Continuous regeneration of degreasing solutions from electroplating operations using a membrane bioreactor

To extend the service life and to improve the quality of degreasing solutions from surface refining processes in the metal working industry a process based on a membrane bioreactor (MBR) with submerged multi-channel flat sheet ceramic membranes was developed. This MBR-based regeneration process combines the retention by membranes and the biodegradation of oils/grease. The objective is to retain the biomass as well as the hydrocarbons in the bioreactor by the microfiltration membranes to enhance biodegradation. Simultaneously, low retention is aimed at for the surfactants since these substances have to be returned to the degreasing bath. The process was tested with a mobile pilot plant over a three-month period on site in an enterprise which carries out electroplating on commission. The multi-channel flat sheet ceramic membranes, which had an average pore diameter of 0.3 µm, fulfilled the objective of retaining the hydrocarbons to a high extent while allowing relatively high permeation of the surfactants (in order to minimise additional dosing). Permeate was free of solid matter and hydrocarbon concentration was reduced by 85–90% (compared to the feed). The reduction in non-ionic surfactants was only 25–40%. Compared to conventional (“open”) biological regeneration, a five fold increase in volumetric biodegradation rate was achieved due to the higher biomass concentration. A preliminary economic analysis for this case study showed, that there was a reduction in costs of roughly one third for the process under study in contrast to conventional hot alkaline degreasing. The main reason for this saving is the lower cost of wastewater treatment.

In the process under study, the quality of the treated process solution was determined by the complex interaction of the membrane separation properties and the biodegradation processes. Owing to the relatively short test period and the specified constellation of the test setup (bypass operation to the existing “open” biological regeneration process), the process could not yet be finally evaluated. Nevertheless, the results obtained in this work are very promising and show that the Fig. 10. Membrane area required for a MBR-based regeneration of the degreasing solution in dependence on target hydrocarbon concentration and permeate concentration.