Wat. Res. Vol. 35, No. 2, pp. 397–404, 2001 # 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter PII: S0043-1354(00)00264-5 AMMONIUM REMOVAL USING A NOVEL UNSATURATED FLOW BIOLOGICAL FILTER WITH PASSIVE AERATION ORI LAHAV, EYAL ARTZI, SHELDON TARRE and MICHAL GREEN* M Faculty of Agricultural Engineering, Technion, Haifa 32000, Israel (First received 20 October 1999; accepted in revised form 3 May 2000) Abstract}A novel vertical bed process for the removal of ammonium from secondary effluents, using a ‘‘passive air pump’’, has been developed. The process is based on convective aeration caused by a fill and draw operational sequence, and combines the advantages of the vertical wetlands concept with the high loading rates typically associated with trickling filters. Experiments were carried out in a 500-l reactor using simulative effluents and actual municipal secondary effluents. A maximal ammonium removal rate of 1100 g N/m 2 reactor/d was achieved using simulative effluents and an effective gravel size of 0.96 mm. At all hydraulic loads applied, the nitrification rate was found to be limited by the oxygen transfer rate. The small-size medium used with simulative effluents clogged when using actual municipal secondary effluents. Two other media (2.46 mm and 4.31 mm) did not clog during the entire experimental period and a maximum removal load of 300 g N/m 2 reactor/d was achieved. This value is still much higher than typical rates reported for conventional vertical beds. # 2000 Elsevier Science Ltd. All rights reserved Key words}vertical bed, unsaturated flow, passive aeration, nitrification, ammonium removal INTRODUCTION Unsaturated flow filters such as trickling filters (TF) or vertical constructed wetlands (VCW) are in use worldwide for the treatment of various types of wastewater. Generally, these systems do not require energy input for aeration purposes. In TF systems, wastewater is mechanically distributed over the top of the bed and trickles downward through an unsaturated, porous filter media. The filtered waste- water is collected by a system of underdrains connected to a secondary sedimentation tank. The underdrain system is designed to allow adequate convective oxygen supply to the filter resulting from temperature differences between the ambient air and the air inside the porous media. Trickling filters are operated at a removal rate ranging from 0.08 to 1.6 kg BOD/m 3 reactor/d (Metcalf & Eddy, Inc., 1991). Vertical constructed wetlands, as opposed to TF, are characterized as ‘‘natural’’ treatment systems and operate with minimal human intervention. As such, they are appropriate for rural areas, where land cost is low and low maintenance and reliable operation are essential. In vertical flow wetlands, wastewater is applied at the top of the bed and flows through an unsaturated porous media, eventually being collected by perforated pipes at the bottom. Typically, the beds are dosed for up to two days and then rested for 4–8 d. The aim of the rest period is to overcome bed clogging by suspended material originating from the influent and/or bacterial growth within the bed. In this period, the organic suspended matter is hydro- lyzed and degraded, allowing further flow through the bed. The unsaturated flow in the vertical bed allows for at least partial aerobic conditions by oxygen diffusion in both the flow and rest periods. Reported oxygen transfer rates for vertical bed can reach values as high as 30–94 g O 2 /m 2 reactor/d (Cooper, 1999). These values are much higher than typical ‘‘extensive’’ units; however, relative to ‘‘intensive’’ units; the land requirement is still large. DESCRIPTION OF THE ‘‘UNSATURATED FLOW BIOLOGICAL FILTER WITH PASSIVE AERATION’’ PROCESS A novel aerobic vertical bed treatment scheme for the removal of BOD and ammonium from waste- water streams is currently being developed at the Technion. The process combines the advantages of the vertical wetland concept (no energy input and therefore low investment and operational costs, low maintenance and high reliability) with the high loading rates typically associated with trickling filters. The process is based on a low-cost method *Author to whom all correspondence should be addressed. Tel.: +972–4–8293479; fax: +972-4-8221529; e-mail: agmgreen@techunix.technion.ac.il 397