrine disruptors and can affect aquatic organisms such as fish. Effects like feminism of male fish have already been observed near WWTP effluents. The most important process to remove the steroids is the microbial degradation. Sorption and to minor extent photo degradation can also play a role in the removal of these hormones. The degradation rates of E2, E1 and EE2 have been studied widely. It was found that E2 is oxidised to E1 in the first step. The half-live of this step is around 4 to 12 hours in aerobic water and soil. However, this step does not significantly reduce the estrogenic potential. Further degradation of E1 needs the cleavage of one ring. Therefore, half-lives of E1 are significantly higher and observed E1 concentrations are normally higher than E2 concentrations. The degradation of E1 shows a linear relationship with the removal of the estrogenic potential. The structure of EE2 is analogue to E2 but there is an ethinyl group at one hydroxyl group containing C-atom. This group is normally vulnerable to microbial attack. A cleavage of this ring is therefore difficult what makes EE2 much more recalcitrant in the environment. Thus, EE2 has a big impact on the estrogenic potential although the secreted amount is much smaller than that of E2 or E1. The degradation rates of steroid hormones in anaerobic sediments are much smaller. Half-lives of anaerobic E2 degradation are in the range of tens of days. E1 and EE2 can often not be degraded under anaerobic conditions. Therefore these substances accumulate in these environments. In the WWTP the half-lives of E2 are in the range of a few minutes under aerobic and denitrifying conditions whereas E1 shows half-lives of up to one hour in the denitrification tank. E2 and E1 are thought to be degraded metabolically, i.e. for the bacteria to gain energy. EE2 is degraded within 2 to 4 hours under aerobic conditions in the WWTP but is quite persistent under denitrifying conditions. EE2 is thought to be degraded cometabolically, i.e. incidentally without an energy gain for the organism, by an enzyme of nitrifying bacteria, but this is not fully understood yet. The much smaller half-lives in the WWTP compared with the natural systems are due to the much higher bacteria density. Nevertheless, it is still controversial which part of the WWTP is most efficient concerning steroid removal. The metabolic degradation pathway of E2 and E1 is still not clear. The metabolic initial cleavage of the aromatic ring of E1 was proposed forty years ago but the cleavage of the five-ring as initial step was recently suggested, too. On the basis of electron density comparisons it was assumed that EE2 is firstly cleaved at the aromatic ring as well. On the other hand, the degradation pathway of testosterone – the so called 9,10-seco pathway is well described and starts with the cleavage of a ring in the middle. To improve the efficiency of WWTP concerning removal of steroid hormones an increased sludge retention time (SRT) and hydraulic retention time (HRT) as well as the introduction of the most efficient bacteria community are of most importance. 1 In der Au 13d, CH-8547 Gachnang, Switzerland. [email protected] 2 Swiss Federal Institute of Aquatic Science and Technology, Eawag, Ueberlandstr. 133, CH-8600 Duebendorf, Switzerland.