Environmental Project, 899 – Degradation af estrogens in sewage treatment processes

Degradation of Estrogens in Sewage Treatment Processes

6 General discussion and conclusions

The existing literature on investigations on the fate of estrogens in sewage treatment plants (STPs) suggests that the central part in the process is the activated sludge system, where removal of nitrogen takes place along with degradation of general organic matter. Most commonly such systems are operated both under aerobic and denitrifying conditions.

The two presumably most important removal processes in the activate sludge part of an STP, sorption to sludge particulate matter and degradation, were studied in the laboratory using activated sludge from a modern Danish STP; Egå STP near the city of Århus (sorption also studied with sludge from Lundtofte STP). Egå STP is considered fairly representative of large STPs in Denmark, which handle the vast majority of the municipal sewage. However, the plant appears to perform slightly better than the average (based on data on effluent quality of general parameters and NPO).

The estrogenic substances studied comprised the natural female steroid hormones estrone (E1) and 17β-estradiol (E2) as well as the active ingredient in most contraceptive pills, 17α-ethinylestradiol (EE2). Furthermore, two conjugated forms of E1, estrone-3-glucuronide (E1-3Glu) and estrone-3-sulphate (E1-3Sul), were included in the study because estrogens are excreted from the human body as conjugates and, thus, enter the sewage system in this form.

The sorption experiments included only E1, E2 and EE2 as free estrogens because the conjugated forms are very hydrophilic and, hence, not believed to be sorbed to any significant degree. The substances were found to sorb almost quantitatively to the sludge within only about 0.5 hour. The (logarithmic) sorption equilibrium constant, Log K_d, was about 2.6 for E2 and EE2 in both types of sludge tested while for E1 it was 2.8 in Egå sludge and 2.3 in Lundtofte sludge.

The latter result is most consistent with observations found in the literature i.e. slightly lower than for E2 and EE2. Values for Log K_d of E1, E2 and EE2 in the literature tend be a little higher than found in this study. The results indicate that at common sludge densities in Danish STPs about 35-45% of E1 and 55-65% of E2 and EE2 (all unconjugated) can be expected to be sorbed to sludge.

The sorbed fractions in the activated sludge tank have implications for the prediction of stability of the steroid estrogens in the activated sludge tank. In the stability experiments with sludge the MLSS was 0.5 g DS/L, which is predicted to give a dissolved fraction of steroid estrogens of 76-92 %. In an activated sludge tank with a typical MLSS of 5 g DS/L only 24-52 % of the total estrogen concentration is dissolved in water.

The degradation of E1, E2 and EE2 and two conjugated forms of E1 was studied under aerobic and anaerobic conditions in a simulated activated sludge system using sludge from Egå STP. An initial study of the abiotic stability in water revealed that practically no degradation took place under such conditions and, hence, the observed degradation can be attributed to the action of microorganisms including enzymatic reactions.

The aerobic degradation was very fast for E1 and E2 i.e. with half-lives of a few minutes or even lower at sludge concentrations commonly found at STPs, while the half-life of EE2 was more than 100 times higher i.e. from 1.4 hours to 11 hours, depending on the sludge concentration (fastest degradation at highest concentration). The glucuronide conjugate of E1, E1-3Glu, was transformed at a slightly slower rate than E1/E2 i.e. with a half-life of 2-16 minutes, whereas the transformation of the sulphate conjugate, E1-3Sul, took place at a rate more resembling that of EE2. In conclusion, the observed order of aerobic degradability of the estrogens was:

E2 > E1 > E1-3Glu >> EE2 > E1-3Sul.

Under anaerobic conditions (not to be confused with anaerobic digestion of sludge), the degradation rates for E1 and EE2 in the activated sludge were considerably (10-20 times) lower than under aerobic conditions while the degradation of E2 was not significantly changed i.e. still in the order of few minutes or lower depending on sludge concentration.

The observations on estrogen degradation are consistent with the international literature in which fast degradation in activated sludge systems at STPs is reported, however, with EE2 and E1-3SUL being more slowly degraded than the others and the degradation of E1 being somewhat variable.

The measured degradation rate constants were used to estimate the stability

of steroid estrogens in Egå STP using the actual hydraulic retention times

in the activated sludge compartments. The estimation predicted that more than 99.9% of E1, E2 and E1-3Glu in the sewage would be removed while about 3.3 % of EE2 would still remain in the effluent. It was not possible to predict the degradation rate of E1-3Sul. Based on these values and influent concentrations measured at other Danish STPs, the effluent concentrations of E1, E2 and E1-3Glu are estimated to be close to zero, while a maximum concentration of 0.6 ng/L is estimated for EE2. These values are somewhat too low compared to monitoring results, probably because of effects of sorption, which have not been possible to include in the predictive model.

The measured degradation rate constants were also used to estimate the stability of steroid estrogens in the effluent of Egå STP in order to predict if samples taken for analysis of steroid estrogens could be transported without preservation to the analytical laboratory. Even with the unusually low concentration of activated sludge in Egå STP of 5 mg DS/L, the concentrations of E1, E2 and E1-3Glu would be reduced significantly after only 3 hours. Therefore, all samples for estrogen analysis should always be preserved, e.g. by acidification with sulphuric acid, prior to transportation.

It is important to recognise that the predictions made for the two removal processes (sorption and degradation) are made independently of each other, while in practice they most likely interact. In practice, degradation of steroid estrogens will only take place in the water phase. If the dissolved fraction of the steroid estrogens is changed, the degradation rate is probably changed proportionally. In the laboratory experiments, the dissolved fraction of steroid estrogens was estimated to constitute only about one third of the total amount.

This implies that the estimated removal in the activated sludge tanks for Egå STP may be overestimated with about 200%. If the sorption to sludge takes place fast and the desorption rate is slower than the degradation rate (neither have currently been measured), then the desorption reaction will be rate limiting for the overall degradation of the estrogens. This means that the degradation rate will be reduced to that of the desorption as soon as the first 25-50 % of the initial concentrations of the steroid estrogens have been degraded.