Hydrodynamics and mathematical modelling in a low HRT inverse fluidized-bed reactor for biological sulphate reduction

Biological reduction of sulphate at low hydraulic retention time (HRT) is presented in this paper. A sulphidogenic inverse fluidized-bed bioreactor (IFBB) was operated successfully at a progressively decreasing HRT from 1 to 0.125 days for a total of 155 days. Synthetic wastewater containing sulphate at a concentration of 745 (± 17) mg/L was used. COD was supplied as lactate in variable concentrations at COD/SO42− ratios of 1.2–2.4. The pH of the feed ranged between 5.2 and 6.2. The highest measured removal rates were 2646 and 4866 mg SO42−/L day at an HRT of 0.25 and 0.125 days, respectively, using a COD/SO42− ratio of 2.3. The biological sulphate reduction was limited by the influent COD concentrations at a COD/SO42− ratio < 2.3. The IFBB ensured biomass retention at a maximum liquid residence time of θ = 3.84 (± 0.013), according to the residence time distribution analysis. Hydrodynamic studies were carried out at recirculation rates of 0, 200, 300, 350, 400, and 500 L/h to measure the relative bed expansion, the mixing pattern, and the fluidization characteristics of the reactor. A dynamic model is also developed based on COD and sulphate as the two limiting substrates in a Monod-type kinetic equation describing the kinetics of lactate oxidation by SRB. A set of the following parameters $$Y_{{{\text{VSS}}/{\text{COD}}}}^{\prime }$$  = 0.23 mg COD of VSS/mg lactate, µmax = 1.758 day− 1, KCOD = 956 mg COD of lactate/L, $${K_{{\text{S}}{{\text{O}}_{\text{4}}}}}$$  = 316 mg SO42−/L, kd = 0.024 day− 1, tres = 5.7 days, and kexchange = 0.4 day− 1 simulated adequately the residual effluent COD and sulphate concentrations, the produced sulphide concentration as well as the pH of the IFBB effluent. Low HRT values, shown efficient in this study, are prerequisite for industrial applicability and economic feasibility of the sulphur reduction process. In addition, the developed model can be used for optimum experimental design and further process upscale and development.