Evaluation of the influence of filter medium composition on treatment performances in an open-air green wall fed with greywater.

According to the European Research and Innovation Policy Agenda, nature-based solutions (NBSs) are key technologies to improve the sustainability of urban areas. Among NBSs, green walls have been recently studied for several applications, among the others the treatment of lowly polluted wastewater flows as greywater (GW, e.g. domestic wastewater excluding toilet flushes). This work is aimed at the evaluation of the influence of four additives (compost, biochar, granular activated carbon, polyacrylate) mixed with a base filter medium made of coconut fibre and perlite, on the performances of a green wall fed in batch mode with synthetic GW. The green wall was operated with a high hydraulic loading rate of GW (740.8 L/m2/day) in open-air winter conditions (3.5-15 °C measured for GW) between January and April. The performances of the green wall have been assessed though the monitoring every 1-2 weeks of physicochemical and biological parameters (pH, electric conductivity, total suspended solids, dissolved oxygen, BOD5 and COD, nitrogen and phosporus compounds, chlorides and sulphates, anionic surfactants and E. coli). Removal performances were excellent for BOD5 (>95%) and E.coli (>98%) for all additives; compared to the base medium, biochar was the best performing additive over the highest number of parameters, achieving removals equal to 51% for COD, 47% for TKN and nitric nitrogen and 71% for anionic surfactants. Compost also achieved high removal performances, but the frequent clogging events occurred during the monitoring period do not make its use recommendable. Granular activated carbon and the combination of biochar and polyacrylate performed better than the base medium, but only about the removal of nitric nitrogen. These results demonstrated that, in the considered experimental boundaries, biochar could improve the overall treatment performances of a green wall fed by GW and operated in challenging conditions.

A review of nature-based solutions for greywater treatment: Applications, hydraulic design, and environmental benefits.

Recognizing greywater as a relevant secondary source of water and nutrients represents an important chance for the sustainable management of water resource. In the last two decades, many studies analysed the environmental, economic, and energetic benefits of the reuse of greywater treated by nature-based solutions (NBS). This work reviews existing case studies of traditional constructed wetlands and new integrated technologies (e.g., green roofs and green walls) for greywater treatment and reuse, with a specific focus on their treatment performance as a function of hydraulic operating parameters. The aim of this work is to understand if the application of NBS can represent a valid alternative to conventional treatment technologies, providing quantitative indications for their design. Specifically, indications concerning threshold values of hydraulic design parameters to guarantee high removal performance are suggested. Finally, the existing literature on life cycle analysis of NBS for greywater treatment has been examined, confirming the provided environmental benefits.

Storage mechanisms in constructed wetlands: Should we modify heterotrophic bacteria modelling?

This study investigated the occurrence of storage mechanisms in biofilm from constructed wetlands (CWs) through respirometric studies and calculated the corresponding heterotrophic growth yields. Respirometric tests were performed in biofilm extracted from horizontal sub-surface flow CWs batch loaded with three different COD mass loads: 7.0., 15.6 and 35.2 g COD/(m2∙day). The bed removal efficiency remained above 96% for all mass loads and COD mass removal rates increased from 6.8 g COD/(m2∙day) for the lowest load to 34.5 g COD/(m2∙day) for the highest load. The percentage of tests with storage evidence decreased from 85% to 10% with increasing mass loads and the responses of the microbial community to the acetate pulse showed an adaptation to the feast-famine conditions, through storage mechanisms, for lower loads, and a metabolic shift to the use of COD for growth for higher loads. Heterotrophic biomass yield values varied from 0.54-0.56 g COD/g COD for low mass loads to 0.69-0.71 for higher mass loads, indicating that greater substrate availably triggers growth and reduces the occurrence of storage. Storage yield values supported this trend varying between 0.89 and 0.94 with increasing mass loads. Given the significant storage evidence obtained in the present study, it is suggested that a modified modelling architecture, which includes storage mechanisms, should be considered in future simulations of CW systems.