Environmental sustainability considerations for remediation of farmland contaminated soil

Similar to the restoration of industrially contaminated sites, the environmental sustainability considerations of farmland restoration are divided into two parts. The first part is the harm to human health caused by the migration and exposure of pollutants in the soil, usually through quantitative analysis of health risk assessment. The second part is the various effects caused by the repair itself, such as resource consumption, pollutant release, waste generation, etc. At present, LCA is widely used for quantitative analysis of the secondary impact of repair. LCA is divided into midpoint index method and endpoint index method. Among them, the midpoint index method can elaborate the impact of various aspects caused by restoration, such as greenhouse gas emissions, atmospheric particulate matter emissions, water eutrophication, land acidification, etc.; and the endpoint indicator method can highly integrate various environmental impacts and classify them as Three categories of human health, ecological environment and resource consumption. The endpoint indicator method is convenient for people to make decision analysis. Therefore, the evaluation index of the LCA endpoint indicator method is introduced as a consideration factor in this analysis framework to comprehensively cover various environmental secondary impacts. At present, there have been studies that successfully combined HRA and LCA to optimize the formulation of remediation targets for contaminated sites.

Social sustainability considerations for remediation of farmland contaminated soil include remediation of workers’ and farmers’ health and safety, public participation and satisfaction, remediation validity, and remediation flexibility. A questionnaire survey of more than 200 restoration practitioners worldwide showed that reducing the health and safety risks of site restoration workers and residents in the communities around the site is considered to be the most important means of improving the sustainability of restoration. Therefore, it is necessary to consider whether the repair method can effectively prevent the release of pollutants and the safety of the repair operation. Public participation and satisfaction mainly depend on the degree of farmers’ participation and feedback on the implementation of restoration during the planning and decision-making process of restoration. Repair elasticity is a relatively new concept, mainly referring to the long-term effectiveness of repair and the sensitivity of the repair effect to changes in the external environment. Many farmland soil remediation methods reduce the bioavailability of pollutants by changing the physical and chemical properties of the soil, but cannot remove the pollutants from the soil. On the other hand, heavy metals in the soil may be activated again due to reasons such as acidification of the soil by acid rain. Therefore, it is particularly important to evaluate the remediation elasticity of contaminated farmland soil remediation.

The economic sustainability of restoration includes the cost and benefits of restoration. The life cycle cost of restoration and subsidies given to farmers for restoration of occupied farmland are the main economic sustainability considerations, and these indicators can be quantified by engineering economic methods. From a regional perspective, the impact of farmland restoration on farmers’ lives and local economic impact also need to be considered. These indicators can be reflected in local people’s living standards and local taxes.

Agricultural sustainability is an important link in the consideration of the sustainability of farmland contaminated soil remediation, and it is also the main factor that distinguishes it from the analysis of the sustainability of industrial contaminated sites. It mainly includes three aspects: soil fertility improvement, agricultural product safety and agricultural product yield increase. Soil fertility as the basic guarantee of cultivated land function can be reflected from soil pH, physical and chemical and biological indicators. Based on these indicators, the US Department of Agriculture’s Natural Resources Defense Agency has established a mature method of soil quality assessment. Although the impact of farmland contaminated soil remediation on agriculture is closely related to local socioeconomic impacts, agricultural sustainability is a larger regional scale and a longer-term consideration of farmland contaminated soil remediation.

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Current status of remediation of farmland contaminated soil in my country

With the rapid development of my country’s social economy, industrial activities and agricultural activities have brought serious soil pollution problems, which have brought huge risks to my country’s ecological environment and human health. According to the “National Survey Report on Soil Pollution Status” issued by the Ministry of Environmental Protection, the actual rate of spot violations in the 6.3 million square kilometers of soil actually surveyed reached 16.1%. Among different types of land, the environmental quality of farmland soil is worrying, and the rate of arable land exceeding the standard reached 19.4%. Facing the severe soil pollution situation, the State Council promulgated the “Soil Pollution Prevention Action Plan” in 2016, which provided an action plan for my country’s soil pollution prevention and control work. For the remediation of farmland contaminated soil, this regulation requires: by the end of 2018, complete the detailed investigation of the soil pollution status of agricultural land; by 2020, complete the treatment and remediation of 10 million mu of contaminated farmland; by 2030, the safe utilization rate of contaminated farmland will reach above 95.

The sources of farmland soil pollution in my country are diverse, including the excessive application of pesticides and fertilizers, sewage irrigation, plastic film disposal, atmospheric deposition, and the discharge of industrial waste. Among various types of pollutants, heavy metal pollution such as cadmium, nickel, copper, arsenic, mercury, and lead is the most prominent, followed by organic pollutants such as DDT and polycyclic aromatic hydrocarbons (PAHs). The annual production of pesticides in my country in recent years is about 3.7 million tons, and the use of pesticides per unit area in my country is 2.5 times the world average. Compared with the heavy metals ingested during natural soil formation, the biological activity of heavy metals migrated to the soil due to human activities is higher. At present, the remediation technologies for heavy metal-contaminated farmland soil include: using biochar, compost, zeolite, clay, lime, etc. as stabilizers to stabilize heavy metal pollutants in situ; plant remediation, using highly enriched plants to absorb and remove heavy metals in soil Pollutants; in situ electric remediation, using the principle of electrochemistry to remove pollution; using silicon nutrients as soil improvers to limit the migration of heavy metals to plant roots; using calcium chloride, ferric chloride and other chemical agents for in situ soil leaching; Optimal management of irrigation during farming. For organic contaminated farmland such as PAHs, remediation technologies include bioremediation and phytoremediation.

By repairing contaminated farmland soil, it can not only inhibit the migration of pollutants from the soil to plants, ensure the safety of agricultural products, but also reduce the ecological and environmental risks and the health risks of land users, bringing huge environmental and social and economic benefits. However, in recent years, remediation practitioners have found that the remediation behavior itself will also cause serious secondary environmental impacts, especially when inappropriate remediation methods are adopted, the negative environmental impact of remediation may exceed the exposure of pollutants in the soil. Impact, so that the “net environmental benefit” of the restoration is negative. On the other hand, when using techniques such as soil leaching and stabilization for remediation of contaminated soil, it is also necessary to consider the effects of remediation techniques on the physical and chemical properties of the soil and biological properties. Therefore, in order to achieve sustainable agricultural land management, comprehensive and comprehensive considerations are required before repairing contaminated farmland.

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Research on Application of Sludge Ceramsite

As an important product of sludge resources, sludge ceramsite has the characteristics of stable chemical properties, small density, good heat resistance and rich pores. It has been widely used in construction, environmental protection, gardening and other fields. In the use of building materials, the use of lightweight ceramsite for the preparation of lightweight aggregate concrete and building materials has been extensively studied. The use of sludge ceramsite to prepare lightweight aggregate concrete has higher porosity and lower density than commercial concrete, and at the same time reduces the thermal conductivity, the concrete compressive strength can reach 11.1MPa. Using sludge ceramsite to prepare concrete, the 28d compressive strength of ceramsite light aggregate concrete can reach 49MPa, which is much higher than 17.2MPa required by ASTM C330 and ACI318 specifications. Surface resistance and ultrasonic pulse testing show that the concrete has excellent properties. How to further increase the amount of sludge, increase the strength of ceramsite, and reduce the water absorption rate will be an important research direction in the field of sludge ceramsite construction and utilization in the future.

In terms of water treatment, sludge ceramsite is widely used because of its low density, rich pores and good biocompatibility. Considering its use as a BAF filler, we have specially studied its treatment effect on pharmaceutical wastewater and soybean product industry wastewater. The rough surface of sludge ceramsite is conducive to the formation of biofilm, thereby improving the wastewater treatment effect. The sludge ceramsite was used as a filler in constructed wetlands, and the removal effect of phosphate on water was studied. The results show that the sludge ceramsite is a continuous and efficient phosphorus removal matrix material. In addition, the use of hydrothermal co-precipitation method to load layered hydroxides (LDHs) on the surface of bioceramics and use them as fillers for constructed wetlands further improves the removal of phosphate in sewage. Co-precipitation method was used to load lanthanum hydroxide onto the surface of porous ceramsite to prepare phosphorus-removing adsorption material, which further expanded the application range of sludge ceramsite.

The use of sludge to prepare light ceramsite is one of the important ways to realize the utilization of sludge resources, which can be widely used in construction, water treatment and other fields. The preparation of sludge ceramsite should be further combined with the use and performance requirements of ceramsite, and the proportion of raw materials should be reasonably adjusted. At the same time, the research on the mechanism of swell and roasting conditions should be carried out to further increase the amount of sludge. In addition, the application range of sludge ceramsite can be further expanded by modifying and loading functional components.

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Analysis of influencing factors of preparing ceramsite from sludge

1. Influence of sludge content

The ratio of raw materials is a key step in the preparation of ceramsite. It is necessary to comprehensively consider the influencing factors such as the amount of sludge, the chemical composition, the plasticity of the blank, and the economy. When using urban sludge and coal gangue as raw materials to prepare porous ceramsite, at the same temperature, the volume density of ceramsite decreases with the increase of the amount of sludge, and the water absorption and porosity increase accordingly. %, the material ball cracked seriously during the drying process; using urban sludge, bentonite and loess as raw materials to burn light ceramsite, the study found that the appropriate amount of sludge is 20% to 50%, as the sludge dries As the weight ratio increases, the internal pores of the ceramsite become larger, the number of pores becomes smaller, the pore cross-connect rate increases, the bulk density becomes smaller, the cylinder pressure strength becomes smaller, and the water absorption increases; the sludge from the water supply plant is the main material and the clay , Fly ash and glass powder are used as auxiliary materials to fire the ceramsite. The results show that when the sludge content of the water supply plant is increased from 40% to 60%, the packing density and apparent density of the ceramsite decrease, and the water absorption rate and The porosity increases accordingly. The increase in the amount of sludge, the increase in the content of organic matter in the raw meal, and the increase in the loss on ignition result in many cracks during the preheating process. Most of the studies on the preparation of ceramsite from sludge use sludge as an organic additive with a small amount (generally between 20% and 30%). At the same time, the problem of high water content of sludge further limits the large-scale utilization of sludge. Based on this, the process of firing ceramsite by using urban sludge as the main material and adding glass powder can increase the amount of sludge to 75%, so as to realize the possibility of large-scale resource utilization of sludge. In the future, the sludge pretreatment technology for the purpose of increasing the amount of sludge and the refined raw material proportioning technology will be one of the important research directions of sludge resource preparation for ceramsite.

2. Influence of raw material chemical composition
The chemical composition of sludge ceramsite raw materials will have a significant impact on the performance of ceramsite apparent density, bulk density, cylinder pressure strength, water absorption and heavy metal sealing. Studies have shown that when the content of raw SiO2 increases, the density of sludge ceramsite will increase accordingly; when the content of Al2O3 is less than 18%, the strength of ceramsite will decrease accordingly; the content of SiO2 has little effect on the strength of ceramsite . The effects of SiO2 and Al2O3 content on the physical properties, morphological structure, crystalline phase and stability of heavy metals of ceramsite were studied. Lightweight ceramsite with the lowest porosity and best sealing performance of heavy metals.

3. Influence of roasting mechanism
The influence of roasting mechanism on the performance of ceramsite mainly includes preheating temperature, preheating time, roasting temperature, roasting time, etc. The preheating temperature mainly affects the moisture and organic matter content of the ceramsite, thereby affecting the strength and apparent density of the ceramsite. Compared with the preheating temperature and preheating time, the roasting temperature and roasting time have a greater influence on the performance of the ceramsite. The higher the calcination temperature, the more vitrified the ceramsite surface and the denser internal structure, which makes the ceramsite particles stronger. As the roasting temperature of the sludge ceramsite increases, the fluxing agent is converted into liquid phase and glassy substance, which increases the volume shrinkage rate of the ceramsite, so that the density of the surface and interior of the ceramsite increases, and the water absorption rate decreases.

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Feasibility of preparing ceramsite with sludge

Sludge preparation of swelled ceramsite must meet two basic conditions:

① The raw material contains enough gas-generating components (such as illite, hematite, pyrite, dolomite, etc.);

② generates enough glass phase under high temperature conditions to encapsulate the generated gas.

The main components of raw materials for firing ceramsite are generally SiO2 and Al2O3. Al2O3 produces mullite and other mineral components under high temperature conditions, which is the main source of ceramsite strength; alkaline oxides Fe2O3, RO (CaO, MgO), R2O (K2O, Na2O), etc. are melting aid components. When the chemical composition of the raw material is located in the swelled area of ​​the three-phase diagram, it can be used as a raw material for firing ceramsite. The suitable range for producing swelled ceramisite is SiO2 48%-65%, Al2O3 14%-20%, Fe2O3 The sum of fluxes such as RO and R2O is 13% to 26%, and m (SiO2 + Al2O3)/m (Fe2O3 + RO + R2O) is 3.5 to 10.
Sludge is similar to clay, shale and other main components, mainly SiO2 and Al2O3, and it has the possibility of resourceful preparation of light ceramsite. Adding too much aluminum-based flocculant to the sludge of the water supply plant results in a relatively low SiO2 content and a high Al2O3 composition, which will cause the strength of the ceramsite to decrease, and at the same time the sintering temperature will increase, resulting in a decrease in the quality of the ceramsite product and energy waste. The sewage sludge also has the problem of low SiO2 content. In addition, its loss on ignition is quite large (LOI>40%). It shrinks significantly during the calcination process, and there is no high temperature liquid phase. It usually does not have the conditions for direct firing of ceramsite. . Therefore, the researchers adopted the method of adding raw materials containing silicon, aluminum and other auxiliary materials to resourcefully utilize the sludge to burn ceramsite. The main chemical components of commonly used additive materials such as fly ash and montmorillonite. After adding silicon and aluminum auxiliary materials, it can basically meet the raw material requirements for firing ceramsite. Sludge as an auxiliary admixture plays a role in promoting the expansion of ceramsite.

 

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Preparation of ceramsite by resource utilization of sludge

Sludge is a by-product produced in the process of urban water treatment. It has the characteristics of high water content and low strength, and often contains harmful components such as pathogenic bacteria, heavy metals and toxic, harmful and difficult-to-degrade organic matter. If not properly disposed of, it will easily cause two Secondary pollution.

Traditional sludge disposal mostly adopts methods such as landfilling of sludge, land use, sludge composting, sludge incineration and ocean dumping. These methods all have certain drawbacks. Among them, ocean dumping has been strictly prohibited, and sludge landfill takes up a lot Land resources and sludge incineration tail gas treatment are difficult to control. Therefore, research on new sludge treatment and disposal technologies to achieve sludge resource utilization has attracted more and more attention.

The chemical composition of the sludge is similar to the raw materials for the preparation of ceramsite, so the researchers consider using dehydrated sludge to replace clay, shale and other non-renewable resources to produce ceramsite. Ceramsite has the advantages of high strength, low density, sound insulation and noise reduction, difficult heat conduction, shrinkage resistance, rough and porous surface, and large specific surface area. It has broad application prospects in construction materials, water treatment, sound absorption materials, horticultural substrates, etc.

Ceramsite can be divided into clay ceramsite, shale ceramsite and fly ash ceramsite according to different raw materials. Clay and shale are non-renewable resources. The state has issued relevant policies to prohibit or restrict mining.

Compared with industrial sludge, water sludge and domestic sewage sludge have large output and relatively simple composition, and have become the key research object of sludge resource preparation at home and abroad. According to the preparation process, ceramsite can be divided into swelled ceramsite, sintered ceramsite and non-fired ceramsite. The sintered ceramsite has a low density, rich internal pores, and a wide range of applications. Therefore, more attention has been paid to research.

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Three differences between solid waste pyrolysis and incineration

Pyrolysis and incineration are two very important methods for processing solid waste. Pyrolysis and incineration have certain differences in terms of processing conditions, products produced, and applicable conditions.

The difference between pyrolysis and incineration method 1: different oxygen supply conditions
Incineration refers to the process of garbage with certain calorific value energy undergoing thermochemical reaction under aerobic conditions. Incineration is the process of releasing heat. Whether garbage can be incinerated depends mainly on whether its calorific value is sufficient to dry itself and maintain a certain high incineration temperature. The general volume of solid waste after incineration can be reduced by 80% -90%. Incineration is an important way to achieve the harmlessness, recycling and reduction of garbage.

Pyrolysis, also known as retorting, thermal decomposition or carbonization, refers to the process in which organic matter is decomposed by heating under oxygen-barrier conditions, and flammable solid waste is decomposed at high temperature, which eventually becomes a form of combustible gas, oil, solid carbon, etc. Pyrolysis is an endothermic process, and the heat stored in solid waste is stored in the form of the above-mentioned substances and becomes a valuable fuel for storage and transportation.

Difference between combustion method and pyrolysis method: different products after combustion
The result of incineration is the production of a large amount of waste gas and some waste residues. Apart from the use of sensible heat (such as the heat generated by incineration can be used for heating and power generation), there is no other utilization method, and the environmental problems are relatively prominent. Pyrolysis produces combustible gas, oil, etc., which can be recycled in various ways. Its energy recovery is good, and environmental pollution is small.

The difference between combustion method and pyrolysis method three: different applicable conditions
Pyrolysis technology is widely used in municipal solid waste treatment, charcoal production, coal retorting, petroleum reforming, and carbon black manufacturing. Combustion technology is mainly used for municipal solid waste treatment.

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Corrosion caused by air preheater

First. Corrosion of air preheater

Air preheater corrosion and ash plugging are mutually reinforcing processes. When the air preheater is corroded, the surface finish of the heated element is seriously deteriorated, making ash more easily adhered to the heated element, which increases the possibility of the air preheater blocking ash. When the air preheater is corroded, the temperature of the primary and secondary air at the outlet of the air preheater under the same load will decrease due to the smaller heat exchange area, and the exhaust temperature will increase.

Second. the cause analysis of air preheater corrosion

1. The flue gas contains water vapor and SO3
During the combustion process, the sulfur in the main fuel coal is mainly converted into SO2 and SO3. Among them, SO3 reacts with the water vapor in the flue gas to generate sulfuric acid vapor. The dew point of sulfuric acid vapor is much higher than that of flue gas, so sulfuric acid vapor easily condenses on the low-temperature heating surface below the dew point of the flue gas, causing low-temperature corrosion;

2. The temperature of the cold end wall of the air preheater is low
In cold weather, the integrated temperature of the cold end of the air preheater is lowered, and the temperature of the wall of the heated element at the cold end is not only lower than the dew point of sulfuric acid vapor, but also lower than the dew point of water vapor in the flue gas. A large amount of water vapor and dilute sulfuric acid liquid condense on the heating element of the cold end of the air preheater, causing low temperature corrosion;

Third. The harm of corrosion to the air preheater
Corrosion of the heat storage element of the air preheater will greatly affect the heat transfer effect of the air preheater and reduce the economic operation of the boiler.

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Structure composition of garbage crane

The structure of the garbage crane is mainly composed of the bridge frame, the operating mechanism of the cart, the main lifting mechanism, the operating mechanism of the trolley, the grab, the weighing device and so on.

Bridge: The main beam and end beam of the bridge are in the form of box beams, which have light weight, and have high vertical and horizontal stiffness, and small wheel pressure is transmitted to the track beam;

Trolley running mechanism: The cart running device is designed on the welded box beam with corner bearing, which is connected to the end beam through a sturdy and reliable horn-type bearing, and the replacement is very convenient;

Main lifting mechanism: one motor drives four steel ropes on two drums to hoist a four-point hydraulic grab through two hard-tooth surface reducers, and the spacing between the four ropes is wide and narrow, making the grab with anti-sway characteristics , Accurate parking positioning;

Car operating mechanism: adopt centralized driving mode and “three-in-one” transmission device, with high transmission precision, light weight, good sealing, low noise and maintenance-free;

Grab: The grapple is planned to use multi-petal hydraulic grabs from foreign brands, with a low center of gravity. The connection between the crane and the grab is made of a four-point “V” arrangement, which reduces the twisting of the grab and the use of the grab and construction Collision between objects;

Weighing: A set of weighing devices (with two sensors) is set on the small frame of the garbage crane, and it has the functions of automatic peeling and semi-automatic weighing, display, accumulation, printing, etc. It is used for process measurement and overload protection.

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The function of garbage hoist of domestic garbage gasification power plant

The garbage crane is located above the garbage pond and is mainly responsible for the feeding, handling, mixing and weighing of garbage.

Feeding: When the garbage at the inlet of the garbage gasifier is insufficient, the garbage crane grabs the garbage
The fermented garbage in the pond runs to the top of the feed inlet and feeds the feed hopper of the garbage gasifier;

Handling: transport garbage close to the discharge door to other places in the garbage pond to avoid unloading
Congestion of the door, adjust the amount of garbage in the pool so that it can store less than 7 days of garbage;

Stirring: Due to the high water content of domestic garbage and low combustion calorific value, garbage
It is necessary to stay in the garbage pond for a certain period of time, through natural draining and fermentation to reduce the water content and increase the calorific value. The mixing of old and new garbage can shorten the fermentation time. In addition, due to the complex composition of domestic waste and large changes in the content of ingredients, in order to avoid excessive fluctuations in the nature of the waste entering the furnace and to ensure the stable combustion of the waste entering the furnace, it is also necessary to stir and mix the waste in the pool;

Fetching: Take out the objects that enter the garbage pond inadvertently but are not suitable for gasification;

Weighing: In order to count the actual gasification amount of garbage, the garbage put into the inlet is weighed before the garbage is put into the inlet of the gasifier.

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