Treatment of membrane retentates
Reverse osmosis and nanofiltration systems are being assessed as final step of effluent treatment trains aiming to recover the final effluent form paper mills and reuse it as process water. But one of the most important drawbacks of these treatments is the production of a retentate, as this effluent is usually high loaded of bio-recalcitrant organic matter and inorganics, and it must be disposed according with the legislation. Therefore, in this project different processes have been tested for dealing with these concentrates.
Evapoconcentration is a unit operation which consists in the separation of two liquid constitutes: the removal of solvent (generally water) as vapour from a solution or slurry. The objective of evaporation may be to concentrate a solution containing the desired product or to recover the solvent. Sometimes both may be accomplished. The aim of evapoconcentration is to obtain a very clean distillate which could be rejected in the nature, sent in a sewage treatment plant or reused. The concentrate is considered as an ultimate waste usually disposed to landfill.
Could be used as a final treatment of nanofiltration concentrates to produce fit-for-use water in the paper industry. However, feed characteristics limit the volume concentration factor (VCF) which can be reached at industrial scale due to scaling risks. If chloride concentration is high (approximately 7 g/L) and the evaporators are made of materials like UB6, surperduplex or hastelloy, a maximum of 45 g/L of chlorides in the concentrate is acceptable. For this reason, the highest VCF which can be reached at industrial scale for this effluent is equal to 6 or 7. Attention should be paid on scaling risk due to the concentration of calcium carbonate, calcium sulphate, calcium phosphate, silica, among others, usually present in paper mills.
On the other hand, advanced oxidation treatments (AOPs) reach an acceptable COD reduction (30-70%) but biological degradation would still be impossible, since the high salt concentration would have an inhibitory effect. AOPs have showed promising results. In the optimization that has been carried out during the AquaFit4Use project Fenton processes, conventional and UV assisted, have showed COD reductions higher than 60% at neutral pH and a total mineralization may be reached with UV assisted Fenton process at optimum conditions. Environmentaly friendly AOPs as solar photocatalysis will permit to increase biodegradability and therefore its combination with biological processes. Moreover, ozone will permit, with no addition of chemicals or necessity of further sludge disposal, to treat the high volumes of retentates coming from paper industry at a reasonable cost.
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