Vegetated greywater treatment walls : design modifications for intermittent media filters

The incorporation of on-site wastewater treatment systems is one means to meet the infrastructure needs of the rapidly expanding urban population. With space at a premium, the existing technologies for on-site systems should be reexamined and adapted to fit the needs of the urban setting. This study examined the potential to combine the existing wastewater treatment technology of the intermittent media filter with the new architectural trend of green walls, creating a treatment system with minimal spatial footprint and with a built-in urban greening component. A novel vegetated intermittent media filter wall was constructed in Ås, Norway and dosed with domestic greywater for a period of three months. Overall treatment performance and removal trends over the 175cm filter depth were monitored. Three separate wall sections were constructed to monitor the treatments effects of containing wall material choice and presence of vegetation. Despite a daily dosing rate of nearly 1000 l/m2 the system achieved average reduction rates of over 95%, 80%, 90%, 30%, and 69% for BOD5, COD, TSS, total nitrogen, and total phosphorus, respectively, as well as approximately two log unit reduction of bacteria indicator E. coli. Examination over the depth of the system showed that most organic (COD) and solids removal takes place in the upper 15cm, but with a sudden increase in loading a greater removal was seen at lower depths. With regard to nitrification, increased nitrate levels did not appear before 100cm filter depth, likely suppressed by high organic loading at the surface. The findings suggest that the great filter height associated with the wall design was useful for buffering sudden increases in hydraulic loading, as well as for facilitating nitrification under extreme loading conditions. Wall material with a more permeable construction (geotextile grid) preformed slightly better in the treatment of organics than non-permeable wall material (plastic liner), but confounding variables reduce the confidence in this finding. The vegetated wall section showed the greatest removal rates in almost every parameter measured, especially removal of E. coli. A difference in hydraulic retention times as shown by (NaCl) tracer tests is the likely cause of this phenomenon, rather than the vegetation itself. The significant reduction of constituents of concern using only a small spatial footprint make this system a worthy candidate for further research and development regarding urban wastewater treatment applications