Integrated Water Cycle Sustainability: Water Reuse in Circular Economy (sonozone technology evaluation)

The work that is presented aims to be a preliminary study on the applicability and potentialities of a low-frequency ultrasonic treatment combined with an ozone disinfection on a primary effluent from an urban wastewater treatment plant for an agricultural reuse (fertigation). The preservation of the nutrients contained in the original wastewater may contribute to the circular economy perspective, reducing the production costs for mineral fertilizers and the freshwater withdrawals. The limited scientific experience towards the hybrid ultrasonic and ozone process, called sonozone, requires an accurate laboratory experimental campaign, focused on the assessment of the regulated physico-chemical and biological compounds, that have to be controlled and maintained below the legislative limits. The importance of the wastewater recovery is nowadays increasing due to the climate change and the population growth. The suitable application of the reclaimed wastewater for fertigation practices could help the process to move towards the integrated water cycle closing, following the circular economy perspective. The primary effluent was appositively selected for the reuse. The work started with a laboratory-scale calibration of the instruments (ultrasonic probe and ozone generator) separately. An initial semi-continuous ultrasonic process and a discontinuous ozonation were performed. A precise characterization of the primary effluent wastewater was needed before the experiments. Then, the combination of processes in a semi-discontinuous setup allowed to test the coupled effect of the sequential ultrasonic pretreatment and ozonation. The hybrid process’ high removal efficiency was matched by an outstanding retention of nutrients (total nitrogen and total phosphate) highlighting the potential for the primary effluent reuse, with possible significant saving of chemical fertilizers. The wastewater’ sonozone recovery could save a meaningful amount of nutrients. However, being a well-known energy-consumer processes, it has to be assessed on the field based on the removal capacity. An economic assessment was given for the hybrid technology and compared to other classic secondary and tertiary treatments. Towards a safe microbial recovery, an in-depth analysis on microorganisms was presented. Being only few the mandatory pathogens required in the legislation, additional microorganisms will be tested with the ultrasonic and ozone processes alone and combined. The kinetic modelling and logarithmic abatement of four microbial species, namely Pseudomonas spp., E. coli, Enterococcus spp. and S. Enteritidis, was performed. Moreover, an ecotoxicological in-silico evaluation indicated the sonozone removal capacity towards several hazardous and persistent chemicals, addressing the eco-toxic concentration in different environments, their persistence and solubility in water. The pilot scale laboratory tests should be intended as the transition from a semi-continuous laboratory scale to a bigger-scale plant running continuously, as a further step for the evaluation of the proposed agricultural reuse. The circular economy concept is strongly related to the possibilities of the conventionally removed compound to be recovered and reused through the fertigation concept. In order to support the laboratory tests, a meaningful case study was analyzed. Two different scenarios were shown for the extent of expressing a tangible recover of water and nutrients. The overall study outcomes showed remarkable opportunities for the primary effluent reuse for agricultural purposes in a circular economy perspective, despite energy costs may still hinder the full-scale applicability of this technology. Moreover, the sonozone application in rural areas, not reached by a suitable sewage and depuration system, may result highly recommended for the prospective of direct effluent reuse, especially when coupled with an electricity generator plant (e.g. photovoltaic panels).