Performance of a Pilot Scale Membrane Aerated Biofilm Reactor for the Treatment of Landfill Leachate

Performance of a pilot scale membrane aerated biofilm reactor for the treatment of landfill leachate

Eoin SYRON, Eoin CASEY,

School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland

Keywords: Oxygen Permeable Membranes, Bubbleless Aeration, Biofilm, Wastewater treatment

 Aeration accounts for over 60% of the total energy spend in wastewater treatment plants, and even with recent technical improvements, bubble aeration can only achieve 50% oxygen transfer efficiency. Due to increasing energy costs and increasing regulatory pressure, there is a substantial growing interest in the application of diffusive gas transfer membranes for bubbleless aeration. These membranes avoid the mass transfer limitations associated with bubble-liquid interactions and increase the gas residence time within the reactor allowing more of the supplied oxygen to be transferred. Unlike conventional membranes in wastewater treatment, the concept of biofouling is not discouraged and the resultant biofilm on the membrane plays an active role in the degradation of COD and Nitrogenous constituents in wastewater.

Landfill leachate is water which has percolated through the waste bed in a landfill and leached organics and nutrients from the solid waste. This wastewater must be treated before being discharged to a water course to prevent pollution of the receiving water.  Typically leachate has a high ammonia concentration due to the anaerobic degradation which takes place in the landfill.  The Membrane Aerated Biofilm Reactor (MABR) is ideally suited for treatment of a high strength ammonia wastewater firstly as to the slow growing nitrifiers are attached to the membranes there is no need of an extra biomass retention operation, secondly the location of the nitrifying organisms on the membrane surface and their high oxygen demand exploit the oxygen transfer advantages of the MABR.  While previous studies have reported on the MABR for ammonia nitrification, use of the MABR for the treatment of leachate has not previously been undertaken.

A 60 litre pilot plant (figure 1) was constructed to treat leachate from Arthurstown landfill, Co Kildare, Ireland. The pilot uses hollow fibre PDMS membranes supplied by SilPro (Broadstairs, UK) which have an internal diameter of 300mm and an external diameter of 500 mm giving a wall thickness of 100 mm and a total surface area of 4.2 m² . PDMS membranes were chosen for this application due to their durability, ease of supply, and high oxygen permeability.   In contrast to porous membranes, PDMS does not suffer from problems such as intra membrane wetting and pore clogging. Although the surface roughness of PDMS is quite low, there have been few reports of poor adhesion or biofilm formation on these membranes.  The hollow fibres were potted in bunches of 60 fibres 0.4m long into 10mm PVC piping using polyurethane as the potting agent. Bunches were then arranged in cassettes of 11 bunches and 10 cassettes were placed in the reactor. The pH is controlled via a feedback loop and maintained between 7.5 and 8 with the addition of Sodium Hydroxide or Hydrochloric acid. Initial Oxygen Transfer Rates OTR experiments determined by re-oxygenation test gave results of 50gO₂ m^-2 bar^-2 day^-1.

Currently the reactor is removing 2.5g N-NH₄ m^-2 day^-1which is comparable with previously reported systems Syron & Casey 2008, but it is foreseen that this rate of reaction will continue to increase with time and thicker biofilm growth, a theoretical maximum of 15 g N-NH₄ m^-2 day^-1 if all of the oxygen supplied is used for oxidation to nitrite. The current pilot is one of the few pilot scale studies of a MABR. Previous attempts have encountered problems with the control of excess biofilm resulting in clogging in the membrane module.  In the present study the biofilm control strategy is allowing the pilot to continue with stable performance over a prolonged period.

References

Casey, E.; Glennon, B.; Hamer, G. Oxygen mass transfer characteristics in a membrane-aerated biofilm reactor. Biotechnol.

Bioeng. 1999, 62 (2), 183–192.

Syron, E.; Casey, E. Membrane-aerated biofilms for high ratebiotreatment: Performance appraisal, engineering principles,scale-up, and development requirements. Environ. Sci. Technol.2008, 42, 1833–1844.