## 1 Introduction  
Increasing use of pharmaceuticals and personal care products, along with population growth, pose new challenges to wastewater treatment that are not met by conventional domestic wastewater treatment plants (DWWTPs). Conventional DWWTPs have been reported to be ineffective barriers to these substances due to their low concentrations and relative resistance to biodegradation or other treatment processes (for example, physical and chemical treatments). Some of these substances may have potentially chronic direct or indirect effects on ecosystems due to endocrine-disrupting chemicals (EDCs) in the aquatic environment (for example, inducing feminization of fish), as well as on public health. EDCs have the ability to mimic, block or compete with endogenous hormones. The presence of such compounds in natural ecosystems is linked to abnormalities observed in wildlife reproduction and development functions [1]. These organic micropollutants (OMPs) can penetrate water bodies, including rivers, lakes and groundwater, and may end up in drinking water through direct discharge or as a result of agricultural irrigation by the treated effluents. For example, 111 types of pharmaceuticals were discovered in DWWTP effluents in Germany, and 23 of them persist in drinking water [2; 3; 4]. A unique biodegradation model for the synthetic steroid sex hormone 17\(\alpha\)-ethynylestradiol (EE2), which is the main ingredient in most oral contraceptive pills, is presented in the present study. EE2 may affect aquatic environments even at an extremely low concentration of 0.1 ng/L [5]. The high biological activity of EE2 is due to its free trafficking through cell membranes and its molecular activity mechanism. All mammals, including humans, excrete steroid hormones which may be transported to the environment through sewage effluents. It has been postulated that domestic treatment plant effluents are among the main sources for EDCs in natural water bodies [6]. [PERSON] et al. [7] showed that effluents contain various hormones, such as natural androgens (androsterone, epiandrosterone and androstenedione), synthetic progestogens (megestrol acetate, notethindrone and medroxyprogesterone noteate) and natural progestogens (progesterone, 20\(\beta\)-dihydroxy-4-progegnen-3-one, 21\(\alpha\)-hydroxyprogesterone, and 17\(\alpha\)-hydroxyprogesterone). EE2 is considered to be the main cause for estrogenic activity in wastewater treatment effluents, which also contain the natural estrone (E1), 17\(\beta\)-estradiol (E2) and estriol (E3) [2; 3; 4]. This endocrine-disrupting chemical has been detected at nanogram per liter levels (Larcher and Yargeau [8]) in both surface waters and domestic wastewater effluents and was reported to cause endocrine disruption and consequently feminization of aquatic organisms [8; 9; 10]. EE2 removal rates in DWWTPs have been investigated and were found to have a broad range, from 34% to 98%, indicating incomplete removal which leads to the presence of a residual amount of EE2 in the discharging effluents and thus might present an estrogenic activity [8]. EE2 appears to have a slower biodegradation rate than the natural hormones, due to its stable chemical structure [11]. The bacterial metabolic mechanism for EE2 biodegradation is not fully understood. In general, microorganisms degrade steroid hormones via two possible mechanisms: Growth-linked (metabolic) and non-growth-linked (co-metabolic). The co-metabolic mechanism is employed by heterotrophic and nitrifying bacterial strains [12]. Only a few microbial strains that possess the ability to degrade EE2 as a sole carbon and energy source have been reported, including _Splingobacterium_ sp. JCR5 [13] and _Rhodococcus equi_[14]. [PERSON] et al. [15] first suggested that ammonium monooxygenase (AMO) can be the key enzyme for EE2 degradation in nitrification tanks. [PERSON] and [PERSON] [16] found a linear relationship between EE2 and NH\({}_{3}\) removal.