Diagnostic investigation of steroid estrogen removal by activated sludge at varying solids retention time

The impact of solids retention time (SRT) on estrone (E1), 17β-estradiol (E2), estriol (E3) and 17α-ethinylestradiol (EE2) removal in an activated sludge plant (ASP) was examined using a pilot plant to closely control operation. Ex situ analytical methods were simultaneously used to enable discrimination of the dominant mechanisms governing estrogen removal following transitions in SRT from short (3 d) to medium (10 d) and long (27 d) SRTs which broadly represent those encountered at full-scale. Total estrogen (∑ EST , i.e., sum of E1, E2, E3 and EE2) removals which account for aqueous and particulate concentrations were 70 ± 8, 95 ± 1 and 93 ± 2% at 3, 10 and 27 d SRTs respectively. The improved removal observed following an SRT increase from 3 to 10 d was attributable to the augmented biodegradation of the natural estrogens E1 and E2. Interestingly, estrogen biodegradation per bacterial cell increased with SRT. These were 499, 1361 and 1750 ng 10 12 viable cells −1 d −1 . This indicated an improved efficiency of the same group or the development of a more responsive group of bacteria. In this study no improvement in absolute ∑ EST removal was observed in the ASP when SRT increased from 10 to 27 d. However, batch studies identified an augmented biomass sorption capacity for the more hydrophobic estrogens E2 and EE2 at 27 d, equivalent to an order of magnitude. The lack of influence on estrogen removal during pilot plant operation can be ascribed to their distribution within activated sludge being under equilibrium. Consequently, lower wastage of excess sludge inherent of long SRT operation counteracts any improvement in sorption.