Evaluating the Effects of Irrigation with Reused Water and Compost from a Pilot Wastewater Treatment Unit on the Experimental Growth of Two Common Ornamental Plant Species in the City of Athens

Since 2019, the NextGen pilot wastewater treatment unit—also known as the NextGen Sewer Mining concept—has been operating at the Athens Plant Nursery, transforming sewage from Athens’ central network into irrigation water and compost. This unit produces resources for plant growth through membrane bioreactors (MBRs) and aerobic sludge digestion. This study experimentally evaluates the effects of NextGen reused water and compost on two common ornamental plant species in Athens, Pittosporum tobira ( Angelica ) and Myrtus communis ( Common Myrtle ), compared to the use of tap water and red soil without additional fertilization. The results indicate that NextGen reused water combined with compost significantly promotes both height and weight growth in these plants. However, by the end of the experiment, compost fertilization had a greater effect on the height and weight growth of both Angelica and Myrtus plants when applied independently and watered with tap water, compared to the use of NextGen reused water combined with red soil. Notably, none of the 96 plants withered throughout the experiment, indicating that promising and sustainable technologies like the concept of Sewer Mining can effectively replace conventional and environmentally outdated methods of plant nutrition and irrigation by producing reused water and compost.