Municipal sewage sludge is solid waste produced in the process of sewage treatment. With the development of domestic sewage treatment, the total amount and degree of treatment of sewage treatment plants will continue to expand and improve, and the amount of sludge will also increase day by day. At present, the capital operation cost of domestic sewage plants accounts for about 20% ~ 50% of the total capital operation cost [1]. Sewage sludge not only contains a lot of organic matter and rich nutrients such as nitrogen and phosphorus, but also contains toxic and harmful components such as heavy metals, pathogenic bacteria and parasites. In order to prevent the secondary pollution caused by sludge and ensure the normal operation and treatment effect of sewage treatment plants, the treatment and disposal of sludge in urban sewage treatment has become increasingly prominent.
China has a population of more than 65.438+300 million, more than 640 cities with an urban population of 270 million. According to the figures provided by the State Environmental Protection Administration of China, at present, China discharges about 406.5438 billion m3 of sewage every year, and more than 400 urban sewage treatment plants have been built and operated, with a daily treatment capacity of 25.34 million m3. According to the sludge output accounting for 0.3%~0.5% of the treated water (calculated as 97% moisture content), the sludge output of China urban sewage plant is between 76,020m3/d ~126,700m3/d (calculated as 97% moisture content). Therefore, China will face enormous pressure of sludge treatment and disposal while continuously promoting sewage treatment.
1 Present situation of municipal sewage sludge treatment and disposal in China
1. 1 Domestic Municipal Sewage Sludge Treatment
1. 1. 1 existing sewage sludge treatment process
According to the urban sewage plants that have been built and put into operation in China, the sewage sludge treatment process can be roughly summarized as 18 processes, as shown in table 1.
1. 1.2 sludge concentration
Sludge concentration is mainly to reduce interstitial water in sludge, and physical treatment methods are usually used, including gravity concentration, air flotation concentration and centrifugal concentration. Their processing performance is shown in Table 2 [3]:
1. 1.3 Sludge stabilization
At present, the commonly used sludge stabilization methods in China are anaerobic digestion, aerobic digestion and sludge composting, and sludge composting is in the stage of continuous research, while pyrolysis and chemical stabilization methods are rarely used due to technical reasons or economic and energy consumption reasons [5]. Figure 2 shows the proportion of the above sludge stabilization methods in China.
1. 1.4 Sludge dewatering
The existing sludge dewatering measures in China are mainly mechanical dewatering, but limited by geographical and climatic conditions, drying equipment is rarely used. Figure 3 shows the proportion of several sludge dewatering technologies in China.
1.2 Problems in Domestic Urban Sewage Sludge Treatment
Domestic urban sewage sludge treatment started late, and there are also many problems, mainly in the following aspects:
1.2. 1 Low sludge treatment rate and imperfect process.
In China, wastewater treatment and light sludge treatment are a development trend. In many cities, sludge treatment is not regarded as a necessary part of sewage treatment plants, and it often takes a long time to build a sludge treatment system after the completion of sewage treatment plants, resulting in a low sludge treatment rate in urban sewage in China. It can also be seen from the process in table 1 that the sludge treatment process of domestic urban sewage plants is very imperfect. Only 25.68% of the above-mentioned urban sewage plants have been treated with sludge concentration, digestion stability and drying and dehydration. This shows that more than 70% of sewage treatment plants in China do not have a complete sludge treatment process. Sewage plants without stable sludge treatment account for 55.70%, and a large number of sewage sludge without stable treatment will cause serious secondary pollution to the environment. About 48.65% sewage plants have no sludge drying and dehydration treatment. After concentrated digestion, the water content of sludge is still about 95%~97%, and the volume is still very large. Such a huge amount of sludge, if not dried and dehydrated, will bring a lot of inconvenience to transportation and subsequent disposal.
1.2.2 Sludge treatment technology and equipment are backward.
At present, the sludge treatment technology adopted by some sewage treatment plants in China has been abandoned by developed countries, and its level still stays at the level of developed countries in the 1970s and 1980s, and some even reach the level of foreign countries in the 1960s. Moreover, some sludge treatment technologies simply do not conform to the characteristics of domestic sewage sludge, and there is a lack of necessary investigation and research on the technologies adopted. Sludge treatment equipment is also relatively backward, with poor performance, low efficiency, high energy consumption and few special equipment, which fails to form standardization and serialization. Therefore, the improvement and development of sludge treatment technology in China are limited.
1.2.3 The management level of sludge treatment is low.
Many completed sludge treatment facilities can't operate normally. In addition to technical level, low management level is also an important factor. The managers and operators of most sewage plants are of poor quality and lack of management experience, so they cannot effectively organize production. In addition, there are few technicians and various occupations do not match. So once there is a problem in production, they don't know how to deal with it. Some sewage treatment plants have to stop running for a long time. Improving the management level of sewage plant and realizing scientific management as soon as possible is the key to ensure the long-term operation of sludge system in sewage plant.
1.2.4 The design level of sludge treatment is low.
China's drainage industry has made great progress, accumulated rich experience in sewage treatment design and trained a large number of designers. However, in terms of sludge treatment, China still lacks practical experience and design experience, especially the overall level of sludge treatment system is still relatively low. Judging from the sludge treatment device of the sewage treatment plant that has been built, the operation condition is not good, and the long-term operation cannot be guaranteed. After the installation of many factories is completed, great technical transformation is carried out, which causes great waste of manpower, material resources and financial resources.
1.2.5 Low investment in sludge treatment.
Domestic investment in sludge treatment only accounts for 20% ~ 50% of the total investment in sewage treatment plants, while developed countries account for 50% ~ 70% of the total investment.
1.3 Status and Analysis of Municipal Sludge Disposal in China
The disposal methods of urban sewage sludge include land use, sanitary landfill, incineration and water digestion. These methods can accommodate a large number of urban sewage sludge, but their applications vary from country to country. After Tianjin Ji Zhuangzi Sewage Treatment Plant, the first sewage treatment plant in China, was completed and put into operation in the early 1980s, the sludge was used for farmland by farmers in the nearby suburbs. Later, sludge from sewage treatment plants such as Gaobeidian in Beijing was also used for farmland. With the increasing output of sewage sludge and sewage treatment plants, China began to use sewage treatment plant sludge for land landfill and urban greening, and made agricultural compound fertilizer with sludge as matrix. In China, the general situation is mainly in the form of sludge land use, and sludge is used for agriculture. However, due to insufficient attention to the control of physical and chemical indicators such as pathogenic bacteria, heavy metals and toxic organic substances and sensory indicators such as odor in sludge management in China, the further disposal and utilization of sludge are limited. Figure 4 shows the proportion of several sludge disposal technologies in China.
There are serious problems in sludge disposal in China, that is, the final way out. As can be seen from the above figure, there is still 13.79% sludge that has not been treated, which will bring great harm to environmental pollution. The odor emitted by sludge pollutes the air, pathogenic bacteria pose a potential threat to human health, and heavy metals and toxic and harmful organic substances pollute the surface and groundwater systems. The reasons for this phenomenon can be summarized as follows: due to the late start of sludge treatment and disposal in China, many cities have not included sludge disposal sites in the overall urban planning. It is difficult for many treatment plants to find suitable sludge disposal methods and sludge disposal sites; The foundation of sludge utilization in China is weak, people's understanding of sludge utilization is seriously insufficient, and they lack attention to the final disposal of sludge, leaving hidden dangers for the final disposal of some harmful sludge; The utilization rate of sludge is not very high. There are also some sludge from sewage treatment plants, which are directly piled up after storage by the sanitation department, especially in some southern cities in China. This disposal method not only affects the normal operation of sewage plants, but also causes secondary pollution and waste of sludge resources when sludge is piled up at will. Therefore, the current problem facing our country is to develop sludge disposal technology as soon as possible to solve the growing sewage sludge.
2 China's urban sewage sludge treatment and disposal countermeasures
2. 1 China city sewage sludge treatment method
Judging from the future development trend of China, its urban sewage treatment will mainly be large-scale sewage treatment plants invested by the state, and sewage treatment plants of different scales will coexist in different regions according to the economic development. Therefore, according to the environmental status, treatment scale, source of funds and economic and technical level of sewage treatment plants, the process methods and technical equipment suitable for China's national conditions should be determined.
The purpose of sludge treatment is to reduce, stabilize, harmlessly and comprehensively utilize sludge. For domestic urban sewage treatment plants, we should constantly improve their sewage and sludge treatment processes, choose more perfect sludge treatment processes such as sludge concentration, anaerobic digestion and dehydration, and actively develop domestic equipment and machinery for sludge concentration, stabilization and dehydration with good performance, so as to improve the solid content of sludge and make the subsequent sludge disposal and comprehensive utilization go smoothly. As far as the selection of sludge concentration technology is concerned, gravity concentration is still an economical and effective sludge reduction method because of the low organic matter content in domestic urban sludge. The main methods of sludge dewatering are natural drying and mechanical dewatering, but due to the limitation of climate and region, natural drying is rarely used. Mechanical dewatering of sludge can effectively reduce the volume of sludge and lay a good foundation for subsequent sludge disposal. At present, the commonly used mechanical dewatering technologies include plate-and-frame filter pressing dewatering, belt filter pressing dewatering and centrifugal dewatering, which have their own advantages and disadvantages in actual operation, and the properties of sludge have a great influence on the dewatering effect. Therefore, the selection of mechanical dehydration mode should be based on the characteristics of sewage plant process and operation and the requirements of sludge treatment and disposal. Different stabilization methods have a decisive influence on the process selection and technical and economic comparison of the whole sewage treatment. Typical stabilization methods include biological stabilization methods, such as anaerobic digestion, aerobic digestion and composting, and chemical stabilization by adding lime. In view of the present situation in China, we should consider adopting a simple and efficient sludge stabilization method with low capital investment and low operation and management cost. Some domestic sewage treatment plants adopt mesophilic anaerobic digestion of sludge, which can not only degrade the organic matter in the sludge, but also kill some pathogenic bacteria and parasites (eggs), thus making the sludge stable and partially harmless, and the biogas produced by digestion can also be used for energy recovery. However, this method has the advantages of large investment, strict operation management, high requirements for process technology and safe operation, and is feasible for large domestic sewage treatment plants, but it is not feasible for small domestic sewage treatment plants that lack technical and economic advantages to adopt anaerobic digestion as sludge stabilization and harmless measures. The author thinks that for small sewage treatment plants, the extended aeration method (such as oxidation ditch) can be chosen when selecting the sewage treatment process first. With the increase of sludge age, the decomposition of organic matter tends to be perfect, the volatile content decreases, its energy gradually decreases, and the sludge tends to be stable. When the sludge age is long enough, the result of aerobic stability is very close to that of anaerobic digestion stability [6]. Secondly, the method of aerobic composting after direct dehydration of raw sludge is adopted. Aerobic composting is a process of transforming sludge into humus by microorganisms, which can eliminate the odor of sludge, and the composted sludge is highly stable and harmless. This is an economical, simple, efficient and low energy consumption alternative technology for sludge stabilization and harmlessness.
2.2 Sludge composting is a sludge stabilization technology in line with China's national conditions.
It is best to compost the sludge before agricultural use. On the one hand, the nutritional forms of plants are more conducive to plant absorption through biodegradation, on the other hand, the odor is eliminated and pathogenic bacteria and parasites are killed.
At present, there are two kinds of composting methods widely used in the world: static composting and dynamic composting, such as natural composting, cylinder closed composting, drum composting, vertical multi-layer reactive composting and strip static ventilation, etc. These methods are constantly developing and improving.
In recent years, some highly mechanized composting plants have been built in China, such as Wuxi, Hangzhou, Wuhan and Shanghai. Mechanized composting technology includes relatively complete pretreatment, fermentation and post-treatment processes and equipment, and its composting technology has reached a high level in product quality, operational controllability and environmental quality. Tianjin Sewage Treatment Research Institute explored a simple and easy-to-operate sludge composting process with little or no regulator in the experimental study of sludge high-temperature composting in Zhuangzi Sewage Treatment Plant, and put forward the process flow and technical parameters, which provided technical basis for the design and construction of production line. The comprehensive toxicity of digested sludge extracts before and after composting to paramecium showed that the difference of semi-lethal concentration between them was nearly 10 times, indicating that composting had a significant degradation effect on toxic organic matter [7].
From 65438 to 0997, Beijing Academy of Environmental Protection summarized the research results for many years, absorbed the advantages of various mechanical composting devices at home and abroad, and designed and developed a dynamic sludge fermentation tank. The device has high efficiency, low energy consumption and easy operation, management and equipment. According to the developed equipment, a new process route of making compound fertilizer from sludge with sludge dynamic fermentor as the core is put forward, and a 1 device with an annual output of 5000t is built. The production line consists of sludge dynamic fermenter, mixer, disc granulator, dryer, screening machine, etc. After operation, the equipment is stable and reliable, and the economic benefit is obvious. The technology of sludge dynamic fermentation and sludge fertilizer production put forward in this study will be applied to the sludge treatment and disposal of sewage treatment plants such as Gaobeidian in Beijing, and will play a very good role in solving the problem of sewage sludge disposal in Beijing. It can be said that the transformation and popularization of this technology has made a good start [8].
2.3 Land utilization of sludge is a disposal method in line with China's national conditions.
Generally speaking, the choice of sludge disposal methods in various countries should consider the balance between environmental ecological benefits, disposal costs and economic benefits. An effective sludge disposal method suitable for the specific situation in China should be environmentally hygienic, socially acceptable and economical. Sludge landfill requires high soil mechanical properties of sludge, which requires a large site area and a lot of transportation costs. The foundation needs anti-seepage treatment to avoid polluting groundwater, and the waste gas generated by landfill may pollute the environment. In recent years, the proportion of sludge landfill disposal is getting smaller and smaller. Incineration technology and equipment are complex, high energy consumption, expensive, and there are air pollution problems; Dumping sludge into the ocean is restricted by geographical location and international maritime conventions, which has threatened the marine ecosystem and human food chain. At the beginning of 1994, the government of China accepted three international agreements and promised not to dispose of industrial wastes and sewage sludge at sea from February 20, 1994. The use of sewage sludge as building materials is a new topic in the research stage in recent years, and there are still many technical problems to be solved. Therefore, the use of the above methods is restricted in our country.
From the composition of sludge, the content of organic matter, nitrogen and phosphorus is higher than that of ordinary farmyard manure, and it also contains trace elements such as potassium [9]. If applied to land, it can improve the physical, chemical and biological properties of soil to some extent. Organic matter in sludge can obviously improve soil structure, reduce soil bulk density, increase porosity, increase soil permeability and field water capacity [101], thus improving soil physical properties. The application of sludge can also increase the cation exchange capacity of soil, improve the buffering capacity of soil to acid and alkali, and provide active places for nutrient exchange and adsorption, thus improving soil fertility [12]. Rich nutrients in sludge can obviously increase soil nitrogen and phosphorus nutrients, effectively provide nutrients for plants [1 1], and reduce the application amount of chemical fertilizers, thus reducing agricultural production costs. In addition, sludge can increase the biomass and metabolic intensity of microorganisms in soil and change the biological characteristics of soil, so land utilization of sludge is an active and productive sludge disposal method [13 14]. At the same time, China is a developing country and a big agricultural country, and its vast land resources are a natural advantage for developing sewage sludge land use. Therefore, the land utilization of sludge, especially the agricultural utilization of sludge, is a disposal method in line with China's national conditions, regardless of economic factors or fertilizer efficiency utilization factors. On the one hand, this disposal method can find a fundamental way out for sludge from domestic sewage treatment plants, on the other hand, it can also alleviate the shortage of rural resources in China.
2.4 Problems needing attention in sludge land use
2.4. 1 Strengthen the control of pathogenic bacteria and parasites.
The sludge of urban sewage treatment plants contains a large number of pathogenic microorganisms and parasites. If not controlled, sludge will do harm to human and animal health in the process of land use or use. Therefore, before sludge is disposed or utilized, it is necessary to adopt medium-high temperature aerobic method, anaerobic method or radiation treatment.
2.4.2 Pay attention to the control of heavy metals and toxic organic substances in sludge.
The content of heavy metals and organic pollutants in sludge has become an important limiting factor for sludge land use. Sludge often contains a lot of heavy metals such as copper, nickel, cadmium, lead, zinc, mercury and many toxic organic substances. If it is used in farmland for a long time, it will lead to soil pollution, be absorbed by crops, and then enter the human body through the food chain, affecting human health. Although the sewage from domestic urban sewage plants is mainly domestic sewage, the content of heavy metals in domestic urban sewage sludge still exceeds the agricultural standard [2 15]. Therefore, when sludge is used as land, special attention should be paid to the problem of excessive heavy metals in sewage sludge. The research on organic pollutants in sludge has been carried out for many years in developed countries, but there is not much research work in this field in China. However, the lack of research work does not mean that sewage sludge in China contains no or almost no organic pollutants. Thirty-five nitrogen-containing aromatic compounds have been detected in the sludge of Beijing Gaobeidian Sewage Treatment Plant, of which seven have been quantified [16]. Therefore, there is still a lot of research work to be done in the field of organic pollutants and heavy metals in sludge, including the manifestations of organic pollutants and heavy metals in sludge, their changes during sludge treatment and their effects on soil-sludge system. Only in this way can the influence of pollutants in sludge on the environment and human health be well solved. However, the quality of sludge is rooted in the quality of sewage treated by sewage treatment plants. Therefore, it is necessary to reduce the concentration of toxic substances such as heavy metals that enter urban sewage from pollution sources, that is, the quality of industrial wastewater discharged into urban sewage pipelines must meet the Water Quality Standard for Sewage Discharged into Urban Sewers (CJ 18-86).
2.4.3 Application amount of sludge
Agricultural utilization of sludge can not only eliminate the pollution of sludge to the environment, but also make sludge resource and improve crop yield. However, the unreasonable application of sludge can easily lead to the accumulation of heavy metal elements in soil, which will pollute soil resources and endanger human health. Generally speaking, the amount of sludge suitable for a certain farmland is limited. When this limit is reached, the agricultural utilization of sludge should be stopped for some time before continuing. On the basis of investigation and study, specific sludge application amount should be formulated according to climatic conditions, geographical environment, crop species and soil assimilation ability to ensure the safety of sludge application in farmland.
2.4.4 Formulate and improve standards and regulations, and popularize environmental protection knowledge.
Many developed countries have formulated laws and regulations on sludge disposal and utilization, and made corresponding regulations on sludge standards, selection of application sites, protection of water sources, control of pathogenic bacteria, allowable application amount of heavy metals and transportation. At present, China's standards and laws and regulations on sludge application are not perfect. For example, the control standard of heavy metals in sludge agricultural utilization is only established on the basis of studying wheat. Obviously, this will be one-sided, so such standards need to be further improved on the basis of scientific research. On the other hand, it is necessary to popularize environmental protection knowledge to all walks of life. An important problem of sludge land is to let the majority of sludge users know the benefits of scientific application of sludge and the harm of blind application of sludge, and consciously abide by environmental laws and regulations and technical specifications for scientific application of sludge land.
3 Conclusion
With the development of industry and cities in China and the improvement of sewage treatment rate, its output will inevitably increase. Judging from the current situation, the domestic sludge treatment and utilization technology is still relatively backward, the sludge treatment rate is still relatively low, people do not know enough about the necessity of sludge treatment and disposal, and there are serious deficiencies in sludge treatment and disposal, and many problems need to be solved urgently. At the same time, China is a big agricultural country, and it should be a promising way to treat and utilize the sludge stabilized by composting. In order to solve the problems existing in domestic sludge treatment and disposal, make full use of sludge resources and reduce environmental pollution, China must vigorously develop various sludge treatment and disposal technologies.
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