Decolorization of heat-treatment liquor of waste sludge by a bioreactor using polyurethane foam-immobilized white rot fungus equipped with an ultramembrane filtration unit

A bench-scale bioreactor using immobilized fungal cells equipped with an ultramembrane filtration unit was developed as a means of decolorizing brown color components (melanoidins) arising from the heat-treatment liquor (HTL) of waste sludge. Artificial HTL containing 4200 color units of synthetic melanoidin supplemented with 1000 mg/ l ethanol was first subjected to decolorization by the fungus Coriolus hirsutus IFO4917 immobilized onto polyurethane foam cubes. Then, the resultant biologically treated HTL was subjected to ultrafiltration to obtain the permeate (filtrate) as the effluent. The retentate (concentrate) of the filtration unit, containing the remaining melanoidin of high molecular weight and extracellular decolorizing enzymes, was returned to the fungal bioreactor to allow further decolorization. This system was operated in a sequencing batch mode under nonsterile conditions. Contamination of the bioreactor with air/water-born microbes markedly lowered the decolorization efficiency. However, this problem was solved by heating the returned concentrate at 50°C for 10 min. Under the almost stable condition of a hydraulic retention time of 2 d in a 1-d cycle sequencing batch mode, about 70% decolorization was routinely achieved using the entire system (bioreactor + ultrafiltration), while the contribution of the fungal bioreactor alone to the decolorization was about 45%.