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It has been reported that about 10-15% of the fresh water intake in a paper mill is used for feeding and diluting retention aids, so important savings could be achieved replacing fresh water with process water. Water from different sources and qualities: fresh water (FW), the outflow from an internal ultrafiltration placed in the machine circuit of a paper mill (UFW), and water from a membrane bioreactor used to treat the final effluent of this paper mill (MBRW), were used to prepare a dual retention system consisting of a cationic polyacrylamide (cPAM) and bentonite. While the behaviour of bentonite was not significantly affected by the quality of the water used in its preparation, the efficiency of the cationic polyacrylamide was reduced around the 12%, when it was prepared with water with high anionic trash content and conductivity due to a partial neutralization of the charged groups. The effect of non-ionic chemical oxygen demand on the efficiency of the polymer resulted negligible.

A novel method to estimate the concentration of Chlamydomonas reinhardtii biomass was developed. The method employs the chord length distribution information gathered by means of a focused beam reflectance probe immersed in the culture sample and processes the data through a feedforward multilayer perceptron. The multilayer perceptron architecture was systematically optimised through the application of a simulated annealing algorithm. The method developed can predict the concentration of microalgae with acceptable accuracy and, with further development, it could be implemented online to monitor the aggregation status and biomass concentration of microalgal cultures.

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