Processes impacting on benzene removal in vertical-flow constructed wetlands Xianqiang Tang a , Paul Emeka Eke b , Miklas Scholz b, * , Suiliang Huang a a College of Environmental Science and Engineering, Nankai University, Tianjin 300071, PR China b Institute for Infrastructure and Environment, School of Engineering and Electronics, The University of Edinburgh, William Rankine Building, The King’s Buildings, Edinburgh EH9 3JN, Scotland, United Kingdom article info Article history: Received 7 February 2008 Received in revised form 20 May 2008 Accepted 23 May 2008 Available online 3 July 2008 Keywords: Fertilizer Petroleum Regression analysis Simulation Water quality abstract The overall aim of this research project was to reduce low molecular weight hydrocarbons such as ben- zene in produced wastewaters. Over 30 months of research was conducted to test the treatment perfor- mance in terms of benzene removal in vertical-flow constructed wetlands. Based on an influent concentration of 1 g L 1 benzene, the results show mean benzene removal efficiencies between 88.71% and 89.77%, and 72.66% and 80.46% for indoor and outdoor constructed wetlands, respectively. A statis- tical analysis indicated that the five days at 20 °C N-allylthiourea biochemical oxygen demand (BOD 5 ), chemical oxygen demand (COD), nitrate-nitrogen (NO 3 -N), dissolved oxygen (DO) and electric conductiv- ity (EC) values of the effluent were positively correlated with the effluent benzene concentrations follow- ing the order COD > DO > EC > NO 3 -N > BOD 5 , and negatively correlated according to the order pH > redox potential (redox) > temperature (T) > turbidity. No strong relationships between benzene and the vari- ables ortho-phosphate-phosphorus (PO 3 4 ) and ammonia-nitrogen (NH 4 -N) were recorded. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Benzene, toluene, xylene, phenol, halogenated aromatic com- pounds, chloroform and trichloroethylene are the major products of the petroleum and fine chemical industries and the most fre- quently used organic solvents (Yeom and Yoo, 1999; Lu et al., 2002). Exploration, production, refining, storage, transportation, distribution and utilization of petroleum hydrocarbons (Atlas and Cerniglia, 1995), accidental spills, improper practices and leaching landfills resulted in the frequent occurrence of these anthropo- genic organic compounds in air, water and soil. Losing these sub- stances to the receiving environments may lead to an adverse impact and might endanger public health and welfare. Therefore, considerable research has been conducted to remove these com- pounds from contaminated environments (Nickelsen and Cooper, 1992; Yeom and Yoo, 1999; Lu et al., 2002). Physical treatment methods such as separation (Descousse et al., 2004; International Association of Oil and Gas Producers, 2002; Uragmi et al., 2006), adsorption (Adachi et al., 2005), and air sparging (Rogers and Ong, 2000) are associated with the disad- vantage of decomposing toxic compounds, resulting in a potential pollution risk. Catalytic destruction (Chuang et al., 1992), ozoniza- tion, chlorination and other expensive chemical methods can effi- ciently remove various kinds of organic compounds. Cost-effective and environmentally friendly alternative biologi- cal methods such as biofilters (Choi and Oh, 2002) and hybrid bio- reactors (Yeom and Yoo, 1999) have been successfully applied for the treatment of wastewater containing low concentrations of ben- zene. In biofilters and hybrid bioreactors, natural organic media such as peat and soil are used as carrier for microorganisms to col- onize the corresponding surface area, forming subsequently a bio- logically active layer, known as the ‘biofilm’ (Choi and Oh, 2002; Lu et al., 2002). Benzene can be transformed to innocuous compounds leaving the system via the effluent (Roychoudhury and Merrett, 2006). Constructed wetlands are designed to use natural wetland processes that are associated with wetland hydrology, soils, mi- crobes and plants to treat wastewater (Drizo et al., 1999; Kadlec and Knight, 1996; Lee et al., 2005). Complex chemical, physical and biological removal processes treat organic pollutants effec- tively (Brix, 1993; Scholz, 2006; Vymazal, 2007). This is also the case for aromatic components in the dissolved water phase (Eke and Scholz, 2008). Due to their relatively small space requirements and good aer- ation conditions (McBride and Tanner, 2000), vertical-flow con- structed wetlands have recently become a common choice for the treatment of various wastewater types (Sun et al., 2005; Proch- aska and Zouboulis, 2006; Eke et al., 2007; Eke and Scholz, 2008). Concerning planted constructed wetland systems, microbes are particularly active in the root zone of the top substrate (Ragusa et al., 2004; Scholz, 2006). In vertical-flow constructed wetlands, the removal of organic compounds and other nutrients were fa- vored through aerobic and/or anaerobic decomposition, microbial 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.05.038 * Corresponding author. Tel.: +44 131 6506780; fax: +44 131 6506554. E-mail address: m.scholz@ed.ac.uk (M. Scholz). Bioresource Technology 100 (2009) 227–234 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech