At present, the treatment and disposal technology of sludge mainly refers to the processing of concentration, regulation, dehydration, stabilization, drying or incineration of sludge to achieve sludge reduction, stabilization and harmlessness. At present, the commonly used sludge treatment and disposal technologies include anaerobic digestion technology, aerobic fermentation technology, deep dehydration technology, thermal drying technology, lime stabilization technology and incineration technology.
1. Anaerobic digestion technology
Anaerobic digestion of sludge refers to the process of decomposing biodegradable organic matter in sludge into carbon dioxide, methane and water by facultative bacteria and anaerobic bacteria under anaerobic conditions to stabilize the sludge. It is one of the commonly used means of sludge reduction and stabilization. Anaerobic digestion of sludge has the advantages of reducing sludge volume, stabilizing sludge properties, and generating methane gas.
Traditional sludge anaerobic digestion has the disadvantages of slow reaction, low degradation rate of organic matter and low methane production, which limits the advantages of anaerobic digestion technology. According to Bryant’s three-stage theory, hydrolysis is the rate-limiting step in sludge anaerobic digestion. Therefore, since the 1970s, various sludge anaerobic digestion enhancement technologies including high temperature thermal hydrolysis, ultrasonic pretreatment, alkaline hydrolysis pretreatment and ozone pretreatment have been studied. The cell wall of the sludge transfers the intracellular organic matter from the solid phase to the liquid phase, promotes sludge hydrolysis, and improves the anaerobic digestion effect of the sludge.
As the amount of sludge in various countries continues to increase and the demand for energy and the quality of treated sludge continue to increase, some of the original sludge anaerobic digestion facilities are facing expansion and transformation. Sludge pretreatment technology can improve the anaerobic digestion effect of sludge, improve sludge dewatering effect and increase biogas production. It can replace the benefit of digestive tank expansion to a certain extent, so it has been widely used in research and application. Among them, high temperature thermal hydrolysis technology is relatively mature. At present, this technology has developed various processes such as Cambi thermal hydrolysis, Biothelysis thermal hydrolysis and Monsal enzymatic hydrolysis. It has been promoted and applied in Europe in recent years, Norway, Britain and Australia. There are cases of successful application.
In view of the limitation of low solid content of traditional sludge anaerobic digestion, research on high-solid sludge anaerobic digestion technology has also become a hot spot. The advantage of high-solids sludge anaerobic digestion is that the biogas production efficiency is higher than the traditional anaerobic digestion, because the solid content of the mud is greatly increased, and the amount of organic matter that can be contacted and digested by the unit microbial biomass in the anaerobic digester is greatly improved. Its gas production efficiency and processing load also increase. At present, a variety of high-solid sludge anaerobic digestion technologies have been developed in foreign countries, and have been applied in practical projects. For example, the Finnish HLAD process controls the sludge solid content into the pre-reaction tank by 10% to 15%. Gas production efficiency is 30% higher than traditional sludge anaerobic digestion.
The anaerobic digestion process can kill some pathogenic bacteria and parasite eggs, so that the sludge is stabilized and not rancid.
The anaerobic digestion process produces biogas, which enables efficient recovery of sludge biomass energy.
Anaerobic digestion can degrade 35%~50% volatile solids in sludge, reduce the dry solids of sludge, and help reduce the cost of subsequent sludge treatment and disposal. At the same time, anaerobic digestion helps to improve sludge dewatering performance, and the sludge volume can be further reduced after dewatering.
Maintaining the temperature required for anaerobic digestion requires a significant amount of heat.
The sludge anaerobic digestion process has a long residence time, usually reaching 20~30d, resulting in a large volume of anaerobic digestion tank and complicated operation and management.
Methanogenic bacteria require high environmental conditions, and must be cultured in the start-up phase, with initial initial debugging time.
After anaerobic digestion, the moisture content of the sludge is still high, and must be followed up. Commonly used methods are heat drying and deep dewatering.
2. Aerobic fermentation technology
Aerobic fermentation is a process in which, under aerobic conditions, microorganisms oxidize a portion of the absorbed organic matter into a simple inorganic substance through absorption, oxidation, decomposition, etc., while releasing the energy required for microbial growth activities; and another part of the organic matter. It is then synthesized into a new cytoplasm, allowing microorganisms to grow and multiply, producing more organisms. High-temperature aerobic fermentation of sludge continuously decomposes organic matter, so that the temperature of the heap is continuously increased, and the pathogenic bacteria and parasitic eggs can be killed and rendered harmless. The high-temperature aerobic fermentation of sludge is called compost and can be used as a soil improver and organic fertilizer. High-temperature aerobic fermentation of sludge should pay attention to the problem of heavy metal pollution of sludge. During the treatment, due to the action of aerobic bacteria, it is easy to produce malignant odor. Therefore, odor pollution should be prevented. The moisture content of sludge after treatment can generally be less than 40%.
At present, the main process forms of sludge composting are divided into static and dynamic, and can be divided into strip type and warehouse type according to the material stacking form. Other reactors in the form of reactors are generally used in small projects.
There are five main aspects that restrict the industrial application of sludge composting technology: floor space, secondary pollution caused by odor discharge, dry material feeding and safe storage and transportation, final sludge disposal and operator health and safety. problem. Compared with the static composting process, the dynamic composting process has made certain breakthroughs in these five aspects. On the basis of mechanical turning technology and forced venting static composting technology, China has developed a complete set of CTB automatic control biological composting technology with independent intellectual property rights. This technology composts short time, small floor space, no odor, waste water, etc. Environmental pollution problems, technical integrity and support. The newly developed sludge aerobic composting process (SACT) is based on the theory of horizontal open fermentation tanks, which is improved and innovated through the construction of structures, improvement of mechanical turning equipment, and adoption of automatic entry and exit systems. The theory of automated composting system further reduces investment and operating costs in engineering applications and achieves good engineering results.
The aerobic fermentation process can kill pathogenic bacteria and weed seeds in the sludge and achieve harmlessness.
The aerobic fermentation process can degrade most of the organic matter in the sludge and reduce the moisture content of the sludge to 40%, which is reduced.
The fertilizer produced after aerobic fermentation can be applied to the land to achieve resource utilization.
No external heating source is required and the operating cost is relatively low.
The sludge mud is unstable, and heavy metals are difficult to stabilize. The surface is narrow and can only be used as a landscaping fertilizer.
A large amount of odor is generated during the composting process, which pollutes the surrounding environment.
3. Deep dewatering technology
In recent years, sludge dewatering technology has been greatly developed, and technological breakthroughs have focused on the development of conditioning agents before sludge dewatering and the improvement of dewatering machinery and equipment. At present, using sludge chemical conditioning + mechanical dewatering technology, sludge conditioning agent is added to the wet sludge raw mud to destroy the microbial structure of the sludge floc and dissolve, and the physical combined water is separated. The moisture of the organism of the microorganism is separated, and finally the majority of the bound water in the sludge is converted into free water, and the free state water is separated by a mechanical dehydration device. The technology has been applied in large scale in sludge treatment of tannery sewage treatment plant. The moisture content of sludge after treatment can be less than 50%. In order to meet the requirements of sludge subsequent treatment and disposal, it is necessary to further reduce the moisture content of conventional mechanical dewatered sludge. .
The quenching and tempering treatment of sludge is the key link and core technology for deep dewatering of sludge. It can be said that sludge conditioning technology determines the success or failure of sludge deep dewatering project. There are many methods for quenching and tempering domestic sludge. It is common to add dehydrating agent, flocculant or coagulant to the sludge to change the existence and structure of water molecules (mainly interstitial water and capillary water) in the sludge. Conducive to the separation of water and mud under certain conditions. Commonly used conditioning agents are ferric chloride (or ferric sulfate, polymeric ferric sulfate) plus quicklime.
The sludge deep dewatering equipment is mainly a high pressure diaphragm plate frame filter press. By applying pressure from the outside of the sludge, the moisture in the quenched and tempered sludge is leached and separated to obtain a mud cake having a high solid content. There are many domestic equipment manufacturers, and the brand awareness is higher. The equipment performance is better in Shandong Jingjin and Hangzhou Xingyuan. The equipment manufacturing process and quality have been improved.
Technical advantages: good reduction effect, low energy consumption, small floor space, short construction period and short processing time.
Technical disadvantages: Ferric chloride is highly corrosive, and quicklime is easy to scale. As a result, the high-pressure diaphragm frame filter press has high operation and maintenance costs, stable and insufficient sterilization, slightly odor, and the organic matter content in the sludge is not reduced.
4. Sludge thermal drying technology
The thermal drying of sludge refers to the process of removing moisture from the sludge through the heat transfer between the sludge and the heat medium. The sludge thermal drying system mainly includes the storage and transportation system, the drying system, the exhaust gas purification and treatment system, the electrical self-control instrument system and its auxiliary system.
According to the degree of drying of the sludge, it is divided into two types: full drying and semi-drying. “Full drying” refers to the type of higher solid content, such as the solid content of sludge after drying is more than 85%; Semi-drying mainly refers to the type of sludge with a solid content of about 45 to 60% after drying. In the form of sludge drying, the sludge is divided into direct drying and indirect drying. Direct drying is to directly contact the sludge with a hot drying medium (such as flue gas) to transfer heat in a convective manner. And take away the evaporated water, also known as the heat convection drying system; indirect drying is the use of conduction means by the heat medium (such as steam) through the metal wall to transfer heat to the sludge, the evaporated water through the carrier gas (such as air Take away and wash the condensation, also known as the heat transfer drying system.
At present, the drying machines used for sludge drying mainly include: convection heat transfer fluidized bed dryer, drum dryer, belt dryer, conduction heating rotary disc dryer, paddle Dryer, turbofilm drying machine combined with convection and conduction heating.
The sludge is significantly reduced in volume and the volume can be reduced by about 4 times.
After the drying treatment, a stable product can be formed, and the sludge property is greatly improved.
The dried product is odorless and free of pathogens, reducing the negative effects associated with sludge and making the treated product more acceptable.
The dried product is used for a variety of purposes, such as fertilizers, soil amendments, alternative energy sources, and the like.
Large investment, high energy consumption and high operating costs.
Drying at high temperatures is prone to odor.
The dust control in the drying process is strict and there are safety hazards.
5. Lime stabilization technology
By adding a certain proportion of quicklime to the dewatered sludge and uniformly blending, the quicklime reacts with the moisture in the dewatered sludge to form calcium hydroxide and calcium carbonate and release heat. Lime stabilization technology can effectively deodorize, sterilize, inhibit corrosion, dehydration, and passivate heavy metal ions.
The typical process is as follows: sludge with a water content of 80% is sent from the screw conveyor to the silo for temporary storage, and the sludge and quicklime are respectively sent to the material reaction system according to the ratio of mass ratio of 4:1 by the metering and conveying device. In the material reaction system, the sludge and quicklime react and react, so that the temperature in the system is rapidly increased to 100 degrees, and the water in the sludge is evaporated in a large amount to complete the drying and dehydration process of the sludge. The dried sludge is transported to an outdoor stacking shed by a double screw mixer for storage and storage. In order to prevent secondary pollution in the sludge drying process, the treatment of the discharged lime dust and malodorous gas can be realized by adding dust removal and deodorization equipment.
In practical applications, in addition to adding lime, other excipients are often added to enhance the effect. Some of these excipients contain N, increase the production of NH3 gas, strengthen sterilization and facilitate land use; some are strong acid iron and aluminum salts. The reaction temperature is increased and the ratio of solid inorganic components is more suitable for building materials utilization; generally it is acidic, in addition to increasing the exotherm, the pH can be moderately adjusted, and the common effect is to reduce the amount of lime added and save costs.
The investment is small, the running cost is low, the floor space is small, and the operation and management are simple.
It can effectively eliminate bacteria and there is no risk of bacterial regeneration.
The dried product is rich in a large amount of calcium hydroxide, silicon oxide, calcium carbonate and the like, and can be used as a base material for building materials, a road infrastructure auxiliary material, a cushion soil for a landfill, and a backfill for road construction.
The lime stabilized sludge has a high pH value and can be used as a desulfurizing agent for incineration equipment.
Especially suitable for emergency or phased disposal.
Due to the large amount of lime added, the degree of reduction is not high compared to other processes.
It is strongly alkaline, with low land use value and narrow surface.
The drug use fee is high.
6. Sludge incineration technology
Sludge incineration refers to the conversion of organic matter in sludge to CO2, H2O, N2, etc. under a certain temperature and sufficient aerobic conditions. The heat released during the reaction is used to maintain the temperature conditions of the reaction. Sludge incineration is the most thorough sludge treatment method. It removes all organic matter, kills all pathogens, and minimizes sludge volume.
Sludge incineration generally adopts fluidized bed process and is divided into fixed (bubble) fluidized bed incinerator, circulating fluidized bed incinerator and rotary fluidized bed incinerator. The flue gas from sludge incineration should be treated and meet the relevant regulations of the Domestic Waste Incineration Pollution Control Standard. The ash from sludge incineration and the fly ash collected by dust removal equipment should be collected, stored and transported separately. The state encourages the comprehensive utilization of slag that meets the requirements: fly ash needs to be properly disposed of after identification. The sludge incineration process is widely used in large and medium-sized cities with more developed economies. Generally, the combined use of dry incineration is used to improve the thermal energy utilization efficiency of the sludge.
The sludge is treated by incineration to maximize the reduction, stabilization and harmlessness.
The ash after incineration can be directly or treated with a heavy metal chelating agent to enter the landfill depending on the heavy metal content, and can also be used as building materials or paving.
The investment is large and the equipment maintenance cost is high.
The sludge itself has a low calorific value and needs to be mixed with domestic garbage and coal, and the running cost is high.
Exhaust gas emissions are large, and it is easy to cause secondary pollution.
7. Comprehensive evaluation
The goal of sludge treatment is to reduce, stabilize, and detoxify. To be used as a resource is a yearning, and there is a long way to go with reality. It is generally “incapable” and cannot be a goal that must be achieved. Each treatment method has different advantages.
The degree of reduction, heat drying and incineration are the highest. Lime stabilization and aerobic fermentation (composting) are at a considerable level, generally up to 35% (aerobic fermentation has a large variation of 15~40 due to different ratios of different processes, different strains and straw-based excipients). %). It is worse than deep dehydration (42%) after chemical conditioning, but the moisture content of the product is lower (because the amount of solids injected is larger), the digestion + semi-drying is 50%, and the drying incineration is 95%, but the economic input is Much bigger.
The degree of stabilization, heat drying and incineration are the most thorough. Lime stability, aerobic fermentation, anaerobic digestion can also achieve a good degree of stability, there is still a gap in deep dehydration.
The degree of harmlessness, heat drying and incineration kill the pathogens most thoroughly. The aerobic fermentation and lime stabilizing ability are second, and the anaerobic digestion and deep dehydration ability are slightly insufficient.
In terms of land occupation, lime stabilization and deep dewatering technology have a short processing time and occupy the least amount of land. Aerobic fermentation and anaerobic digestion generally take a long time and occupy a large area of land.
In terms of economy, lime stabilized construction investment and operating costs are the lowest, anaerobic digestion, aerobic fermentation, deep dewatering, thermal drying and incineration technology investment and operating costs are higher.
There is no best in the choice of sludge treatment technology, only the most suitable. According to different regions and different sludge types, considering the climate, regional characteristics, construction conditions, etc., it is an ideal choice to combine various processes to achieve the best results.