Study On Advanced Treatment And Reuse Of Washing/Bleaching Effluents Of Pulp & Paper Industry

The shortage and pollution of water has become a serious problem in the world. In order to solve this problem, advanced treatment and recycling of sewage are indispensable except economy.Because of complex component, high load of pollutant and large sewage capacity, the screening/bleaching effluent from paper mills has become the focus in pollution controlling. Now sewage treatment projects in most mills just include the first and second treatment phases, a majority of chromomeric lignin ramifications are difficult to be decolored. The effluent with deep color and COD_Cr content, can't meet the newest'discharge standard of water pollutants for pulp and paper industry'. So it's imperative to further reduce COD_Cr and chroma, improve effluent quality and study advanced treatment methods of the washing and bleaching effluent.Screening/bleaching effluent from paper mills was treated with"magnetic pre-treatment + catalyzed+ coagulation"combined process, the main work contents: the optimum terms of magnetic pre-treatments, catalyzed and coagulation; pilot experiment and reuse experiments had been done. The results show that:(1)The best intensity magnetic field is about 550mT, the speed of water is about 1.5m/s, under these conditions, the removal of COD_Cr is about 6.1%, surface tension reduces about 35.8%, conductivity rise about 1.6% and pH rise 4.1%.(2)The mechanisms and operating conditions of catalyzed polymerization were mainly studied. The results show that under the conditions of COD_Cr 368mg/L and color 500 times for the influent, when m(H₂O₂)∶m(catalyst )equals 1/3, the dosage is 150mg/L, pH is 5.0, reaction time is 30min, effluent water with COD_Cr below 100mg/L and the color below 50 times can be reached with the removal ratios of COD_Cr and color up to 72.8% and 90% respectively.(3)Al₂(SO₄)₃ is superiority compared with Al₂(SO₄)₃ and PAC The suitable condition of Al₂(SO₄)₃ is that pH=7.0, input dosage is 250 mg/L, speed grads is 25s-1, the removal ratios of COD_Cr and color up to 51.2% and 81.2% respectively.(4)Pilot experiment shows that: When the input wastewater's COD_Cr is about 300mg/L, the color is about 200 times, the output water's COD_Cr is about 50mg/L, the color is about 10 times. The removal ratios of COD_Cr and color are up to 83.3% and 95%. The operation cost is about RMB 1.35 yuan/m³. (5)The influence of wastewater recycling during cooking on pulping process and the bleach ability and physical properties of pulp and paper making was discussed. Effluents after advanced treatments were used to replace fresh water in the kraft cooking of aspen wood. The results show that: the most capabilities of pulp and paper are just as fresh water, but sizing degree reduced a lot, and COD_Cr, BOD₅ of effluents can become higher, so, the reuse ration must low than 60%.