Specialist Group on Use of Macrophytes in Water Pollution ... - IWA

EE2) were spiked into vessels containi
with double-distilled water supplemented with nutrients (HACH BOD nutrient cat. No.
14861-66, APHA formulation), in the presence or absence of wetland biofilm (1.5g wetland
gravel). Significant removal of estrogens was found only in the presence of wetland biofilm
and was highest under aerobic conditions. For example, aerobic half-life of E2 in filtered
secondary effluents and in water supplemented with nutrients were 48 and 36hrs,
respectively, whereas it was an order of magnitude longer under hypoxic conditions.
Biotransformation of E2 in the presence of wetland biofilm was evident by the appearance
and accumulation of E1 in the medium. Seven and 15% removal of total estrogens (E2+E1)
was found under aerobic conditions, in water and filtered effluent, respectively, whereas the
concentration of estrogens under hypoxic conditions remained unchanged.
Discussion and conclusions
Only a few studies on the removal of estrogens or estrogenicity from municipal effluent by
wetland systems have been reported, all from 2000 on (7 on SF and 4 on SSF). The estrogens
examined are mostly the natural ones E2 and E1 and a synthetic one EE2. These estrogens
were commonly found in domestic sewage effluent and are considered the most potent ones
among the endocrine disruptors (e.g. Khanal et al., 2006).
In most cases direct evidence for the pathway of estrogen removal is lacking. Measurable
attenuation as well as failure of significant removal of estrogens was reported in SF wetlands.
The attenuation of estrogens in SF is attributed mostly to adsorption and biotransformation. It
is suggested that increasing vegetation density will improve estrogen removal by providing
greater surface area for biofilm development, enhancing adsorption and biotransformation
processes (Brix, 1997; Gray and Sedlak, 2005). Decreasing wetland depth so that more water
flows through the roots and sediments as well as increasing hydraulic retention time were
also recommended (Gray and Sedlak, 2005). However, achieving high removal of estrogens
by the latter will require an HRT of months (Metamoros and Bayona, 2008), making most SF
CW impractical. Additional improvement of estrogen removal in SF wetlands can be attained
by enhancing oxygenation of the system to support a higher rate of biodegradation
(Laboratory experiments, Milstein et al., in preparation; Lee and Liu, 2002; Andersen et al.,
2004). We found no evidence for removal of estrogens in SF CW by photo-degradation. In an
experiment in which E2 was dissolved in secondary effluent and exposed to sunlight for
24hrs in open glass beakers, only 10% was photo-degraded, compared to none in the dark
(Mansell et al., 2004). Little or no photo-degradation may be expected in turbid or shaded SF
systems (Gray and Sedlak, 2005; White et al., 2006). Existing evidence suggests that in subsurface
flow wetlands estrogens are mostly attenuated under aerobic conditions by
biotransformation and degradation (Song et al., 2009; Milstein et al., in preparation). This
conclusion is supported by findings that in conventional wastewater treatment plants too,
estrogenic compounds are mostly degraded under aerobic, nitrifying conditions (e.g., Liu et
al., 2009). Indeed, high estrogen removal within a short HRT (hours) was found in vertical
flow wetlands, which are characterized by aerobic conditions. Lower removal rate may be
expected in horizontal systems which are characterized by a network of aerobic, hypoxic and
anoxic zones (Vymazal, 2003). Indeed, no attenuation of estrogens was recorded in both the
HSSF and SF systems, where hypoxic conditions prevailed (Peterson and Lanning, 2009;
Milstein et al., in preparation). This conclusion is also supported by the observation that
estrogen attenuation was reduced in vertical saturated wetlands, compared to that measured in
unsaturated conditions (Song et al., 2009). High (>90%) removal of estrogen in HSSF CW
reported by Masi et al. (2004) contradicts the above conclusion. However, Masi et al. (2004)
examined estrogen removal from primary effluent in which estrogen concentration as well as
the concentration of suspended solids was extremely high. It is highly probable that the
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20 IWA <strong>Specialist</strong> <strong>Group</strong> on Use of Macrophytes in Water Pollution Control: Newsletter No. 35