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

Degradation of Estrogens in Sewage Treatment Processes

Summary and conclusions

Scientific and public concern about the possible impacts of steroid hormones (estrogens) being released into the aquatic environment has been increasing in the last few years. Public sewage treatment plants (STPs) are believed to be a major source of discharge of these substances, but knowledge about the specific processes regulating the fate in STPs is still sparse.

In recognition of this, the Danish Environmental Protection Agency (DEPA) has funded the present project, which has the objective to generate more knowledge about the ability of the key treatment processes at typical, modern Danish STPs (i.e. sorption and degradation in the activated sludge stage) to remove estrogens from the water phase.

The report mainly presents the documentation and results of the experimental laboratory work performed, but two literature based introductory chapters describe the present situation regarding treatment of public sewage in Denmark and the present knowledge about estrogens in sewage and their fate in STPs, respectively.

Most of the 1,267 Danish STPs bigger than 30 PE have been upgraded significantly since 1987 when the first Water Environment Action Plan was launched by the Danish Parliament. The plan included more strict emission standards for nutrients and organic matter for STPs > 5,000 PE and, thus, rendered technological upgrading of the majority of Danish STPs necessary.

Today, about one fifth of the STPs treat almost 90% of the total volume of sewage in Denmark. Typically, these plants have mechanical treatment and biological treatment including removal of nitrogen and organic matter in activated sludge systems, a chemical precipitation step and finally settling of suspended particles in a clarifier tank (MBNDK plants). Many are, in addition to this, equipped with a filter or lagoon after the settling step.

The STP delivering the activated sludge for the main part of the experimental work, Egå STP near the city of Århus, is fairly representative of the MBNDK type of STPs in Denmark. The designed treatment capacity of the plant is 90,000 PE and the effluent quality with respect to general parameters is somewhat better than the average indicating that Egå is a well-run STP.

The substances chosen for the experimental study were the natural estrogens estrone (E1) and 17β-estradiol (E2), and the synthetic estrogen used in most contraceptive pills, 17α-ethinylestradiol (EE2). These are the substances exhibiting the highest estrogenic activity. In addition to these, two conjugates of E1 (a glucuronide- and a sulphate conjugate; E1-3GLU and E1-3SUL) were included as estrogens are excreted from the human body and enter the sewage systems as conjugates. The conjugates possess in themselves virtually no estrogenic activity but they can be transformed back into the free, active form in the sewage system.

The studies published of the fate of estrogens in STPs are new and relative few, but they indicate that the primary treatment step has only very limited effect while the main removal of estrogens take place in the activated sludge system. E2 disappears quickly (is transformed into E1) while the removal of E1 takes place a little more slowly and more variably, and the rate by which EE2 is removed is even lower. Conjugates, in particular the glucuronides, are cleaved easily in sewage systems to produce the free, active forms of the estrogens. Based on the literature, a long sludge retention time appears to improve the treatment efficiency.

The experimental study comprised sorption experiments with free estrogens using sludge from two different STPs (Egå and Lundtofte), testing of the abiotic stability in sterile water, and the degradation under aerobic and denitrifying conditions, respectively (conjugates only aerobically).

The adsorption isotherm was determined using a modified version of the US EPA procedure while biodegradation experiments were performed in glass reactors with 0.50 g of activated sludge/L being fed daily with artificial peptone sewage according to OECD. To maintain homogeneity and aerobic conditions in the reactors, oxygen was added via an inlet in the bottom of the reactor. In the anaerobic experiments the oxygen was replaced by nitrogen but otherwise the set-up was similar to the aerobic experiment.

The sorption experiments showed that the substances were sorbed 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 sludges 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. The results indicate that at common sludge densities in Danish STPs about 35-45% of E1 and 55-65% of E2 and EE2 can be expected to be sorbed to sludge in the aeration tanks.

The test of abiotic stability in water revealed that practically no degradation took place under sterile conditions and, hence, any 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 was: E2 > E1 > E1-3Glu >> EE2 > E1-3Sul.

Under anaerobic conditions in the activated sludge, the degradation rates for E1 and EE2 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 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. The predicted removal efficiencies are higher than typically observed in monitoring studies at STPs.