Coalescence Filtration Performance and Mechanism of Fibrous Media for Offshore Oilfield Produced Water Treatment

Produced water from offshore oilfields contains dispersed oil droplets and oil-in-water (O/W) emulsions that require effective treatment before discharge. Compared with conventional retention-dominated membranes, fibrous coalescence filter media can promote droplet coalescence and continuous oil drainage, but the underlying filtration mechanisms remain insufficiently understood. In this study, six glass fiber filter media with different structural parameters were evaluated using O/W emulsions with four droplet size distributions. Based on the evolution of pressure drop and filtration efficiency, the filtration process was identified as an “interception–filling–drainage” staged mechanism. Laser scanning confocal microscopy showed oil accumulation within the inter-fiber pores and the formation of “liquid sail” structures during the filling stage. At steady state, with an increase in average pore size from 1.97 to 16.37 μm, filtration efficiency decreased from 85.5% to 46.4%, while steady-state pressure drop fell from 13.4 to 0.2 kPa. Droplet-size-switching experiments further showed that oil filling reduced the downstream oil concentration by up to 46.44% relative to the corresponding single-droplet-size condition. Based on the proposed staged mechanism, empirical correlations were established to quantify the effects of the ratio of emulsion droplet size to average filter pore size and filter structural parameters on pressure drop and filtration efficiency. These correlations provide an empirical means of estimating overall filtration performance under the conditions examined.