Role of microbial accumulation in biological sulphate reduction using lactate as electron donor in an inversed fluidized bed bioreactor: Operation and dynamic mathematical modelling

This study evaluated the impact of substrate accumulation (sulphate and polyhydroxybutyrate (PHB)) on bioprocess control of a sulfate reducing inversed fluidized bed bioreactor. To investigate the impact of substrate accumulation, step feed changes were induced to an inversed fluidized bed bioreactor performing biological sulphate reduction. A first step feed change set both the chemical oxygen demand (COD) and sulphate influent concentration to zero. As hypothesised, sulphide was still being produced after 15 days of operation without electron donor and sulphate supply. This suggests that accumulated and/or sorbed COD and sulphate supported the continued biological sulphide production. PHB was indeed found present in the sludge and batch tests showed PHB can support the sulphate reduction. A second step feed change of adding solely COD (and no sulphate) to the bioreactor influent resulted in a continuous production of sulphide, suggesting that sulphate had accumulated in the inversed fluidized bed bioreactor sludge. A mathematical model that includes microbial growth, PHB and sulphate storage as well as metabolism of lactate oxidizing sulphate reducing bacteria was developed, calibrated and validated. The model was able to simulate the accumulation of both PHB and sulphate in the inversed fluidized bed bioreactor.