Cultivation of green algae Chlorella sp. in a stirred tank bioreactor

Algae, as a renewable raw materials, are valuable sources of biomass for many branches of industry. In the area of environmental protection they are used among others for biological treatment of water, removal and recovery of heavy metals from industrial wastewater, production of bio-fertilizers and fodder additives. Due to their rapid growth, they are able to efficiently use less fertile areas more than energetic plants (e.g. rapeseed, soya). To properly grow, under in vitro conditions, algae need access to water, nutrients (i.a. nitrogen and phosphorus) and carbon dioxide as well as appropriate temperature, pH, mixing and lighting. In our climate conditions, algal cultivation in natural water reservoirs is ineffective, due to daily and annual temperature and sunlight changes. An efficient alternative is the use of bioreactors that limit or eliminate the possibility of bacterial infection in algae culture. The aim of the study was to investigate the biomass yield of algae Chlorella sp., cultivated in BG-11 synthetic medium, inside the tank bioreactor, under the artificial lighting and aeration conditions. The periodic culture was conducted at 25°C with mechanical stirring (150 rpm). Compressed air was supplied to the suspension (in alternating cycles: 5 successive aeration days, 2 days without aeration). The process of algal cell multiplication was supported by the light exposure for 16 hours per day (8 h - dark phase). The source of light was LED lamp (30 W), suitable for stimulating the growth of chlorophyll concentration (blue light 420÷480 nm, red light - 630÷690 nm), located in the length of 10 cm from the culture vessel. Separation process of the dried biomass from culture medium was consisted in: sedimentation, centrifugation (8000 rpm, 10 min), freezing with liquid nitrogen and lyophilisation (–80°C, 3. The efficiency of biomass production was &#8764 3190 mg/dm3 (&#8764 63 g/dm3 per day), while optical density &#8764 2.13. Based on the results, it was found that high efficiency of biomass production was influenced by light with a well-chosen wavelength spectrum, forced air circulation, limited evaporation of water from the vessel and sterile conditions during initiation of the culture. Further studies are needed to involve determining the concentration of different forms of nitrogen (nitrates, nitrites, ammonium nitrogen) in the culture medium and investigating the impact of process conditions on enhanced assimilation of biogenic compounds by algae, at following stages of their growth. A profound knowledge on these relationships and estimated costs will allow to continue experiments on increasing the productivity of algae biomass, which will be commonly recognized as economically reasonable.Zbadano wydajność produkcji biomasy alg Chlorella sp., hodowanej na bazie pożywki syntetycznej BG-11, w zautomatyzowanym bioreaktorze zbiornikowym z mieszadłem mechanicznym w warunkach sztucznego naświetlania i napowietrzania. Kontrola hodowli polegała na spektrofotometrycznym pomiarze gęstości optycznej (OD) oraz badaniu zmian stężenia związków biogennych w podłożu hodowlanym, będących wynikiem zachodzących procesów metabolicznych. Hodowlę prowadzono przez 46 dni do momentu wyczerpania fosforu. Końcowa wydajność produkcji biomasy przekroczyła 3 g/dm3 przy jej średnim dobowym przyroście > 60 mg/dm3, a OD686 osiągnęła wartość 2,13. Na podstawie przeprowadzonych badań stwierdzono, że na wysoką efektywność produkcji biomasy alg kluczowy wpływ miały kontrolowane warunki hodowli, m.in. naświetlanie światłem o specyficznej długości fali (420÷480 nm, 630÷690 nm), wymuszony obieg powietrza, ograniczenie możliwości odparowania wody z naczynia hodowlanego oraz zapewnienie sterylnych warunków podczas inicjowania hodowli