There are many methods to treat and dispose of sludge at home and abroad. Sludge is generally treated by concentration, digestion, dehydration, drying, etc., and then effectively utilized (mainly used in agriculture), landfilled and burned, or some combinations thereof. The final outlet of sludge is resource utilization or returning to the environment in some form. With the prohibition of ocean dumping, the proportion of sludge disposal is gradually decreasing, and land landfill is increasingly restricted. In China, agricultural sludge accounts for 44.3%, sanitary landfill accounts for 3 1%, other methods account for 1 1%, and untreated sludge accounts for 13.7%. People realize that the key point of sludge treatment is reduction, recycling and abandonment, and sludge reduction, stabilization and innocuity have become the mainstream as a kind of resource reuse.
The choice of sludge treatment and disposal scheme should be based on the nature and quantity of local sludge; Investment situation and operation and management expenses; Environmental protection requirements and relevant laws and regulations; After comprehensive consideration of urban development trend and sludge resource utilization, it was selected [5]. With the enhancement of people's awareness of environmental protection, the sludge recycling technology that makes full use of useful components in sludge and improves the added value of sludge is heating up. Sludge recycling is an important way for the sustainable development of sludge treatment.
1. Selection of sludge recycling process
1. Making ceramsite with sludge
Making ceramsite from sludge is a kind of sludge disposal method, which uses the principle that the composition of sludge is similar to that of clay, mixes sludge with clay or fly ash in a certain proportion, and burns it into ceramsite at high temperature. When firing ceramsite, the chemical composition of raw materials is the main factor affecting the expansion coefficient of ceramsite. The materials (pellets or pellets) for firing ceramsite must have two basic conditions for expansion: first, they can produce appropriate viscosity and surface tension at the expansion temperature; Second, enough gas is generated at the same time. Only by meeting the above two conditions at the same time, it is possible to obtain homogeneous porous ceramsite with good expansibility. Using biological sludge from different sewage treatment plants to replace part of clay ingredients, light ceramsite meeting national standards can be produced. The sintering temperature of sludge burden for firing lightweight ceramsite is about 65438 0200℃. According to the different components of biological sludge in sewage treatment plant, the formula should be adjusted appropriately. Under normal circumstances, the amount of fired ceramsite sludge is low, and the proportion of mixed firing is about 1%.
2. Sludge brick making
There are two ways to make bricks from sludge, one is to make bricks directly from dry sludge, and the other is to make bricks from sludge ash. When using dried sludge to make bricks, the inorganic composition of sludge is similar to the chemical composition of brick-making clay, which can partially replace clay as brick-making raw material. Sludge brick is made by mixing crushed and screened sludge with auxiliary materials, drying, and baking at high temperature by tunnel kiln brick making process. The process route is shown in figure 1-2. The best ratio of sludge to clay is 1: 5, and the strength of sludge brick can reach that of ordinary red brick.
When making bricks with sludge ash, the process of making bricks with sludge ash is basically the same as that with dry sludge, and the chemical composition of ash is basically similar to that of brick-making clay. When making bricks, just add a proper amount of clay and admixture, and directly mix sludge ash with other brick-making raw materials to form and sinter bricks. The suitable weight ratio of ingredients is: ash: clay: sludge = 100: 50: (65438+. During the firing process, toxic heavy metals are sealed in the sludge, which also kills all harmful bacteria and organic matter, and the fired brick has no peculiar smell. Brickmaking with sludge not only realizes waste utilization, but also reduces the burden of sludge treatment, which is a better way for sludge resource utilization.
Produce fertilizer
Sludge fertilizer production is the main way of sludge resource utilization. Sludge is rich in organic matter, nitrogen, phosphorus, potassium and various trace elements needed for plant growth, which is very suitable for producing soil improvers or other high-quality slow-release compound fertilizers. However, untreated sludge has high moisture content, high viscosity and a large number of pathogenic bacteria, so it cannot be directly applied to soil. Generally, biological fermentation technology is used to produce fertilizer.
Because of its negative characteristics such as odor, pathogenic bacteria and viscosity, the quality of dried sludge has been significantly improved, and it can be directly applied to soil, or refined fertilizer can be developed through aerobic fermentation technology and compound fertilizer production technology. The technical route is shown in figure 1-3. The dried sludge is first sent to the storage bin for fermentation and decomposition. In the fermentation process, the sludge is decomposed more fully, and the conditions of organic matter structure, particle size and moisture content are more suitable for agriculture, and it can be directly packaged into refined organic fertilizer for sale. The decomposed sludge can be dried and crushed, mixed with inorganic raw materials such as monoammonium phosphate, potassium chloride and calcium superphosphate in a certain proportion, and can also be used as high-quality slow-release compound fertilizer after granulation to maximize resource utilization.
Fired cement
Burning cement with sludge can realize partial cost recovery to a certain extent, which is one of the more thorough industrial schemes for sludge disposal at present. The chemical characteristics of inorganic part of sludge are basically similar to those of raw materials used in cement production. Compared with ordinary portland cement, the particle size and specific gravity of cement made of sludge are basically similar, but the stability, expansion density and curing time are better. As an alternative raw material, sludge needs to be dried to a solid content of more than 92%-95%, and the finer the particles, the better. The sintering process of directly adding cement requires that the sludge used as cement raw material must reach more than 87% of the ground sludge with particle size less than 90 microns, and the solid content after grinding is more than 98%. When the proportion of dry sludge is 3-8%, the calorific value of sludge (3000 kcal/kg) can meet the heat demand of 28% in the process of cement firing. In the cement sintering process with indirect addition of sludge, sludge is used as alternative fuel and residual ash is used as cement raw material, so the heat utilization rate is lower than the former.
Under normal circumstances, sludge, as the raw material of cement, must be mixed with fly ash, gypsum, clay and other raw materials before firing cement. The firing temperature of cement is 1450 ~ 1700℃, and the fuel consumption and carbon dioxide emission are lower than those of traditional cement firing. See figure 1-4 for the process principle flow. The technology of producing ecological cement from sludge does not need pretreatment of raw materials, and pollutants in sludge can be destroyed. Therefore, using cement rotary kiln to treat the sludge of municipal sewage plant not only has the characteristics of volume reduction and reduction by incineration, but also the residue after combustion becomes a part of cement clinker, which is a better sludge treatment and utilization method reflecting the principles of harmless, reduction and resource utilization of solid waste treatment.
5. Biomass fuel technology
Biomass fuel technology is a technology to make low-quality fuel from sludge. The process adopts indirect heating and decompression drying technology, and the waste edible oil is directly mixed into sludge to be heated as a heat medium, which destroys biological cells and evaporates water in tissues. In this technology, a heat exchange plate is installed on the air hole shaft inside the drying device, and the steam generated by heating the heat exchange plate is used to improve the drying efficiency, so that the water content of the sludge with 80% water content reaches about 65,438 0% after treatment, and the calorific value of the produced biomass fuel is 3,700-6,200 kcal/kg, which can be used as an auxiliary fuel for incineration facilities, cement plants and power plants.
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