Notes
The aim of this research was to study the removal of nitrogen compounds and heavy metals from drinking water by algae Chlorella sorokiniana. Firstly, in the experiments on the removal of nitrogen compounds (nitrate, nitrite and ammonium) from drinking water by alginate immobilised C. sorokiniana, besides the impacts of the types of carbon source on the removal efficiencies of nutrients, the impacts of several other parameters were studied such as the impact of cell number, pH, light intensity, amount of bio-sorbent, etc. The experiments showed that algae C. sorokiniana in the presence of an appropriate carbon source efficiently removed nitrogen compounds from the water samples at all tested concentrations of nutrients. Amongst the four tested carbon sources, the highest nitrate removal efficiencies were achieved when using grape juice, acacia honey or molasses as carbon sources, while sucrose gave poorer results. The experiments without any external carbon source added were less successful. As grape juice and acacia honey until this date have never been used as carbon sources in such kinds of studies, the obtained results represent an important contribution for further research. At an initial nitrate concentration 20 mg/L in the presence of grape juice, honey or molasses, the nitrates were removed from drinking water after one day at rates of more than 93 %, while at 100 mg/L of initial nitrate concentration by more than 95 % over three days. The main reason that these carbon sources gave better results than sucrose arises from their compositions, as they contain various nutrients that could positively impact the algae growth and thus nitrogen removal. The complete removal of ammonium (at initial concentration of 30 mg/L) in the presence of 50 mg/L of nitrate was achieved in less than three days. The nutrient removal efficiency was highly correlated with the bio-sorbent amount, the concentration of added carbon source, the concentration and type of nutrient, and with the light intensity. Otherwise, sufficient nitrate removal efficiencies could be achieved with a beads/water ratio of 1:6.7 or smaller. The study on the impact of algae C. sorokiniana on the biological removal of nitrate from drinking water by activated sludge showed that this alga had a noticeably negative effect on the denitrification process. At an initial nitrate concentration of 100 mg/L, only 65 % %70 % of the initial nitrate was removed in the presence of algae, whilst conventional biological denitrification enables above 95 % removal. The presence of ammonium or urea slowed down the denitrification process. Furthermore, experiments regarding the bio-sorption of heavy metals were performed, wherein the potential was studied of alginate immobilised C. sorokiniana for removing Cu2+, Ni2+, Cr6+ and Cd2+ ions from drinking water. It was found that immobilised algae C. sorokiniana could efficiently remove up to 97 % Cu2+, 51 % Ni2+ and 74 % Cd2+. Removal of Cr6+ ions was unreasonable due to low efficiencies achieved and due to the strong toxic effects of these ions on algae. The bio-sorption of the metals was strongly affected by the initial metal ion concentration and initial pH. At an initial metal concentration of 25 mg/L alginate immobilised C. sorokiniana cells could be used within 5 or even more successive bio-sorption-desorption cycles and still sufficiently remove metal ions. The highest toxicity effect on the algal cells was noticed in the case of Cr6+ ions, whilst Ni2+ and Cu2+ ions showed quite lower toxicities to C. sorokiniana. The lowest toxicity was observed in the case of Cd2+ ions, which among all metals also shown the negligible impact on the nitrate removal. As bio-sorption of the multi-metal systems of heavy metals from drinking water by alginate immobilised C. sorokiniana had not been studied until now, the bi- and multi-metal systems of Cu2+, Ni2+, and Cd2+ were investigated at initial metal concentrations of 30, 50 and 100 mg/L.