At present, the sludge produced by urban sewage treatment plants is divided into three levels, with the majority having a moisture content of 80%, a small portion having a moisture content of 50-60% after deep treatment, and a very small portion having a moisture content of less than 40% after drying treatment. Below is a discussion on common sludge conveying equipment in China, analyzing and comparing the technical performance and practicality of sludge conveying processes. Belt conveyor is a mechanical device driven by friction to transport materials continuously. It can be used for conveying fragmented materials or for conveying individual items. Used for horizontal or inclined transportation, it is very convenient to use. It has the advantages of large conveying capacity, simple structure, convenient maintenance, low cost, and strong universality. However, when transporting sludge, due to the high moisture content and viscosity of the sludge, it is easy to stick to the belt, which can cause the conveyor belt to deviate and slip during the transportation process, seriously affecting the service life of the belt. Moreover, belt conveyors are greatly affected by the site, and generally require the construction of longer belt corridors for their layout. The lifting height is relatively limited, and the inclination angle generally does not exceed 20 degrees. Choosing this transportation method requires higher civil engineering costs and a larger site. Belt conveyors are suitable for conveying dried sludge with a moisture content of less than 40%. Suitable for transporting short distance (less than 50m) and low head (less than 20m) sludge, commonly used to transport thermally dried sludge to designated locations. Belt conveyors can achieve changes in conveying inclination angle with a single belt conveyor by setting a turning device; But when the conveying direction on the plane changes, it is generally necessary to add a first level conveying device. Spiral conveyor belongs to non pressure conveying and is one of the most commonly used sludge conveying methods. The screw conveyor pushes the sludge forward through the rotating spiral blades. The force that maintains the sludge from rotating with the spiral blades is the self weight of the sludge and the frictional force between the sludge and the casing. Spiral conveyors can be divided into axial and non axial types based on the presence or absence of a spiral shaft; According to the number of screws, it can be divided into single screw conveyor and double screw conveyor. It can transport materials horizontally, obliquely, or vertically, and complete mixing, stirring, and cooling operations while transporting materials. For sludge, screw conveyors are suitable for conveying sludge with a range of 60% to 85%, which has a relatively loose structure and moderate viscosity; Suitable for short distance (less than 25m) and low head (less than 8m) sludge transportation, commonly used in sludge dewatering machine rooms of small and medium-sized urban sewage treatment plants to transport dewatered sludge to sludge storage tanks or truck tankers. A single screw conveyor device can only achieve horizontal or inclined conveying. When the conveying direction and angle change, a first level conveying equipment needs to be added. When conveying sludge with a moisture content of about 80%, the inclination angle of the spiral conveyor should not exceed 25 degrees. The spiral conveyor trough is enclosed, making it convenient for short distance transportation of materials that are prone to flying, and for continuous and uniform transportation of loose, moderately viscous, and less prone to clumping materials. It can be loaded at any point on the line or unloaded at many points; Mixing, stirring, and cooling operations can also be carried out during the conveying process. However, at the same time, it consumes a large amount of power per unit, is prone to material breakage and wear, and is sensitive to overloading. Due to these disadvantages, screw conveyors can generally only transport bulk materials in the form of frictional powder, granular particles, and small pieces over short distances. The conveying length is generally 5-30m, and the energy consumption is particularly high when it exceeds 40m, which is not economical.
Screw pumps in sludge pumping systems usually use eccentric screw pumps, whose main working components are eccentric screw (rotor) and screw sleeve (stator). When the electric motor drives the pump shaft to rotate, the screw rotates around its own axis on one hand, and rolls along the inner surface of the liner on the other hand, thus forming the sealing chamber of the pump. Every time the screw rotates once, the sludge in the sealing chamber advances forward by one pitch. With the continuous rotation of the screw and rod, the transported sludge is pressed in a spiral form from one sealing chamber to another, and finally squeezed out of the pump body. The vulnerable parts of the pump, stator and rotor, are usually replaced every 2 months to 6 months. Eccentric screw pumps are suitable for short distances, low flow rates, low conveying pressure, and continuous conveying of sludge. The screw pump does not produce turbulent pulsation when transporting sludge, and has almost no shear force on the medium. The screw pump has a simple structure and low one-time investment. Stable operation without any problems while ensuring the cleanliness and uniformity of materials. In engineering applications, the working pressure of screw pumps should be controlled at 1/2-1/3 of the rated pressure; The speed of the pump should be controlled below a relative sliding speed of 0.5m/s between the stator and rotor; The horizontal conveying distance of sludge by screw pump generally does not exceed 100m; The vertical critical height is 50m, and the theoretical maximum pressure can reach 4.8 megapascals. The biggest flaw of the screw pump itself is that the stator and rotor rely on sliding friction to form a moving closed chamber for conveying the medium, so the stator and rotor are extremely prone to wear. The sediment content of sewage sludge in our country is relatively high, which will accelerate the wear rate of the stator and rotor. This not only increases maintenance costs, but also directly affects the normal operation of the sludge conveying system. The sediment content in sludge directly affects the service life of screw pumps. Screw pumps have a small adaptability range for sludge moisture content. Many sewage treatment plants have sludge with a minimum moisture content of around 75%, which can reduce pumping capacity by at least 20-40%. As the pumping capacity decreases, the delivery capacity of the screw pump will also decrease, making it impossible to ensure stable operation at the designed flow rate continuously. Due to the fact that sludge viscosity is an important parameter affecting transportation, and viscosity is directly proportional to concentration, meaning that the higher the concentration, the greater the viscosity. The sludge concentration is high, and the viscosity also increases accordingly. In high viscosity conditions, the working efficiency of screw pumps will be significantly reduced, and the operating energy consumption will be significantly increased. If the sludge contains flexible fibrous substances (such as hair, plant stems, rods, and plastic bags), after passing through the front-end crushing device of the pump body, it will still wrap around the rotor, causing leakage in the sealing chamber, and the system working pressure can generally be reduced by 50-90%. Sludge inevitably contains a certain amount of flexible fibrous substances, and the structural characteristics of screw pumps themselves determine that they cannot guarantee continuous and stable transportation.
The plunger pump relies on the reciprocating motion of the plunger in the cylinder body to change the volume of the sealed working chamber to achieve the transportation of sludge. Piston pumps have the advantages of high rated pressure, compact structure, high efficiency, and convenient flow regulation, and are widely used in high-pressure, high flow, high solid content, and material transportation with a large amount of debris. For sludge with a moisture content of 80%, a hydraulic dual cylinder plunger pump is used, designed for viscous materials with high solid content. The system has high conveying pressure (0-24Mpa), large conveying flow rate (0-80m3), long conveying distance (0-1000m), high tolerance for debris (can transport debris with an outer diameter not exceeding 1/2 of the pipeline diameter), low operating costs (with fewer vulnerable parts, long service life, and low price), and unparalleled advantages over traditional conveying methods. The instantaneous flow rate of a plunger pump is pulsating because in the plunger pump, the suction and discharge processes of sludge alternate, and the speed of the piston is constantly changing during displacement. The double cylinder plunger pump, due to the alternating operation of two cylinders, has a constant average flow rate regardless of whether it flows continuously in the pipeline. In theory, the flow rate of a pump depends only on its main structural parameters n (number of reciprocating times per minute), S (piston stroke), D (piston diameter), and is independent of discharge pressure, as well as the physical and chemical properties such as temperature and viscosity of the conveying medium. So the flow rate of the pump is constant. The discharge pressure of a plunger pump cannot be limited by the pump itself, but depends on the pipeline characteristics of the pump device and is independent of the flow rate. That is to say, if the transported liquid is considered incompressible, then theoretically, the discharge pressure of the reciprocating pump can be assumed to be unrestricted, and any required discharge pressure of the pump can be established based on the pipeline characteristics of the pump device. Of course, all reciprocating pumps have a regulation on the discharge pressure of the pump. This does not mean that the discharge pressure of the pump will not rise again, but rather that due to the limitations of the rated power of the prime mover and the structural strength of the pump itself, it is not allowed to use it beyond this discharge pressure. Piston pumps can transport any medium in principle, almost without being limited by the physical and chemical properties of the medium. By optimizing the configuration of the pump hydraulic system and the structure of the pump body, as well as selecting materials for production and strengthening sealing technology, there are no technical issues with using a plunger pump to transport sludge.
