Biological treatment of waste semi-synthetic metalworking fluid using a fluidized-bed bioreactor

Date of Completion

January 1997


Engineering, Chemical|Engineering, Environmental




A fluidized bed bioreactor (FBB) was studied to treat wastewater originating from the aqueous phase of metalworking fluids (MWFs). This wastewater had high levels of chemical oxygen demand (COD) and biochemical oxygen demand (BOD) owing to water soluble organic compounds in semi-synthetic MWFs. The goals of the research were two-fold: first, to establish an operable aerobic FBB to treat semi-synthetic metalworking fluid wastewater, and second, to characterize the bioreactor to better control and predict its performance.^ To establish an operable FBB, a recipe for simulated semi-synthetic MWF wastewater was developed that allowed a microbial population to develop in the FBB and degrade the water soluble organic compounds. Once a microbial population was established, controlling biofilm thickness on fluidized sand particles was crucial to stabilizing the FBB bed and producing effluent of relatively consistent quality. Effluent COD and BOD$\sb5$ concentrations remained below 300 and 30 mg/L, respectively, when the organic loading rate was gradually increased from 300 to 900 mg COD per hour. Effluent pH remained relatively constant at about 5, and a mean concentration of suspended solids was measured to be 44 mg/L.^ The FBB was characterized by analyzing substrate uptake rates. Biofilm-coated sand particles were found to stratify in the fluidized bed in such a way that the particles with thicker biofilm coatings segregated toward the top of the bed. Sand particles with thick biofilm (thickness $>$ 100 $\mu$m near the top of the bed) were found to consume substrate at significantly lower rates per unit biomass than for particles with thin biofilm (10-20 $\mu$m near the bottom of the bed); this suggested that substrate mass transfer resistance through biofilm may limit biodegradation rates in the upper portion of the FBB. Dissolved oxygen was found to be the limiting substrate in the biodegradation process. A model was developed that predicts COD removal and effluent quality based on the rate of oxygen transfer into the FBB and the ratio of O$\sb2$ uptake rate to COD uptake rate (Y$\sb{\rm O2/COD})$. ^