Carbon dioxide (CO2) is often a limiting factor for the growth of microalgal biomass. Consequently, the search for new CO2 sources that do not contain components inhibitory to microalgal metabolism remains a priority. An alternative to the solutions tested thus far may involve the use of CO2-rich gas derived from microbial fuel cells (MFCs). This concept served as the basis for the original experimental work described in this study. The objective of the research was to evaluate the effect of using gases from the anode chamber of an MFC as a CO2 source in the autotrophic cultivation of Tetraselmis subcordiformis. The highest biomass growth efficiency was observed when the CO2 concentration in the culture medium was maintained at 220.0 ± 8.0 mg/L. Under these conditions, the microalga proliferation rate reached 0.52 ± 0.03 g VS/(L∙day) and 11.54 ± 0.42 mg chl-a/(L∙day), with a final biomass concentration of 2.68 ± 0.10 g VS/L and 63.53 ± 2.44 mg chl-a/L at the end of the cultivation cycle. Moreover, the highest total hydrogen (H2) production of 312 ± 38 mL was achieved in the same experimental variant, corresponding to an H2 production rate of 62.4 ± 6.1 mL/day. The removal efficiency of ammonium nitrogen (N-NH4) was notably high in experimental variants using MFC-derived biogas, ranging from 97.0 ± 2.2% to 98.2 ± 1.8%. Additionally, the growing microalgal biomass effectively utilized phosphate phosphorus (P-PO4) and iron, further highlighting its potential for nutrient recovery.

The development and implementation of innovative production technologies have a direct influence on the creation of new sources of pollution and types of waste. An example of this is the wastewater from soil-less agriculture and the effluent from microbial fuel cells. An important topic is the development and application of methods for their neutralisation that take into account the assumptions of global environmental policy. The aim of the present study was to determine the possibilities of utilising this type of pollution in the process of autotrophic cultivation of the biohydrogen-producing microalgae Tetraselmis subcordiformis. The highest biomass concentration of 3030 ± 183 mgVS/L and 67.9 ± 3.5 mg chl-a/L was observed when the culture medium was wastewater from soil-less agriculture. The growth rate in the logarithmic growth phase was 270 ± 16 mgVS/L-day and 5.95 ± 0.24 mg chl-a/L-day. In the same scenario, the highest total H2 production of 161 ± 8 mL was also achieved, with an observed H2 production rate of 4.67 ± 0.23 mL/h. Significantly lower effects in terms of biomass production of T. subcordiformis and H2 yield were observed when fermented dairy wastewater from the anode chamber of the microbial fuel cell was added to the culture medium.

Research to date has mainly focused on the properties and efficiency of the production of selected, individual types of biofuels from microalgae biomass. There are not enough studies investigating the efficiency of the production of all energy sources synthesised by these microorganisms in a single technological cycle. The aim of this research was to determine the possibilities and efficiency of the production of hydrogen, bio-oil, and methane in the continuous cycle of processing T. subcordiformis microalgae biomass. This study showed it was feasible to produce these three energy carriers, but the production protocol adopted was not necessarily valuable from the energy gain standpoint. The production of bio-oil was found to be the least viable process, as bio-oil energy value was only 1.3 kWh/MgTS. The most valuable single process for microalgae biomass conversion turned out to be methane fermentation. The highest specific gross energy gain was found after applying a protocol combining biomass production, hydrogen biosynthesis, and subsequent methane production from T. subcordiformis biomass, which yielded a total value of 1891.4 kWh/MgTS. The direct methane fermentation of T. subcordiformis biomass enabled energy production at 1769.8 kWh/MgTS.

Microalgae have gained attention due to their higher reproduction rate and lipid productivity. In particular, various stress conditions lead to an overproduction of lipids in microalgae cells. The study investigated the influence of additional CO2 introduced with air into the reactor during biomass growth of Chlorella sp. Additionally, increased phosphorus concentration in the medium under stress cultivation (low nitrogen concentration) was examined. The partial pressure of CO2 and its increased availability to Chlorella sp. in the cultivation medium increased biomass growth (1.4 times) and chlorophyll content (2.5 times) in microalgae cells. A high phosphorus fertilizer significantly increased lipid production under stress conditions with CO2 supply to 85.2 mg/g (2.6 times) and without CO2 to 73.8 mg/g (2.2 times). A high concentration of phosphorus in the culture medium stimulated the synthesis of C16:0 (about 38–45%) and C18:1 CIS9 (about 24–30%). The results confirm that the fertilizers can be used as a culture medium to induce stress and stimulate lipid production. Adjusting the composition of the fertilizers and controlling the additional CO2 supply could prove beneficial to increase the content of the desired fatty acids.

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Photosynthetic microbial fuel cells (pMFCs) are hybrid systems that enable simultaneous wastewater treatment under anaerobic conditions and the generation of electricity by utilizing the potential difference in the anaerobic anode chamber and the oxygenated cathode chamber. Dairy wastewater with a concentration of 2000 mg COD/L was treated in the anode of a batch pMFC. In the cathode chamber, Chlorella vulgaris or Arthrospira platensis was cultivated in synthetic medium, and next in diluted effluent from the anode chamber. The highest power density of 91 mW/m2 was generated by the pMFC with the cultivation of Arthrospira platensis. Higher values of dissolved oxygen remained during the dark phase in the cathodic medium with Arthrospira platensis cultivation than with Chlorella vulgaris. This depletion of oxygen significantly decreased voltage generation, which during the light phase increased again to the maximum values. The COD removal achieved in the anodic chamber was 87%. The efficiency of nitrogen removal in the cathode chamber during the cultivation of Arthrospira platensis and Chlorella vulgaris was about 78% and 69%, respectively. The efficiency of phosphorus removal in the cathode chamber with the cultivation of Arthrospira plantensis and Chlorella vulgaris was 58% and 43%, respectively. This study has shown that the introduction of Arthrospira platensis into the cathode chamber is more effective than that of Chlorella vulgaris.