Recycling of wastewater from small paper machines

　　White water refers to the water removed during paper sheet formation, as well as the water further removed by the vacuum box and press. The recycling of white water involves using various methods to treat paper machine white water, reducing its suspended solids content, and reusing it in the pulp and papermaking process as a substitute for fresh water. The purpose is to reduce fresh water consumption and lower wastewater discharge and solids loss. An effective method in paper mills is to use a paper machine white water recycling system to treat paper machine white water. So, what methods are there?
　　There are three methods for white water recovery:
　　1. Filtration methods, such as concentrators, dehydrators, and multi-disc filter presses;
　　2. Flotation methods, such as flotation white water recovery machines;
　　3. Sedimentation methods, such as sedimentation tanks, sedimentation towers, and settling tanks.
　　I. Flotation Method
　　Due to the varying difficulty of solids floating in water, the solids in papermaking white water can be divided into three categories: solids with higher density that are not suitable for flotation separation; fine suspended matter and fine flocculates on which buoyancy has little effect; and floating solids with high buoyancy, low relative density, or flocculation during flotation.
　　Traditional flotation recovery equipment is only suitable for separating and recovering easily-floatable solids from water, while the Poseidon pressurized flotation machine can effectively remove all three types of solids.
　　Traditional flotation white water recovery devices are based on first saturating the white water with air, allowing the solids in the white water to adsorb air, reducing their apparent density, and thus floating to the surface for separation.
　　Factors such as air supply, white water concentration, type and amount of flocculant will affect its recovery efficiency. Among them, the air supply has a relatively large impact, and the dissolved gas ratio needs to be controlled. Too much air will cause fluid turbulence, destroying flocculation, and reducing the flotation effect; insufficient air will reduce the gas-solid ratio, thus reducing the recovery efficiency.
　　Traditional dissolved air flotation equipment uses rectangular flotation tanks. The Krofta Sandfloat dissolved air flotation equipment that has emerged in recent years uses circular flotation tanks, which have higher recovery efficiency. Its operation is based on the principle of zero velocity, giving the white water a short residence time and shallow (400㎜) operating conditions.
　　The white water enters the purification tank through a rotating multi-tube feeder. The feeder rotates around the center in the opposite direction to the feed direction, which reduces the white water velocity to a minimum, creating a relatively still water column in the flotation tank. Bubbles carry solids vertically to the liquid surface, which are then removed with a sludge scraper.
　　II. Filtration Method
　　Filtration is a type of method that uses various forms of filter media to treat white water and wastewater.
　　Currently, a representative piece of equipment in widespread use is the multi-disc vacuum filter.
　　The multi-disc vacuum filter has a compact structure, wide adaptability, high filtration efficiency, and convenient installation and maintenance, therefore, it is widely used. In papermaking, it is mainly used for the concentration of various pulps and white water recovery.
　　It uses the vacuum generated by the filtrate water leg pipe as the filtration driving force, with a net water leg height generally of 5~7 meters. During operation, the liquid level in the tank is 100~200 mm higher than the center line of the main shaft.
　　The various sectors on the disc are in different operating states during rotation. When a sector is brought into the liquid surface, it first enters the atmospheric filtration zone. The liquid in the tank is filtered under the action of static pressure difference, and the turbid filtrate is discharged through the main shaft and the distribution valve atmospheric filtrate outlet. At this time, the sector begins to accumulate pulp, then enters the vacuum zone, and a large amount of filtrate passes through the already pulped filter screen via the main shaft and the distribution valve vacuum zone filtrate outlet, discharging clear filtrate and forming a filter cake on the sector.
　　When the sector rotates out of the liquid surface, under the action of vacuum suction, the filter cake is further dewatered, and the dryness increases to 10%~15%.
　　When the sector leaves the vacuum zone and enters the pulp stripping zone, pressure water peels off the pulp layer on the sector, which falls into the pulp receiving hopper and is further diluted with water (3%~4%) before being transported to or directly falling into the pulp tank by a screw conveyor.
　　III. Sedimentation Method
　　The sedimentation method uses sedimentation to separate fibers and fillers, achieving the purpose of recovering fibers and fillers and clarifying white water. Sedimentation towers are currently more commonly used sedimentation equipment. White water enters the inner casing of the tower from the top. The inner casing and the distributor cause the white water to form a swirling fluid with a certain intensity and form a stable centrifugal sedimentation form to sink.
　　When the white water concentration is high, the white water will continue to settle in the tower cone section. At this time, under the action of the downward flow rate formed by the continuous discharge of tail pulp from the bottom of the tower cone, it gradually accelerates, and the sedimentation rate of the concentrated white water entering the cone section makes the tail pulp concentration in the cone section form a stable concentration gradient according to the cone height, which is the key to the stable operation of the sedimentation tower.
　　Most of the preliminarily settled water bypasses the edge of the inner casing, allowing the tower body wake section to continue sedimentation at a slow upward movement speed until it reaches the overflow ring and enters the overflow trough.
　　At the same time, closed-loop white water circulation will also produce some adverse effects. White water contains various dissolved organic and inorganic substances, some of which are harmful. When the white water system is closed, these substances will enter the system, and the amount of fresh water and discharged wastewater will be greatly reduced. These harmful substances will accumulate in the system, causing water quality degradation, or producing foam, or producing sediment and fouling, clogging the mesh and felt, etc., affecting the wet end of the paper machine and paper properties.
　　In addition, closed white water circulation can also have varying degrees of impact on paper properties. According to relevant research reports, after closed white water circulation, the wet paper strength and tensile strength are reduced to some extent, which leads to increased paper machine breaks and reduced operating performance.
　　In addition, when the dissolved salt content exceeds a certain value, the main performance indicators of the paper will decrease. The increase in the dissolved solids content in the white water also has a negative impact on the tensile strength of the paper.