Efficiency of chitosans for the treatment of papermaking process water by dissolved air flotation

•Papermaking process waters were treated with chitosans in dissolved air flotation.•At neutral pH, native chitosans were more efficient than quaternary derivatives.•A high synergy is obtained when native chitosans are combined with bentonites.•Chitosan dosage can be reduced by half if bentonite is added at a ratio 1:2 by weight.•FBRM technique allows monitoring flocculation and predicts separation efficiency. Interest has grown in bio-polymers as being environmental friendly alternatives to synthetic additives. In this work, two native chitosans with different molecular weights have been evaluated on a laboratory scale for their effectiveness for the removal of contaminants from papermaking process waters by dissolved air flotation (DAF). The use of chitosan quaternary derivatives and the use of the native chitosans in combination with anionic bentonite microparticles have also been tested. Results demonstrate a high efficiency of the native chitosan products at intermediate dosages and furthermore, their efficiency is enhanced by the combined addition of bentonite. For an equivalent removal of contaminants, the required dosage of chitosan is about half that the dosage required in absence of bentonite. Quaternary derivatives have not improved the efficiency of the native chitosan in this case. The optimum treatment would be 50mg/L of nativechitosan and 100mg/L of bentonite where this treatment is capable of the removal of 83–89% turbidity (residual turbidity 210–320 NTU), 68–71% dissolved turbidity (residual dissolved turbidity of 22–24 NTU), 18–22% total solids (residual total solids of 2750–2900mg/L) and 19–23% COD (1440–1525mg/L). The low molecular weight native chitosan is more efficient than the medium molecular weight chitosan in all cases. The Focused Beam Reflectance Measurement (FBRM) is used to assess the aggregation process and to predict the separation efficiency of DAF units either with single or dual systems. The efficiency predicted through the FBRM technique is very similar to that obtained later in the DAF tests.