1. Sludge disposal status
The sludge after urban domestic sewage treatment generally has a moisture content of about 80%, which is rich in nutrients such as organic matter, as well as a certain amount of heavy metals and harmful substances such as viruses, pathogens and parasitic eggs. Improper disposal of sludge will cause serious secondary environmental pollution. At present, the main disposal methods of sludge in foreign countries are sludge landfill, incineration and reclamation. In European and American countries, it is mainly landfill and farmland utilization. The sludge from land reclamation in Japan accounts for more than 70%; The main disposal methods of sludge in China are sludge landfill and incineration. Its disposal methods can not realize the comprehensive utilization of sludge, and there are certain social and environmental problems, which is not the best way out of sludge disposal.
For a developing country like China, comprehensive utilization, especially agricultural utilization, is undoubtedly a better choice. Realizing the effective utilization of sludge, turning waste into treasure, making it resource and generating economic benefits will become the development direction of sludge disposal.
2. Technical process route of sludge reuse
In the sludge of urban domestic sewage treatment, the water content is generally about 80%, and the content of organic matter is about 45% ~ 55%. The process of making organic compound fertilizer from sludge is to dry the sludge at a high temperature of 800 ℃ ~ 1000 ℃, kill bacteria and insect eggs, preserve the organic part from damage and remove harmful bacteria (harmless treatment), connect beneficial bacteria for culture, eliminate the odor of sludge, increase the nutrient elements in sludge, add effective components of nitrogen, phosphorus and potassium, increase the nutrient content in sludge, granulation, low-temperature drying and other processes, The sludge is made into an organic compound fertilizer with biological activity, complete nutrition and no pollution. Thus, it finds a good way out for the sludge after water treatment, increases the economic and social benefits of sewage treatment plants and equipment manufacturers, turns sludge into treasure, reduces environmental sludge, protects the natural environment on which human beings depend, and is conducive to the development of ecological agriculture.
In addition, according to the actual situation, the air drying or retting process of sludge can be increased before the high-temperature drying process of sludge. The air drying or heap retting process of high humidity sludge can reduce the moisture content of sludge and effectively save drying energy, so as to reduce the drying cost; Sludge can also be directly dried in the dryer to reduce the use site of drying or composting, so as to save the floor area.
3. Sludge odor solution
The odor of untreated sludge is a prominent problem. The reason for the odor is that the sludge of urban sewage treatment plant contains some substances with odor, such as sulfide, ammonia, putrescine, etc., so when it is processed into fertilizer and turned into commodities for storage and application, it will still emit odor around and continue to pollute the environment. This will greatly reduce the commercial value of sludge fertilizer, and will directly affect the survival and development of fertilizer in the market, so it must be solved.
At present, the removal methods of this odor mainly include: chemical method, physical deodorization method and biological deodorization method. Biological deodorization is to decompose and transform odorous substances through the physiological metabolism of microorganisms to achieve the role of deodorization. The advantages of the first two methods are relatively simple, but their disadvantages are that the deodorization effect is not very good, and they either increase the alkalinity of sludge or increase the volume of sludge. The advantages of biological deodorization are: good deodorization effect; After microbial deodorization, it can further degrade various macromolecular elements in the sludge and increase the nutrients that are conducive to plant absorption; Through the deodorization of sludge, the biological heat generated by microbial fermentation can further dry the sludge and reduce the drying cost of sludge; These microorganisms with deodorization function are mostly beneficial microbial flora in soil. They have the functions of improving soil aggregate structure, improving soil ecological environment and resisting harmful microorganisms. Disadvantages: deodorization takes a long time and covers a large area.
Considering that many domestic small and medium-sized sewage treatment plants have no digestion process and produce raw sludge, the biological deodorization method can play the role of sludge digestion process, so the biological deodorization method is better. The test shows that the use of beneficial microorganisms can remove the odor of sludge in sewage treatment plant and increase the nutrients that are conducive to plant absorption (such as the content of available organic carbon). At the same time, the bio heat generated by microbial fermentation in the process of sludge deodorization can further dry the sludge, reduce the cost of sludge drying and increase the market competitiveness of sludge fertilizer.
4. Experimental analysis of organic matter loss in sludge drying
Through the drying test, the sludge is dried at high temperature by the dryer. The dried high nitrogen content is 2.7%, phosphorus content is 2.45%, potassium content is 0.67%, and organic matter content is 43.7%. After drying, the nitrogen loss is 9.3%, organic matter loss is 5.7%, and other nutrients change little. Therefore, the nutrient loss of sludge before and after drying is small. The drying of the material after granulation, the loss of nutritional components of the material after drying, the reduction of crude protein < 6%, the reduction of amino acid < 2%, the reduction of nitrogen (n) < 2%, and the reduction of phosphorus (P2O5) < 0.5%, can achieve the expected effect.
5. Analysis and test of sludge drying flue gas emission
The high-temperature drying production process of sludge uses the flue gas furnace as the drying heat source, the crushing dryer as the drying equipment, and the wet water film dust removal system is used for dust removal. The moisture content of sludge is about 80%, and the moisture content of sludge after drying is 13%. Therefore, the main component of flue gas is steam. Because the flue gas furnace is used as the heat source and coal is used as fuel, a small amount of sludge dust is produced in the drying process, Therefore, the flue gas contains a small amount of sulfur dioxide and dust. Through environmental protection monitoring, the concentration of sulfur dioxide emitted by 2T flue gas furnace is 342 mg / standard m3, the concentration of smoke and dust is 140 mg / standard m3, and the blackness of flue gas is less than lingman blackness Level I. All results meet the emission standard of air pollutants for boilers (gwpb3-1999).
6. Field test
The field experiments of corn, wheat and cotton with sludge, organic compound fertilizer and inorganic compound fertilizer showed that the number of grains per ear, 1000 grain weight and yield per mu of corn with sludge as the only fertilizer source increased by 5.1%, 4.7% and 10.1%; The effect of organic compound fertilizer was the best. The number of grains per panicle increased by 11.5%, 1000 grain weight increased by 11.6%, and the yield per mu increased by 20.75%. The number of grains per ear, 1000 grain weight and yield per mu of wheat with sludge as the only fertilizer source increased by 10.4%, 2.15% and 26.0%; The effect of organic compound fertilizer was the best, the number of grains per ear increased by 22.32%, the weight of 1000 grains increased by 4.3%, and the yield per mu increased by 44.2%; For cotton with sludge as the sole fertilizer source, the boll growth rate per mu was 5.1%, the boll weight increased by 4.7%, and the yield per mu increased by 10.1%; Taking organic compound fertilizer and inorganic compound fertilizer as fertilizer source, the effect of cotton is basically the same, but the use of organic compound fertilizer can reduce the use of inorganic chemical fertilizer, so as to reduce the toxic effect of chemical fertilizer on soil, and organic compound fertilizer has the long-term effect of organic fertilizer, and the effect is better year after year. After the application of sludge, the heavy metals harmful to human beings in corn, wheat and cotton meet the food hygiene standard (gbb2715-81), and their contents in grains also meet the national food hygiene standard. The test shows that the sludge is safe to be used as fertilizer.
Therefore, the raw sludge produced by the secondary treatment of the sewage treatment plant can be used for agricultural production after high-temperature sterilization and drying. Control the amount of use, that is, it will not cause heavy metal accumulation in plants. Whether applied alone or mixed with inorganic fertilizer and bacterial fertilizer to make active organic compound fertilizer, it can significantly promote plant growth. Among them, the active organic compound fertilizer made with inorganic compound fertilizer has the best effect. It has the comprehensive advantages of inorganic fertilizer, organic fertilizer and bacterial fertilizer. It not only has high nutrient content, strong fertilizer supply intensity, but also has comprehensive nutrients, appropriate proportion, long fertilizer efficiency period and no toxic effect on Soil and crops. It can improve soil micro ecological environment and soil fertility.
Therefore, the high humidity sludge with moisture content of about 80% and organic matter content of about 40 ~ 55% is dried by air drying, composting and then drying, or directly drying at 800 ℃ ~ 1000 ℃, so as to kill viruses and insect eggs, preserve the organic components from damage and remove harmful bacteria (harmless treatment), and then biological deodorization, add effective components of nitrogen, phosphorus and potassium to increase the nutrient content in the sludge. After granulation, low humidity drying and other processes, The best way of sludge disposal is to make sludge into organic compound fertilizer with biological activity, complete nutrition and no pollution. At the same time, we have successfully carried out the research and development of complete sets of sludge fertilizer making equipment. At present, a production line with an annual output of 5000 tons of organic compound fertilizer has been made and installed and operated in Baoding sewage treatment plant. It is a beneficial way to benefit the country and the people, protect the natural ecological environment on which mankind depends, and improve the economic benefits of the sewage treatment plant.
7. Economic benefit analysis
Waste water can be disposed of by landfill, and the utilization rate of sewage can be improved. The research of the ecological environment research center of the Chinese Academy of Sciences shows that the environmental loss caused by the sludge of urban sewage treatment plant per square meter is 38 yuan, excluding the transportation cost. The annual sludge output of Baoding sewage treatment plant is about 2.7% × 104m3, the annual environmental loss value is 1.03 million yuan. If the sludge is processed into activated organic compound fertilizer, the annual output will be 5000 tons and the annual profit will be more than 1 million yuan. Therefore, it is economically feasible to develop and manufacture organic compound fertilizer with sludge and has remarkable economic benefits.
8. Social benefit analysis
At present, China has nearly a thousand sewage treatment plants that have been built, under construction and proposed to be built. With the improvement of national requirements for environmental protection, the enhancement of people's awareness of environmental protection and the acceleration of urbanization, the treatment and utilization of sludge has become an urgent problem to be solved. Poor sludge treatment will cause secondary pollution to the environment and a series of social problems. Using sludge to produce organic compound fertilizer can make sludge turn harm into benefit, which not only reduces the economic burden of sludge treatment, but also plays a positive role in promoting agricultural production, and has a broad development prospect; At the same time, the secondary pollution caused by sludge incineration and landfill is eliminated.
9. Engineering application
At present, the complete set of equipment production line has been put into formal production in Baoding sewage treatment plant. The sludge treatment capacity of the complete set of equipment is 3t / h, calculated as 20 hours per day, 60 tons of sludge are treated per day, 20000 tons of sludge are treated per year, and 5000 tons of organic compound fertilizer are produced. The production line of the complete set of equipment has the advantages of advanced technology, compact structure, good integrity, convenient transportation, installation and commissioning, high degree of automation, reducing workers' labor intensity, improving productivity and increasing economic benefits. At the same time, the project complies with the national energy and environmental protection policy.