introductory

A large-scale enterprise, with three newsprint production lines and a whiteboard production line, while supporting the construction of a 30,000t / d and a 15,000t / d sewage treatment system. As a result of the use of all waste paper for deinking pulp, so the loss of short fibres in the pulping process and waste paper in the dust, ink and other materials in the production process along with the discharge of production wastewater into the back of the sewage treatment system, which produces a large amount of sewage sludge. The traditional process generally adopts double-net filter press for sludge gravity dewatering after landfill or composting, but due to the lower dewatering efficiency of double-net filter press, the water content of the treated sludge still reaches about 75%, so the volume of sludge is huge, and it needs to take up a large amount of land resources for landfill, and generates a large amount of transport and treatment costs and is prone to cause secondary pollution. As the national environmental protection requirements become more and more stringent and the land resources are reduced, the supervision of solid waste is getting stronger and stronger, and it must be handled by qualified professional units and supervised throughout the whole process, so the treatment cost increases significantly. In order to long-term development, the company uses the opportunity of retreating from the second into the third, environmental protection relocation, the construction of the plate and frame filter press, the sludge of the sewage treatment system for deep dewatering, and supporting the construction of the comprehensive utilization of waste residue power plant, specializing in the incineration of deep dewatering of sewage sludge, so as to achieve harmless treatment and the purpose of resource recycling (Note: the company's sewage sludge contains a certain amount of fibers and inks, depth). (Note: Our sewage sludge contains a certain amount of fibre and ink, which can be recycled after deep dewatering and still contains a certain amount of heat).

2 Diaphragm type plate and frame filter press working principle

The sludge deep dewatering equipment used isFully automatic diaphragm type plate and frame filter presses. Fully automaticdiaphragm filter pressIt is a kind of intermittent pressurised filtration equipment, which can be used for solid-liquid separation of various suspensions. The principle of dewatering of this equipment is mainly through high-pressure feeding and high-pressure pressing, under pressure, the water in the suspension in the filter chamber enters into the filtrate side through the filtering cloth, while the solid particles are retained in the filter chamber under the interception of the filtering cloth, so as to realise the purpose of solid-liquid separation. The filter plate equipped with plate and frame filter press is divided into diaphragm plate and frame plate, and the two plates are arranged at intervals. The diaphragm plate has a hollow inner cavity with a certain degree of expansion, while the frame plate is a hard plate solid design, playing the role of top pressure. When the press liquid is injected into the inner cavity of the diaphragm plate, the diaphragm plate gradually swells under pressure, and the volume of the filter chamber becomes smaller and smaller, thus further squeezing out the sludge water in the filter chamber. The operation of the filter press is intermittent feeding, and a cycle mainly includes several steps of feeding, pressing, draining and discharging. The moisture content of sludge processed by diaphragm-type plate and frame filter press can generally reach less than 50%, and its dryness is related to the high pressure feeding pressure and pressing pressure, but considering the economy of operation and the safety of the equipment, the design of the high-pressure feeding pressure and pressing pressure is 1.6MPa, and the dryness of the sludge is 50% to 60%.

3 System design and operation and maintenance

The sewage filter press treatment system is a three-stage treatment process, the process is: physical sedimentation a (Ic anaerobic + SBR aerobic) biochemical treatment a shallow air flotation, so the sludge produced by the system in addition to the physical sludge produced by the sedimentation, and the residual sludge produced by the biochemical treatment of the SBR and air flotation sludge. The process of sludge treatment system is as follows: the physical sludge from the sedimentation tank and the residual biochemical sludge from SBR enter the sludge thickener for thickening, and then sent to the filter press for treatment. The air flotation sludge, however, is directly into the sludge conditioning tank because it combines a large number of fine bubbles with low specific gravity, which makes it difficult to be thickened by gravity settling. The thickening tank has a buffer and can be processed intermittently, but the air flotation, due to continuous operation, produces sludge continuously and enters the sludge mixing tank continuously. The plate and frame filter press is an intermittent feed, with the change of operating conditions, the sludge handling capacity will change, so the sludge concentration in the conditioning tank will fluctuate, and the flocculation effect of the chemicals will also fluctuate, which will cause interference in the normal operation of the filter press. In view of the above problems, we have focused on optimisation in the system design.

3.1 Increase pre-dewatering facilities Plate and frame filter press dewatering system design in the past is generally a variety of sludge into the sludge conditioning tank mixing and dosing conditioning by the feed pump into the plate and frame filter press filter dewatering. The concentration of the feed depends on the mixing concentration of the various sludges entering the conditioning tank. Air flotation sludge is difficult to be concentrated by conventional gravity concentration method due to carrying fine air bubbles and low specific gravity, therefore, the system is generally designed to send the air flotation sludge directly into the sludge mixing tank for dewatering. Due to the low concentration of air-float sludge, and due to the carrying of small bubbles, the mud quality is relatively loose, so when the proportion of air-float sludge is larger, the concentration of mixed sludge will be lower and the dewatering effect is poor, and it is easy to be adsorbed on the surface of the filter cloth and block the filter channel of the filter cloth. In order to eliminate the influence of sludge concentration fluctuation on the operation, we designed a gravity bed for pre-dewatering before the sludge conditioning tank. The principle of gravity bed is: after flocculation, the suspension enters the gravity bed, and under the effect of gravity, the external water of the material is discharged through the filter screen into the collecting tank, while the concentrated material after preliminary dewatering stays on the filter screen, and then enters the next process after conveying. After coagulation of the suspension through the gravity bed dewatering, the concentration is raised to 5% 10%, and then into the plate and frame filter press system, which can greatly improve the efficiency of the plate and frame filter press.

3.2 Optimise the dosing system plate and frame treatment system design generally set the dosing point in the sludge conditioning tank, that is, the agent is added directly in the sludge conditioning tank stirring flocculation. In order to ensure the flocculation effect and stable material quality, the general system will be designed with two conditioning tanks, but need to occupy a lot of space, in addition, the operation and control requirements are higher. Due to the site constraints of our company, it is difficult to arrange two sludge conditioning tanks, so we chose to use one conditioning tank in the system design and borrowed the original sludge mixing tank of the system as a transfer tank. In order to eliminate the influence of sludge composition fluctuation on pharmaceutical flocculation, we adjusted the flocculant addition point. After mapping and drawing on previous operating experience, we adopted a pipe mixer for flocculation dosing and set the dosing point on the pipe in the range of (3-5) m before entering the gravity bed. The flocculant and material in the pipeline after (2~3)S rapid mixing, flocculation reaction into the gravity bed. As the inlet of gravity bed is open, the flocculation effect of materials can be observed at any time, so the dosage of chemicals can be adjusted in time according to the flocculation effect of materials and changes in the composition to ensure good flocculation effect and to lay the foundation for the normal dewatering of the plate and frame filter press in the latter part of the process.

3.3 Mixer form selection material after gravity bed pre-dewatering, the concentration greatly increased from the operating data, mixed sludge concentration from 1.5 to 3 per cent before dewatering to 5 to 7 per cent, the plate and frame machine processing capacity greatly increased. However, after increasing the concentration, the fluidity of the material deteriorates, therefore, in order to ensure that the material enters the plate and frame with uniform concentration, but also to avoid flocs being dispersed again due to excessive mixing efforts, so the design of the plate and frame machine into the mud pool configuration of the mantle speed mixer, vertical mounting, rotational speed is selected as (5-8) r/min, so that the high concentration of the material mixing and homogeneity of the material into the filtering system smoothly.

3.4 System operation and maintenance Plate and frame filter press adopts polypropylene monofilament woven mesh as the filter bag, and there is a gap between the mesh and the mesh of the prepared mesh, the material is squeezed under high pressure, and the liquid in the material enters into the filtrate area through the gap between the mesh and the mesh, and the sludge is detained in the mesh bag. In order to facilitate the collection of filtrate, the surface of the frame plate is designed with a raised nail point, and the filtrate is collected by the concave channel between the nail points and discharged into the main collecting pipe. However, due to the high operating pressure, the fine particles in the material will partially penetrate the filter cloth into the filtrate side, and deposit in the concave channel of the frame plate. et accumulates, the concave channel will be blocked by the piled up particles, and the disappearance of concave and convex points causes the frame plate to be poorly drained, which will have a certain impact on the filtration efficiency of the filter cloth. Although the plate and frame machine is equipped with an online cleaning machine, the cleaning machine can not clean the accumulated mud on the filter plate, so it is necessary to regularly clean the filter cloth and frame plate offline. Cleaning method is to stop the filter press offline, remove the filter cloth, the use of high-pressure water gun on the exposed filter plate piece by piece rinsing, will be deposited in the concave channel sludge cleaning off, to restore the hydrophobic channel of the frame plate. Similarly, the filter cloth is also sprayed on both sides using a high-pressure water gun. If the filter cloth is scaled, it can also be soaked in citric acid and then rinsed with high-pressure water to restore the filtering capacity of the cloth. The general cleaning cycle is four months. Because the filter cloth is a consumable product, the general use of the cycle is four months, and the use of offline cleaning, the use of the cycle can be extended to 9 ~ 10 months, greatly reducing operating costs.

4 Conclusion The diaphragm-type plate and frame filter press system, through reasonable system design and effective operation and management, can give better play to the depth dewatering efficiency of the equipment, so that the dryness of the sludge reaches more than 50%, which is directly sent to the boiler for incineration, providing technical support for the reduction and harmless treatment of solid wastes, as well as laying a solid foundation for the enterprise to achieve the energy-saving and environmental protection goals.