Anaerobic digestion of starch particulates in an upflow sludge blanket filter reactor

Abstract

The effect of starch particulates in wastewater on an anaerobic upflow sludge blanket filter (UBF) reactor was evaluated at 37°C and pH 7.0-7.5 over a period of 415 days, during which the volumetric chemical oxygen demand (COD) loading was increased from 3 kg.m-3.d-1 to 50 kg.m-3.d-1. The initial organic substrate was a mixture of sucrose and starch. Upon reaching the constant loading rate of 10 kg-COD.m-3.d-1, starch gradually replaced sucrose in the wastewater and eventually became the sole substrate at loading rates of 10 kg-COD.m-3.d-1 and higher. Results showed that, despite the insoluble nature of the starch, only about 1.2% of starch in wastewater remained unhydrolyzed and about 5.9% became soluble COD in the effluent for loading rates up to 40 kg-COD.m-3.d-1. Of the COD removed, 86.1% was converted to methane and the rest was presumably converted to biomass with a sludge yield of 0.10 kg-VSS.kg-COD-1. The suspended solids in the effluent were mostly composed of the washed-out biomass, the amount of which increased with loading rate, because of the turbulence generated by the increased gas production and the insufficient depth of the reactor. There was no accumulation of starch in the reactor throughout this study. The starch-degrading granules had a higher methanogenic activity than granules in the previous studies degrading soluble carbohydrates. The bacterial populations of acidogens, syntrophic acetogens, and methanogens, were about 108-109 per ml.