Influence of the number of levels and system age on greywater treatment in a green wall

The use of green walls for greywater treatment represents a great opportunity to reduce potable water consumption and achieve multiple benefits. This study aims to analyse the efficiency of a pilot scale open-air green wall and evaluate its durability over time. The green wall consists of two modular panels (P1 set up in 2020, and P2 in 2018), each including nine pots organised in three independent columns. Coconut coir (80%) and perlite (20%) were used as filter media for three ornamental plant species (Carex morrowi, Hedera helix, Lonicera nitida). Each column was individually fed in batch mode with 24 Lday􀀀 1 (hydraulic loading rate - HLR 740.8 Lm􀀀 2 day􀀀 1) of greywater from January to March 2021. 190 water samples were analysed for 13 physical-chemical parameters. Both panels exhibited very good removal efficiencies for TSS (P1: 97.4 ± 4.6%, P2: 89.1 ± 13.8%), BOD5 (P1: 98.8 ± 2.4%, P2: 94.5 ± 9.2%), COD (P1: 78.5 ± 5.2%, P2: 80.1 ± 4.2%), TN (P1: 49.2 ± 30.4%, P2: 48.0 ± 23.31%), MBAS (P1: 92.7 ± 3.8%, P2: 75.3 ± 8.0%). P1 reached satisfying pollutants removal after two levels, while P2 showed higher variability and frequent clogging phenomena, needing three levels (e.g., cumulative BOD5 removal along the three levels in P1: 45.2 ± 36.2% | 86.5 ± 14.8% | 98.8 ± 2.4%; in P2: 16.5 ± 20.8% | 51.8 ± 32.3% | 94.5 ± 9.2%). This highlighted that the third level of P2 was necessary to guarantee good performances after over 2 years of operation. This study confirmed that green walls have high potential in greywater treatment also in challenging conditions (i.e. winter climate, high HLR and operating times), opening to further studies devoted to extending their lifetime.