Anaerobic digestion of triglyceride emulsions in expanded granular sludge bed reactors with modified sludge separators

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Abstract
Modification of the gas‐solids separator of expanded granular sludge bed (EGSB) reactors is necessary to prevent excessive sludge wash‐out during anaerobic treatment of lipid emulsions. This paper describes two modifications: a hybrid reactor with a layer of floating carrier material (reticulated polyurethane foam) above the expanded sludge bed and a novel EGSB reactor equipped with a sieve‐drum separator (EGSB‐SDS). Replacement of the conventional sludge separator by a layer of floating polyurethane foam does not prevent sludge wash‐out. Therefore, the hybrid reactor is unreliable in treatment of emulsified lipids. An EGSB‐SDS reactor allows stable anaerobic digestion of emulsified lipids. An increase of the amount of granular sludge in the reactor can be achieved, provided that sludge granules with a sufficiently large diameter are used for inoculation. Especially if easily degradable organic contaminants are present in the waste water besides the lipids, maintaining a sufficiently large inventory of granular sludge is no problem. The EGSB reactor with sieve‐drum separator can accommodate higher organic and hydraulic space loading rates than previously described anaerobic filter reactors. The relatively poor conversion of organic material removed from the waste water to methane as a result of the flotation of lipids requires further research.