Study On Photocatalytic Treatment Of Paper Mill Effluent And Its Process Control System

Paper industry is one of the most closely related industries with the national economy, which is developing rapidly. The paper industry brings us not only tremendous social development opportunities, but also the great pressure of environmental pollution. Up to now, more than 300 organic chemicals have been detected in the paper mill wastewater, a great part of which are hard to be degraded or toxicants. At present, some traditional methods are used in wastewater treatment. But the COD and BOD removal rates of the flocculation and sedimentation method are limited, which are hard to reach to fisrt grade standard of domestic wastewater emission. The alikali recycling method isn't suitable for paper mills of middle or small size, who are familiar in China. And the process of biochemical treating method is too complex to be used with much higher cost. In order to overcome these disadvantages of traditional methods, in this paper, gas chromatography-mass spectrometry (GC-MS) analytical method was employed for the determination of organic pollutants in recirculating water of a recycling paper mill of Guangzhou. Then, photocatalytic method with nanosized TiO₂ colloids was studied for treatment of mixed wastewater and the water treated by biochemical method. Based on these chemical experiments, an intermittent process controlling system of the photocatalytic reaction was developed.The water samples from different workshop sections were analyzed. About 50 kinds of organic chemicals, such as resin acids, fatty acids, additives and their ramifications were detected. And esin acids were mainly existed in the pulping water; the fatty acids, aromatic acids and their ramifications were mainly existed in the white water and the mixed water coming out of the mill. Comparatively, the mixed water contained the most kinds of organic chemicals, and the vacuum pumping water contained the least organic chemicals. Origins of the detected organics and infections which may play on the environment and human were discussed according to their molecule structures and chemical characteristics. Their contents in different production stages water were compared.The mixed wastewater coming out of the paper mill and the water which had already been treated by biochemical method were studied. They were treated by photocatalytic method with nanosized TiO₂ colloids. As a result, their best reaction conditions were same. The pH value was 3.0, the TiO₂ dosage was 0.5g/L, and the most rapidly reaction speed was occurred within 6 hours. At last, the COD value of the mixed wastewater was degraded to 103.0mg/L, with the removel rate 88.7%, and COD value of the water treated by biochemical method was as low as 75.2mg/L, with the removel rate 73.7%, which is up to the national standard of sewage discharge. At the same time, metal halide lamps of 35W and 70W were used to simulate the sunlight. It was proved that the sunlight was feasible to be used as the light sauce in the photocatalytic reaction. The definition of photocatalytic degradation capability, Capphoto was brought forward. Through calculating, Capphoto of photocatalytic system of the mixed wastewater was 4.45×10-6J-1, Capphoto of photocatalytic system of the SBR wastewater was 1.58×10-7J-1, which were both better than the data from orther studies. In this study, it cost about $83 when 1 ton of wastewater was treated, which means that the photocatalytic cost was greatly reduced. But it was still too much to be used in practice. Fortunately, since the sunlight feasibility was already proved as an additional light and the good TiO₂'s sedimentation characteristic when pH=7, the cost of photocatalytic treatment of paper mill wastewater will be further reduced. The result was optimistic for factual application of photocatalytic method in wastewater treatment plant.Basing on the technical studies, a set of process control system of the photocatalytic treatment was developed with Kingview 6.53 and Siemens S7-200 PLC. Detecting and controlling online and automatically of switches, pumps, producting parameters, such as pH value and water level were realized, as well as the functions of long-distance controlling, data basing inquiring, network issuing and so on. A PID control algorithm with three-segment non-linear variable gains was chosen to control the pH value. It exhibits such advantages as good robustness, quick response, high control accuracy, small pH surge and less consumptions of acid and alkali.