Constructed wetroofs: A novel approach for the treatment and reuse of domestic wastewater

Abstract

The lack of space in urban areas can be a reason for their lack of green areas and sanitation provisions. Since roofs represent common unused space, they become an option to locate natural treatment systems such as constructed wetlands (CW). A conventional CW is too heavy to be placed on a roof, hence its design must be adjusted without altering its treatment properties. Thus, the concept of a constructed wetroof (CWR) was developed, a 9-cm-depth system composed of sand, organic soil, light expanded clay aggregates (LECA), polylactic acid beads (PLA) and roots. Due to the shallow depth and unsheltered location, the CWR is susceptible to changes in its physical and hydrological properties (such as a hot day causing severe evapotranspiration that dries out the filter media and the plants) that may lead to treatment failures. Therefore, this study investigates the capacity of a CWR to treat (low organic loaded) domestic wastewater (DWW) from an office building in The Netherlands. The results showed that the CWR is a highly aerobic system that treats DWW beyond the levels required by local discharge quality guidelines for organic matter, solids, NH4+-N, total nitrogen and total phosphorus (>79% for all parameters tested). Further investigation is needed to test the CWR performance at higher organic loading rates. The roots, organic soil and sand provided the majority of the treatment; while the LECA and PLA were used mainly to provide volume without increasing significantly the weight. LECA also played a significant role in phosphorus removal. Although the hydrology of the CWR was highly weather dependent, however the treatment efficiency was not affected by it. Moreover, the rain had a minor effect on washing away the nutrients retained in the CWR media. The CWR provided a resilient and efficient DWW treatment that requires, as it is placed on a roof, 0m2 per population equivalent.