In the process of urban sewage treatment, it is inevitable to produce sludge with high moisture content, which contains a lot of organic matter, rich nitrogen, phosphorus and heavy metals. With the improvement of sewage treatment rate and degree, the sludge output will inevitably increase, resulting in the increasingly serious problem of sludge disposal. Sludge is the inevitable by-product of urban sewage treatment plant. The common feature of sludge is that the water content is very high, generally up to 25 ~ 98%. It is huge and difficult to treat. The harm of sludge to the environment mainly includes deteriorating the water quality of water sources, polluting soil and crops, etc. With the improvement of people's living standards and the expansion of production scale, the harm of sludge as a pollutant is becoming more and more obvious. The applicable treatment technology of sludge should make the sludge volume reduced, stabilized and harmless.

2、 Sludge treatment technology and its development trend at home and abroad

1. Development trend of sludge treatment technology in EU countries

At present, the sludge disposal technology widely used abroad can be divided into three categories: ① land disposal, including sludge agriculture and application in forest or horticulture; ② Landfill alone or together with domestic waste; ③ Heat treatment.

Due to the available land area, treatment cost, increasingly stringent environmental standards and the popularity of resource recovery policies, EU countries generally recognize that the landfill disposal of sludge is not a sustainable development method, and many member states have formal or experimental restrictions on the physical properties of landfill sludge, This restriction is based on the consideration of promoting the utilization of sludge organic matter and reducing landfill methane gas and leachate. There are also views that thermal drying of sludge is a solution to overcome this limitation, but in countries that limit the total amount of organic matter in landfill sludge, this is only a short-term remedy. Therefore, in the near future, Germany, France, Denmark and other countries, only sludge incineration ash may be the form of sludge suitable for landfill.

2. Shidongkou sludge drying incineration system

Shanghai Shidongkou sludge drying incineration plant is the first project example of sludge incineration treatment in China. It incinerates 220 tons of dry sludge per day. It adopts the process route of sludge concentration mechanical dehydration fluidized bed drying fluidized bed incineration. The equipment is mainly imported equipment. It was officially put into operation at the end of 2004, opening up a new route of domestic sludge treatment and disposal.

Because the combination of sludge drying and sludge incineration has less one-time investment and low treatment cost than single sludge incineration, sludge drying is often the pretreatment of incineration. Sludge drying can control the moisture content of sludge at 10% ~ 40%, reduce the volume and weight of sludge, reduce the transportation fee and landfill fee, and greatly reduce the odor of sludge.

The sludge treatment system of Shanghai Shidongkou sludge drying incineration plant is mainly composed of fluidized bed sludge drying device, fluidized bed sludge incinerator and flue gas purification device. The heat required for sludge drying is provided by the waste heat of sludge incineration after drying. Without auxiliary fuel, the system can realize heat self balance. From the perspective of sludge harmlessness and reduction, the incineration scheme has obvious advantages. After incineration, a small amount of mud ash can be used for concrete, brick and tile products, aggregate of subgrade and pavement and backfill of engineering construction.

According to the sludge characteristic test that has been carried out, the sludge (dry) analysis base has many similarities with lignite in physical properties, elemental analysis, industrial analysis and low calorific value, especially the ash content and low calorific value are similar, and the content of fixed carbon is much lower, so it can be used as low-grade fuel.

At the same time, the incineration characteristics of sludge are different from that of coal. It has lower decomposition temperature, ignition temperature and burnout temperature, and the time required for complete combustion is also less. The main reason for this difference is that they have different composition and structure. Sludge is mainly composed of low-level organic substances, such as amino acids, humic acids, bacteria and their metabolites, polycyclic aromatic hydrocarbons, heterocyclic compounds, organic sulfides, volatile isoodors, organic fluorides, etc. its structure is relatively simple, and has undergone secondary biological oxidation, which is decomposed and destroyed to varying degrees and is easy to decompose at high temperature; Coal is mainly composed of polycyclic aromatic hydrocarbon network, with dense structure and high carbon content. It is not easy to decompose under high temperature and takes a long time to decompose.

In addition, the release temperature of SO2 and NOx in sludge is lower than that of coal. Generally, the sulfur in sludge exists in the form of organic sulfur. The content of pyrite sulfur and sulfate sulfur is small, while the content of organic sulfur in coal is low. Pyrite sulfur and sulfate sulfur account for the majority. Organic sulfur is easy to decompose and volatilize under high temperature, while pyrite sulfur is firmly combined, and decomposition requires a process. Therefore, sludge is easy to release SO2. The test shows that under the condition of the same sulfur content, the SO2 released by sludge is 4 times higher than that of coal. Similarly, nitrogen in sludge mainly exists in the form of organic nitrogen (protein nitrogen, low-grade fatty amine, etc.), and organic nitrogen is easy to volatilize at high temperature; The nitrogen in coal mainly exists in the form of heterocyclic nitrogen. The decomposition of heterocyclic nitrogen needs a process. The difference of this structural form determines the difference of nitrogen decomposition temperature.

From the above analysis, it can be seen that dry sludge is easier to burn than coal. However, the dehydrated sludge contains 70% ~ 80% water, and the combustion conditions are completely different (the low calorific value of dehydrated sludge is only 2713 ~ 971 kJ / kg). In order to make the dehydrated sludge burn directly in the boiler, it is generally necessary to add an appropriate amount of auxiliary fuel (coal or oil) to stabilize the combustion, but this requires a large amount of coal or oil and other energy.

The sludge disposal of most domestic sewage treatment plants is mainly carried out for landfill, mainly including:

Some are transported to the domestic waste landfill for combined treatment. Due to the high moisture content, the operation of the landfill is difficult, and a large amount of landfill capacity will be occupied; Some are dumped everywhere, resulting in secondary pollution; Some are also directly applied to farmland as agricultural fertilizer. Because the directly dewatered sludge has not been treated at high temperature, it contains a large number of pathogenic bacteria, which will affect people's health. In short, a separate dehydration and outward transportation process can not be called a qualified sludge treatment and disposal process.

The process is relatively improved. After digestion and treatment, the sludge kills most pathogenic bacteria, and can also produce biogas for production and domestic use. It is basically harmless, and it also has certain resource significance. However, the problem of disposing dewatered sludge with high moisture content still needs to be solved by this process.

Although the theoretical calculation of the process is feasible and has been adopted in some small sewage plants, the actual operation effect is not good. The main reason is that the equipment is immature, the operation management is difficult, a large amount of auxiliary fuel needs to be added, and the operation cost is high.

The thermal method of drying dewatered sludge has been adopted in some sewage treatment plants, and the operation is acceptable. Because the drying process needs a lot of heat, it has a relatively high operating cost.

2. Sludge drying and incineration is a feasible and reasonable scheme at present

Whether from economic factors or fertilizer utilization factors, sludge land use, especially sludge agriculture, is a disposal method in line with China's national conditions. However, the content of some heavy metals in sewage sludge is close to or exceeds the pollutant control standard of agricultural sludge in China, which partially limits the agricultural application of sewage sludge. Therefore, while considering the use of agricultural sludge, we must find other suitable sludge disposal methods.

The drying and incineration of sewage sludge is an important research topic at home and abroad in recent years. According to the heating mode, thermal drying can be divided into direct and indirect heating. The direct heating mode has high thermal efficiency. Its drying process is generally realized by rotary cylinder dryer and belt fluidized bed.

Thermal drying is to dry the sludge with heat energy. The dried sludge is in the form of particles or powder, and the volume is only 1 / 5 ~ 1 / 4 of the original. Moreover, since the moisture content is below 10%, the microbial activity is completely inhibited to avoid the mildew and odor of the product, and its high-temperature sterilization is relatively complete.

Therefore, according to the successful experience of Shidongkou sewage treatment plant in the combined treatment of low-temperature drying and high-temperature incineration of dewatered sludge, the relatively mature fluidized bed drying process is adopted to evaporate the water in the sludge at a lower temperature and reduce the water content with less heat. The circulating fluidized bed incinerator is adopted for incineration. The heat in the flue gas is recovered in the form of heat transfer oil (or steam) by incineration of dried sludge, and the recovered heat is used in the drying system to improve the thermal efficiency of the system and keep the heat of the whole system in a balanced state, which is a feasible and reasonable scheme at present.

In order to completely change the practical problems of large investment and high operation cost of imported equipment for sludge treatment, on the basis of summarizing the existing projects and existing technologies, according to the successful experience of Shidongkou sewage treatment plant in the combined treatment of low-temperature drying and high-temperature incineration of dewatered sludge, and after localization improvement, summarize its advantages of long-term engagement in boilers and drying equipment, and learn from the experience of foreign sludge treatment, Based on the pragmatic principles of "treating waste with waste" and "tailor-made", a sludge resource utilization device suitable for China's national conditions has been developed, which has obtained the national patent (zl2004 2 0054749.6). After practical application of several sludge treatment projects and passing the national environmental protection test, it has been determined as one of the sludge treatment technologies to be promoted at the national level.

Purpose of sludge treatment: completely harmless, no long-distance transportation is required, the internal heat is fully utilized to produce benefits, and the volume is greatly reduced (more than 95%), so as to truly achieve harmless, reduction, stabilization, resource-based and low-cost treatment, without worries about environmental protection.

e. Minimize the floor area of sludge treatment system and arrange flexibly according to the site conditions of the enterprise.

e. Designed combustion calorific value of auxiliary coal: 4500 ~ 5500kcal / kg (ordinary fuel coal in the market).

a. Sludge feeding system (2 sets): it is composed of sludge belt conveyor, sludge chute, special sludge feeder, etc. Sludge conveyor and feeder adopt remote stepless speed regulation control;

b. Sludge drying system (2 sets): it is composed of sludge drying chamber, mechanical crushing device, enhanced heat transfer device, etc. Firstly, the sludge enters the sludge drying chamber with enhanced heat transfer device and enhanced mixing device, the hot flue gas contacts with the wet sludge for heat transfer and drying, and the dried sludge automatically enters the collection system;

c. Sludge collection and conveying system (2 sets): the dried sludge powder is collected by the high-efficiency fine powder collection system, continuously discharged into the intermediate bin by the automatic discharge valve, sent to the feed inlet of the incinerator by the dry sludge conveyor, and sent to the incinerator for incineration by primary air;

d. Sludge incinerator and secondary combustion regulating greenhouse (1 set): the most advanced "shallow bed fluidized bed technology" (SFBC) is adopted to make the particles of sludge dry powder and hot bottom material in the fluidized bed boiling and fully mixed. The sludge can be incinerated rapidly in a short time to form self-sustaining and stable combustion with high combustion efficiency. The secondary combustion air and mixed air are injected into the secondary combustion greenhouse with high-speed swirling flow, resulting in strong disturbance, forming strong secondary combustion and further improving the combustion efficiency.

e. Auxiliary coal feeding system (1 set): it is mainly used for the initial stage of system startup and bottom material circulation supplement. It is composed of vertical lifting device, hopper, screw feeder, etc. The auxiliary coal and circulating bottom material are poured into the lifting bucket by the trolley, and the materials are sent to the hopper by the vertical lifting device. The feeding amount of the feeder is remotely controlled by the console.

f. Balanced ventilation system (1 set): it is composed of incinerator blower, primary and secondary air supply system, induced draft fan, chimney, etc. The drying device, fine powder catcher, induced draft fan and chimney are equipped with fully sealed metal flue. The external of high-temperature flue is equipped with thermal insulation materials, and the internal and external anti-corrosion treatment of low-temperature flue.

g. Electrical instrument system (1 set): a TV monitor is set above the sludge chute, and the operator can start the conveyor according to the sludge level in the chute. Temperature measuring points are set in the sludge incineration chamber (3 places), secondary combustion regulating greenhouse and drying chamber (3 places) to monitor the sludge incineration and drying in real time. Pressure monitoring devices are set in the incineration room and drying room to ensure the negative pressure operation of the system. The induced draft fan is depressurized and started, and all electric equipment is centrally controlled by the console.

h. Flue gas purification and washing system (1 set): after the dusty flue gas passes through the sludge dryer and fine powder collector, the dust content has been very low. In order to adapt to the increasing environmental protection emission standards, a flue gas washing device is set in front of the induced draft fan to extract the clarified wastewater for wet washing, desulfurization and dust removal of the flue gas, so as to further remove the small particle dust and further improve the dust removal efficiency, At the same time, adjusting the alkalinity in the wastewater can react with SO2 in the flue gas to achieve the triple effect of deacidification, deodorization and dust removal, and ensure that the flue gas is discharged up to the standard.