Heat treatment method of waste incineration slag

Heat treatment of municipal solid waste incineration ash is a promising technology, which can not only fix heavy metals, but also reduce the volume of waste. When the conditions are well controlled, it can also produce materials suitable for further utilization, which is a cost saving treatment method. However, the high energy consumption of heat treatment method limits its use. Generally, there are two types of heat treatment: melting and sintering.
(1) Melting.

The melting method is to melt the incinerator slag at 1200 ~ 1400C, so that the organic components can be decomposed and combusted for gasification, while the inorganic components can be melted into the glass slag to form a stable glassy substance. In the melting process, the heavy metals and salts with low boiling point are collected, and the nonvolatile heavy metals react with silica and are encapsulated in the silicate network matrix structure, or are adsorbed on the aluminosilicate layer on the fused product artificial glass during leaching. After melting treatment, the slag mass and volume are greatly reduced, and high density and stable melting products are produced. Not only can the storage capacity of landfill be saved, but also the slag can be used as filling material, subgrade, concrete, asphalt and cement raw materials and other building materials. If heavy metals are further removed, the slag can be reused, such as cement, decorative tile, asphalt and paving brick. It is found that when the melting temperature is higher than 1300 ℃, the melting effect of the sample is better, and the leaching rate of Zn, Cr, Pb, Cu, CD and other heavy metals is significantly reduced. The volatile heavy metals Cd and Pb are collected in the form of gas. Z, Cr and Ni are dissolved in the slag.

In recent years, some new technologies have been developed in the original melting treatment technology:

1) secondary melting. Through various secondary melting processes, the incineration ash is further vitrified.
2) recrystallization of molten slag. Because the strength is not enough to limit the utilization of molten slag, some scholars have recrystallized the molten slag to make high-strength stones, which are mainly used in asphalt binder, concrete upper binder and permeable sidewalk.
3) control the nucleation and crystallization process. It is a promising technology to melt and promote controlled crystallization to form glass ceramics because it can transform complex chemical composition into useful materials with better engineering properties at relatively low cost.

(2) Sintering.

Sintering is the heat treatment of bonded particles to enhance the strength and other engineering properties of compacted particles. Sintering has been used in many aspects, such as ceramics, metals and various composite materials. Recently, it has been applied in the treatment of incineration ash to remove dioxins and stabilize heavy metals in the sintering matrix for safe disposal or recovery, as well as the application of incineration ash in building materials, subgrade aggregates, pavement bricks and other purposes.
When sintering products are used in building materials, it requires very low leaching characteristics of heavy metals and sufficient compressive strength. However, due to the volatilization of alkali chlorides during sintering, the porosity increases, and the glass phase between grains which is not fully formed will lead to the low degree of solidification and insufficient mechanical strength of compacted samples. However, the leaching rate of Cr in the sintering product is obviously increased because the water-insoluble Cr3 + is oxidized to water-soluble Cr6 + in the sintering process.

In general, sintering products are suitable for aggregate of Portland cement, asphalt concrete of subgrade, pavement brick, garden brick, high temperature mineral insulation material

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Industrial solid waste

Introduction

With the development of industrial production, the quantity of industrial waste is increasing day by day. In particular, metallurgy, thermal power generation and other industries have the largest emissions. The amount of industrial waste is huge, the variety is various, the composition is complex, and the treatment is very difficult. Today, only a limited number of industrial wastes are used, such as steel slag in the United States, Sweden and other countries, and fly ash and coal slag in Japan, Denmark and other countries. Other industrial wastes are still mainly in passive storage. Some harmful industrial solid wastes are disposed by landfill, incineration, chemical transformation, microbial treatment and other methods; some are put into the sea.

Harm

The passive storage of industrial waste not only takes up a lot of land, resulting in the waste of human and material resources, but also many industrial wastes contain water-soluble substances, which pollute the soil and water through leaching. Powdery industrial waste, flying with the wind, pollutes the atmosphere, and some emit odor and poison. Some wastes even silted up river channels, polluted water systems, affected biological growth and harmed human health.

 

Treatment and utilization

Industrial waste can be used as industrial raw material or energy after proper process treatment, which is easier to realize resource utilization than waste water and waste gas. Some industrial wastes have been made into a variety of products, such as cement, concrete aggregate, brick and tile, fiber, cast stone and other building materials; extraction of metals such as iron, aluminum, copper, lead, zinc and rare metals such as vanadium, uranium, germanium, molybdenum, scandium, titanium; manufacture of fertilizers, soil improvers and so on. In addition, it can also be used for waste water treatment, mine fire fighting, chemical filler, etc. Almost all industrial wastes can be processed into building materials, or energy and industrial raw materials can be recovered from them Nowadays, most countries mainly deal with industrial waste by industrial departments, that is to say, under the management of the government, the industrial departments and factories that discharge the waste will deal with and use it by themselves. With the increase of industrial waste discharge, Japan and other countries have developed specialized contract treatment, aiming at the final treatment.

Industrial waste is affected by industrial production process and other factors, and its composition often changes, which makes it difficult to treat and utilize. Industrial waste can only be used after a certain treatment process. For example, the slag formed by high temperature must be cooled, the slag generated by wet method must be dried, and the dust must be collected, so the cost is high. During this period, many countries are committed to the research of recycling.
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Factors determining the total cost of sludge disposal

1. The sludge disposal plan determines the sludge disposal cost
Nowadays, China’s sludge disposal program tends to be sludge resource utilization, and sludge composting, sludge incineration heating power generation, sludge brick making, and cement making are the mainstream. At the time of the sludge disposal plan, the approximate sludge treatment cost range is basically determined. For example, sludge landfill includes sludge drying, sludge transportation costs, landfill use fees, etc., the landfill disposal cost of the standard is about 200 yuan / ton; sludge incineration is about 100-300 yuan / ton, But it is accompanied by high resource value; sludge composting treatment costs are 90-150 yuan / ton.

2. Composition of disposal costs during sludge disposal
In the sludge disposal process, the disposal costs include sludge transportation costs, site costs, and sludge drying costs. The sludge drying costs include electricity charges, depreciation costs of sludge treatment equipment, workers’ wages, and management expenses. Due to the different sources of sludge in the sludge treatment project, if there is sludge near the sewage treatment plant, the transportation cost is low, and some sludge is transported from a remote place, so there are some differences in this part.

3. The selected sludge treatment equipment has a great impact on the overall cost.
For example, if the selected sludge treatment equipment consumes very little energy, it will save a lot of money for sludge disposal projects with a daily treatment capacity of more than 100 tons. There are also many sludge treatment production lines for automated sludge treatment equipment that require only one person to supervise. In today’s labor cost increase, the cost of sludge treatment is virtually saved. Another example, if the company chooses deep dewatering equipment, not only will the volume of sludge be reduced, but the transportation costs and sludge disposal costs will be reduced accordingly.

The sludge is mainly produced after the wastewater treatment. In the early stage, the heavy water and light mud were used. As the country pays attention to sludge pollution, the sludge treatment market is also expanding year by year. The sludge drying and reduction technology was introduced, which reduced the sludge treatment cost to 40%, which greatly reduced the processing cost of the enterprise.

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Sludge expansion measures

For: long-term low sludge load and insufficient nutrient salt

If the degree of expansion has not yet reached a high degree of expansion, adjusting the sludge load and replenishing a sufficient amount of nutrient salt can gradually return the sludge to a normal state.

Among them, the adjustment of sludge load should be based on increasing the amount of sludge, supplemented by increasing the influent load, so that the sludge load can reach 0.2 KGBOD/ KGMLSS.d or above. Under the premise of satisfying the demand of microbes for nutrients such as N and P, the load can reach a reasonable interval, which can promote the proliferation of bacteria micelles, and the growth rate is higher than that of filamentous bacteria, thereby inhibiting sludge expansion; At the same time, increasing the discharge of excess sludge can not only improve the sludge load of the system, but also discharge a large number of filamentous bacteria, which is beneficial to the advantage of the bacteria micelles in the growth of activated sludge during the optimization and adjustment process. status.

For: high water temperature, low dissolved oxygen
If the degree of expansion has not yet reached a high degree of expansion, the sludge can be gradually returned to a normal state by adjusting the sludge load while increasing the amount of aeration.

If the oxygen supply amount is difficult to increase greatly due to equipment reasons or water temperature, the adjustment of the sludge load can be achieved by increasing the sludge discharge and reducing the sludge concentration of the aeration tank.

Since the decrease in sludge concentration is beneficial to reduce the demand for oxygen, the increase in the ratio of food to food is beneficial to the utilization efficiency of oxygen.

For: low pH
In this case, the sludge load is often insufficient. If the degree of expansion has not reached a high degree of expansion, in addition to adjusting the pH value of the influent water, adding liquid alkali to the aeration tank, increasing sludge discharge and increasing sludge load It is a necessary adjustment tool.

Precautions
Through process adjustment, the required time period is longer, and the effluent water quality deteriorates for a long time and even causes substandard discharge, which is not worth the loss for the enterprise. At this point, the expansive sludge in the system can be emptied, and it is a reasonable choice to inoculate new activated sludge for re-cultivation.

In addition, it should be noted that people often do this: adding inert substances, bacteriostatic agents to the system and raising the pH to above 10 to crush filamentous mycelium and kill filamentous fungi. These measures are not recommended because not only the high cost, but also the inadvertent operation will lead to further deterioration of the system’s effluent, and eventually have to choose to re-culture the activated sludge, extending the processing cycle.

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The cause of sludge expansion

Wastewater quality

In the biological treatment of wastewater, the wastewater itself is the medium of microorganisms. Therefore, the relationship between wastewater quality and the physiological activities of microorganisms is very close. From the two types of sludge expansion mentioned above, it is related to the physiological activities of microorganisms. That is, sludge swelling is related to microbial growth (such as filamentous bacterial sludge expansion), or due to accumulation of metabolites (such as non-filamentous bacterial sludge expansion). It can be seen that the quality of wastewater is an extremely important factor in the cause of sludge expansion.

Wastewater quality: organic content

Filamentous bacteria have an advantage over bacterial micelles in the competition for carbon sources. When the carbon source in the water is sufficient, the bacteria and filamentous bacteria can get enough food; when the carbon source in the water is lacking, the filamentous bacteria can compete for food more easily because of the huge surface area of ​​the filamentous bacteria. Filamentous bacteria expansion often occurs in activated sludge systems with low organic loading.

Wastewater quality: nitrogen and phosphorus nutrients

In order to carry out normal growth and reproduction, activated sludge microorganisms require nutrients such as nitrogen and phosphorus in addition to carbon sources. There should be an appropriate ratio between nitrogen, phosphorus and carbon. The general experience suggests a ratio of BOD5:N:P=100:5:1.

When the nitrogen and base content in the wastewater is insufficient, sludge swelling is likely to occur. For example, in activated sludge, the surface area of ​​filamentous bacteria is larger than that of other microorganisms, and it is easier to take up the substrate. Therefore, when the ratio of nitrogen and phosphorus to BOD5 is insufficient, filamentous bacteria can use the substrate more easily than other microorganisms, and can still grow and reproduce normally.

When the nitrogen and base content in the wastewater is insufficient, other microorganisms in the activated sludge are gradually degraded due to the failure of nitrogen and phosphorus, and the filamentous bacteria are greatly increased, resulting in the expansion of filamentous bacterial sludge.

In addition, when the nitrogen and phosphorus sources in the wastewater are insufficient, the carbon source is relatively large. In this case, if there are many saccharides, the high-viscosity substance of the metabolite polysaccharide increases, so that the activated sludge is prone to non-filamentous bacterial expansion.
Dissolved oxygen content of wastewater.

In the aeration tank operation, the dissolved oxygen concentration of the mixture has a great influence because different microorganisms have different requirements for dissolved oxygen.

From the past practical experience, if the dissolved oxygen concentration in the aeration tank is too low, it is unfavorable, and sludge swelling is easy to occur. Although the filamentous bacteria are aerobic bacteria, they and other aerobics in the activated sludge. Different bacteria, most of the aerobic bacteria can not continue to grow and breed under the low dissolved oxygen condition of activated sludge, but the filamentous bacilli can still adapt to this environment and continue to grow and reproduce, thus making the filamentous bacterial sludge swelling easy to occur.

Filamentous bacteria do not lose their vitality during a long period of anaerobic conditions. If they return to aerobic conditions, they will re-grow. According to actual experience, the dissolved oxygen should generally be controlled at a level of not less than 2 mg/L, such as 2-4 mgL. If it is too high, it is not necessary to increase the power consumption, resulting in excessive wastewater treatment costs.

Other influencing factors
The effect of pH and water temperature, the pH is too low, and the temperature below 5 ° C or higher than 35 ° C easily causes the growth of filamentous bacteria.

The influent water quality has changed. For example, the treatment of sulfide-containing papermaking wastewater, when the biochemical device in the water contains hydrogen sulfide, the concentration of more than 1-2mgL will cause sludge expansion.

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Type of activated sludge expansion

Activated sludge expansion can be divided into: filamentous bacterial sludge expansion caused by excessive proliferation of filamentous bacteria in the sludge and non-filamentous bacterial sludge expansion without the presence of a large number of filamentous bacteria. In fact, more than 90% of sludge expansion is caused by filamentous bacteria, and less than 10% is caused by non-filamentous bacteria.

Non-filamentous sludge expansion

The non-filamentous bacteria expansion mainly occurs when the wastewater water temperature is low and the sludge load is too high. At this time, the bacteria adsorb a large amount of organic matter, which is less than metabolism, and accumulates a large amount of highly viscous polysaccharide substances outside the cell, so that the surface attachments are greatly increased. It is difficult to precipitate compression. When nitrogen is seriously deficient, it is also likely to cause swelling. Because if nitrogen is deficient, the microorganisms cannot fully utilize the carbon source to synthesize cellular material, and the excess carbon source is converted into polysaccharide extracellular storage. This storage is a high-density hydrophilic compound, which easily forms bound water, thereby affecting the pollution. The sedimentation performance of the mud produces a highly viscous sludge expansion.

When the non-filamentous sludge is inflated, its biochemical treatment effect is still high, and the effluent is relatively clear. Microscopic examination does not show excessive proliferation of filamentous bacteria. In actual operation, non-filamentous sludge expansion occurs less, and harm Not very serious.

Filamentous sludge swelling

Sludge swelling caused by excessive proliferation of filamentous bacteria is common in daily work, and the cause is very complicated. There are many factors affecting the expansion of filamentous sludge. Activated sludge is a mixed culture system in which at least 30 filamentous bacteria that may cause sludge swelling exist. Filamentous bacteria are an indispensable class of microorganisms in the relationship with the active bacteria micelle system. Its presence plays a very good role in purifying sewage: it maintains the floc structure of the sludge and maintains biochemical treatment. The purification efficiency and the filtration of suspended solids in the precipitation are of great significance.

When the filamentous bacteria are in equilibrium with the bacteria micelles, no sludge swelling will occur. Only when the filamentous bacteria grow more than the bacteria micelles will the sludge expand.

For normal activated sludge, there is an appropriate proportional relationship between filamentous bacteria and fungal micelles. If the filamentous bacteria grow and multiply, the growth and reproduction of the fungus mass will be inhibited, and the excessively proliferating filamentous bacteria will extend beyond the surface of the sludge, causing the flocs to loosen, the sedimentation performance to deteriorate, the sludge volume to expand, and the sludge to settle. Both the ratio and the sludge volume index are high, which is the filamentous bacterial sludge expansion.

Ammonia nitrogen increases with the expansion of the sludge, and when the sludge expands severely, a large number of filamentous bacteria can be observed under the microscope to protrude from the surface of the micelle. Sludge swelling is not only a high incidence, it is common, and once it is difficult to control, it usually takes a long time to adjust.

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Definition and consequences of sludge expansion

1. What is sludge expansion?

Sludge expansion refers to the phenomenon that the sludge structure is loose, the volume is increased, the SV value is increased, and it is difficult to settle and separate the water quality. The performance is as follows: the activated sludge suddenly increases in volume, and the structure is loose and not dense. It floats on the surface of the secondary settling tank and cannot be precipitated normally. SVL>200 mL/g (SVL=activated sludge volume/mixed liquid suspended solids concentration (MLSS) and continued to rise, normal activated sludge SVL is 50-150 mL/g).

2. The consequences of sludge expansion

Sludge expansion is a common problem in wastewater treatment plants. Most sewage treatment plants using activated sludge process have sludge swelling in varying degrees. But it is more difficult to handle. First, there are too many reasons for the expansion; second, the adjustment period is longer.
Sludge expansion will lead to sludge loss, BOD removal rate, effluent suspended matter, COD, and ammonia nitrogen exceeding the standard. Poor treatment may result in paralysis of the entire sewage treatment system.

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New direction of resource utilization of paper sludge industry

First, the development trend

1. Preparation of activated carbon
The sludge is rich in internal pore structure and is a good material for the preparation of activated carbon. Through the pyrolysis treatment, the sludge can be made into a carbonaceous adsorbent. The carbonaceous adsorbent under this process has lighter texture, more internal pores, strong adsorption capacity, and low cost of sludge activated carbon, which not only solves the sludge. The pollution problem has also realized the utilization of sludge resources.

2. Preparation of ethanol
Sludge can be used as a raw material for ethanol fermentation. Ethanol is subject to certain constraints in terms of production due to major raw material problems. At the same time, since most of the main raw materials are crops, pretreatment is required to be put into production, which greatly increases production costs. The sludge does not need to be pretreated, saving costs while saving energy.

3. Hydrogen production
At present, foreign countries are studying sludge production technology for hydrogen production. The use of organic matter in paper sludge for biohydrogen production produces hydrogen that can generate electricity and form electrical energy. Compared with the traditional power generation technology, the sludge hydrogen production power generation technology is more advanced, and the electric energy output is higher, which has certain advantages.

4, processing briquette
Studies have shown that the use of sludge cohesiveness can support anthracite coal and process coal. When the sludge is treated at high temperature, it can effectively improve the internal pore structure of the briquette and reduce the amount of residual carbon in the ash. At the same time, the sludge briquette has strong pressure resistance and good stability, and the sludge briquette does not cause secondary pollution.

Second, the treatment utilization plan
At present, China is still in the exploration and research on the utilization of paper solid waste. Both the application technology and the processing mode have a certain gap with foreign countries, and there is also a huge room for improvement.

From a macro perspective, China needs to continuously strengthen the environmental awareness of enterprises, improve the technical level of solid waste utilization, and improve the overall proportion of solid waste resource utilization, in order to fundamentally solve the problem of solid waste pollution.

From the perspective of national policies, the state should formulate corresponding support policies to provide certain technical support, policy support and promotion support in the treatment and utilization of solid waste in the paper industry, in order to achieve a sustainable development path for the paper industry.

At present, China’s problems in the solid waste treatment of paper industry are mainly due to the low utilization rate, general treatment technology and small promotion scope. The future development trend of solid waste treatment should be biased towards the two directions of reduction and industrialization of resource utilization. The recycling technology of solid waste requires the joint efforts of experts and scholars from all walks of life.

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Five-step method to identify whether sludge is hazardous

According to the National Development and Reform Commission of the Ministry of Environmental Protection and the Ministry of Public Security, the National Hazardous Waste List and the Hazardous Waste Identification Standards, which were implemented since August 1, 2016, are combined with the Ministry of Housing and Construction, the Ministry of Environmental Protection, and the Ministry of Science and Technology in 2009. On June 28th, the “Technical Policy for Sludge Treatment and Pollution Prevention and Control of Urban Sewage Treatment Plants (Trial)” was issued in five steps to determine whether sewage sludge is hazardous waste.

One、Classification

Sewage sludge

Two、Judgment basis

Judgment basis

Three、How to regulate

regulate

Four、Utilization

utilization

Five、How to exempt

Exemption

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Interpretation of whether sludge is dangerous or not

Is the sludge hazardous? Many people have such doubts. The following is an explanation of whether the sludge is dangerous or not, for your reference!

1. The public sewage treatment plant used solely for the treatment of urban domestic sewage, the sludge produced by it usually has no dangerous characteristics and can be used as general solid waste management;

2. Sludge from treatment facilities that specialize in the treatment of industrial wastewater (or a small amount of domestic sewage at the same time) may have hazardous characteristics and should be in accordance with the National Hazardous Waste List and the National Environmental Protection Standards Hazardous Waste Identification Technical Specification (HJ/ T298-2007) and hazardous waste identification standards for the identification of hazardous properties of sludge;

3. Public sewage treatment plants that treat domestic sewage as the main function, if receiving and treating industrial wastewater, and the industrial wastewater can stably meet the national or local pollutant discharge standards before being discharged into the public sewage treatment system, public sewage The sludge of the treatment plant can be managed in accordance with the provisions of Article 1. However, when there is a major change in the discharge of industrial wastewater, the identification of the hazardous characteristics shall be carried out in accordance with the provisions of Article 2;

4. If the enterprise discharges industrial wastewater directly or indirectly outside its legal boundary, the effluent water quality shall comply with national or local pollutant discharge standards; the sludge generated during the wastewater treatment process belongs to the solid waste being generated, and it is dangerous. For the identification of characteristics, samples shall be taken in the wastewater treatment process according to the “Technical Specifications for Identification of Hazardous Wastes”, and the minimum number of samples shall be determined according to the amount of sludge generated.

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