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New difficultiestechnical analysis of sludge treatment

2021-05-28 11:22:03

First of all, the process of sewage treatment does not simply transfer pollutants to sludge. The so-called sludge is actually a combination of various microorganisms and inorganic substances. In the sewage treatment process, most organic pollutants are transformed into their own cellular components such as protein, CO2, nitrogen and other substances through microbial degradation and conversion. However, there are many substances that are only adsorbed into microbial micelles.

This also has a lot of negative effects on sludge. If it is not handled properly, it is washed by rainwater or its moisture content is relatively high (for example, more than 80%), the pollutants in the water will return to the water body, and the microorganisms in the water will also have a negative impact on human body, animals and ecological environment. If the sludge produced by printing and dyeing wastewater is not treated properly, the whole river will be dyed; If the sludge produced by other sewage such as chromium containing wastewater and heavy metal wastewater is not treated properly, the consequences can be imagined. Therefore, sludge also belongs to hazardous solid waste. I am used to referring to hazardous waste for short.

I have experienced several urban sewage treatment plants. Of course, the sludge produced by urban sewage treatment plants has relatively small negative impact compared with the whole sewage treatment industry, but it can not be ignored. Among these water plants, the vast majority of water plants can only treat the sludge with a moisture content of less than 80%.

The main reason is the investment and operation cost. The traditional belt filter press has the advantages of low treatment cost, small investment, relatively simple maintenance and mature process. The plate and frame filter press can treat the sludge moisture content below 50%, and the volume will be greatly reduced. Moreover, when the sludge is used as combustion, the calorific value can also meet the requirements. However, due to the government's restrictions on the total investment of the project, BOT companies cannot increase their own additional investment unless the government agrees to increase the water price and realize the reporting and recovery of additional investment.

The decrease of sludge moisture content can be realized through equipment upgrading, and the investment is relatively small. However, the subsequent co production projects such as burning materials, brick making and fertilizer making need preferential policies from the government to attract investment. Now there are more than n such enterprises and they are doing very well. Now a vigorous sludge disposal alliance has been formed to jointly develop the sludge market.

Taken together, the so-called difficulties are the support of the government, the supervision of local environmental protection departments and the policy guidance to force environmental protection enterprises to upgrade and transform. For enterprises, if they are given a good return on investment, the output value of this market is definitely more than several hundred million if they choose honest and authoritative enterprises and the joint production of relevant enterprises.

The sludge produced by sewage plants is generally called sewage sludge or urban sludge in academia. Many people have mentioned the composition of sludge before. I won't elaborate here, but focus on two aspects:

The first is the drying of sludge. The water content of sludge dewatered by sewage treatment plant is about 80%, roughly four parts of water for one part of sludge. If it can be dried to 20% of water content, its volume will be reduced to 1 / 5 of the original. For such a large sludge output every year, the volume reduction rate is considerable; In addition, the prospect of resource utilization of dried sludge is also better.

In view of the high content of organic matter in sludge, the calorific value is relatively high. The heat content of sludge is about 10.3kg / MJ, which is as high as the heat value of untreated sewage (MJ / MJ) after being tested, and the heat content of sludge here is as high as 10.50% of that of untreated sewage (MJ / MJ).

If the drying cost of sludge is reduced, the sludge can be thrown into the power plant for combustion and power generation.

Secondly, it is the resource utilization of sludge. At present, the more mature technology is incineration. The heavy metals in sludge limit its agriculture. Although pyrolysis technology can be used to produce bio oil and activated carbon, the smell is unbearable, and the fluidized bed pyrolysis technology is actually immature (it is found in the practical operation stage that the fluidized bed can not supply heat by itself, the double bed operation invented is difficult, and the cost of wrapping electric heating wire is too high).

1、 Sludge source

First, we should define the concept of sludge. In a broad sense, sludge includes sewage sludge (i.e. municipal sludge, the products of municipal sewage treatment plant after sewage treatment, including primary sludge, surplus sludge and some sludge in grit chamber), industrial sludge (the products of industrial sewage treatment have different properties due to different sources of industrial wastewater, such as possible oil and high content of certain substances), pipe network sludge (sludge obtained from pipeline desilting) Water supply sludge (sludge produced by waterworks) and river and lake sediment (such as sediment excavated from river dredging).

In a general sense, we refer to sewage sludge, which is the most common type with similar properties and huge output. The so-called "sludge siege" is also him.

2、 Necessity of disposal

The nature of sludge has two sides: on the one hand, it contains a large amount of organic matter, nitrogen, phosphorus, potassium and other nutrients, which makes it useful (which is also the basis of sludge land use); On the other hand, some pollutants contained in sewage are transferred to sludge, resulting in high water content, strong odor, and a large number of pathogenic bacteria, parasitic eggs, toxic, harmful and carcinogenic substances that are difficult to degrade such as chromium, mercury and other heavy metals and dioxins. Without effective treatment and disposal, it is very easy to cause secondary pollution to groundwater and soil, which directly threatens environmental safety and public health, This has greatly reduced the environmental benefits of sewage treatment facilities and triggered a number of public events.

3、 Why is it difficult to handle

That's a good question. More than a decade ago, people asked, "why is sewage treatment so difficult?", No one asks this question now. Why? Because people didn't pay attention to it more than a decade ago, they later found that sewage must be treated, but it can't be treated without treatment, so they invested human, material and financial resources (the most important) to deal with the problem of water. The same is true of sludge.

The lack of attention in concept and heavy water and light mud are the root causes of the current situation of sludge treatment and disposal.

No one paid attention to this thing. I think the sewage plant only needs to treat the water. As for the generated sludge, let him go. "The day is divided and the night is close together", that's what I mean (there have been several typical cases of secretly discharging sludge in recent years, but they haven't been in the past? I just don't think they're enough for the news). As a result, a series of disadvantages such as lack of policies, lack of funds, weak awareness and backward technology have been exposed, resulting in serious lag in the construction of sludge treatment and disposal facilities (you can see the gap in capital investment by looking at the previous relevant plans).

For example, the problem of inconsistent standards: in order to reduce the operation difficulty of the sewage treatment plant, it was simply required that the moisture content of the sludge leaving the factory should be 80%. The problem is: where can 80% of the dewatered sludge go? There are hidden dangers in landfill, and the calorific value of incineration is not enough. As for composting and digestion, it doesn't work well at all. In this way, the process before and after is not connected. How can we do it well? The only advantage is that no one is responsible for the problem. Anyway, the sewage plant is qualified, and the landfill and incineration facilities also have their own access standards. In the end, the problem is not solved, but all parties are not responsible. Just pour it into the river!

4、 Treatment and disposal methods

The problem of sludge treatment is to solve the problem of decreasing water content. At present, the common treatment methods include deep dehydration, composting, digestion, etc., and the final disposal includes land use, incineration, landfill, etc.

After deep dehydration, the water content of sludge is about 60%, which can be directly landfilled. In case of emergency, a small amount of mixed combustion can be carried out (together with domestic waste). However, at present, most of the iron salt + lime for chemical conditioning is used, and the increment of inorganic matter is high. The introduction of chloride ion has a certain impact on the incineration conditions and the furnace itself. However, the cost of this technology is low, and chemical conditioning agents are very common. What is needed is only a pressure filtration equipment (many domestic products). As long as there are landfill facilities, the downstream can be connected. At present, it is a process launched in many areas. The odor problem of composting is too serious, but the most fatal thing is that the product chain is not connected, and there is no way out for the final product. Anaerobic digestion is suitable for large-scale sewage plants. The equipment and automatic control requirements are very high and the operation is difficult. In particular, due to the high sand content of domestic sludge, I don't know how many equipment manufacturers and operators have been damaged.

From the perspective of final disposal, both at home and abroad, land use has been given the first priority, that is, the nutrients in sludge should be used as much as possible. However, land use must go through composting, digestion and other pretreatment to ensure that it can be applied only after reaching the standard. The biggest problem is the problem of heavy metals. China has strict standards for several aspects of land use (gardening, greening, soil improvement, agriculture, etc.), but in the actual process, many areas are not aware of this. They think that dumping sludge into the ground is land use, which has serious consequences. Sludge can not be used directly!!!

Incineration is a common practice in foreign countries and has been explored in China. Sludge can be burned alone (many in foreign countries and basically none in China), or with domestic waste (the flue gas emission standard is the strictest, and the heat source and flue gas purification equipment can be used together) Coal fired power plants (which must undergo strict environmental impact assessment, because the flue gas purification of power plants is different from that of domestic waste incineration facilities, and pollution may occur after adding sludge) and cement kilns (which is still a problem of flue gas standard. In addition, beware of some projects driving the production capacity of the cement industry in the name of sludge treatment). Drying treatment is generally required before incineration. When the moisture content is reduced to 40%, the incineration working condition will not be affected. The calorific value is very close. We have done experiments in the incineration plant and can also refer to the literature. I personally recommend this.

Landfill is the simplest method with low technical difficulty, but it must be noted that the current admission standard is 60%, and 80% of the sludge cannot be landfilled, which may cause geotechnical risks such as landslide and settlement.

With the development of China's social economy and the improvement of urbanization level, the sludge output of urban sewage plants in China has increased rapidly. It is estimated that the sludge output (80% moisture content) in China exceeded 30 million tons in 2011. Sludge disposal technology includes sanitary landfill, incineration, aerobic fermentation, resource utilization and other methods. Sludge aerobic composting is the recommended technical route in the technical guide for sludge treatment and disposal of urban sewage treatment plants (Trial Implementation) because of its moderate investment and operation cost and the resource utilization of organic matter and nutrient elements in sludge.

At present, the common sludge aerobic fermentation technology at home and abroad mainly adopts the trough or bin structure constructed of concrete, which maintains the aerobic state in the reactor through manual or mechanical turnover, combined with natural or forced ventilation, so as to ensure the temperature of the reactor and the normal growth of microorganisms. The odor, steam and leachate produced in the composting process are difficult to control, which seriously affects the surrounding environment and damages public health. In addition, there are many problems in composting, such as long composting time, large floor area and so on.

On August 14, 2012, the 2012 urban blue book "China urban development report 2012" issued by the Institute of urban development and environment of the Chinese Academy of Social Sciences and the "China urban status report 2012 / 2013" jointly issued by the China Science Center of the International Eurasian Academy of Sciences, China Mayors Association and UN Habitat pointed out that at the end of 2011, there were 657 cities in China, with a total administrative area of 5.216 million square kilometers, It accounts for 54.3% of the total area of 9.6 million square kilometers. The number of established towns increased to 19683. In 2011, the population of Chinese Mainland was 1347.35 million, including 690.79 million urban population and 656.56 million rural population. The urban population accounted for 51.27% of the total population. The urban population exceeded the rural population for the first time, which is a historical change in China's social structure.

From the report of the 18th CPC National Congress to the central economic work conference, from the 12th Five Year Plan to the premier's speech, new urbanization has been given an important position. The central government has identified it as an important national development strategy and has become the greatest potential for China's long-term economic development.

According to the research results of China's low-carbon ecological city development strategy, by 2050, China's urbanization level will reach 70% ~ 75%, and the contribution rate of urban economy in the total national economy will reach 90%.

The blue book points out that facing the quiet arrival of the urban era dominated by an urban society, China's urban development is facing severe challenges including environmental and ecological pressure, urban security, social differentiation and a series of problems.

Resources and environment are important contents of contemporary urban construction and management, especially land resources and water resources, which are related to the sustainable development ability of cities, human settlements and public security.

According to the new standards, 2 / 3 of China's urban air quality does not meet the standard; 57.2% of the monitoring points in China have poor or very poor urban water quality; Two thirds of large and medium-sized cities are surrounded by garbage; Urban natural vegetation coverage is low, the area of reinforced concrete jungle is expanding, and urban green space accounts for only 10% of urban construction land. The area of urban wetlands has decreased sharply, biodiversity has continued to decrease, urban groundwater has been overexploited, and land subsidence has accelerated.

At present, the overall deterioration trend of environmental conditions has not been fundamentally curbed, environmental contradictions are prominent, and the pressure continues to increase; Water pollution in some key river basins and sea areas is serious, atmospheric haze is prominent in some regions and cities, and the emission of major pollutants in many regions exceeds the environmental capacity.

Urban infrastructure is an important support for the level of urban development and civilization, and the material basis for the coordinated development of urban economy and society. Driven by urbanization, China's urban infrastructure construction has been accelerating. In 2010, the investment in urban municipal infrastructure reached 143.5 billion yuan, an increase of 155% over 2005.

It can be predicted that in the next two or three decades, urban infrastructure construction such as urban water supply, sewage treatment, garbage treatment, gas, heating, roads and landscaping will show large-scale growth with the continuous advancement of the new urbanization process. At the same time, the world is facing the huge pressure of climate change and resources and environment. The epitaxial growth development has been difficult to meet the various needs of the new situation. Therefore, the green, ecological and low-carbon sustainable development model has become a new direction of China's new urbanization.

In his speech at Seoul University in South Korea, Ma Yun boldly predicted that great companies in this century should be committed to solving "social problems".

The sludge is enriched with organic matter, nitrogen, phosphorus, potassium and other nutritional elements that cause water pollution. At the same time, it contains a large number of pathogenic microorganisms, which are easy to rot and stink. If it is not handled properly, it will cause very serious environmental pollution and ecological damage, and it is also a great waste of resources. According to statistics, only about 10 ~ 20% of sludge in China has been safely and innocuously disposed, and a large number of sludge has not been effectively disposed of. The large output and improper treatment of sludge will cause serious pollution to the environment. At the same time, sludge contains a lot of nutrients, which will be a new resource after proper treatment.

Since 2007, under the leadership of the Ministry of housing and urban rural development, a number of domestic units engaged in the design and operation of urban sewage plants have jointly carried out standard research, and a total of 9 industry standards have been issued in four years. In the following years, four technical guidelines were issued successively. According to the national plan for the construction of urban sewage treatment and recycling facilities during the 12th Five Year Plan period issued by the State Council on April 19, 2012, China plans to add about 430 billion yuan of sewage treatment and related investment during the 12th Five Year Plan period, including 34.7 billion yuan of sludge treatment and disposal facilities. This series of technical documents have made some guiding provisions on the technical selection, technical development trend, specific implementation scheme and investment and financing methods of sludge disposal, pointing out the development direction for sludge treatment and disposal of urban sewage treatment plants in China.

At present, the main methods of sludge disposal in use at home and abroad include landfill, incineration, composting, resource utilization and so on. Landfill technology has low treatment cost, but it covers a large area, and there are many landfill gases and leachate, which are difficult to treat. The volume reduction effect of incineration technology is remarkable, and the calorific value of sludge can be utilized, but the operation cost is high, and there is the problem of secondary air pollution. Anaerobic digestion technology can recover energy materials, but it is difficult to operate and manage, and there are few successful cases. The investment and operation cost of aerobic fermentation technology is low, and the products can be made into fertilizer. The resource effect is remarkable, the management and operation is simple, and it is suitable for a wide range of sewage plants. Through the analysis and investigation of the existing sludge treatment and disposal technology, the R & D team chose the technical route of sludge aerobic fermentation to achieve the goal of low treatment cost and product recycling.

The sludge from the municipal sewage treatment plant has little risk, but the sludge classified as hazardous solid waste such as electroplating sludge and printing and dyeing sludge has great environmental risk if it is not reasonably disposed of. At present, there are good sludge treatment technologies at home and abroad, which is not difficult, but the key lies in the management level and understanding level. At present, the main difficulties in sludge treatment are that the management system is not detailed enough and the sludge generating units do not pay enough attention to it Sludge is also in contact with municipal sewage. The general direction of sludge is to classify municipal sludge and industrial sludge. Many industrial sludge include heavy metals, oils, toxic and harmful substances, etc., which are often treated with hazardous waste. There are corresponding hazardous waste treatment processes, incineration, landfill, etc. Most of the municipal sludge we contact is to be transported to the landfill, but the current requirement of 60% moisture content is more embarrassing. The conventional sludge dewatering process, after treatment, the moisture content is 80%, to achieve 60%, there is no particularly mature, feasible and low-cost process. For a long time, there has been a phenomenon of heavy water and light mud in sewage treatment, but the pollutants are only transferred to the mud. Therefore, in order to avoid secondary pollution, it is necessary to recycle, stabilize, harmless and reduce the sludge. The common technologies include landfill, incineration, composting and anaerobic digestion. However, due to various reasons such as technology, cost and ideas, the treatment of sludge is not satisfactory.