Crossflow microfiltration of marble processing wastewaters

The wastewaters generated during the marble processing (cutting and grinding operations), the average flow rates of which are ca. 15 m3/h for a plant producing 45 m2 of polished marble/h, contain a high load of particulate matter. Furthermore, crushed stone operations sometimes use flotation agents, such as organic amines, fatty acids, and pine oils, in order to remove impurities from the marble. Therefore, the marble processing wastewaters should not be disposed directly into the environment without a suitable treatment in order to prevent negative impacts and comply with the national legislations currently in force. On the other hand, bearing in mind the severe water shortage in Mediterranean countries, which are intensive producers of carbonaceous rock and stone, it would be of utmost importance to recover and reuse the processing wastewaters generated in these plants. In this work, the treatment of marble processing wastewaters by microfiltration (MF) was investigated. Wastewaters from a marble processing plant located in Pero Pinheiro, Portugal, were characterised in terms of pH, conductivity, total solids (TS), and total suspended solids (TSS). A Micro Carbosep 60 module equipped with a mono-tubular mineral MF membrane, Carbosep M45 (nominal pore size = 0.45 µm, ID = 6 mm, L = 0.60 m), was tested at ambient temperature, natural effluent pH, a broad range of transmembrane pressures and crossflow velocities, and up to a 9-fold concentration. MF eliminated the suspended matter from marble wastewaters, allowing the treated water to be recycled into the process, whereas the concentrated stream (rich in particulate matter) can be used elsewhere, e.g. in the ceramic industry. The results from this work clearly show that MF is an efficient and ecologically suited environmental technology for decontamination and recycling of the wastewaters generated by marble processing plants, besides the environmental pollution abatement involved.

The microfiltration of wastewaters from a marble processing industry was investigated. A tubular mineral membrane 0.45 µm nominal pore size, Carbosep M45, was tested in this work. For moderate transmembrane pressures and high velocities, the MF permeate flux of marble wastewaters does not decay with respect to pure water permeate flux, remaining constant either with time and a 9-fold concentration rise, revealing the total absence of concentration polarisation and/or cake build-up. This is a remarkable finding, meaning that there is no need for membrane washing during crossflow MF of marble wastewaters, at least up to a 9-fold concentration. The high permeate fluxes and particles separation achieved, especially throughout the concentration run, clearly points out that MF is technically suited to accomplish the aimed goals, i.e. environmental pollution abatement from the marble wastewaters, recycle of water into the process and potential reuse of the concentrate stream elsewhere, e.g. in the ceramic industry. Continuous experiments should be carried out at higher concentration factors and longer operation periods, in order to get better insight of the concentration effect upon the concentration polarisation phenomena and/or cake build-up and thereby optimise the water recovery rate during microfiltration of marble wastewaters.