**1. Introduction**

Discharge of petroleum hydrocarbons into the environment whether unintentionally or due to anthropogenic sources is the main cause of surface water and ground water pollution. Because of this alarming situation and the hazardous effect of hydrocarbons on the aquatic ecosystem, much attention is being focused on the remediation of hydrocarbons [1]. Moreover, during the petroleum refining process, raw crude oil is converted into various useful end products such as gasoline, diesel fuel, kerosene, and fuel oils. Purification of crude oil utilizes huge volumes of water which causes the generation of wastewater enriched with toxic organic compounds [2,3]. Treatment of such wastewater is very essential to achieve safe environmental water quality standards before discharge into any water body [4,5]. Traditional wastewater treatment processes such as electrochemical oxidation, membrane filtration, coagulation, flocculation [6,7] require highly skilled man power and high operational and maintenance costs. Moreover, application of these techniques generates toxic waste that further needs treatment before disposal [8,9].

It has been reported by many studies that various types of domestic and industrial wastewaters are widely treated using floating treatment wetlands [10–14]. This innovative technology has low installation, operational and maintenance costs, along with aesthetic value and environmentally friendly quality [15,16]. In floating treatment wetlands, plants are grown on a floating mat, whereas roots are hanged in the water column [17]. The extended roots in the water body offer plants the ability to create a direct contact between contaminants and the roots-associated microbial community. In addition, the suspended roots in water accelerate the sedimentation process by trapping suspended particles and reducing the water turbulence [18]. The roots grow horizontally and vertically to provide a large surface area for nutrient uptake and biofilm enlargement [13]. The associated microbial community degrades complex organic matter into simple components which are removed through the combined action of plants and microbes [19,20]. Floating treatment wetlands in assistance with bacterial consortium can be a promising alternative and green technology for remediation of oil refinery effluent. Many bacterial genera have been reported to degrade the hydrocarbons by their metabolic process [21]. Bacteria enhance the solubility, bioavailability, biodegradation, and uptake of hydrophobic compounds by production of biosurfactants which also facilitate microbial growth and hydrocarbon emulsification [22]. Plants provide nutrients, metabolites, phyto-hormones, and habitat for bacteria [23]. In FTWs, the plants augmented with hydrocarbon degrading bacteria could be an effective methodology for the remediation of diesel oil from water. Floating treatment wetlands augmented with *Cyperus laevigatus* L and hydrocarbon-degrading bacteria have not been widely tested for treatment of water polluted by diesel oil. So, considering this, the current study was undertaken with the main objective to assess the synergistic potential of *Cyperus laevigatus* L and hydrocarbons degrading bacterial strains in the FTWs system to remediate diesel oil from water.
