(Schwede et al. 2013; Zhong et al. 2013). The maximum dosing rate of
self-sufficient co-substrates such as food waste or sewage sludge is typically limited
by the anaerobic digestion plant capacity and co-substrate availability. Regardless
of the co-substrate, anaerobic co-digestion stands as a suitable option to reach OLR
higher than 2 g VS/L/d in algae digesters, since the operation of algae
mono-digesters at OLR higher than 2 g VS/L/d has resulted in inhibitory ammonia
concentrations and caused the accumulation of volatile fatty acids (VFAs) (i.e.
higher risk of process failure) or even process failure (Yen and Brune 2007; Park
and Li 2012; Herrmann et al. 2016).
The integration of algae cultivation in WWTP to substitute the conventional
activated sludge reactor, to treat the anaerobic digestion supernatant, or to polish the
WWTP final effluent followed by their co-digestion with sewage sludge is attracting
a lot of attention (Sahu et al. 2013; Beltran et al. 2016; Peng and Colosi 2016). The
cultivation of algae on anaerobic digestion supernatant (diluted or pretreated) is of
special interest since it (1) reduces the nutrient load to the headworks, which
represents about 20% of the WWTP nutrient load; (2) mitigates greenhouse gases
emissions by using CO 2 from biogas combustion for algae growth; and (3) produces algae as on-site co-substrate, which lowers the uncertainty about co-substrate
availability and seasonality (Rusten and Sahu 2011; Yuan et al. 2012). Even though
this scenario appears very promising, it remains uncertain if the amount of algae
able to grow on digester supernatant is enough to make a significant difference on
the WWTP methane production (Hidaka et al. 2017). Conversely, the addition of
large amounts of algae (or any other nitrogen-rich co-substrate) should be carefully
evaluated since it will increase the digester and supernatant nitrogen concentration.
In this regard, Mahdy et al. (2017), who co-digested algae and cattle manure,
showed that inoculum acclimation could provide anaerobic digestion stable performance at nitrogen concentrations as high as 4 gNH 4
+
-N/L and 700 mgNH 3 -N/L.
Likewise, Arnell et al. (2016) plant-wide simulation study warned of the impact of
co-digesting nitrogen-rich waste on the WWTP water train, e.g. aeration requirement, methanol consumption, effluent quality. Finally, the cultivation of microalgae
on pig and cattle manure effluent supernatant, and its subsequent co-digestion, has
also been studied with the aim of increasing the methane production and moving
the nutrients from the supernatant to the biosolid (Wang et al. 2016a, b; Mahdy
et al. 2017).
5 Design and Operational Considerations
Biogas production using microalgae as substrate has been studied since the 1950s.
The first report addressing the anaerobic digestion of microalgal biomass was
published by Golueke et al. (1957). This early study reported a biogas production of
0.5 m
3
/kg of volatile solids of algal biomass. During the last decade, an intensive
research has been conducted in order to develop solar energy fixation processes
256
F. Passos et al.
self-sufficient co-substrates such as food waste or sewage sludge is typically limited
by the anaerobic digestion plant capacity and co-substrate availability. Regardless
of the co-substrate, anaerobic co-digestion stands as a suitable option to reach OLR
higher than 2 g VS/L/d in algae digesters, since the operation of algae
mono-digesters at OLR higher than 2 g VS/L/d has resulted in inhibitory ammonia
concentrations and caused the accumulation of volatile fatty acids (VFAs) (i.e.
higher risk of process failure) or even process failure (Yen and Brune 2007; Park
and Li 2012; Herrmann et al. 2016).
The integration of algae cultivation in WWTP to substitute the conventional
activated sludge reactor, to treat the anaerobic digestion supernatant, or to polish the
WWTP final effluent followed by their co-digestion with sewage sludge is attracting
a lot of attention (Sahu et al. 2013; Beltran et al. 2016; Peng and Colosi 2016). The
cultivation of algae on anaerobic digestion supernatant (diluted or pretreated) is of
special interest since it (1) reduces the nutrient load to the headworks, which
represents about 20% of the WWTP nutrient load; (2) mitigates greenhouse gases
emissions by using CO 2 from biogas combustion for algae growth; and (3) produces algae as on-site co-substrate, which lowers the uncertainty about co-substrate
availability and seasonality (Rusten and Sahu 2011; Yuan et al. 2012). Even though
this scenario appears very promising, it remains uncertain if the amount of algae
able to grow on digester supernatant is enough to make a significant difference on
the WWTP methane production (Hidaka et al. 2017). Conversely, the addition of
large amounts of algae (or any other nitrogen-rich co-substrate) should be carefully
evaluated since it will increase the digester and supernatant nitrogen concentration.
In this regard, Mahdy et al. (2017), who co-digested algae and cattle manure,
showed that inoculum acclimation could provide anaerobic digestion stable performance at nitrogen concentrations as high as 4 gNH 4
+
-N/L and 700 mgNH 3 -N/L.
Likewise, Arnell et al. (2016) plant-wide simulation study warned of the impact of
co-digesting nitrogen-rich waste on the WWTP water train, e.g. aeration requirement, methanol consumption, effluent quality. Finally, the cultivation of microalgae
on pig and cattle manure effluent supernatant, and its subsequent co-digestion, has
also been studied with the aim of increasing the methane production and moving
the nutrients from the supernatant to the biosolid (Wang et al. 2016a, b; Mahdy
et al. 2017).
5 Design and Operational Considerations
Biogas production using microalgae as substrate has been studied since the 1950s.
The first report addressing the anaerobic digestion of microalgal biomass was
published by Golueke et al. (1957). This early study reported a biogas production of
0.5 m
3
/kg of volatile solids of algal biomass. During the last decade, an intensive
research has been conducted in order to develop solar energy fixation processes
256
F. Passos et al.