Bioreactor innovation for efficient anaerobic bilge water treatment
Ship bilge water is a mixture of seawater and waste liquids such as fuels, detergents, solid particles and oils. The discharge of these oil residues into marine environments is prohibited by MARPOL.aspx (International Maritime Organization) regulations and EU legislation, so various methods are used to treat bilge water. These include membrane separation, electrocoagulation, centrifugation and chemical agents. However, these often incur secondary pollution, alongside high costs. While anaerobic biological treatment is more sustainable, the EU-supported ElectroSAnMBR project found that the organic compounds in bilge water inhibit anaerobic microorganisms, especially methanogens, reducing performance. ElectroSAnMBR has developed an innovative technology that integrates a single-chamber electrolysis cell (EC) inside a Submerged Anaerobic Membrane Bioreactor (SAnMBR), called e-SAnMBR. “As the membranes retain the anaerobic biomass, rather than them being washed away, the microorganisms, including methanogens, adapt to the recalcitrant wastewater, effectively becoming inoculated, and so remain active,” explains Ioannis Vyrides, project coordinator. This effect of the EC was demonstrated through two pilot bioreactors.
Microbial Electrolysis Cell-anaerobic bioreactor
e-SAnMBR’s anaerobic bioreactor consists of two electrodes. The anode helps create micro-aerobic conditions, contributing to the biodegradation of so-called recalcitrant compounds, those difficult to biodegrade; the cathode helps produce hydrogen and then microorganisms convert carbon dioxide to methane. The e-SAnMBR also contains a submerged microfiltration membrane which retains the microorganisms and organic materials. To create this electrochemical system, the team first tested how bilge water interacts with the biomass which contains the biodegrading anaerobic microorganisms, in this case granular sludge. As the e-SAnMBR biomass granules were typically larger than 500 micrometres in diameter, oxygen could not penetrate deep enough for the methanogens in the granules to help create the necessary aerobic conditions. “Happily we found that adding the Microbial Electrolysis Cell (MEC) to the sludge enabled the bioreactor to produce the hydrogen and oxygen needed for micro-aerobic conditions, accelerating the biodegradation of recalcitrant compounds,” says Vyrides. Two pilot bioreactors were operated over 3 months across five experimental cycles. e-SAnMBR contained a membrane and stainless steel electrodes under 1 volt, while SAnMBR contained only a membrane. Both SAnMBRs showed relatively high organic compound removal rates – known as chemical oxygen demand (COD) removal rates – of around 75 %, when operated at a high hydraulic retention time (HRT) of around 12 days – that is the total time the bilge remains inside the bioreactor. However, under a lower HRT (around 4 days), the e-SAnMBR showed a COD removal rate of around 50 %, with around 40 % for the SAnMBR. While the microfiltration submerged membrane in both bioreactors contributed only around 10-15 % COD removal, the effluents were then particle-free.
Future opportunities
European Directive 2000/59/EC states that vessels over 400 tonnes should separate oil from water, releasing the water overboard with the leftover effluent under 15 g/L, to be treated onshore. Vessels below 400 tonnes should hold bilge water on board for treatment at port facilities. The e-SAnMBR could be installed on vessels to pretreat bilge water, reducing the volume treated offshore. “Also, since the anode’s biofilm partially biodegrades bilge water and as undiluted bilge water inhibits methanogenesis at low HRTs, we could produce hydrogen as a final cathode gas – a valuable renewal energy source, instead of methane, a greenhouse gas,” adds Vyrides. This would mean that the system could operate at low HRT, under 24 hours, with no extra chemicals needed, to effectively treat bilge while producing hydrogen. Currently, Ecofuel Cyprus is piloting the e-SAnMBR. This research was undertaken with the support of the Marie Skłodowska-Curie Actions programme.
Keywords
ElectroSAnMBR, bioreactor, biodegradation, bilge, ship, methanogens, microorganism, hydrogen, methane, electrolysis cell, biomass, recalcitrant compound
