Evaluating the conversion of an automotive paint spray-booth scrubber to an activated-sludge system for removing paint volatile organic compounds from air

Volatile organic compound emissions from painting operations are of primary concern in the automotive industry and require costly control measures using vapor-phase activated-carbon adsorption. At Ford Research Laboratory, Dearborn, Michigan, a process modification has been proposed as a supplemental or an alternative control measure to convert an existing paint-sludge pit, part of a spray-booth scrubber system, to an activated-sludge reactor. Under this scheme, water-soluble paint solvents, captured by the scrubber water from the booth air in the spray booth, are to be biologically degraded in the sludge pit. For this paper, the technical feasibility of the conversion was evaluated by conducting a laboratory experiment using a bench-scale activated-sludge reactor fed with nine selected paint solvents; analyzing the experimental data with Monod kinetics for substrate utilization and a first-order kinetics for endogenous respiration; and estimating the performance of the paint-sludge pit as a bioreactor at a typical assembly plant. Results showed that a paint-sludge pit is sufficiently large to biologically destroy all captured paint solvents at a typical paint-spraying rate ii the pit is converted to a bioreactor; most of the oxygen-supply requirement for degrading paint solvents, which is a significant operating cost item for activated sludge, could be provided by the existing spray-booth air circulation at no extra cost; the rate of biomass production was estimated to be comparable to the rate of accumulation of overspray paint polymers; and the data agreed well with the Monod and first-order respiration kinetics.