TO THE PROBLEMS OF WASTEWATER TREATMENT FROM MINERAL POLLUTANTS WITH THE USE OF MICROALGAE

. This article presents the results of experimental studies on the use of Chlorella microalgae for wastewater treatment from the most common mineral pollutants. The influence of the mineral pollutants, namely anions HSO 3− , NO 3− , H 2 PO 4− on the dynamics of wastewater treatment has been established. Analytical dependences of microalgae biomass growth rate depending on pollutant concentration HSO 3− , NO 3− and H 2 PO 4− are obtained. The values of biomass growth coefficients at the corresponding pollutant concentrations are determined. Based on the obtained experimental research results, mathematical models have been constructed that allow predicting the maximum values of mineral pollutants concentrations at which effective wastewater treatment by chlorophyllsynthesizing microalgae is possible. The technological scheme of wastewater treatment from mineral pollutants and the mechanism of the obtained application biomass are proposed.


Introduction
Due to the expansion of production and the imperfection of treatment technologies, environmental pollution has reached its peak. The hydrosphere is significantly affected by pollutants. Currently, the biological method of wastewater treatment is the promising one (Manakov et al., 1990). The disadvantage of biological purification methods such as activated sludge, granular sludge, biofilm is that the necessary condition is oxygen enrichment, mixing and, as a consequence, carbon dioxide evolution. The advantage of our technology is that the sorption of sulfur anion (HSO 3 − ), nitrogen anion (NO 3 − ), phosphorus anion (H 2 PO 4 − ) and carbon dioxide (CO2) by Chlorella microalgae occurs with the release of oxygen as the product of photosynthesis.
The problem of the current system of biological wastewater treatment in aerotanks is the enormous cost of oxygen for the bacterial processes of organic and mineral substances removal in wastewater, and carbon dioxide (CO2), as a product of bacteria living abilities, is released into the air. It means that the present existing treatment plants are the oxygen consumers and air pollutants with carbon dioxide (CO2). It should also be noted that the biological feature of the bacteria is that they are narrowly specialized, that is, there is no one type of bacteria that could purify the entire spectrum of contaminants present in wastewater (Hubskyy, 2007).
The proposed technology provides a method of wastewater treatment with different concentrations of contaminants using a specific strain of microalgae. Ecologically, this is justified by the fact that microalgae consume carbon dioxide (CO2) and release oxygen for their living abilities (Statsenko, Vynohradova, 1972). Evolutionary microalgae are at a higher level of development than bacteria, and therefore many of their species are universal consumers of many organic and mineral substances which are pollutants of wastewater.
In the burning of fuel (solid, liquid, gaseous), besides carbon dioxide, sulfur, nitrogen and phosphorus oxides are released into the air. With precipitation, these pollutants return to the hydrosphere in the form of anions and cause acidification of reservoirs. Therefore, these anions were chosen for the experimental study.
The literature contains little data on the effect of anions HSO 3 − , NO 3 − and H 2 PO 4 − on the dynamics of wastewater treatment by chlorophyllsynthesizing microalgae (Dyachok et al., 2018;Dyachok et al., 2019a). Therefore, it is important to study the influence of these anions concentrations on the dynamics of wastewater treatment.

Theoretical part
Using the strain of the Chlorella type microalgae a high degree of biological purification from mineral pollutants is achieved. The biomass, accumulated in this way, can be used as an organic "green" fertilizer or converted by biomethanization to methane gas (Zolotaryova et al., 2008). The use of this microalgae strain does not require reorganization or major construction of new treatment plants.
During the experimental research, culture of green microalgae -Chlorella was the object of observation. For this purpose, the culture of microalgae -Chlorella was placed into wastewater containing anions  Fig. 3 and Fig. 5. Accordingly, the control solution, which did not contain the corresponding anions in three variants of the research, it should be also noted that the conditions of the experiment involved the presence of natural light and the temperature of 30±5 ºС. The increase in the biomass of chlorophyllsynthesizing microalgae under these conditions was determined by the photocolorimetric method using a blue light filter according to the law of Bouguer-Lambert-Behr (Poltorak et al., 1972). Since the optical absorption of light at a given wavelength is proportional to the concentration of microalgae, the obtained experimental data on the accumulation of microalgae biomass depending on the time within the studied concentrations of anions HSO 3 − , NO 3 − and H 2 PO 4 − are proportional to the values of optical densities. The optical density value of the researched and control solutions was measured in relation to the reference solution. In our case, it was water.

Presentation of the main material and discussion of the results
During the experimental data processing, dependencies were obtained which illustrate the change in microalgae concentration over time at different values of anion HSO 3 − in solution with its single introduction − . As can be seen in Fig. 3, at the anion NO 3 − concentration, 68 mg/ml (4th sample) on the second day behaves in the same way as others, it means that the adaptation phase takes place up to two days, with a slight increase starting on the third day, which is higher on the 5th day than in the control sample, but from the sixth day there is a decline and the next five days there is a decrease in growth. At higher upper values of anion NO 3 − concentrations, there is a dying of microalgae compared to the control.
The values of the growth coefficients -μ were found similarly, by the same methodology and they were: 0.075 d -1 ; 0.076 d -1 ; 0.077d -1 . increase in the biomass of microalgae cells compared to the control. The main parameter is the value of the growth coefficients -μ which was determined according to the presented methodology Fig. 6. It should also be noted that in the experimental study, there is a change in the alkaline acid balance of the aquatic environment, namely from acidic (pH = 4.7) to neutral (pH = 6.8).   (Fig. 9).  Fig. 9. Combined wastewater treatment scheme using Chlorella type microalgae. 1anoxide zone; 2oxide zone; 3wastewater after the process with activated sludge; 4 -secondary sedimentation tank; 5precipitate secondary sedimentation tank; 6microalgae biomass; 7tank; 8cultivation of the Chlorella type microalgae; 9hydrothermal liquefaction or anaerobic digestion; 10 -biogenes and СО2; 11 -СО2, P2O5 and NxOy from other systems; 12biochar; 13protein/nutritional supplements; 14fertilizers; 15soil amendment; 16biofuels Fig. 9 presents a scheme of wastewater treatment using the Chlorella type microalgae. The scheme consists of a biofilm reactor with a movable layer of nozzle for air bubbling 1, and an open pond for the cultivation of microalgae as a biological reactor 2. Wastewater after the process with activated sludge 3, fall into the container 8, where the cultivation of the Chlorella type microalgae happens. The wastewater sludge enters the secondary sedimentation tank 4 and from there the precipitate of the second sludge 5 enters the hydrothermal liquefaction or anaerobic digestion 9. Since microalgae cultivation requires СО2, there is an opportunity to use its emission at the enterprises of other industries (for example, power plants), reducing the emission into the atmosphere. Therefore, СО2, P2O5 and NxOy from other systems 11 fall into the tank 8 for the cultivation of microalgae. From tank 8, the biomass of microalgae enters tank 6, from which it enters tank 9 for hydrothermal liquefaction or anaerobic digestion, or tank 7 where organic "green" fertilizer is made from it. From tank 9 biogenes and СО2 enter tank 10, and from it again into tank 8 of microalgae cultivation. Everything else after hydrothermal liquefaction or anaerobic digestion 9 is also converted into fertilizers 14, protein or nutritional supplements 13, soil amendment 15, and biofuels 16 (Khlorella…, 2020).

Conclusion
Chlorella microalgae are the most appropriate option for wastewater treatment. They are also a promising substrate for biofuel production since obtaining biomass creates the potential for energy production.
Based on the results of the experimental research, mathematical models have been created that allow predicting the maximum values of anions NO 3 − and H 2 PO 4. − concentrations and at which effective wastewater treatment with chlorophyll synthesizing microalgae is possible.
The result of wastewater treatment by this method is the change of alkaline acid balance from acidic to neutral after the experiment.
The scheme of wastewater treatment using chlorophyll synthesizing microalgae of the Chlorella type is proposed. Limit values of concentrations of anions HSO 3 − , NO 3 − and H 2 PO 4. − are set for the successful operation of this treatment plant.
The use of the Chlorella type microalgae for wastewater treatment creates new opportunities for environmental safety improvement by designing and building a reliable system.