Page 195 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Biorefinery of microalgae biomass cultivated in wastewaters 165

P and iron (Fe) limitations in LL cultures were the probable reasons for the lower
growth in LL.
Limited amounts of phosphorus to grow algae are a common matter found
in LLs.
Aside from PO 3 -P assimilation by microalgae cells, other mechanisms can par-
ticipate in the P removal in LLs (Pereira et al., 2016). The P precipitation at high
pH levels is the main reason for the abiotic P removal from algae cultures
(Paskuliakova et al., 2018; Zhao et al., 2014). P precipitates mostly consist of cal-
cium carbonate (CaCO 3 ) with small amounts of P, N, and Mg. An improved growth
of C. vulgaris, and NH 3 -N removal following the addition of phosphorus to the cul-
ture media, was observed by Pereira et al. (2016). The utilization of phosphoric
acid (H 3 PO 4 ) as P supplement and to control the pH of LL-based growing medium
was effective in improving the growth of Chlamydomonas sp. and in reducing NH 3
losses by volatilization (Paskuliakova et al., 2018).
The loss of NH 3 from the cultures can be due to NH 3 stripping caused by aera-
tion under alkaline conditions. Zhao et al. (2014) reported that only 52% of the
NH 3 removed from cultures of microalgae bacteria consortium in LL was
absorbed biologically. The remaining was lost to the surrounding by stripping due
to the increased alkalinity provoked by the addition of NH 3 at high levels.
The toxicity level of NH 3 to the cells is highly variable among species and it is
affected by the composition of the culture media. Lin et al. (2007) found that three
strains were inhibited by increasing the proportion of leachate in the culture media.
This was apparently linked to high concentrations of NH 3 -N ( . 670 mg/L). In their
study the maximum growth of C. pyrenoidosa grown was obtained in a growing
media consisting of 10% LL in water (134 mg/L of NH 3 -N). Similarly, Zhao et al.
(2014) observed the maximum biomass productivity of a microalgae bacteria con-
sortium (mainly C. pyrenoidosa) in growing media containing 10% LL (183 mg/L
of NH 3 -N).
1
There is a pH-dependent equilibrium between soluble NH and dissolved ammo-
4
nia gas [NH 3 (aq)]. The concentration of NH 3 in the cultures increases with increas-
ing pH and temperature. High pH, with high NH 3 concentration in the leachate,
1
causes NH to be deprotonated into its neutral form NH 3 , which can then be vola-
4
tilized (Sniffen et al., 2018). However, the increase in pH not only promotes NH 3
volatilization but also increases leachate toxicity as free NH 3 gas is more toxic than
the soluble form of NH 1 (Edmundson and Wilkie, 2013). Another reason for the
4
raise in pH in the cultures is the photosynthesis process, since the consumption of
1
CO 2 (carbonic acid in its hydrated form) by algae cells releases H ions. The fluc-
tuation of the pH in LLs-based culture media must be controlled to achieve optimal
growths.
Sniffen et al. (2018) conducted a 1-year experiment in a large-scale open pond
to assess the nitrogen pathway in LL during microalgae growth. Based on their
study, the loss and the gain in N in the culture were mainly due to NH 3 volatiliza-
tion and N transformation, respectively. They also proposed that the apparition of
2
NO could indicate that ammonia-oxidizing bacteria were present and active.
2