Experimental study of the treatment of greywater through green walls

In recent years, water availability is becoming a main issue, due to population increase, urbanization and extensive water pollution. In this context, Nature Based Solutions (NBSs) represent an alternative solution to traditional wastewater treatment technologies that can be effectively applied in urban areas. Among different NBSs, green walls can be efficiently applied on unused building surfaces in urban areas. Furthermore, they provide multiple benefits, such as thermal, naturalistic, economic, environmental and social. Greywater is the portion of wastewater with the highest potential in terms of in situ water reuse for non-potable purposes, since it is available in high quantity with low pollutant content. In fact, greywater (e.g. the portion of household wastewater coming from sinks, showers, bathtubs and washing machines, excluding toilet flushes and kitchen sinks) represents over 70% of urban wastewater. Considering the available literature data, green walls’ efficiency and design for greywater treatment still needs to be improved. The aim of this thesis is to evaluate the efficiency and durability of a specific green wall design (a vertical flow modular system) along time, in terms of pollutants’ removal and hydraulic properties. Two modular panels installed in the Hydraulics lab courtyard at DIATI, were compared: the first one (“old”) was installed in December 2018 and the second one (“new”) in December 2020. Each panel consisted of nine planted pots organized in a matrix of three rows and three columns, fed with 24 l/day of synthetic graywater (GW) prepared weekly. Each column worked as an independent vertical system with a hydraulic loading rate of 740.8 L/m2/day, in which water flows from the upper pot to the bottom one. During a monitoring campaign lasting 3 months, all 18 pots and the input GW were sampled weekly for physic-chemical analysis. Temperature (T), pH, electric conductivity (EC) and dissolved oxygen (DO) were measured on site with a multiparametric probe; total suspended solids (TSS), biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total nitrogen (TN), nitrate, ammonium, total phosphorus (TP), anionic surfactants (MBAS), and chloride were all measured in the Circular Economy lab at DIATI. Both panels showed an increase in DO, pH and EC as water flows down the columns, without relevant differences over the monitoring campaign. Excellent organics’ removal was achieved for COD, BOD5 and MBAS already after two treatment rows in both panels, even if the “new” one exhibited more constant removal performances. Also, TN removal showed good performances, possibly due to microbial degradation and biofilm uptake. TP and chloride concentrations are quite constant between input and output in both panels showing a negligible removal. Even if the two panels showed similar behaviors, panel age seems to adversely affect hydraulic and removal properties of the “old” system. The “new” panel showed better performances and greatest removal efficiencies compared to the “old” one, suggesting that only two levels of pots are sufficient for greywater treatment, but the third level can compensate possible losses in efficiency over time. The old panel was more subject to occasional clogging compared to the new one. This study demonstrated the great potential of green walls systems for greywater treatment in terms of treatment efficiency and water saving, and highlighted potential issues of their durability and resilience.