The horizontal screw feed settling type, which is widely used in sludge dewatering of sewage treatment plant, has been widely used. Although different manufacturers with different specifications or models of horizontal screw with different equipment structure, material, specifications and operation adjustment mechanism, but the basic principle is similar to the equipment, the paper gives a simple introduction of the present, to use and convenient on-site adjustment of users better.

1、Structure and dewatering principle

Decanter centrifuge is mainly composed of drum, spiral, differential system, liquid baffle, driving system and control system etc..

Horizontal decanter centrifuge is the use of the density difference between solid and liquid, under the action of centrifugal force, accelerate the sedimentation rate of solid particles to achieve solid-liquid separation. The separation process for sludge and liquid flocculant by the entrance of the pipeline was put into the mixing chamber within the drum, then mixed flocculation (if sludge pump or pump pipeline before dosing after dosing, has been ahead of the flocculation reaction), the rotor (and spiral drum) high speed rotation and friction resistance, the sludge is accelerated in the rotor and the internal the formation of a cylindrical liquid ring layer (liquid ring area), under the action of centrifugal force, a larger proportion of solid particles to the inner wall of the drum forming a mud layer (solid ring layer), the relative velocity and the phase difference of spiral drum to drum cone, launched after the level (shore area or dry mud area) cinder can be dehydrated, to the dregs outlet, was discharged from the drum, realize the solid-liquid separation.

2、Factors influencing the effect of horizontal decanter centrifuge

Centrifuge use effect, influence the mechanical part brought can be divided into regulatory factors and non regulation factors, are described, first understand its principle, can be in use for its effective control.

2.1Non adjustable mechanical factor

A、Drum diameter and length

The drum diameter is larger, the effective length is longer, the settlement area is greater, the greater processing capacity, residence time of material in the drum is longer at the same speed, the separation factor is bigger, the better the separation effect. But limited by the material, can not limit the centrifuge drum diameter increases with the increase of the diameter, because the maximum permissible speed with the material firmness decrease, thus the centrifugal force is reduced. Usually the drum diameter between 200 ~ 1000mm, the ratio of length to diameter in 3 ~ 4. At present, the development of the horizontal decanter centrifuge tends to the high speed and large aspect ratio, which is more suitable for the treatment of low concentration of sludge, and the quality of the mud is better.

In addition, in the same capacity under the condition of large diameter drum centrifuge can run at low speed difference, the reason is that the diameter of the spiral drum slag conveying capacity is large, to achieve the same slag conveying capacity, small diameter drum centrifuge must be realized by increasing the speed difference.

B、The half cone angle of the drum

The settlement in centrifuge inside the sediment along the drum cone end is pushed to the outlet, due to centrifugal force by the return force sliding downward. The half cone angle is an important parameter of drum centrifuge design. To clarify the effect, the cone angle as large as possible; and from the slag and dehydration effect, the cone angle as small as possible. As the slag is a necessary condition for the normal operation of the centrifuge, the optimal design must first meet the conditions of slag. For the difficult separation of materials such as activated sludge in the half cone angle of 6 degrees or less, in order to reduce the return rate of sediment. The general material in the half cone angle less than 10 degrees to ensure the smooth transport of sediment.

C、pitch

The pitch is the distance between two adjacent spiral blades, which is a very important structural parameter, which directly affects the success or failure of the slag. When the diameter of the screw is fixed, the larger the pitch, the greater the helix angle, the greater the chance of the material blockage between the spiral vanes. The number of circles at the same time will reduce the pitch of spiral vane, the drum cone material distribution is not uniform and increase caused by the vibration of the machine. So for the separation of difficult materials such as activated sludge, slag conveying difficult pitch should be smaller, the diameter of the drum is generally 1 / 5 to 1 / 6, in order to delivery. For the easy separation of material, pitch should be bigger, generally 1 of the diameter of drum / 2 to 1 / 5, in order to improve the transmission capacity of sediment.

D、Spiral type

Is the main component of spiral centrifuge, its role is to transport in the inside of the drum settlement and smoothly discharged sediment sediment, it is not only the unloading device, also determines the production capacity, service life and separation effect.

Solid and liquid in the drum relative movement of the different ways into countercurrent and flow according to the spiral type. Countercurrent centrifuge feeding spiral cavity in the middle, is also located near the boundary between the dry zone and sedimentation zone, to ensure that the liquid phase sedimentation distance enough, but can only stay through the solid cone part of the time required, so the centrifugal force is high; material from here into this area will cause the settlement of solid particles due to the disturbance to float again within the drum, will produce additional turbulence and vortex, reduce the separation effect.

The concurrent centrifuge inlet on the drum end, to avoid the turbulence of counter flow, ensure undisturbed sediment, centrifuge length plays a role of subsidence, expand the settlement area, suspension stay in the machine time increases, thus the separation effect is improved. Due to the extension and settlement can effectively reduce the amount of flocculant without interference, so that the flow of the fluid in the state machine has been greatly improved, and to improve the centrifugal force by increasing the diameter of the drum, it can significantly reduce the speed, save power consumption, while reducing, prolong the life of machine.

Centrifuge downstreaming spiral structure is especially suitable for the solid-liquid density difference is small, the particle sedimentation performance is poor, difficult to be separated with low solid content. But the concurrent centrifuge filtrate by leaving the liquid discharge pipe, will be deposited in the separation of solid particles in liquid skimming pipe filtrate through the write liquid pipe is not isolated, they will block the skimming pipe needs to be washed periodically.

In recent years, with the increasing requirement of sludge dewatering, high efficiency helical structure. Such as the Swedish AlfaLaval BD plate technology, namely in the centrifuge cone spiral discharging end is provided with a special baffle, can make the centrifuge in ultra deep pool, in order to increase the pressure force on the slag mud cake, and only the upper and lower sediment transport, high water content sludge retention in the lateral pressing cone section, implementation of dewatering, with more dry mud. The Swedish NOXON adopts inclined plate sedimentation principle of Lamella technology, the screw pusher centrifuge blade design into optimum tilt state, the blade angle, blade pitch, pitch and other parameters were optimized design, processing ability, reduce the rate of water containing flocculant consumption and mud cake.

2.2、Adjustable mechanical factor

A、Drum rotational speed

Adjust the rotational speed usually by frequency motor or hydraulic motor to achieve. The greater the rotational speed, the greater the centrifugal force, which is helpful to improve the solid content of the cake. However, excessive speed will make the sludge floc is destroyed, but reduce the dehydration effect. At the same time, the higher the speed of the material requirements, the wear of the machine increases, power consumption, vibration and level will increase accordingly.

B、The speed difference (difference ratio)

Differential speed directly affects the discharge capacity, mud filtrate quality and biscuits, is the need of adjusting parameters according to the operation situation of important centrifuge operation.

To improve the speed difference, is helpful to improve the ability of slag, but sediment dewatering time will be shortened, high rate of moisture after dehydration, while excessive speed difference will make spiral perturbation on the clarification zone liquid pool increase, filtrate the relatively poor quality of some (commonly known as "mixing").

Lower the speed will increase the thickness of sediment, sediment dewatering time growth, dehydration after the moisture content of the cake is reduced, at the same time to clarify the material spiral disturbance area is small, the filtrate quality is relatively good, but will increase the helical feeding load, should prevent the decrease of sediment discharge amount of slag in the centrifuge can not timely caused by blocking material a large number of mud filtrate to prevent the phenomenon, then we must reduce feed or improve speed, some types of equipment with automatic speed up discharge function, not only when the set torque reaches a certain limit value, the device will automatically reduce the dosage and enter into mud volume, increase the speed difference, the rapid accumulation of mud ring layer to be launched, torque is reduced to a certain value, the difference of flow and then automatically return to normal. This is a kind of effective protection measures of the equipment, but in the long run, should avoid frequent emergence of this situation, because it's easy to make the equipment often in unstable flow and unstable situation in the process of difference degree, fluctuation will affect the treatment effect and the treatment capacity decreased.

Therefore, it is necessary to determine the size of the differential velocity according to the properties of the material, the size of the treatment, the processing requirements and the structural parameters of the centrifuge. That is to say, in the scene to find the best treatment according to the situation, the effect of the processing needs of the differential value range, in order to meet the requirements to meet the mud biscuit as high as possible

Put simply: the processing capacity and effect of contradictions, in order to improve the processing capacity, will increase the differential ratio, but may reduce the degree to improve the mud mud cookies; biscuits will reduce the degree of difference, thus reducing the processing ability, so, the debugging work is to find the site with their respective the actual conditions of sludge properties the best equipment operating parameters, to achieve the highest efficiency of equipment operation and the best treatment effect of double objective. This is not a simple data can be calculated, only rely on long-term experience in the actual debugging experience, and timely adjustments in accordance with changes.

At the same time, in a certain range, the difference degree of control and control the dosage of flocculants are complementary, required to achieve a certain degree of mud in the biscuit case, when the number of degrees lower, can save the dosage of flocculant. Simply speaking is to increase the pressure on the equipment to reduce the pressure on the use of flocculants. Therefore, the proper use of the lowest possible degree of difference can reduce the consumption of flocculant in a certain extent, saying "good medicine, equipment operation, equipment operation is not good for medicine, design and machining precision on the device itself will not only depend on the equipment, but also relates to the control of equipment operation the operating parameters.

With a degree of centrifuge automatic adjusting function, parameter setting according to the difference of the use of long-term, and according to the changes that may occur at any time correction.

C、Liquid ring thickness

The thickness of liquid ring is an important parameter in the optimization of the equipment, which directly affects the effective settling volume and the length of the drying zone (Shore), and then affects the effect of sludge dewatering. The general level of the baffle by manual adjustment to achieve in shutdown state, must ensure that each level adjustment baffle or consistent, otherwise it will lead to severe vibration centrifuge operation, also can achieve automatic adjusting thickness of liquid loop part of foreign products can.

With the increase of the thickness of the liquid layer, the settlement area will be increased, the residence time of the material in the machine will also be increased, and the filtrate quality will be improved, but the length of the drying zone (shore area) in the machine will be shortened, which will lead to the decrease of the biscuit. On the contrary, reducing the thickness of the liquid ring layer can obtain higher cake solids content, but at the expense of filtrate quality.

Therefore, it is necessary to adjust the level of the liquid baffle reasonably to make the best combination of the quality and the quality of the filtrate. In general, many equipment suppliers will baffle in the factory equipment level is adjusted in advance, but due to the use of different field conditions are different, if the condition is not ideal, please cooperate with equipment manufacturers engineers on-site level adjustment of the baffle, the foot with the actual demand.

2.3、Technological factors

The centrifuge is the use of solid-liquid density difference to realize solid-liquid separation, so the proportion of the more easy separation of granular sludge. In general, the sewage treatment plant primary sludge is easy to dehydration, the residual sludge is difficult to dehydration, and dewatering performance of sludge mixing between the two, different sewage sludge and using different water treatment process of sludge will have a greater difference, so the sludge dewatering will have different performance.

In order to improve the performance of sludge dewatering, mechanical dehydration before adding organic polymer flocculant amount should be uniform, such as polyacrylamide (PAM), to reduce the specific resistance of sludge, the sludge solid phase and liquid phase separation easier after dehydration, must be suitable to the type of flocculant and sludge characteristics and the type of equipment and operating conditions to adapt. In many cases, the better drug in the selection of flocculating agent beaker, and did not have a better performance in practical applications, a very important reason is that although the chemical characteristics to some extent to meet the characteristics of the sludge, but the operating conditions and equipment can not fully meet the.

According to the actual operation, it is shown that the dosage of flocculant has little effect on the solid content of the cake, and has a great influence on the quality of filtrate. Therefore, sludge dewatering, sludge to meet the requirements and quality requirements of biscuits was case, continue to increase the use amount of flocculant is completely unnecessary. The main reason is the waste caused by flocculant. In addition, with the increase of dosage of flocculant, the supernatant of better quality, but in many cases, excessive pursuit of quality and the supernatant of the flocculant is increased more than just the The loss outweighs the gain., a few percentage points of the sludge rate and consuming more flocculant consumption is not (just as spent 10 yuan to buy 5 yuan of goods). As long as the solid content of the supernatant can be controlled within a certain range.

In general, the equipment can meet the requirements of a certain range of sludge concentration, sludge concentration is too low or too high will consume more flocculant. In the case of the normal operation of the equipment of sludge concentration, the dosage of flocculant and the solid content of sludge to be treated approximately proportional relationship, so the sludge flow conditions, flocculant dosage should be adjusted according to the concentration of the sludge sludge because most of the time, changes in the concentration and dosage of flocculant without and make timely adjustment of field operation or poor performance of drug consumption increased.

If the sludge concentration increased and flocculant dosage does not increase will affect the treatment effect, will show a lower degree of mud cookies, supernatant turbidity; on the contrary, if the sludge concentration is reduced, flocculant dosage did not reduce the formation of flocculant waste, and increase the treatment effect is not obvious.

In addition, if the dissolving condition is not good to the actual amount of insufficient or low concentration of effective ingredients of the flocculant configuration of liquid supply, it is difficult to form the corresponding dry cake degree, quality and effect of supernatant; flocculant concentration is too large, active group of flocculant polymer chain will make effective components to make full use of the mutual effect shielding, package, resulting in chemical waste; the dosage of flocculant excessive more, redispersion of flocs will also destroy the stability of floc flocculation effect, also not good.

The flocculant dosage is too large, not only waste, but the treatment effect is not significantly improved. Municipal sludge treatment, concentration of organic polymer flocculant solution is generally 1 per thousand to 5 per thousand, flocculant dosage is 3 ~ 5kg / TDS, some industrial sludge flocculant dosage may reach or exceed 10kg / TDS, depending on the nature of sludge and sludge dewatering performance. Due to the difference in the performance of the dewatering machine, the same properties of the sludge in the use of the same type of flocculant in the case of different flocculants consumption.

Many factors affect the centrifuge dewatering effect, and various factors which influence each other, so the treatment effect is the comprehensive effect of the above factors results should be combined with the actual situation of the project of the centrifuge selection, operation parameters should be adjusted after dehydration of mud cake final disposal method required the optimum water content of sludge cake considering rate solid rate and economic and other factors.