Biodegradation of real bilge water by designed microbial consortia and their performance in pilot Moving-Bed Biofilm Reactors

Abstract

Bilge waters that are located at the bottom of the ships are greasy wastewaters high in organic load and recalcitrant organic compounds that originated from metal working fluids, diesel oil, oily sludge, spills from the engine room, water leaks from internal pipes, and seawater filtrations. Although different processes and technologies, such as electrochemical methods, membrane separation technologies and combined technologies, have been examined in bilge water treatment, only a few studies focus on biological treatment of bilge water. According to the international standards, vessels above 400 gross tonnage (GT) should use oily-water separator (OWS) to separate oil from the water and discharge the water overboard, provided the effluent contains less than 15 mg/L of oil. The remaining oily water should be consequently discharged and treated in port reception facilities. Vessels below 400 GT can sail in two ways: either treat their bilge water with the same systems as those vessels above 400 GT or they can be equipped to retain on board bilge water for subsequent discharge at reception port facilities. Bilge water is a recalcitrant wastewater with varying Chemical Oxygen Demand (2-15 g/L) and high salinity (10-35 g NaCl/L). While many bilge water treatment companies try to optimize their efficiency to treat bilge water, such procedures are costly and energy consuming. Particularly, EcoFuel LTD based in Zygi, Cyprus, uses three steps to treat bilge wastewater: a) Dissolved Air Flotation (DAF) followed by b) aerobic biological treatment in Moving Bed Biofilm Reactor and then c) the ozonation process. However, the biological treatment is inefficient requiring the extensive operation of the ozonation unit. The aim of this study is to isolate microorganisms that can biodegrade bilge water, and to use these microorganisms into pilot (200 L) Moving Bed Biofilm Reactors for bilge water treatment. To achieve this goal, a series of bacterial isolations and identifications were achieved, and a series of experiments were performed to identify and select the best bacterial strains for bilge water biodegradation. Bacterial strains were examined and selected based on several parameters: a) biodegradation of hydrocarbon (toluene, phenanthrene, hexadecane and dodecane) at various conditions (high salinity, pH and high/low temperatures), b) production of extracellular polymeric substances (EPS) with emulsification and/or flocculation properties and c) bilge biodegradation (soluble Chemical Oxygen Demand (COD) removal in bilge water). Based on these criteria a microbial consortium (A), consisting of 5 strains (Pseudomonas aeruginosa, Enterobacter sp. SW, Citrobacter sp. D2, Citrobacter sp. S1, Citrobacter sp. S6) was constructed and tested under various conditions. Then, this consortium (A) was inoculated at three pilot scale MBBRs (200 L) in different filling fractions of Mutag biochip carrier (10%, 20% and 40%) and a fourth MBBR was operated which was inoculated with the commercial consortium (filling ratio of Mutag biocarrier 10%). The MBBR with a filling fraction of 40% resulted in the highest COD decrease (60%) compared to the operation of the MBBRs with a filling fraction of 10% and 20%. The comparison of the tested consortium (A) with the commercial in 10% biocarrier showed similar COD removal. A second set of experiments was carried out to optimize the biological process and a new consortium (B) was formed with three strains taken from consortium A (P. aeruginosa LVD-10, Enterobacter sp. SW, Citrobacter sp. D2) and addition of three isolated strains from bilge water: Stenotrophomonas sp. PE, Exiguobacterium sp. Ex-Ind2 and Halomonas sp. Hal-CG. The pilot MBBRs in the second time of operation were tested with consortium B and a commercial consortium; under these conditions, consortium B had about 20% higher COD removal compared to commercial consortium.