Municipal wastewater treatment by combining in series microalgae Chlorella sorokiniana and macrophyte Lemna minor: Preliminary results

Abstract

Conventional treatment of municipal wastewater requires large inputs of energy, and high operation and maintenance costs. On the other hand, the use of microalgae and macrophytes on wastewater treatment offers an alternative approach that combines removal of pollutants with production of valuable biomass. An integrated sequencing batch reactor system that combines in series microalgae Chlorella sorokiniana UTEX 1230 and macrophyte Lemna minor was tested for municipal wastewater treatment in the current study. The average removal of COD, TKN, NH4–N and PO4–P was 99%, 88%, 90% and 91%, respectively, while the concentrations of major pollutants in the effluents complied with European Directive 91/271 for wastewater discharge to the aquatic environment as well as with national legislation for wastewater reuse for limited irrigation. The microalgae reactor was capable of fully removing COD and partially removing N and P nutrients, while Lemna minor reactor contributed mainly to nitrogen removal. Batch experiments with Chlorella sorokiniana and different types of municipal wastewater (raw sewage, anaerobically treated wastewater and aerobically treated wastewater) with no pH adjustment and CO2 addition showed that the optimal microalgae growth and the highest pollutants removal were achieved when raw sewage were used. Comparison of mixotrophic and heterotrophic conditions showed highest biomass growth under mixotrophic conditions. Except for nitrates, the higher removal of other pollutants was also achieved under mixotrophic conditions. Further experiments should be conducted to examine the characteristics of produced microalgae and macrophyte biomass and the alternative ways of their valorization under the frame of circular economy.