Outdoor cultivation of an autochthonous Chlorella-strain during spring season for urban wastewater phycoremediation and subsequent biostimulation tests

The use of water for anthropic activities leads to the production of wastewaters (WWs). WWs are rich in pollutants such as nitrogen (N) and phosphorus (P), whose release into the environment can cause eutrophication, and Escherichia coli, an indicator of faecal contamination that can pose a health risk to humans. For these reasons, it is important that WWs are treated before their release to prevent environmental harm. In the city of Ferrara, urban wastewater (UWW) treatment takes place at the Hera SpA wastewater treatment plant (WWTP), where the UWW is subjected to conventional water treatments, which can be classified into physical, chemical, and biological treatments. During the purification process, sludge is produced; part of which is recirculated within the plant, and another part undergoes further treatments, such as thickening, anaerobic digestion, and dewatering, aimed at reducing the water content before being properly disposed. The obtained water, still rich in pollutants, is then recirculated in the WWTP. In this context, it is possible to overcome WW recirculation exploiting microalgae and their photoautotrophic metabolism to reduce, not only N and P content, but also management costs. Many studies on wastewater phycoremediation using microalgae have been conducted, with the aim to identify the best species for this purpose. Most of the trials have been carried out on a laboratory scale and have shown promising results. However, regarding outdoor scale up systems further research is needed, especially considering the environmental conditions to which the process is subjected. In the present work, an indigenous strain of Chlorella isolated from the thickening wastewater provided by Hera WWTP was used. This strain was previously employed in preliminary tests in laboratory scale showing encouraging results. Thus, it was deemed interesting to test the same algae in a 600L prototype system located at the Hera WWTP and study the phycoremediation activity of the strain exposed to environmental condition. The tests conducted in late summer and winter season showed different results due to the variability in the composition of the wastewater, which is subject to seasonal fluctuations, and the unique environmental conditions typical of each season. To gain more detailed and comprehensive information on the applicability of microalgae as a biological wastewater treatment, the Chlorella-strain was used for a phycoremediation test in the spring season, evaluating the effectiveness of the treatment under the typical climatic and environmental conditions of this period. During the trial biomass production, N and P removal, E. coli load, organic and inorganic carbon concentrations were monitored. The weather during the spring test was mild, with minimum temperatures within the normal range and maximum temperatures slightly above the seasonal average, leading to an average temperature variation of 11°C. Following an initial acclimatization phase where the algal biomass slightly decreased, it gradually increased and had nearly doubled by the end of the test. Regarding nutrients, N experienced a moderate reduction, while P concentration was halved after just 7 days of treatment and reduced by 90% by the end of the test. At the end of the experimentation, the treated water was separated from the algal biomass and stored for future tests to evaluate the potential application of this product in agriculture, as a natural biostimulant or antifungal agent, exploiting the bioproducts released by the algae into the medium during cultivation. The results obtained in this trial are consistent with those from previous trials conducted across various seasons of the year. Analyses using linear regression models and the first-difference model have shown that this treatment technology has a particularly positive impact on P and can therefore be considered for integration with other pollutant treatment technologies.