tested. The wetland system seems to possess a buffering capacity, as the pH is
stabilized in the slightly basic region. The increase in the electrical conductivity and
also TDS is more or less expected, due to both the interactions between the
wastewater and the substrate media that possibly results in the release of salts in
the water, as well as to the water loss through evapotranspiration in the two wetland
beds. It is calculated that this water loss in the wetland system through both
evaporation and plant transpiration accounts for approximately 15% of the inflow
water as an average annual value. Higher values are expected during summer months
and lower in winter.
As it is obvious, the effluent quality complies with the national irrigation
standards (Table 7.2). Most of the organic solids are retained above the filter layer,
as indicated by the low effluent TSS concentration. Moreover, the system achieves
almost complete organic matter degradation and ammonia nitrification, mainly as a
result of the high oxygen provided via the artificial aeration means. Similar high
efficiency results are also reported in other studies (Butterworth et al. 2013, 2016;
Fan et al. 2013). The complete and fast degradation of organic matter might be
attributed to the easily biodegradable carbon in the domestic wastewater and the high
temperatures of the area (Li et al. 2014). As it is known, the intensity of the metabolic
activities of microorganisms that degrade organic matter is largely dependent on
oxygen supply (Stefanakis et al. 2014). Artificial aeration provides sufficient oxygen
amount, which favours these microbial activities, resulting in high COD and BOD
removal.
The removal of ammonia nitrogen was almost complete and remained quite stable
during the whole monitoring period. Nitrification is an aerobic process; thus the
higher oxygen supply via the artificial aeration provides adequate dissolved oxygen
for ammonia nitrogen oxidation (Li et al. 2014; Stefanakis et al. 2014). The nitrifying bacteria obtain energy from oxidizing NH4+-N and use oxygen as an electron
acceptor. The ambient temperatures are also ideal for nitrification, since it is known
that this process has an optimum temperature range between 25
C and 35
C
(Kuschk et al. 2003; Stefanakis et al. 2014), as well as optimum pH values around
7.5 to slightly alkali values close to 7.8 (Cooper et al. 1997; Stefanakis et al. 2014).
In addition to that, it is also noticeable that nitrate effluent concentration remains
below the legal limit. This implies that despite the aerobic conditions in the system
and the almost complete nitrification, nitrate removal also takes place (denitrification). This interesting result has also been found in other studies (Tee et al. 2011;
Saeed and Sun 2011; Fan et al. 2013). Denitrification is a known anaerobic process,
since excess oxygen suppresses the enzyme system required for this process. More,
sufficient organic carbon supply is crucial as an electron donor for nitrate reduction
and provides an energy source for denitrification microorganisms. In this design,
sufficient carbon supply is maintained through the step-feeding of raw wastewater to
the ACW, which apparently contributes to the nitrate transformation. Additionally,
the aeration regime possibly alters the microbial community composition and
characteristics, enabling nitrate removal even under these conditions. Such changes
in the operational mode and even wastewater composition have been found to impact
the microbial community composition and patterns (Stefanakis et al. 2016). Further
208
A. I. Stefanakis
stabilized in the slightly basic region. The increase in the electrical conductivity and
also TDS is more or less expected, due to both the interactions between the
wastewater and the substrate media that possibly results in the release of salts in
the water, as well as to the water loss through evapotranspiration in the two wetland
beds. It is calculated that this water loss in the wetland system through both
evaporation and plant transpiration accounts for approximately 15% of the inflow
water as an average annual value. Higher values are expected during summer months
and lower in winter.
As it is obvious, the effluent quality complies with the national irrigation
standards (Table 7.2). Most of the organic solids are retained above the filter layer,
as indicated by the low effluent TSS concentration. Moreover, the system achieves
almost complete organic matter degradation and ammonia nitrification, mainly as a
result of the high oxygen provided via the artificial aeration means. Similar high
efficiency results are also reported in other studies (Butterworth et al. 2013, 2016;
Fan et al. 2013). The complete and fast degradation of organic matter might be
attributed to the easily biodegradable carbon in the domestic wastewater and the high
temperatures of the area (Li et al. 2014). As it is known, the intensity of the metabolic
activities of microorganisms that degrade organic matter is largely dependent on
oxygen supply (Stefanakis et al. 2014). Artificial aeration provides sufficient oxygen
amount, which favours these microbial activities, resulting in high COD and BOD
removal.
The removal of ammonia nitrogen was almost complete and remained quite stable
during the whole monitoring period. Nitrification is an aerobic process; thus the
higher oxygen supply via the artificial aeration provides adequate dissolved oxygen
for ammonia nitrogen oxidation (Li et al. 2014; Stefanakis et al. 2014). The nitrifying bacteria obtain energy from oxidizing NH4+-N and use oxygen as an electron
acceptor. The ambient temperatures are also ideal for nitrification, since it is known
that this process has an optimum temperature range between 25
C and 35
C
(Kuschk et al. 2003; Stefanakis et al. 2014), as well as optimum pH values around
7.5 to slightly alkali values close to 7.8 (Cooper et al. 1997; Stefanakis et al. 2014).
In addition to that, it is also noticeable that nitrate effluent concentration remains
below the legal limit. This implies that despite the aerobic conditions in the system
and the almost complete nitrification, nitrate removal also takes place (denitrification). This interesting result has also been found in other studies (Tee et al. 2011;
Saeed and Sun 2011; Fan et al. 2013). Denitrification is a known anaerobic process,
since excess oxygen suppresses the enzyme system required for this process. More,
sufficient organic carbon supply is crucial as an electron donor for nitrate reduction
and provides an energy source for denitrification microorganisms. In this design,
sufficient carbon supply is maintained through the step-feeding of raw wastewater to
the ACW, which apparently contributes to the nitrate transformation. Additionally,
the aeration regime possibly alters the microbial community composition and
characteristics, enabling nitrate removal even under these conditions. Such changes
in the operational mode and even wastewater composition have been found to impact
the microbial community composition and patterns (Stefanakis et al. 2016). Further
208
A. I. Stefanakis