Coupling membrane filtration to anaerobic primary sludge digestion

Anaerobic digesters are widely employed at wastewater treatment works for the destruction of volatile solids present in settled primary and waste activated sludge. Membrane separation techniques afford an effective method to separate solids from the digester suspension and recycle them to the digester. A commercially available polymer ultrafiltration and ceramic microfiltration membrane were tested for filtration of anaerobic sludge. For the filtration of suspensions with similar solids concentrations, the short term fluxes reached in ceramic microfiltration (200 tot 250 l m(-2) h(-1)) were on the average tenfold those for polymer ultrafiltration (25 l m(-2) h(-1)). The permeate quality of the ceramic microfiltration and polymer ultrafiltration was similar. Discontinuous coupling of the ceramic microfiltration to an anaerobic primary sludge digester increased the total solids concentration of the reactor from 22 g l(-1) to 35 g l(-1), but the reactor efficiency was not improved in terms of volumetric loading rates. The high frequency of displacement of the anaerobic sludge through the filtration unit disrupted the interaction between the different species in the anaerobic consortia due to shear stress. Ceramic microfiltration of digested sludge was estimated to represent an extra cost of 1.53 ECU (57.2 BEF) m(-3) permeate compared to polymer ultrafiltration. In line implementation of membrane filtration in sludge digestion is not recommended in terms of improved microbial process performance. However, coupling membrane filtration to the final treatment of the digested liquor by polymer ultrafiltration or ceramic microfiltration opens perspectives because high fluxes can be achieved and a high quality permeate is produced.