Removal of nitrogen and phosphorus is a priority task when treating wastewater from different sources (Shamanskyi & Boichenko 2018). Reducing the concentration of these nutrients in wastewater is extremely important to prevent anthropogenic eutrophication of water bodies and/or to achieve their satisfactory ecological status (European Commission 2016; Mohsenpour et al. 2020). Considering the negative consequences of eutrophication (harmful algal blooms), strict regulations have been established for the content of nitrogen and phosphorus compounds in wastewater (Silva et al. 2015). Traditional wastewater treatment systems cannot provide sufficient nutrient removal. Therefore, alternative methods of wastewater post-treatment for nitrogen and phosphorus removal are necessary (Mohsenpour et al. 2020).
The most promising alternative technology for wastewater treatment is phytoremediation, which consists in the removal or biotransformation of pollutants, including nutrients, in wastewater using plants and associated microflora (Renuka et al. 2015). Microalgae have become attractive biological systems for wastewater treatment of various origins (Eladel et al. 2019; Mohsenpour et al. 2020). This is due to the fact that they are characterized by higher photosynthetic activity and lower solar energy requirements than higher plants (Bhatnagar et al. 2011; Razzak et al. 2013). In addition, they play a key role in removal of organic and inorganic pollutants from the aquatic environment (Abdel-Raouf et al. 2012). According to Eladel et al. (2019), the rate of nitrogen and phosphorus assimilation by microalgae can reach 24 kg N ha−1 day−1 and 3 kg P ha−1 day−1. In the process of remediation, microalgae effectively remove nitrogen and phosphorus from wastewater to synthesize organic substances necessary to maintain their metabolism (Markou et al. 2014; Madkour et al. 2017). Another advantage of using microalgae to remove nutrients from wastewater is the possibility of recycling assimilated nitrogen and phosphorus into algal biomass, which can be used as a biofertilizer (Solovchenko et al. 2013) or as feedstock for the production of various types of biofuels, including biodiesel, bioethanol, biomethane, biosyngas and biohydrogen (Thomas et al. 2016). It should be noted that the use of microalgae for the production of biodiesel is considered a more promising technology for its production than oilseed crops (sunflower, rapeseed, and soybean), since the yield of oils obtained from algal biomass is much higher (Abou-Shanab et al. 2010). Furthermore, the accumulation of algae biomass in wastewater can significantly reduce the cost of their cultivation due to the presence and high availability of all nutrients there needed for algae growth, which makes this technology cost-effective.
The purpose of this study was to assess the efficiency of nitrogen and phosphorus removal from different types of wastewater using
Cultures of green
Artificial wastewater with different loads of nitrogen and phosphorus was used in this study (Table 1). The composition of the synthetic wastewater was based on a modified formulation of the BG-11 medium. NH4Cl and KH2PO4 were added to the culture medium for microalgae in varying amounts to obtain typical concentrations of ammonium nitrogen and phosphate found in wastewater (Hence et al. 2002; Rawat et al. 2011; Acevedo et al. 2017).
Concentrations of N-NH4+ and P-PO43− in wastewater
Variant of the experiment | Initial concentration, mg l−1 | |
---|---|---|
N-NH4+ | P-PO43− | |
1 | 30.00 ± 1.12 | 7.00 ± 0.18 |
2 | 50.00 ± 2.25 | 14.00 ± 0.56 |
3 | 90.00 ± 2.58 | 26.00 ± 0.47 |
4 | 140.00 ± 2.68 | 41.00 ± 0.67 |
Cultures of microalgae in the late exponential phase of growth (18-day culture) were used to prepare the inoculum. The ratio of inoculum to wastewater was 1:10. Test vessels were flasks with a volume of 500 cm3. Microalgal cultures were incubated in wastewater for 14 days under artificial lighting (using Philips TL-D 18W 54-765 G13 daylight fluorescent lamps). The light intensity was 50–54 μmol photons m−2 s−1; light regime – 16 h of light and 8 h of darkness. The pH and temperature of the test solutions were monitored throughout the exposure period. The pH of the solutions remained in the range of 7.5 to 8.2 and did not change during the exposure period by more than 0.7 units in any test; the temperature of the solutions was 25.0–26.2°C.
The concentration of ammonium nitrogen and phosphate in water was analyzed according to Arsan et al. (2006).
The nutrient removal efficiency was calculated as follows:
The functional activity of the cultures under the studied conditions was assessed based on changes in dry weight and the content of photosynthetic pigments (chlorophyll
To determine the dry weight, the filters with concentrated microalgae were dried in a thermostat at a temperature of 105ºC to constant weight. Dry matter content was calculated according to the formula (Lim et al. 2010):
The specific growth rate (μ) of microalgae was calculated using the equation (Krzemińska et al. 2014):
The content of pigments was analyzed by an extractive spectrophotometric method (SCOR-UNESCO, 1966). For extraction of pigments, the filter with algae was thoroughly homogenized with the addition of quartz sand and 90% acetone. The resulting extract was separated by centrifugation. In samples of green microalgae, which contained chlorophyll
All measurements were conducted in triplicate. Statistical processing of the research results was performed using SPSS Statistics software (version 17). The Dunnett test was used to assess the differences between the values of the indicators. A difference was considered significant at p ≤ 0.05. All data were presented as mean ± standard deviation (M ± SD).
During the cultivation period, the efficiency of ammonium nitrogen and phosphate removal by
Final concentration of nutrients in wastewater (mg l−1) during the microalgae cultivation period, M ± SD
Experiment variant | ||||
---|---|---|---|---|
N | P | N | P | |
1 | 11.00 ± 0.25 | 5.70 ± 0.13 | 12.00 ± 0.50 | 3.15 ± 0.11 |
2 | 20.03 ± 1.00 | 13.00 ± 0.36 | 19.50 ± 1.21 | 8.00 ± 0.44 |
3 | 40.00 ± 2.50 | 24.00 ± 1.48 | 62.00 ± 3.45 | 19.30 ± 0.82 |
4 | 71.00 ± 2.25 | 38.00 ± 1.66 | 168.10 ± 5.40 | 50.00 ± 2.55 |
For
A reliable indicator of the metabolic activity of microalgal cells is the growth rate. Figure 1 shows the changes in dry weight of
An opposite trend was observed for the blue-green alga
When microalgae are used for wastewater treatment, their high photosynthetic activity is of paramount importance, because the rate of photosynthetic processes in algal cells determines the rate of biomass accumulation, nutrient assimilation, and photosynthetic aeration (Solovchenko et al. 2013). An important characteristic of the photosynthetic apparatus, determining its functional activity, is the content of photosynthetic pigments, chlorophyll
Changes in the content of photosynthetic pigments in
Experiment variant | Chlorophyll | Carotenoids, mg g−1 DW | ||
---|---|---|---|---|
0 days | 14 days | 0 days | 14 days | |
1 | 4.89 ± 0.25 | 8.66 ± 0.53 | 1.08 ± 0.04 | 2.17 ± 0.10 |
2 | 4.89 ± 0.25 | 9.01 ± 0.45 | 1.08 ± 0.04 | 2.22 ± 0.11 |
3 | 4.89 ± 0.25 | 10.47 ± 0.58 | 1.08 ± 0.04 | 2.48 ± 0.11 |
4 | 4.89 ± 0.25 | 14.89 ± 0.66 | 1.08 ± 0.04 | 3.50 ± 0.20 |
In the biomass of the
Changes in the content of photosynthetic pigments in
Experiment variant | Chlorophyll | Carotenoids, mg g−1 DW | ||
---|---|---|---|---|
0 days | 14 days | 0 days | 14 days | |
1 | 4.00 ± 0.12 | 7.00 ± 0.33 | 0.62 ± 0.02 | 2.40 ± 0.09 |
2 | 4.00 ± 0.12 | 6.76 ± 0.24 | 0.62 ± 0.02 | 2.10 ± 0.10 |
3 | 4.00 ± 0.12 | 4.60 ± 0.16 | 0.62 ± 0.02 | 1.12 ± 0.04 |
4 | 4.00 ± 0.12 | 1.03 ± 0.04 | 0.62 ± 0.02 | 0.47 ± 0.01 |
The results of the study show that
As reported in the literature (Markou et al. 2014), the ability of microalgae to remove nutrients from the aquatic environment depends on various factors. The rate and efficiency of these processes are first determined by physiological characteristics of species, in particular their requirements for various nutrients to maintain their vital activity (Fernandes et al. 2017). Thus, for example, the nitrogen content can range from 1% to 14% of dry weight, depending on the species of microalgae (Markou et al. 2014), whereas the phosphorus content can range from 0.05% to 3.3% of their dry weight (Grobbelaar 2004). Nitrogen and phosphorus are involved in many metabolic pathways in algal cells (Madkour et al. 2017). Nitrogen is a component of proteins (amino acids), nucleic acids (DNA, RNA), and pigments (Markou et al. 2014). Phosphorus is a structural component of phospholipids, nucleic acids and an integral component of the unique “energy currency”, ATP (Borowitzka et al. 2016). It was shown that the total protein content in
Low efficiency of phosphate removal by
According to the results obtained,
It should be noted, however, that
Wastewater from different sources is characterized by varying nitrogen and phosphorus content, as well as the ratio of these nutrients, so it is necessary to carry out preliminary screening of microalgae cultures before using them in wastewater treatment systems with the appropriate chemical composition.
The results of this study indicate that
The culture of