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Introduction to model selection of sludge dewatering equipment

Large quantities of sludge shall be produced after sewage is subject to sediment treatment. Even though the sludge is treated by means of concentration and digestion, the water content is still as high as 96%, and the volume is too big to be digested and disposed. So it must undergo dewatering treatment to increase the solid content of the mud cake and reduce the floor area where the sludge is stacked.

  Mechanical dewatering is used commonly in large and medium wastewater treatment plants. There are many types of dewaterers. According to dewatering principles, they are classified into three major types: vacuum filter dewatering, pressure filter dewatering and centrifugal dewatering. This article introduces the working principles of filter press (including belt filter press and plate-and-frame filter press) that are often selected for use in wastewater treatment plants, major considerations in selection of equipment models and their maintenance and operating costs.

  1、Belt filter press dewaterer
  Belt filter press dewaterer carries a sludge layer between an upper and a lower strained filtering belt. The sludge layer passes through a series of rollers that are arranged regularly in an “S” shape, where a pressing force and a shearing force are formed to the sludge by the tensile force of the filter belt itself to squeeze capillary water out of the sludge layer and obtain mud cakes with higher solid content, thereby the sludge is dewatered.

  General belt filter press dewaterer is composed of belt, rollers, belt tensioning system, belt adjusting system, belt flushing system and belt driving system. When selecting the models, considerations must be made in the following aspects:

  (l)Filter belt. The filter belt must be characteristic of higher tensile strength, resistance to folding, acid, alkali and temperature change. Meanwhile, specific property of the sludge must also be taken into consideration to select suitable weaving texture so that the belt has good air permeability and good performance to intercept sludge particles.
  (2)Roller adjusting system. It is usually performed by means of a pneumatic device.
  (3)Belt tensioning system. Usually it is also controlled with a pneumatic system. Belt tension is usually controlled between 0.3-0.7MPa,the commonly used value is 0.5MPa.
  (4)Belt speed control. Sludge with different nature requires different belt speed, that is, any specific sludge has an optimum control range of belt speed, within which the dewatering system can have certain treatment ability for high quality mud cakes.

  Belt filter press dewaterer is less affected by sludge load fluctuation. It is also featured by low water content in produced mud, stable operation, less starting consumption, relatively simpler management control and less quality requirement for the operator. At the same time, belt filter dewaterer was introduced into our country earlier, so there are quite a number of manufacturers can produce this type of equipment. When making decision for construction of a wastewater treatment project, belt filter dewaterer may be selected to reduce the project investment. At present, belt filter press dewaterers are used in most of the newly-built wastewater treatment plants in China. For example, all the five sets of dewaterers used in the first phase project of Beijing Gaobeidian Wastewater Treatment Plant are belt filter dewaterers. The belts, rollers and filters have been working well ever since they are put into operation, that’s why the same model of equipment has been selected again for the second phase of the project.

  2、Centrifugal dewaterer
  Centrifugal dewaterer mainly consists of a turning drum and a spiral conveyor with a hollow spindle. After being sent to the rotor drum from the hollow spindle, sludge is immediately thrown into the turning drum cavity under the centrifugal action generated by high-speed revolution. The specific gravity of sludge particles are larger, the generated centrifugal force is also larger, so the sludge particles are thrown onto the inner wall of the turning drum and form a solid layer; water density is small, and the centrifugal is also small, so the liquid layer is generated only on the inner side of the solid layer. The sludge of the solid layer is pushed slowly by the spiral conveyor to the conic end of the turning drum and discharged continuously through the exits around the turning drum while the liquid is discharged out of the turning drum by the overflow weir and out of the dewaterer after collection.

  The most critical component of centrifugal dewaterer is the turning drum. The larger the diameter of the turning drum is, the greater the treatment capacity is, but the manufacturing and operating costs are quite high, so it is not very economical. The longer the turning drum is, the solid content of the sludge is, but an excessively long turning drum may decrease its cost performance. In the course of use, the rotating speed of the turning drum is one of the important control parameters. It is the critical to the operation of the centrifugal dewaterer to control the rotating speed of the turning drum such that it can have both higher solid content and lower energy consumption. At present, it is more common to use low-speed centrifugal dewaters. In selection of centrifugal dewater models, special requirements should be considered to the materials because the turning wheel or the outer edge of the spiral is worn out easily. In new-type centrifugal dewaters, the spiral outer edges are mostly made into assembling blocks for easy replacement. The material of the assembling block is usually tungsten carbide, which is expensive. Such types of centrifugal dewaterers are characteristic of high noise, high energy consumption, low treatment capacity and so on. Only a few domestic manufacturers can produce small centrifugal dewaterers. If large centrifugal dewaterers are selected, they shall be only imported, which shall increase investment of the project. At the same time, centrifugal dewaterers are seriously impacted by the fluctuation of sludge load and they have high quality requirements for the operators, therefore, centrifugal dewaterers are not generally used in wastewater treatment plants. However, in recent years, with the development of science and technology, the dewatering technology has been considerable improved in foreign countries. For example, the solid content of the mud cakes produced with the spiral centrifugal dewaterers manufactured by Alfa Layal in Sweden can be over 30%, and the operation is within a fully enclosed environment, so there is no sludge or wastewater existing around nor there is any bad smell, the working environment of operators is greatly improved, therefore they are preferred by people of the industry.

  3、Plate-and-frame filter press dewaterer
  Plate-and-frame filter press performs dewatering by means of squeezing of the plate frame to discharge water in the sludge through the filter cloth. It is mainly made up of recessed filter plate, frame, automatic-pneumatic closing system side plate suspension system, filter plate vibration system, air compressor, filter cloth high pressure flushing unit and body-side photoelectric protection unit. The following aspects must be taken into account in selection of the model:

  (1)Requirements for solid content of mud cake. When compared with other types of dewaters, general plate-and-frame filter press has high solid content of mud cake, which can be 35%. If reduction of floor area for sludge stack is considered, plate-and-frame filter press shall be the preferred alternative.
  (2)Material of the frame.
  (3)Materials of the filter plate and filter cloth. Corrosion resistance is required. The filter cloth must have certain tensile strength.
  (4)Movement mode of filter plate. It is required that the movement is achieved either by way of hydraulic-pneumatic system or semi-automatically so as to reduce labor intensity of operators.
  (5)Filter cloth oscillation device enables filter cake to fall off easily. When compared with other types of dewaterers, the biggest disadvantage of plate-and-frame filter press is larger floor area.

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