With the widespread use of biotechnology in municipal wastewater treatment plants, the sludge produced has also increased significantly, resulting in a certain degree of secondary pollution. In this case, the sludge treatment of municipal wastewater treatment plants has become a common concern of society. But at present, although sludge treatment accounts for about 20%-50% of the construction cost of sewage treatment plants, which is an important part of sewage treatment system, most sludge is still not properly treated, but disposed at will, which will cause environmental pollution and even endanger human health.
1. Characteristics of sludge from municipal sewage treatment plants
One is the large amount of sludge. With the development of social economy and urbanization in China, the generation and quantity of urban sewage are increasing. With the development of sewage treatment in China, the problem of sludge treatment and disposal has become increasingly prominent in urban sewage treatment.
Secondly, the sludge property is safe and stable. Municipal sludge generally contains a certain amount of heavy metals, and its content often determines whether it can be used for agriculture. There are many kinds of heavy metals in sludge of sewage treatment plants in China. The content of heavy metals is relatively stable, mainly Zn and Cu. The content of other heavy metals is relatively low. The harmful components of sludge in general municipal sewage treatment plants are relatively low. Therefore, the land use of sewage sludge from municipal sewage treatment plants after aerobic composting is appropriate and safe. The pH value of sludge is 6-7, which generally does not affect the subsequent disposal and utilization. With the enhancement of people's awareness of environmental protection, the content of heavy metals in sludge shows a downward trend.
Third, the fertilizer efficiency is higher. The sludge produced by municipal sewage treatment plants contains a large amount of N, P, K and organic matter. Moreover, N and P are mainly organic and contain many trace elements necessary for plants. The average organic matter content of sludge in China is 37.18%, and the average contents of N, P and K are 3.03%, 1.52% and 0.69%, respectively. They all exceed the nutrient standards required by the state for composting. In China, sewage treatment plants in different regions are polluted. In the long run, the content of N and P in sludge of sewage treatment plants in China will increase with the increase of secondary sewage treatment processes such as nitrogen and phosphorus removal, which will be beneficial to sludge land use and composting treatment.
2. Traditional sludge treatment methods and disadvantages
2.1 Landfill Treatment
Sludge landfill treatment technology is a mature and widely used method of sludge treatment, which is to transport sludge to mountain, flat and other areas needed, and then cover inert soil after compaction, and finally form a sound land cover, which can grow green plants. This treatment technology has the advantages of less investment, simplicity and quick effect.
However, this technology also has many drawbacks, mainly in the following aspects: after sludge landfill, it is easy to pollute groundwater, and needs to occupy a large amount of land. With the increase of sludge volume after municipal sewage treatment, under the current situation of dense population in China, large-scale site selection has become a difficult problem, because the problems in landfill site selection limit the wide application of landfill treatment technology. In addition, compared with developed countries, sludge treatment technology in our country started late and developed slowly. There is a lack of planning for municipal sludge treatment, and the phenomenon of random mixed landfill is more common, which leads to the deterioration of the ecological environment around the landfill area to a large extent.
2.2 Sludge Incineration
Sludge incineration mainly disposes of sludge through the fact that sludge contains a large amount of organic matter, which is easy to burn. Sludge combustion is a relatively thorough method of sludge disposal, which is mainly divided into two categories: one is that dehydrated sludge is directly sent to incinerator for incineration, the other is that dehydrated sludge is first dried to remove its moisture, and then incinerated. As for the choice of incineration mode, it should be based on the specific situation of incineration.
Compared with other sludge disposal methods, sludge incineration has great advantages. For example, sludge incineration can rapidly reduce the existing sludge stock, and the products after sludge incineration contain no bacteria and odor. However, sludge incineration by incineration method requires strict incineration equipment and conditions, and is difficult to operate. The investment of funds is relatively large, so it does not meet the practical conditions for popularization and application.
At the same time, because sludge contains a lot of organic matter, although the products after incineration will not pollute the environment, many harmful gases, such as sulfur dioxide, carbon dioxide and so on, are produced in the process of incineration. These gases will cause secondary pollution to the atmosphere, and useful nutrients will be incinerated in the process of sludge incineration, thus no sludge resources can be realized. Change. These drawbacks limit the wide application of sludge incineration method. Generally, only when other methods are limited, sludge incineration method will be considered for sludge treatment.
3. Comprehensive Utilization of Municipal Sewage Treatment Plant Sludge
3.1 Energy and Thermal Energy Utilization
Sludge oil production technology can be realized by two processes: one is sludge pyrolysis oil production technology. The principle is to heat the sludge to a certain temperature under the condition of no oxygen or less than theoretical oxygen. Under the high temperature environment, the solid matter in the sludge can be decomposed into oil, non-condensable gas and carbon by pyrolysis and thermochemical conversion of organic matter. Some products can be used as energy source for drying and pyrolysis, and the surplus can be recycled as energy. Direct oiling technology. Under the conditions of high temperature, high pressure and catalyst, the macromolecule compounds in sludge are transformed into low-molecular-weight oily substances through a series of reactions such as decomposition, condensation, dehydrogenation and cyclization, which have been widely used in the United States, Britain, Japan and other countries.
3.2 Sludge Energy Utilization
(1) Biogas production by anaerobic digestion of sludge. Sludge anaerobic digestion to produce biogas is the use of anaerobic bacteria growing in sewage and sludge under anaerobic environment, which can decompose organic matter into other stable substances after change, so as to achieve the purpose of sterilization and harmlessness. Biogas is the flammable gas with methane as the main component, which is produced by the decomposition of these organic substances under anaerobic conditions by anaerobic bacteria. Biogas can provide energy for urban residents and factory production activities. It is reported that the combustion calorific value of 1 m3 biogas is equivalent to that of 1 kg coal, which shows a good prospect of resource utilization. At the same time, sludge anaerobic digestion can also be combined with wastewater treatment to form a complete sewage treatment system to achieve the purpose of harmless treatment of organic matter. At present, anaerobic digestion of sludge to produce biogas has become the most widely used sludge treatment process.
(2) Low temperature pyrolysis of sludge to produce oil. Low-temperature pyrolysis of sludge to produce oil is a new technology of heat energy utilization. It mainly refers to the heating of sludge to a certain temperature in the absence of oxygen or hypoxia, and the conversion of organic matter into hydrocarbons through distillation and thermal decomposition, and finally to the conversion of sludge into combustible products that can interact with oil or carbon. In terms of sludge pyrolysis technology, a lot of research has been carried out in China. It is found that low temperature pyrolysis of sludge has the advantages of low cost and good application prospects. In recent years, microwave pyrolysis technology has been developed in China, which is more convenient and fast, and the liquid oil produced contains higher fat substances.
(3) Use sludge to produce synthetic fuel. Municipal sludge contains 70% - 80% organic matter, and the dewatered sludge has the characteristics of high calorific value, so it is scientific and feasible to make it into synthetic fuel. The results show that the thermal efficiency of synthesizing fuel from 35% digestive sludge, 50% coal and 15% additive is much higher than that of coal. At the same time, the sludge also has a strong viscosity. Mixing activated sludge with anthracite pulverized coal can reduce coal pollution, improve coal reactivity and reduce residue in the process of combustion and utilization.
With the development of sludge treatment technology, some achievements have been made in the utilization of municipal sludge in China. However, compared with developed countries, China is still in a relatively backward stage. In order to promote the reuse of municipal sludge and build a resource-saving and environment-friendly society, cities should follow the characteristics of municipal sludge and combine with the reality of urban development to realize the effective utilization of sludge and make it play a huge environmental protection benefit.
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