Silica is present in paper mill effluents in a significant concentration. Silica mainly comes from the sodium silicate (Na2SiO3) added during the deinking process. Na2SiO3 is required in the process to perform several functions:

1. it stabilizes hydrogen eroxide used to bleach pulp in the pulper;
2. it has buffering and saponification properties;
3. it assists ink particles dispersion and influences their size;
4. it appears to act as an ink collector, and it reduces fibre losses and suppresses the flotation of fillers.

Replacement of Silica by other products is rather difficult, due to these different functions. One of the research lines addressed in the AquaFit4Use project comprised testing a pilot technology train aimed to enable the reuse of the HOLMEN Paper Madrid factory effluent as fresh water. The train integrated a biological treatment, a membrane bioreactor and a reverse osmosis. The relatively high concentration of silica in the effluent reaches much higher values in the retentate side of the reverse osmosis (RO) membrane, what makes it precipitate and form a layer on the membrane surface that obtrudes the pores and limits the RO recovery (see picture).

In order to have a cost effective RO, the recovery should be above 80%. Silica solubility ranges from 120 to 150 mgGL-1 (25ºC, pH<8.0-8.5). Considering these conditions, the UCM group calculated theoretically that more than 90% of silica should be removed from the water feeding the RO to avoid scaling, whenever working at a recovery higher than 80%. One possible alternative to prevent silica precipitation is to limit its solubility through the increase of pH, although in the case concerned it would lead to carbonate scaling. The research team of UCM focuses therefore on coagulation as a previous stage to the RO to remove silica. The coagulation mechanisms induced by six inorganic coagulants was studied in real time at different pH levels. 100% removal of silica was obtained although at non economical conditions for industrial application. A reduction of over 50% was achieved without pH regulation, what is still not reaching the 90% reduction required for a feasible RO. Although promising, the results indicate that further studies are required to improve silica removal. The researchers from UCM points out that the key to achieve a better removal may lay on finding the adequate coagulants/flocculants and pH conditions.

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