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Comparison of sludge disposal methods
Mar 22, 2018

Common disposal methods for sludge include methods such as landfill, fertilizer production, and desiccation. Various methods have advantages and disadvantages. In order to achieve “minimization, stabilization and harmlessness” to the greatest extent, most Western developed countries adopt sludge incineration technology to properly treat sludge, which will be introduced in this article.

Sewage treatment and sludge treatment are two systems that are equally important and closely related to solving urban water pollution problems. Urban sewage treatment plants produce a large amount of biological activated sludge while purifying sewage. Many chemical and pharmaceutical industries also produce sludge wastes, which are characterized by high water content, instability, perishableness, and malodor. Unsuitable treatment of sludge into the environment will cause secondary pollution to water bodies and the atmosphere, posing a serious threat to the ecological environment. Therefore, the sludge must be professionally treated using effective methods and mature technologies.

Sludge common disposal methods

At present, there are four types of sludge disposal methods:

Landfill

Sanitary landfilling is simple in operation, low in cost, and the organic matter content of the sludge after digestion is reduced, the performance is relatively stable, and the total volume is reduced. After dehydration, landfilling has become an economical sludge treatment method. In view of the current domestic economic development, sludge landfill will continue to exist for a long time.

However, the moisture content of dewatered sludge is often much higher than the 30% moisture content required in sanitary landfills for general household waste. It must be reprocessed before it can be sent to a landfill site for landfill, or based on the moisture content and physical and chemical properties of the sludge. Characteristics and other factors, set a dedicated sludge landfill. At the same time, the dedicated sludge landfill will have problems such as large area, difficult site selection, and difficulty in treating leachate, which may affect groundwater quality and other hidden safety problems. Once improperly disposed, it may cause secondary pollution.

Fertilizer utilization

Sludge-making fertilizer used to be the main way of sludge utilization. Its essence is the use of aerobic microorganisms in the sludge to oxidize and decompose many organic substances in the sludge and convert it into humus which is easily absorbed by plants. Therefore, biological energy can be obtained. Use, energy can be saved.

However, in recent years, with the requirements of green foods and vigilance against soil pollution, the standards for sludge fertilizers for agriculture have become increasingly stringent, and due to their inconvenience in use and poor fertilizer efficiency, they cannot compete with chemical fertilizers. Sludge has been used as agricultural fertilizer. Unsustainable. In addition, due to concerns about heavy metals, detergent additives, etc., this type of disposal is shrinking.

Drying

Sludge drying technology refers to the use of heat to destroy the gel structure of sludge and sterilize and sterilize the sludge. Drying temperatures as high as 95°C or more, in addition to effectively killing pathogens, can also significantly reduce sludge volume and eliminate odors.

However, the drying treatment process has high energy consumption, complicated equipment, high investment and operating costs, and incomplete reduction. Moreover, measures such as condensation and deodorization of evaporative substances are also required, and the overall cost is high. Sludge after drying or sent to incineration or fertilizer, there are still problems, generally less used.

Incineration

Sludge incineration is a high-temperature heat treatment technology that utilizes high-temperature oxidation combustion reaction. Under the conditions of excess air, the entire organic matter and pathogens of sludge are oxidized, pyrolyzed and completely destroyed at 850~1100°C.

Sludge incineration has the advantages of small area, rapid treatment, large amount of treatment and obvious reduction, and the reduction capacity can be more than 90%. The incineration ash can be directly or after treatment with heavy metal chelating agent according to its heavy metal content. Landfills can also be used as building materials or paving. The use of incineration to treat sludge can achieve “minimization, stabilization, and harmlessness” to the maximum extent, and is the most thorough method for sludge treatment. This method is commonly used in western developed countries. Sludge incineration challenges and difficulties

With the continuous development of the economy and the strengthening of environmental protection, sludge treatment will become another emerging industry. However, the current domestic sludge incineration technology is still immature and needs to be improved; at the same time, governments, enterprises, and other social organizations are needed. The close cooperation of policies, technologies, funds and other series of industry elements.

The moisture content of the sludge is a key factor that restricts the operating cost of the sludge disposal system. Therefore, it is necessary to reduce the moisture content of the sludge through pretreatment.

Sludge incineration will produce environmental pollutants such as acidic flue gas, ash and fly ash, which require follow-up treatment, must be strictly controlled and meet discharge standards, and are also key technologies.

According to the difference in heavy metal content, ash can be considered as follows: direct landfill or cement green brick; after adding heavy metal chelating agent to landfill.

Fluidized bed incineration process

The sludge incineration system is mainly composed of the following operation units: sludge transport system, sludge pretreatment system, fluidized bed incinerator, waste heat steam boiler, flue gas treatment system, ash collection and transportation system and so on.

Sludge transport system: to achieve the transfer of sludge operation

For sludge from sewage treatment plants, because of its high moisture content, the sewage sludge is in a liquid state. Therefore, a screw pump with a continuously adjustable flow rate is generally used, and the sludge is sent to a centrifuge for dehydration. For the grating slag and mud cake dewatered by the sludge, the state is generally paste-like and is operated by a conveyor. There are generally two types of shaft-less screw conveyors and belt conveyors.

Sludge pretreatment system: reduce the moisture content of sludge

The moisture content of sludge is a key factor restricting the cost of sludge treatment. In terms of the incineration disposal method, the higher the moisture content of the sludge, the lower the calorific value, the greater the energy consumption of the system, and the higher the operating cost. Commonly used methods of sludge dehydration are belt-type filter press dewatering, plate and frame filter dewatering and centrifugal dewatering.

After the sludge is mechanically dewatered, its water content can be reduced to 80% to 85%, and can be directly sent to the fluidized bed incinerator for treatment.

Fluidized Bed Incinerator: Key Equipment for Sludge High Temperature Incineration Oxidation

The primary incinerators used for sludge incineration mainly include bubbling fluidized bed incinerators, circulating fluidized bed incinerators, and rotary kiln incinerators.

In the initial stage, the front-end burner heats the quartz sand (ie, bed material) in the fluidized bed incinerator and the furnace to a suitable operating temperature of about 850 to 870° C., and then the sludge is stabilized and evenly distributed by specially designed feeding equipment. Into the fluidized bed for processing.

The lower part of the fluidized bed incinerator is equipped with a wind box made of multiple nozzles. Combustion air is blown upward into the incinerator through the nozzle, and the bed material quartz sand and sludge are evenly fluidized under the appropriate air volume. Sludge is evenly decomposed by heat to create conditions. On the other hand, the sludge enters the fluidized bed and is immediately blown away by a large amount of high-temperature inert bed material, so that the bed sand does not have problems of adhesion.

There is a secondary combustion zone with a secondary air introduction system in the middle of the fluidized bed incinerator. The introduction of secondary air forms a staged combustion, plus enough space in the combustion zone to ensure that there are enough particles of flue gas and fly ash. The residence time reaches full combustion. The introduction of the secondary air adopts toroidal layout, and the proper ratio is beneficial to strengthen the combustion effect of the incinerator, increase the boiler output and reduce the generation and emission of NOx.

At the same time, the fluidized bed incinerator can increase or decrease the bed material and remove the slag at the bottom of the bed in real time according to the working requirements and working conditions to avoid parking, and the continuity of the system operation can be realized.

If the sludge is defined as hazardous waste, it is necessary to set up a second combustion chamber so that the flue gas stays at 1100°C for more than 2s to meet the requirements of the "Technical Requirements for Centralized Incineration and Disposal of Hazardous Wastes" of the Huanfa 2004-15. .

Waste Heat Steam Boiler: Recycling heat energy from incineration

The waste heat boiler is used to recover waste heat and generate saturated steam from the high-temperature flue gas generated by the incineration. The designed non-standard boiler can adapt to the characteristics of sludge incineration products. The waste heat boiler adopts a vertical flue, and the high-temperature flue gas flows through different flues in sequence, and a membrane-type water-cooled wall is used between each flue. The water in the water wall pipe absorbs waste heat and turns into wet steam. After the steam and water separation, saturated steam is formed, and it enters the steam pipe network for use.

Flue Gas Treatment System: Effectively treats post-incineration flue gas to meet emission standards

The complete flue gas follow-up treatment system is composed of cooling unit, desulfurization unit, dust capture unit and other control units. Combined with the corresponding control scheme, various controlled substances such as nitrogen oxides, sulfides, acidic substances, heavy metals, and dust generated from incineration, or even two Efforts such as treatment of odors, emissions can meet local regulatory requirements.

The advantages of Longsha Project

In terms of sludge incineration technology, Longsha Engineering can provide overall solutions, including basic research, process design, project management, construction and construction.

The Lonza project is committed to the application of Western advanced technology, to maximize the realization of domestic construction, to meet user needs with reasonable cost and excellent quality.

With the existing process innovation technology and more than 110 years of experience in chemical production, Longsha Engineering has successfully completed many important projects, and has cooperated with some advanced European companies in environmental technology and has reached a concession agreement.

Longsha Engineering has strong process design capabilities and technological integration capabilities. It has established partnerships with leading companies in the industry. In foreign countries, especially in Europe, there are a large number of projects of the same type that can be visited and inspected.

Taihu Boiler, a domestic partner, has strong design and manufacturing capabilities for equipment localization and engineering construction capabilities, providing a strong guarantee for reducing system investment.

summary

Fluidized bed burning sludge is a better solution. Due to strong turbulence, long gas residence time, uniform temperature distribution, high efficiency incineration at relatively low temperature and less excess air, high damage and removal rate of harmful substances, removal of harmful substances The use efficiency of the agent is high, and through staged combustion, the generation of NOx can be greatly controlled; by adding basic substances such as limestone in the bed, the content of acid gases (SOx and HCl) in the flue gas can be greatly reduced. By recovering sensible heat in the combustion flue gas as much as possible, the amount of auxiliary fuel added can be reduced, thereby minimizing the cost of sludge treatment.