Textile printing and dyeing wastewater has characteristics of large water volume, high content of organic pollutants, high alkalinity, changeable water quality etc, it belongs to one of the intractable industrial wastewater, which contains dye, sizing agent, additives, oil, acid and alkali, fiber impurities, sand material, inorganic salts, etc. Currently the main methods of printing and dyeing wastewater treatment are physicochemical method, biochemical method, chemical method and combining several processing methods, and the pretreatment in wastewater treatment is mainly to improve wastewater quality, remove suspended solids and directly sedimental impurities, adjust effluent water quality and water volume, reduce waste water temperature, etc., thus to improve the overall effect of wastewater treatment, and ensure the stability of the entire process system, so the pretreatment has anis extremely important status in printing and dyeing wastewater treatment.
Currently pretreatment technologies used in printing and dyeing wastewater treatment mainly include: grilling, screen mesh, riffling, regulating water flow and water quality, cooling etc. Different pretreatment methods are taken in accordance with different printing and dyeing wastewater quality, remove part of pollutant to improve the treatment effect of subsequent improved wastewater quality processing unit.
Printing and dyeing wastewater contains large amounts of hair cloth, thread, fiber scraps etc. Tiny suspended solids, for example, boiled bleaching wastewater of knitting weaving, jeans rinse wastewater contains a large number of small fibers suspended solids, mixed printing and dyeing wastewater often contains many larger suspended substances. These substances will cause damage to pump and affect subject handling, so they must be intercepted before entering the pump and the main body structure. Set grille to intercept larger suspension, set the screen mesh to intercept tiny suspended solids.
A, Grilling
Grilling is generally used in comprehensive printing and dyeing wastewater treatment where water volume is huge and water quality is complex, such as textile printing and dyeing wastewater treatment of industrial park above ten thousand tons. For this kind of wastewater volume is huge, and suspended particles are larger. Set up the grille can effectively intercept larger suspended solids; processing capacity is high, and also not easily jammed. According to the characteristics of printing and dyeing wastewater, the engineering practice of our company do not set the coarse grid, generally only use fine grille. Grille space usually adopts 1-5 mm. Grillage machine mainly have rotary machine grillage, mesh type chain grillage machine, fixed grillage, conversely rotary fine screen machine, etc.. The main and commonly used in our company are conversely rotary fine screen machine, mesh type chain grillage machine, fixed grillage, etc.
B, Screen Mesh
Screen mesh is commonly used when water volume is relatively small, wastewater contains a large number of fine suspended solids such as cloth hair, thread etc., it also can remove large particles of pumice slag at the same time. The suspended solids and large particle removal rate can reach above 90%. Engineering practice shows that the mesh clearance generally is 30 ~ 60 mesh, installation form is fixed installation, the installation Angle is 30 ~ 45 °. Setting Angle can be neither too large nor too small. If too large, it will reduce the water load, reduce processing ability also increase some investment meanwhile, if too small, it will easily cause congestion, increase the difficulty of slag removal, influence the treatment effect.
Rinse wastewater (such as jeans rinse wastewater) of printing and dyeing wastewater, contains a large number of sand clay material such as pumice slag. If do not riffling the wastewater treatment, it often can cause a large amount of sand deposition for subsequent structures, reduce the molten bath of subsequent processing structures, the hydraulic retention time, and make the hydraulic characteristics cannot meet the design requirements, which seriously influence the treatment effect of wastewater, especially on the pump abrasion. This will reduce the service life of the pump, increase the operating costs. So it is necessary to set the riffing in dyeing and printing wastewater treatment. Grit chamber in general can be divided into advection grit chamber, aerated grit chamber and vortex gift chamber. Our company applies the advection grit chamber mostly, mainly due to the pumice slag surface after jeans rinse does not contain large amounts of organic matter in wastewater, so it is not necessary to use the aerated grit chamber, or vortex grit chamber. The advection grit chamber handling is simple, operation and management is convenient.
During the design process of grit chamber, the water quality characteristics of rinse water are fully analyzed. Considering the characteristic of small sand particles, grit chamber can be divided into 2 ~ 3 level grit, so it can make mud sand particles sediment step by step according to grit number, thus ultimately achieve the goal of removing mud sand. The total stay time can be designed for 1.5 hours. Sand draining modes are gravity sand draining and mechanical sand draining, which can be determined according to the engineering actual situation.
Due to its unique production process of textile printing and dyeing industry, which causes the discontinuity and variability of wastewater discharge, the discharge of waste water quality and quantity has a very big change in one day, or even within each turn. Therefore, it is required to regulate the waste water, balance water quality, and allows it to evenly inflow the subsequent processing units, to improve treatment effect. The regulation of printing and dyeing wastewater is divided into: water quantity regulation and water quality regulation.
Wastewater treatment equipment and structures are designed according to certain water flow standard. It requests inflow water evenly, especially more important on the biological treatment system. In order to guarantee the normal operation of the subsequent processing system, you should regulate the water in advance before entering in wastewater treatment system, thus to make processing system meet the design requirements.
Printing and dyeing wastewater contains high organic pollutant, deep chromaticity, high alkalinity and changeable pH value and water quality, so it is necessary to regulate wastewater quality, especially the pH value of wastewater. Before entering into wastewater biological treatment, please adjust pH value to 6-10 in order to meet the requirements of wastewater biological treatment.
Practice has proved that regulating reservoir standing time is different according to the different water quality and quantity of printing and dyeing wastewater. When processing water amount is small, the standing time optional can choose larger, when processing water amount is larger, the standing time can be chosen smaller according to the specific circumstances, 4 to 10 hours on usual occasions.
For some printing and dyeing wastewater, in order to make the regulating reservoir has certain removal efficiency and increase the uniformity of wastewater, especially when the wastewater contains more reducing agents, you can consider to add preaerator device in regulating reservoir, which can effectively improve the water quality characteristics of wastewater. Such as denim warp pulp dyeing wastewater contains a large number of sulfide (300-500 mg/L), pre aeration of wastewater can make them partly S - oxidation.
Most of the water temperature of printing and dyeing wastewater is pretty high (except sizing dyeing and printing wastewater), such as bleaching dyeing waste water temperature of knitting fabric, knitting thread is 40-45 ℃, bleaching waste water temperature of lint and woolen is 40-50 ℃, waste water temperature of returned woven cloth is 40-50 ℃, etc.. When the water temperature is too high, it will lead to the abnormal operation of the wastewater biochemical treatment system, and directly impact the standard sewage discharging. Therefore to cool down the high temperature wastewater must be considered. Then make the wastewater into biochemical treatment system so as to meet the water temperature requirements of biochemical treatment, and to ensure the normal operation of the system. At the same time, the heat energy in the waste water is also a kind of renewable resources.
The cooling method for waste water is by heat transfer, after mixing the industry processing wastewater of different temperature, then come into the heat exchanger for cooling. General temperature controls to below 42 ℃, which is conducive to the growth of organisms, and improve treatment effect.
Cooling water can use fresh process water, thus to use part of the heat energy for production process water preheating. Then on one hand, it reduces the waste of water temperature, on the other hand, also improves the production process water temperature to save steam from heating the fresh process water, which reach the purpose of saving production cost. Heat transfer scheme can consider using multiple heat exchangers in parallel design; it can guarantee the normal use of heat exchange system. Heat exchanger can be divided into the plate heat exchanger and tubular heat exchanger, etc.; the most widely used in our company is high efficient plate heat exchanger.
Anyway, printing and dyeing wastewater is a kind of refractory organic wastewater with huge water volume, high chroma, complicated components, changeable water quality range and high temperature. Through a large number of engineering practices has proved that the wastewater pretreatment technology during comprehensive technology of printing and dyeing wastewater treatment is very important. It relates to the stable operation of the whole system and the standard discharging, also involves the high or low operation cost. It can significantly improve the wastewater quality after wastewater pretreatment, which is conducive to further processing, and finally reach the goal of removing pollutants. So pretreatment technology in printing and dyeing wastewater treatment is one of the key indispensable technologies.
