**2. Material and methods**

#### **2.1 Study area**

The lake system of Xochimilco is located south in Mexico City, surrounded by a mountainous area formed by the hills Xochitepec, Cantil and the volcanoes Teoca, Zompole and Teutli [12]. It is in the geographic coordinates 19° 00<sup>0</sup> and 19° 20' North Latitude; 99° 00<sup>0</sup> and 99° 16' West Latitude, with an approximate surface of 2657 ha, at an altitude between 2240 and 2500 m [13–15].

The climate is sub-humid temperate, with rains in summer and an average annual temperature varying between 8 and 18°C. The average rainfall is of 620 mm/year, the most abundant rains occur between the months of June and September and the minimum from December to February [16].

Among the most important channels are Cuemanco, Canal Nacional, Chalco, del Bordo, Apatlaco, San Sebastián, Apampilco, Texhuilo and Japón. Also, the main lagoons are Tlilac, del Toro, Huetzalin, Apampilco, Texhuilo and the Lake of conservation of flora and fauna of San Gregorio Atlapulco [17].

#### **2.2 Field work**

Sampling was carried out at the dump of water from Cerro de la Estrella wastewater treatment plant, located in the old channel of Cuemanco. The samples of microalgae were collected directly from the outlet of the dump water pipe, just at the drop and at the distances of 10, 20, 40 and 60 m (**Figure 1**).

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

**Figure 1.** *Study zone and sampling points map, based on Google earth, 2020.*

For microalgae study two types of samples were taken, for the quantitative and qualitative analysis. For the first, samples were taken with the aim of a Van Dorn bottle, placed in 500 mL containers with lugol at 1% solution. For qualitative samples it was used a trawl net with a mesh opening of 54 μm, this samples were placed in amber jars of 30 mL and it was added formalin at 4%. In each sampling point it was recorded the pH, temperature, conductivity, depth, and turbidity. Also, were taken water samples of 100 mL to determine nutrients concentrations of NO2-, NO3-, NH4+, and PO4 <sup>3</sup> in laboratory.

On the other hand, water samples with live organisms were stored for the isolation of three species for use in wastewater purification bioassays from the Cerro de la Estrella treatment plant.

#### **2.3 Laboratory work**

The sample review of microalgae was carried out in Phycology and Phytopharmacology laboratory from UAM Xochimilco, using a Zeiss optical microscope model Axiostar. Aliquots of 0.1 mL were taken and reviewed with the scanning technique [18], which consist on locate a starting point and make the revision in the form of "transects", from each sample the necessary aliquots were revised until no new organism was observed.

For the isolation of microalgae different techniques were used, including capillary pipetting, seeding in agar plates, and reseeding in liquid medium.

The nutrients (NO2-, NO3-, NH4+, PO4 <sup>3</sup>, TP, and TN) were evaluated using a multiparametric photometer HI 83200 [19]. Initial parameters were valuated, which were the different forms of inorganic nitrogen (NO2-, NO3-, and NH4+), phosphate as orthophosphate and heavy metals (Pb, Ni and Cu), using a multiparametric photometer HI 83200 and a spectrophotometer HACH 3900.

Bioassays of removal of nutrients and heavy metals were conducted, for which the isolated microalgae were used in the samples of treated wastewater from the Cerro de la Estrella treatment plant. To bioassays, manual agitation twice a day were given to avoid sedimentation of microalgae and water nutrients.

To provide the necessary amount of light, white light bulbs were used and controlled at intervals of 12 hours light and 12 hours dark with a timer clock.

### **2.4 Data analysis**

In order to know the differences in the values of nutriment concentrations and parameters taken *in situ* in the sampling points located at different distances, were obtained a coefficient of variation, standard deviation, and arithmetic mean, using the program Excel 2013, in addition to a linear correlation analysis to know the interaction with the physicochemical parameters registered in field.

On the other hand, counts were made for 10 days and plotted to know the growth curve of each isolated microalgae.

Regarding bioassays, with the obtained values population growth graphs were made for each microalga; also, it was made a comparison between physicochemical parameters measured in each bioassay. In addition, this data was analyzed to observe the change in nutrient and heavy metal concentrations in each bioassay and evaluated the purification capacity of each microalga.
