Discussion on comprehensive treatment technology of sludge in municipal sewage treatment plant.

The following is the comprehensive sludge treatment process of urban sewage treatment plant brought by Zhong Da Consulting for your reference.

Since 1980s, modern sewage treatment plants have been built on a large scale in China, but sludge treatment has been neglected. In sewage treatment plants in developed countries such as Europe and America, the investment in sludge treatment and disposal accounts for 50% ~ 70% of the total construction investment. China, only 20% ~ 50%, China's current investment in sludge treatment is still far from enough compared with western countries.

1. Introduction

The output of sludge is easily influenced by factors such as sewage treatment capacity, sewage source, treatment technology, treatment level and sludge dehydration degree. Combined with the rapid development of sewage treatment industry in China, the construction and operation of sewage treatment plants are expected to increase substantially. In the next decade, sludge production will increase, and sludge treatment will become a more prominent environmental problem. Only by solving this problem can China achieve sustainable development.

2. The characteristics of sludge are mainly pollutants.

The definition of sludge has always been controversial. The original definition of EPA in the United States: sludge refers to solid, semi-solid or liquid residues obtained during sewage treatment. The World Water Environment Organization renamed sewage sludge as "biosolids" to accurately reflect the utilization value of sewage sludge.

2. 1 Characteristics of sludge.

(1) is rich in organic nutrients, such as nitrogen, phosphorus and potassium, and various trace elements needed by plants, such as calcium, magnesium, zinc and iron.

(2) It contains more pathogenic bacteria and parasite eggs.

(3) The content of heavy metals is high. Most sludge is rich in copper, zinc, cadmium, chromium, mercury, lead and so on.

(4) Organic matter and silicate clay minerals constitute the main components of sludge, which can be divided into three categories according to their proportions: one is organic sludge silicate clay minerals; Second, soil sludge, silicate clay minerals and organic matter are more than organic matter; Third, organic soil sludge, silicate clay minerals and organic matter are equivalent. When there is organic matter in sludge, there is effective calorific value. The research shows that the dry basis calorific value of sludge is11~16mj/kg, and it can spontaneously ignite when the water content is lower than 75%.

(5) Black or dark brown, with small solid particle size, mostly less than 25 microns, with a dense bulk density of about 0.7g/cm3 and a ratio of 60% to 70% with water. When the water content is less than 50%, it is regarded as clay. After drying, it is a relatively hard solid, which is not easy to deteriorate.

2.2 Main pollutants of sludge.

(1) pathogenic microorganisms. Sewage treatment process mainly includes pathogenic microorganisms and parasitic pathogens. According to monitoring, there are thousands of pathogens in fresh sludge, which contain a large number of harmful parasites, including salmonella, ascaris and enterovirus. There are four ways for pathogens in sludge to pollute people or animals: direct contact with sludge; Infected by direct contact with sludge through food chain; The water source is directly polluted by pathogens; Pathogens first pollute the soil, and then enter the water body, causing water pollution.

(2) nitrogen, phosphorus and other nutrients pollution. When sludge is applied to soil, if the rainfall is heavy and the soil is loose, the decomposition rate of organic matter may be faster than the absorption rate of nitrogen and phosphorus by plants. At this time, nutrients such as nitrogen and phosphorus easily enter surface water and groundwater with water loss, resulting in eutrophication of water bodies.

③ Heavy metal pollution. The heavy metals in the sludge of municipal sewage treatment plants mainly include 70% ~ 90% of heavy metals such as copper, lead, zinc, nickel, chromium, mercury and cadmium, which are transferred to the sludge by adsorption or precipitation during sewage treatment. Heavy metals mainly come from industrial production and domestic pipeline system. The accumulation of heavy metals in sludge on the ground is an important factor limiting the large-scale land use of sludge. In addition, heavy metal elements flowing into rivers, lakes or oceans, or into soil can not be biodegraded, but can be enriched under the biomagnification of food chain, which will cause greater impact and damage to the environment.

(4) Organic polymer pollution. Due to the diversity of sewage sources, any organic compounds entering the environment may be found in sludge. At present, 332 kinds of organic pollutants that may harm human body and environment have been found in sludge, of which 42 kinds can be detected frequently. The most common pollutants are benzene, chlorophenol and so on.

3. Study on sludge drying process

3. 1 fluidized bed drying process. Before the sludge enters the dryer, the cutting machine specially cuts the sludge into fine particles, and then sends them to the fluidized bed dryer. The fluidized bed dryer basically consists of three parts from bottom to top. It is the bottom of corrugated pipe, which produces fluidization gas and forms fluidization layer. Each dryer is equipped with a special gas distribution plate, which is used to stir inert fluidizing gas to keep dry particles suspended evenly. The middle part is a heat exchanger, which circulates hot steam or hot oil to the heat exchanger by evaporating the heat of water; Sucker is used to dry sludge particles in fluidized bed, which is responsible for the evaporation of annular gas circulation gas, fine sludge particles and moisture in the dryer at the upper left. The internal temperature of the dryer is 85 C. As the sludge gradually dries, the density decreases, rises to the upper part, and then is pumped out of the fluidized bed with the gas pumped out from the upper part. The wear of fluidized bed and pipeline is very serious, and the energy consumption of the system is also very high. The drying capacity of fluidized bed dryer depends on the temperature of hot oil or steam. After cooling, the dried particles are sent to the dry particle storage bin through a conveyor belt sealed in an inert gas environment. A small amount of waste gas generated by the drying system is sent to the subsequent treatment system and then discharged into the atmosphere.

3.2 Vertical disc drying process. This process is a construction process of heating the heat transfer oil of the heat medium, exchanging heat between the heat transfer oil and the hot oil furnace, and indirectly transferring heat to sludge particles to dry the sludge. Working principle: The mechanically dewatered sludge is sent to the sludge bin, transported to the coating machine by the sludge pump, and mixed with the back-mixed sludge to form particles. Under the rake action of the rake arm connected with the rotating spindle, sludge particles circulate on the upper plate, then are thrown out from the middle and scattered on the second plate. Driven by the rotating rake arm, sludge particles are repeatedly rotated from the upper tray of the dryer through the dryer until they reach the chassis. When the particles run on the tray, they are directly and evenly distributed. Sludge particles gradually increase downward and finally form solid particles, also known as pearl process. After leaving the dryer, it is sent up to the separation hopper by the bucket elevator, some particles are recovered and mixed, and the rest particles are cooled to 40°C by the cooler and sent to the particle storage bin.

3.3 Direct drum drying process. The dryer is mainly composed of two parts: drum and fin screw, which heat sludge at the same time. Among them, the drum is heated by an external combustion furnace, and the fin screw transfers heat through circulating hot oil. The drum and blade spiral rotate in opposite directions, and under their action, the sludge can be continuously dried and moved forward. The whole drum runs under negative pressure, so there will be no water vapor and dust escaping. The sludge is gradually dried into particles by screw driving and heating, and sent to the sludge storage bin. The evaporated water vapor is sent to the condensation, washing and adsorption system for treatment by the exhaust fan. The device has that advantage of low energy consumption. But it also has the following disadvantages: the energy loss at the outlet of the dryer is more; Follow-up treatment is expensive; There is a certain risk of fire and explosion.

3.4 Indirect drum drying process. After back-mixing, the dehydrated sludge is conveyed to the drum by the screw conveyor, mixed with the hot air flow from the same end and heated. Under the rotation of the drum, the sludge moves forward in the airflow and is transferred from the inner drum to the outer drum. During the movement, the sludge water evaporates and forms stable round particles. The dried sludge is sent to the separator by the screw conveyor, and the steam discharged from the separator is collected for heat energy reuse, and the biological filter treats the tail gas. After the particles are formed, they are separated from the circulating gas in gas-solid separation, cooled in a screw cooler, and the cooled dry particles pass through a screen, and the fine dry sludge and wet sludge are mixed again. System features: running in oxygen-free environment, less dust; The dried sludge is granular and the particle size is controllable; After the waste gas leaves the drum, the energy can be recovered and treated reasonably.

3.5 Centrifugal drying process. Dehydrated sludge is discharged from the discharge port at high speed in the form of fine powder, and high-temperature air is introduced into the centrifugal dryer to dry the fine powder sludge to a water content of about 20% in the shortest time. System features: simple process, short reaction time, large capacity, omitting sludge dehydrator and storage, transportation and conveying devices from dehydrator to dryer.

4. Concluding remarks

Because of the importance of sludge drying, more and more attention has been paid to the study of sludge drying. Through a series of studies on sludge characteristics and sludge drying process, this paper has achieved certain results, which provides a certain theoretical basis for future research and engineering examples.

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