In this paper, sludge conditioning and plate-and-frame pressure filtration dehydration technology are expounded, and the accumulated experience in actual construction is applied to the project under construction, and the corresponding sludge conditioning methods are put forward, which has certain guiding significance for sludge deep dehydration technology.
To solve the problem of sludge treatment, we must first solve the problem of sludge dehydration. Deep dewatering of sludge refers to the dewatering method of conditioning sludge, destroying cell wall, releasing bound water, adsorbed water and intracellular water, improving sludge dewatering performance, and making the water content of dewatered sludge below 60%. Commonly used sludge conditioners include ferric chloride and lime.
1 characteristics of sludge and difficulty in dewatering
Sludge dewatering is very important to realize the reduction, stabilization, harmlessness and comprehensive utilization of sludge, and to achieve the disposal goal of energy saving and emission reduction and developing circular economy. Only when the water content of sludge is reduced below 60% can comprehensive utilization of resources be possible. The difficulty of sludge dewatering is not only related to the existing form of water in sludge, but also related to the size of sludge particles, sludge specific resistance and organic matter content. The finer the sludge particles, the higher the organic matter content, the greater the sludge specific resistance, and the more difficult it is to dehydrate.
2 Currently commonly used sludge deep dehydration process
Although all parties claim that their processes have proprietary technical characteristics, to sum up, the existing deep dehydration processes all have the following characteristics: chemical agents need to be added to stabilize sludge before dehydration; Most of them are dehydrated by plate-frame diaphragm, and a few are dehydrated by belt conveyor.
Three engineering cases
Shanghai Tianshan Sewage Treatment Plant is located at the intersection of Tianshan Road and Shuangliu Road in the west of Shanghai central city, serving the upper reaches of Suzhou River. The design scale of sludge deep dehydration technical transformation project in this plant is dry sludge 13tDS/d, wet sludge 1300 ~ 1500m3/d, and the sludge moisture content is 99 ~ 99. 13%, which is converted into sludge with 80% moisture content of 65t/d.
3. 1 Deep dehydration process flow of this project
This project adopts iron salt lime plus plate and frame filter pressing process. In this process, lime and ferric chloride are added to the sludge for conditioning, and then pressed by a high-pressure diaphragm to reduce the water content of the concentrated sludge to 60%.
3.2 Comparison and Selection of Deep Sludge Dewatering Technical Schemes
The filter pressing process of lime belt conveyor has good sludge dewatering effect, but the amount of lime is huge, the operating environment is poor and the amount of sludge produced is large. At present, the filter pressing process of air-conditioning plate frame is a confidential technology of manufacturers, and it must be operated in the mode of packaging, construction and transportation, which has great uncertainty. In addition, due to the influence of conditioners, the pathology and toxicity of sludge are still unclear. In contrast, at present, ferric chloride and lime plate-frame filter pressing technology are widely used, which has mature technology, good dewatering effect and stable operation. The chemicals used are easily available, cheap, non-toxic and harmless, and will not cause other potential pollution, and the filtrate has no adverse effect on biological treatment.
3.3 The importance of sludge conditioning
At present, almost all sludge deep dehydration methods need to pre-condition the sludge, which weakens the binding water mechanism of solid particles and homogenizes the sludge structure. Through sludge conditioning, the cell wall based on protein was destroyed, and the bound water, adsorbed water and intracellular water in sludge were released, which overcame the specific resistance of sludge, greatly reduced the viscosity of sludge and improved the sludge dewatering effect. Therefore, sludge conditioning technology determines the success or failure of sludge deep dehydration project and is the core technology of key links.
The main factors affecting sludge conditioning effect are: (1) sludge properties; (2) Selection of conditioner; (3) Sludge conditioning conditions (temperature, pH, conditioner concentration, stirring conditions, etc.). ).
3.4 Selection of sludge conditioning scheme for this project
According to the sludge properties of Tianshan plant, combined with the results of sludge debugging and exploration in the early stage, lime and ferric chloride were selected for conditioning after comprehensive consideration of various factors. Because the calcium carbonate particle structure generated by lime and heavy calcium carbonate in sewage can also increase the porosity of sludge, promote the separation of sludge and water, and significantly improve the dehydration effect of sludge. At the same time, the added ferric chloride and lime also have the functions of passivating heavy metals and sterilizing and deodorizing.
After our small-scale experiments, we got the following experiences:
(1) Hardware facilities and equipment need to be improved. First of all, the sludge conditioning tank should be well treated for anticorrosion, because the sludge is very corrosive. The bottom of the pond should be considered to be equipped with a sludge discharge and emptying device to discharge sludge regularly to prevent sediment accumulation from affecting the conditioning effect; Secondly, the mixing equipment in the conditioning tank should adopt vertical mixing equipment to reduce the corrosion of sludge to the equipment. Its power should meet the requirements of ensuring full mixing of sludge and conditioner.
(2) According to the sludge with different properties, select the appropriate conditioner. The difference of sludge properties directly affects the conditioning effect. For sludge with high organic content, the more effective conditioner is cationic organic polymer conditioner. For inorganic sludge, anionic organic polymer conditioner can be considered. In order to achieve a certain conditioning effect, the number of conditioners needed is obviously different. Generally speaking, the more difficult sludge dewatering is, the larger the dosage of conditioner is, and the smaller the sludge particles are, which will lead to an increase in the dosage of conditioner, and the higher the organic content and alkalinity of sludge will also lead to an increase in the dosage of conditioner. In addition, the solid content of sludge also affects the dosage of conditioner. Generally, the higher the solid content of sludge, the greater the dosage of conditioner.
(3) temperature. The temperature of sludge directly affects the hydrolysis of conditioner. When the temperature is low, hydrolysis will slow down. If the temperature is lower than 10℃, the conditioning effect will be obviously worse, and the conditioning effect can be improved by appropriately prolonging the conditioning time. Therefore, when the temperature is low in winter, attention should be paid to the insulation link of the sludge conveying system (the temperature of the sludge discharged from the sewage treatment system is generally not lower than 15℃) to reduce the heat loss in the sludge conveying process. If necessary, the dissolution conditions can be improved by heating the conditioner dilution tank or appropriately prolonging the mixing and dissolution time and increasing the stirring intensity.
(4) the concentration of the preparation. The preparation concentration of conditioner not only affects the conditioning effect, but also affects the dosage and mud cake output, among which the influence of organic polymer conditioner is more significant. Generally speaking, the dosage range of ferric chloride is 2 ~ 10% ds of sludge, and the dosage range of lime is 5 ~ 40% ds of sludge, both of which are calculated as dry sludge.
(5) Administration sequence. When multiple conditioners are used, the order of adding conditioners will also affect the conditioning effect. When iron salt and lime are used as conditioners, iron salt is usually added first, and then lime is added, so that the floc formed is easier to separate from water, and the total amount of conditioner is less. In this project, 38% FeCl3 _ 3 _ 3 solution and lime with particle size greater than 100 mesh are used for sludge conditioning. Ferric chloride solution is added by dosing pump and measured by electromagnetic flowmeter; After being measured by an electronic scale, lime is automatically added. The optimum dosage of ferric chloride and lime depends on the characteristics of sludge.
(6) Sludge should be dehydrated as soon as possible after conditioning. The experiment shows that the conditioned sludge should not be left for too long. Only when the intensity and time of mixing reaction are within a certain range can better conditioning effect be achieved, and the conditioning effect will decrease with the increase of residence time. In other words, the time and intensity of conditioning must be strictly observed in actual operation after being determined by experiments. On the one hand, the stirring reaction time cannot be prolonged or shortened at will, on the other hand, the conditioned sludge should be sent to the frame machine for dehydration as soon as possible, otherwise the filtration performance will be poor. The moisture content of conditioned sludge is generally controlled at 94 ~ 96%.
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