## 4 Discussion  
Sustainable production of feedstocks for different commercial purposes such as fuels, plastics, dyes, and others [5] is one of the main pillars to improve the sustainability of microalgae and cyanobacteria biomass-based bioprocesses [47]. The physicochemical characterization of the leachate shows that it has a pH of 9.74, which would allow optimal growth of most microalgae and cyanobacteria [37]. According to [PERSON] et al. [48], leachates less than one-year-old (young leachates) contain high COD concentrations (>15 g.dm\({}^{-3}\)). On the contrary, leachates from stabilized landfills (older than five years) contain shallow COD concentration (<3 g.dm\({}^{-3}\)), which is comparable to the COD observed in Table 2 with a value of 2.93 g.dm\({}^{-3}\), thus indicating that the leachate employed corresponds to old landfill leachate. During the last 20 years, different researchers have evaluated the ability of different strains to grow in culture media enriched with landfill leachate; however, different results have proved that microalgae could not withstand the toxicity of the medium due to the high concentrations of contaminants, causing inhibition in their growth due to a deficiency in their metabolization process [23]. High nitrogen concentrations ensure efficient phosphorus removal in the leachate [47]. According to [PERSON] et al. [28], some algae adapted to leachate can remove high concentrations of Nitrate and Phosphate. In their case, strains belonging to _Chlorella_, _Scenedesmus_, and _Oscillatoria_ sp. isolated from Jeram sanitary landfill (Malaysia) can remove up to 380 mg:dm\({}^{-3}\) of NO\({}_{3}\). In another research, [PERSON] et al. [26] used 5% of leachate and removed up to 65% of NO\({}_{3}\) and 31% of PO\({}_{4}\) using _C. vulgaris_. On the other hand, when _Tetradesmus obliquus_ was grown in medium with 15% \(v/v\) of leachate, only 5% of NO\({}_{3}\) and 29% of PO\({}_{4}\) present could be removed. However, the medium of T. obliquus contained  
Figure 5: Production of metabolites of industrial interest at different carbonate and bicarbonate concentrations in _Chlorella_ sp. (**a**) and _Scenedesmus_ sp. (**b**). 3 times more leachate, so the concentrations removed by the two strains are relatively close. Other results such as those presented by [PERSON] et al. [21] demonstrate the ability of _Chlamydomonas_ sp. SW15 aRL to remove 97% of the PO4 present in a system with 30% leachate. On the other hand, [PERSON] et al. [36] were able to remove 99% of NH4 and 100% of PO4 by growing _C. vulgaris_ in a culture medium with 50% leachate. Other works such as [PERSON] et al. [32] evaluated the growth of _C. vulgaris_ in undiluted leachate. Their results show that it is possible to remove up to 96% of NH4+NO3 and 100% of PO4 present in undiluted leachate. It is noteworthy that [PERSON] et al. [32] designed a scalable membrane-based tubular photobioreactor (SM-PBR), which allowed reducing the contact of the cells with the culture medium, thus reducing the toxic effect of the leachate on the cells. The addition of a carbon source is one of the most important parts of algal production, since this microorganism is considered a sustainable carbon sink. CO2 is the most frequent source. In the photoautotrophic culture, the CO2 reacts with the different salts in the media producing carbonate ions (HCO3\({}^{-}\)). These ions can flow through the algal membrane and be used in the chloroplast [47]. Therefore, supplementing the leachate with different concentrations of sodium carbonate or sodium bicarbonate will eventually improve the growth of the microalgae and reduce the polluting capacity of the leachate. According to the results reported in this work, the addition of NaHCO3, especially in _Chlorella_ sp. significantly increased the final biomass concentration; however, the removal of NO3 and PO4 did not increase in the same way as the biomass.