1According to current environmental legislation concerned with water scarcity, paper 2 industry is being forced to adopt a zero liquid effluent policy. In consequence, reverse 3 osmosis (RO) systems are being assessed as the final step of effluent treatment trains 4 aiming to recover final wastewater and reuse it as process water. One of the most 5 important drawbacks of these treatments is the production of a retentated stream, which 6 is usually highly loaded with biorecalcitrant organic matter and inorganics; and this 7 effluent must meet current legislation stringent constraints before being ultimately 8 disposed. The treatment of biorefractory RO retentate from a paper mill by several 9 promising advanced oxidation processes (AOPs) -conventional Fenton, photo-Fenton 10 and photocatalysis-was optimized considering the effect and interaction of reaction 11 parameters; particularly using response surface methodology (RSM) when appropriate 12 (Fenton processes). The economical cost of these treatments was also comparatively 13 assessed. Photo-Fenton process was able to totally remove the COD of the retentate, and 14 resulted even operatively cheaper at high COD removal levels than conventional 15 Fenton, which achieved an 80% reduction of the COD at best. In addition, although 16 these optimal results were produced at pH = 2.8, it was also tested that Fenton processes 17 are able to achieve good COD reduction efficiencies (> 60%) without adjusting the 18 initial pH value, provided the natural pH of this wastewater was close to neutral. 19Finally, although TiO2-photocatalysis showed the least efficient and most expensive 20 figures, it improved the biodegradability of the retentate, so its combination with a final 21 biological step almost achieved the total removal of the COD. 22Keywords: Advanced oxidation processes; Fenton; photo-Fenton; photocatalysis; 23 reverse osmosis retentate; paper mill effluents; response surface methodology. 24 AOPs treatment of RO retentate from a paper millDaphne Hermosilla et al.3

A further closure of the water circuit in paper mills with a relative high optimization of their water network is limited by the increase of contamination in the water and runnability problems of the paper machine. Therefore, new strategies for saving water must be focussed on the treatment of final effluents of the paper mill, aiming to obtain high quality water that may replace fresh water use in some applications. An appropriate treatment train performed at pilot scale, consisting on a previous clarification stage followed by anaerobic and aerobic treatments, ultrafiltration, and reverse osmosis, made possible producing the highest water quality from the final effluent of the mill. Anaerobic pre-treatment showed very good performance assisting the aerobic stage on removing organics and sulphates, besides it produced enough biogas for being considered as cost-effective. Permeate recovery depended on the silica content of the paper mill effluent, and it was limited to a 50-60%. The reject of the membranes fully met the legislation requirements imposed to effluents arriving to municipal wastewater treatment plants.

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13Coagulation is presented as an efficient alternative to reduce the silica content in 14 effluents from recovered-paper mills that are intended to be recycled by a final reverse-15 osmosis (RO) step. Coagulation pretreatment by several polyaluminum chlorides 16 (PACls) or FeCl 3 was optimized prior to the RO process. PACls with low alumina 17 content and high basicity achieved almost a 100% removal of silica at pH 10.5. A good 18 reduction of the silica content was attained without regulating the pH by adding one of 19 these PACls. Silica removal was related to the structure of the produced clots in which 20 cylindrical particles produced higher silica removal. All coagulants removed more than 21 50% of the chemical oxygen demand (COD). 22

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