**4. Discussion**

#### **4.1 Microalgae**

According to obtained results a total of 88 species of microalgae were determined, from which 55 have been previously reported by diverse authors, in different points in the Xochimilco channels and at different seasons. On the other hand, 24 species were new registers for the study zone, which indicates that nowadays there's no full knowledge of the species present in this place, this can be due to the fact that over time the conditions of the environment are changing, making changes in the microalgae community composition, as well as the introduction of new species, that come from treated water.

## **4.2 Physicochemical parameters**

Regarding the parameters taken on field, it could be observed that most of them did not vary significantly, only depth had a variance higher than 30%, which decrease according to distance, one of the reasons can be the movement that the waterfall generates on the lake, because the greater the distance the movement of the water is less, which favors a higher deposit of sediments.

Nutriments values were compared with the NOM-001-ECOL-1996 [20], where the maximum permissible limits for basic pollutants in wastewater discharges are established. It was observed that the values of total nitrogen (TN) for use for agricultural irrigation do not exceed the maximum permitted limits (40–60 mg/L), because in the present investigation a maximum value of 14.6 mg/L was obtained. Nevertheless, this value is near the maximum limits permitted for urban public use which is of 15 mg/L.

Regarding to obtained total phosphorus values (TP), the values were in a range of 5 to 9.4 mg/L and they did not exceed the maximum permitted limits for use for agricultural irrigation which are of 20 mg/L daily average and 30 mg/L monthly. However, for urban public use, the obtained values are above the monthly average (5 mg/L) and very close to the permitted daily average which is of 10 mg/L [20].

As for nitrogen, it was observed a higher concentration in the form of Nitratos (NO3 ) in the site of the dump (14 mg/L), observing a decrease as the sites were farther away from the waterfall, this can be due to the higher density of microalgae found in those sites, which could be using this nutrient, because NO3 are one of the main forms of nitrogen that absorb microalgae [21].

The above is complemented with the linear correlation analysis because the higher correlation values were obtained in the NO3 and NH4+ on species richness, which points out that the presence of these nutrients is essential for the growth and formation of biomass, as microalgae absorb them directly [22].

## **4.3 Isolation and growth of microalgae**

After work with isolation techniques, the species *Chlamydomonas* sp. and *Chlorella* sp., responded better to isolation in liquid medium, because both species are more of planktonic character, in addition to being species with wide ranges of tolerance regarding temperature and nutriments.

*Nitzschia* cf. *ampphibia* being a specie of benthic character, was isolated in solid medium with the Pasteur pipette spraying technique, so when growing it produced brown spots, which is a characteristic of the species of Bacillariophyta Division.

It should be mentioned that the species *Chlamydomonas* sp. and *Nitzschia* cf. *amphibia* did not were very abundant in in reviewing field samples, only *Chlorella* sp. was.

#### **4.4 Bioassays**

## *4.4.1 Growth curves*

Regarding the growth of the three microalgae in the wastewater from the dump, it was observed that none of them presented a normal growth curve [23], in such a way that in the case of *Chlamydomonas* sp. and *Chlorella* sp. they had an exponential growth until the fifth day of experiment without having a stationary phase, but continued directly to death phase during the sixth day for *Chlamydomonas* sp. and tenth day for *Chlorella* sp., to then observe ups and downs in the number of cells until the end of the experiment. These results differ from the study conducted by [24] which worked with *Chlorella* sp. and obtained a more normal growth curve, and it was observed that the maximum growth value was obtained at 15 says, having its death phase between 17 and 18 days of their experimentation.

*Nitzschia* cf. *amphibia* also showed a different growth than conventional curves, because it had an exponential growth during the firsts days having its maximum growth on day four without having an stationary phase, but as in the other two species went directly to the death phase during the five and six days of the experiment, followed by various phases of exponential growth and death in few days, which could be explained due to the rapid growth that is given by this species, that according to Brennan and Owende [25], some species of microalgae can duplicate its biomass in less than 24 hours.

Additionally, it was observed that in some time the cells remained glued to the glass in the flask and despite the care a smaller number of cells was counted. When the flask was vigorously agitated it was not counted the same number of individuals as the days before.

#### *4.4.2 Comparison between treatments*

Regarding to comparison of the nutriment evaluation it was observed that *Chlamydomonas* sp. and *Chlorella* sp. use NO3- and NH4+ as source of nitrogen because they reduced the NO3 in 61% for *Chlamydomonas* sp. and 57% for *Chlorella* sp. at the end of the experiment, and both species reduced up to 100% the NH4+ at day six, in this regard. Oliveros and Wild [26] point out that *Chlorella* sp. was capable of removing up to 95% of NO3 in wastewater and mentioned that this microalga is suitable for this type of treatment because it has a great ability to remove nutrients in wastewater. Meanwhile, Chacón et al. [27], mentioned in their study that the highest reductions by *Chlorella* sp. were of NH4+, reducing it by 100% as in this study. However, their experimentation time was 27 days while in this investigation was 15 days.

In the case of *Nitzschia* cf. *amphibia*, it was observed that this specie use NH4+ as source of nitrogen, because it reduced this nutriment in a 97% at 12 days of experimentation and according to Pérez [28], some microalgae prefer nitrogen in form of NH4+, so when looking at the graphs of the nitrite and nitrate values an increase was noticed instead of decreasing.

On the other hand, in the case of phosphates, the method of using it was more dynamic, which was reflected in increases and decreases during the experiment by the three microalgae, so a removal of this nutriment could not be found. However, it was observed that *Nitzschia* cf. *amphibia* and *Chlamydomonas* sp., presented higher percentages of removal of this nutriment at 12 days of experimentation with 36% and 52% respectively. This is like the reported by Oliveros and Wild [26], which worked with *Chlorella* sp. and obtained a removal percentage of 20% at 8 and 12 hours of experimentation.

About heavy metals, as mentioned above the values of copper were zero, so they were discarded from the subsequent analyses. Meanwhile, the levels of nickel were reduced by *Nitzschia* cf. *amphibia,* after six days of experimentation by 63%. Nevertheless, after that time the values increased again, which might suggest that microalgae adsorb and retain it for some time, but when it dies, the metal is released again, and the value increase. Instead, *Chlamydomonas* sp. had a slight increase in the values on the first six days; however, from day nine it reduced the value of nickel by 75% at the end of experiment, being this specie that show the highest removal percentage of removal. On the other hand, *Chlorella* sp. at the third day of

*Water Cleaning by Means of Microalgae in the Channels of Xochimilco, Mexico DOI: http://dx.doi.org/10.5772/intechopen.104711*

experimentation had an increase in nickel values, and at sixth day there was a slight decrease of 63%, nevertheless, after that it was observed that the values increased util the end of the experiment, contraire to the experiment made by Hammouda et al. [29] which reported a removal of 77.3% up to 81% of Ni by *Chlorella* sp.

The lead was the metal that presented highest removal by the three microalgae, with 49% by *Nitzschia* cf. *amphibia,* 64% with *Chlorella* sp. and 79% with *Chlamydomonas* sp., being this last one the one with the highest capacity of absorption of this metal. It is worth mentioning that the experiment lasted 15 days and it was not possible to decrease the totality of this metal by the microalgae. Perhaps, if the experimentation time was longer, the totality of this metal could have been removed, but would be necessary to remove the produced biomass, because the metal would remain in the microalgae and could be transferred to the next food chain levels, including reaching humans.

Of the three microalgae study, *Chlamydomonas* sp. appears to be the one that have better capacity of nutrients and heavy metals removal with 50%, except in NO2-, so it is reiterated that apparently, they do not use this form of nitrogen for their growth. Secondly, there is *Chlorella* sp. which although with some nutrients such as phosphate had a low percentage of removal (26%), in other nutrients such as NH4+ reached to remove from 95 to 100% throughout the experiment. This match with what was reported by Martínez et al. [30], which mention that some species of chlorophytes are capable of remove 98% of the phosphorus and up to 100% of nitrogen in wastewaters.

Meanwhile*, Nitzschia* cf. *amphibia* resulted being the microalgae with lowest efficiency regarding the quantity of removed compounds, because of not decreasing nitrite and nitrate values and had low removal of heavy metals, nevertheless, it can be considered as highly efficient in the removal of NH4+ since it reached a 97%.
