Treatment of hydrocarbon-rich wastewater using oil degrading bacteria and phototrophic microorganisms in rotating biological contactor: Effect of N:P ratio Anal Chavan, Suparna Mukherji ∗ Centre for Environmental Science and Engineering (CESE), Indian Institute of Technology (Bombay), Powai, Mumbai 400076, India Abstract Treatment of hydrocarbon-rich industrial wastewater in bioreactors using heterotrophic microorganisms is often associated with various oper- ational problems. In this study, a consortium of phototrophic microorganisms and a bacterium is developed on the discs of a rotating biological contactor (RBC) for treatment of wastewater containing diesel oil. The reactor was fed with oil degrading bacterium, Burkholderia cepacia and oil tolerant phototrophic microorganisms. After biofilm formation and acclimatization to 0.6% (v/v) diesel, continuous-mode operation was initiated at 21 h hydraulic retention time (HRT). Residual diesel in the effluent was 0.003%. Advantages of this system include good total petroleum hydro- carbon (TPH) removal, no soluble carbon source requirement and good settleability of biosolids. Biofilm observations revealed the predominance of B. cepacia and cyanobacteria (Phormidium, Oscillatoria and Chroococcus). The N:P ratio affected the relative dominance of the phototrophic microorganisms and bacterial culture. This ratio was a critical factor in determining the performance efficiency of the reactor. At 21h HRT and organic loading of 27.33 g TPH/m 2 d, the N:P ratio 28.5:1 and 38:1 both yielded high and almost comparable TPH and COD removal efficiencies. This study presents a feasible technology for the treatment of hydrocarbon-rich wastewater from petrochemical industries and petroleum refineries. Keywords: Bacteria; Bioreactor; Cyanobacteria; N:P ratio; Total petroleum hydrocarbons 1. Introduction Petrochemical industries and petroleum refineries generate large amounts of priority pollutants. The major pollutants found in these industries are petroleum hydrocarbons, specifically aliphatic hydrocarbons, arising from storage of crude oil, spills, wash downs and vessel clean-outs from processing operation. Treatment of this wastewater is performed through a series of on-site treatment technologies, such as, American petroleum institute (API) separators, tilted plate interceptor (TPI) separa- tors and dissolved air floatation (DAF) units. These operations are followed by biological treatment in a suspended growth pro- cess. However, these processes are typically associated with numerous operational problems, which include: poor settleabil- ity of the sludge due to low F/M (food to microorganism) ratio; production of extra-cellular polymers consisting of lipids, pro- teins and carbohydrates that adversely affect sludge settling; biological inhibition due to toxic compounds, which necessi- tates very long sludge retention time; long period of acclimation or start-up and production of large amount of biological sludge [1,2]. Some of these problems can be avoided by replacing the suspended growth process by fixed film biological reactors, such as, the trickling filter and the rotating biological contac- tor (RBC). The inherent advantages of RBC include good COD removal efficiency and capability for handling toxic pollutants [2]. Moreover, the rotating discs facilitate oxygen transfer in the system, thus, eliminating the need for additional aerators. Hydrocarbon degradation has been widely reported in laboratory-scale batch studies [3–5]. However, studies demon- strating biological treatment of hydrocarbon-rich wastewater in continuous flow bioreactors are limited and need attention. Some researchers have reported the degradation of low concentration of hydrocarbons using bacterial cultures [2,6] and algal cultures in bioreactors [7]. Compared to bacterial systems, bioreactors utilizing a symbiotic association between algae and bacteria have been reported to yield higher treatment efficiency [8–10].