# The Fate of MTBE and BTEX in Constructed Wetlands
## 4 Effect on MTBE and BTEX Fate by Other Compounds  
MTBE and BTEX are often present with other compounds in contaminated water. MTBE is often found in groundwater with other gasoline contaminants (e.g., BTEX [98]), which increases the water complexity since various interactions and/or interferences occur between the different contaminants. Ethylbenzene or xylenes (BTEX compounds) have been found in water containing MTBE, resulting in complete inhibition of MTBE degradation, while partial inhibition could be the result of benzene and toluene's presence [99]. MTBE degradation increased only after the complete elimination of benzene and toluene. Results indicated that MTBE and BTEX degradation occurs via two independent pathways [99]. Another study on biotrickling filters also found that a single BTEX compound or BTEX mixtures partially inhibited the degradation of MTBE [98]. Microbial community composition was found to regulate the simultaneous removal of BTEX and MTBE [71]. MTBE degradation was completely inhibited by BTEX in batch reactors inoculated with MTBE and BTEX enrichment cultures. However, MTBE biodegradation in a semibatch reactor was almost three times higher when BTEX was present, though it occurred at a slower rate than MTBE biodegradation when BITE is not present in the solution. The results of that study indicated that MTBE can be biodegraded when BTEX is present depending on the reactor configuration and the microbial culture composition [99]. In another study, MTBE degradation was not significantly affected by BTEX in batch conditions [100]. Additionally, MTBE, BTEX, and TBA can also be biodegradable when an appropriate bacterial mixture is utilized [98]. The same has also been reported for CWs used to treat water-containing MTBE and BTEX. Many studies implemented at the experimental facility in Leuna, Germany reported that the presence of BTEX partially inhibits MTBE biodegradation. Lower MTBE removal rates (17%) than benzene (81%) were measured in an HSSF CW with gravel [28], possibly due to hindered microbial activity [58]. Smaller differences were found in an HSSF CW (i.e., 24%-100% for benzene and 16%-93% for MTBE [42]). A higher benzene mass removal rate (98%; 544 mg/m\({}^{2}\)/day) compared with MTBE (78%; 54 mg/m\({}^{2}\)/day) was also detected in a plant root mat [96]. Volatilization of both accounted for less than 10% of the total mass removed in gravel-based, planted HSSF CWs [40], whereas in an aerated trench, system volatilization accounted for 1% and 53% of benzene and MTBE removal, respectively [24]. Additionally, benzene and MTBE were completely eliminated in a two-stage VF CW [44]; the first stage was an unplanted vertical roughing filter followed by a planted vertical flow CW for final polishing, resulting in effluent concentrations of \(5\pm 10\) and \(0.6\pm 0.2\) mg/L for MTBE and benzene, respectively (influents were 2970 \(\pm\) 816 and 13,966 \(\pm\) 1998 \(\upmu\)g/L, respectively). Similar results are reported for CWs [48; 89] and aerated trench systems [24] treating groundwater contaminated with petroleum hydrocarbons, whereas MTBE removal was lower than that of benzene. However, similar removal rates were found for benzene and MTBE in other CWs (between 30% and 40%), possibly because the influent concentrations were near the detection limits [90]. The simultaneous fate of benzene, MTBE, as well as phenols (i.e., phenol and m-cresol) has also been studied in HSSF CWs [1]. The two phenolic compounds were completely removed (inflow of 15 and 2 mg/L, respectively) without changing MTBE and benzene removal (20.2 and 334.6 mg/m\({}^{2}\)/day, respectively). The planted CW showed a higher removal efficiency, implying the positive contribution of plants. It should also be noted that most of this removal was measured in the first 1/3 part of the CW length, which indicated the potential of the HSSF CW to accept increased influent loads.