Effects of design and operational parameters on ammonium removal by single-stage French vertical flow filters treating raw domestic wastewater

Four pilot-scale single-stage vertical flow filters (of 2.25 m2 each), treating raw domestic sewage, were studied over 20 months in order to assess the impact of different designs and operational conditions on treatment efficiency. One of them was designed and operated as a standard 1st stage “French” vertical flow constructed wetland unit. The other 3 pilots differed from the standard pilot with respect to the filtration depth, the loading rate or the partial replacement of gravel by zeolite (chabazite), respectively. The pilots were monitored by analysing 24-h flow-weighted composite samples for TSS, CODtot, CODd, ammonium, nitrate and carbonate. All pilots showed a high ability to remove TSS and CODtot, with average removal of 81% and 75%, respectively. Increasing the depth of the filtration layer from 40 to 100 cm allowed to significantly improve ammonium removal (81%), whereas the simultaneous increase in hydraulic and organic loads resulted in a deterioration of ammonium and CODd removals (44% for both parameters). Using zeolite did not induce any observable improvements in ammonium removal under the conditions of the study.