Treatment of paper pulp and paper mill wastewater by coagulation–flocculation followed by heterogeneous photocatalysis

In this work is investigated the combined treatment of post-bleaching effluent from a cellulose and paper industry. The biodegradability index determined by the biochemical oxygen demand (BOD)/chemical oxygen demand (COD) ratio of in natura sample was 0.11, which implies little biodegradability and that it may not be discharged to the environment without previous treatment. First, the effluent was submitted to the coagulation–flocculation treatment applying FeCl3 as the coagulating agent and chitosan as an auxiliary. In sequence, the aqueous soluble phase obtained from the first treatment was submitted to a UV/TiO2/H2O2 system using mercury lamps. The optimized coagulation experimental conditions were chosen: pH 6.0, 80 mg L−1 of FeCl3·6H2O, and 50 mg L−1 of chitosan. The optimized photocatalysis conditions were: pH 3.0 in 0.50 g L−1 of TiO2 and 10 mmol L−1 of H2O2. COD values for the in natura sample was 1303 mg L−1 and after the optimized conditions of coagulation without chitosan and in chitosan presence were 545 and 516 mg L−1, respectively. Effluent turbidity decreased sharply after coagulations (from 10 FTU of in natura samples to 2.5 FTU without chitosan and 1.1 FTU with chitosan). Similarly, a decrease was observed for concentrations of N-ammoniac, N-organic, nitrate, nitrite, phosphate, and sulfate ions after coagulation. Additionally, it was observed an absorbance reduction of 90% at the wavelength of 500 nm and of 70–80% in regions corresponding to aliphatic and aromatic groups (254, 280, and 310 nm). The use of chitosan for quantitative purposes was not so efficient; however, it improves sedimentation and compaction. COD results of photolyzed samples by UV/H2O2 were 344 mg L−1, UV/TiO2 326 mg L−1, and UV/TiO2/H2O2 246 mg L−1. The reduction in absorbance intensity was approximately 98% for aliphatic and aromatic chromophores, and 100% for chromophores absorbing at 500 nm with color disappearance. During photodegradation, SO42− was formed (∼340 mg L−1 for the coagulated sample to ∼525 mg L−1) suggesting again the mineralization of the pollutant. The combined method (coagulation followed by photocatalysis) resulted in a biodegradability index of 0.71, transparency, and absence of color and odor in the treated water, suggesting again good water quality. This result is reinforced by the toxicity studies employing Artemia salina bioassay, which showed that an expressive decrease in toxic pollutants in effluents after treatment, mainly by combined processes. The wastewater treatment carried out in association at optimized experimental conditions provided good results.