# The Fate of MTBE and BTEX in Constructed Wetlands
## 3 Fate of MTBE in Constructed Wetlands
Four parallel pilot subsurface flow CWs with upward flow were operated with a hydraulic residence time (HRT) of 1 day for the removal of petroleum hydrocarbons from refinery wastewater [48; 89]. The CW units (10 m\({}^{2}\) area, 1 m depth) contained sand; were planted with willows, bulrush, rush, and dogwood; and contained an artificial subsurface aeration system. The pretreated inflow had an MTBE concentration up to 1.5 mg/L and average total hydrocarbons (including BTEX) of 45 mg/L. The MTBE removal was within the range of 15%-30%, lower than for other compounds. Higher removal rates were reported in the units with artificial aeration, while another layer of mature wetland sod improved the performance. Moreover, 40% MTBE removal was measured in systems of 1 ha area treating urban wastewater [90]. However, the MTBE inlet concentration was low (0.1 ug/L) (i.e., approaching the detection limit). Another study investigated pilot-scale CWs located next to an industrial area with many industrial facilities and an old refinery, where intense groundwater contamination occurred with benzene, MTBE, and ammonium, among other organic compounds [91; 92]. The experimental setup included several designs, such as aerated ponds [24], HSSF CWs, and VF CWs. Two VF CWs units were tested [44]: one unplanted with expanded clay as substrate media and one planted with willows (_Salix alba_) containing zeolite. The surface area of each VF CW was 4 m\({}^{2}\) and the depth was 1.75 m. Groundwater was fed every hour and was introduced 15 or 25 cm below the top of the substrate media surface, resulting in an unsaturated downflow. The hydraulic loading rate (HLR) applied was from 60 to 480 L/m\({}^{2}\)/day, respectively, resulting in HRTs of a few hours in the control bed and up to 2 days in the planted bed [28; 91; 93]. Contaminated groundwater was pumped onsite and fed both beds, with 3 mg MTBE/L, 14 mg benzene/L, and 51 mg ammonium/L on the average in the inflow. The removal of MTBE in the unplanted unit dropped from 97% (lowest HLR) to 75% (highest HLR). On the other hand, the unplanted bed reached a minimum 93% at all HLRs [44]. Higher removal rates were found for benzene (as previously shown). A study of the flow patterns, transport, and reactions in these CWs and numerical model simulations indicated that the measured MTBE removals occurred mainly due to biodegradation, while emissions due to volatilization were practically insignificant [28; 91]. For this, an upper substrate layer in the unsaturated CW was needed to filter the vapor phase of the organic compounds, which was confirmed in the laboratory [94]. Considering the numerical model results, the filter media and the water injection point are important parameters that affect the efficiency of these systems [95]. The pilot HSSF CWs (5 m\({}^{2}\) area, 0.5 m depth) tested in the same experimental facility contained fine gravel, were planted with _P. australis_, and were operated under an HLR of 30 L/m\({}^{2}\)/day and 6 days HRT [29]. A plant root mat of the same area was also examined. The inflow concentrations were 4 mg MTBE/L, 20 mg benzene/L, and 45 mg ammonium/L. The CWs performance presented high variations with seasons; the highest values occurred in summer months. The CW containing gravel showed an MTBE removal of 17% in summer, which was slightly higher than the MTBE removal in the unplanted system (8%) but lower compared with benzene and ammonium, without any detected improvement when some media additives were used (charcoal and iron(III)) [29]. On the other hand, the plant root mat reached an MTBE removal rate of 82% for the same period. The results indicated that volatilization was promoted by wetland plants [40], though it was not a significant contributor to the measured removals in the gravel-based CW beds compared to biodegradation and plant assimilation [41]. In the plant root mat, most of the removal was attributed to volatilization initiated by the direct contact between the aqueous and vapor phases.