With the development of social economy and urbanization in China, the production and quantity of municipal wastewater are increasing. According to the forecast, China's urban sewage volume in the next 20 years will have a greater growth in 2010, the sewage discharge will reach 4.4 x 1010m3 / d-1. At present, China's annual output of dry sludge nearly 3 x 106t. With the development of domestic sewage treatment, sewage treatment plant total content and degree of treatment continues to expand and improve, the amount of sludge generated is on the increase. A large number of sludge can not be timely and reasonable treatment to become a heavy burden on the sewage treatment plant.

Sludge also contains a large number of N, P, K, Ca and organic matter, and P, N in the organic state, can slow release, has a long-term effect. Sludge is a useful biological resources, if can reasonable use can not only turning waste into treasure, but also to increase economic benefit. Therefore, to explore the suitable for national conditions of our country effectively dispose and use sludge technology has important practical significance.

1, the composition, characteristics and classification of sludge

Sludge is the product of sewage treatment, composition is very complex. It includes the sewage or industrial wastewater in the mud and sand, fiber, animal plant residues and other solid particles and condensation floc, mixed with the formed by a variety of microorganisms zoogloea and adsorption of organic matter, heavy metals and salts, a small amount of pathogenic microorganism and parasite eggs and other solid material. The main characteristics of the sludge is high moisture content (up to 99%), high organic matter content, easy to corruption stinks, and the particle size is small, only a small proportion, a colloidal liquid.

There are many methods for the classification of sludge. According to the properties of the sludge, can be divided into organic sludge and sediments; according to the method of sewage treatment can be divided into: (1) at the beginning of the precipitation sludge activity sludge 3 humus sludge chemical sludge; according to the sludge produced in different stage can be divided into: life sludge digestion sludge 3 sludge concentration 4 dehydration dry sludge drying sludge.

2, sludge treatment of new technologies

At present, the sludge treatment methods of domestic and foreign sewage treatment plants mainly include regulation, concentration, dewatering, drying, digestion, composting, etc.. These methods have exposed some shortcomings and deficiencies in engineering practice, so there are some new methods of sludge treatment to be applied in practice.

2.1 sludge wet oxidation process

Wet air oxidation (WO) is the sludge in an airtight reactor, under the conditions of high temperature and high pressure in air or oxygen as oxidant, by submerged combustion principle for the decomposition of sludge in the oxidation of organic matter, organic matter transformation into inorganic. The method is mainly applicable to the treatment of various kinds of refractory organic sludge, the disadvantage is that the equipment is complex, operation and maintenance costs are high.

2.2 earthworm treatment sludge

Poland BnrskO and Zambrow using earthworm effective treatment of the waste water sieve residue and sludge, a no smell, resemble the humus and earthworm fertilizer containing high nutrition. The digestive sludge of Changsha sewage treatment plant was treated by using earthworm, and the effect was good. After earthworm purification treatment, sludge of Cu, Zn and Ni contents were significantly lower; accumulation of heavy metals in sludge after treatment with no odor, loose and efficient organic granulated fertilizer, but the treatment of sludge after the earthworm, and therefore should not be used as feed lest enter the human food chain.

2.3 membrane bioreactor

Membrane bioreactor is a new technology which combines the membrane system in the membrane separation technology and the biological reactor in wastewater treatment engineering. Sludge retention time in membrane bioreactor is very long, and even can avoid sludge. However, the application of the method is limited by the blocking of membrane and the price of membrane material.

2.4 sludge acidification

Sludge acidification is a treatment method which is developed based on the characteristics of the high content of residual sludge organic matter. The basic principle is to return to the waste water treatment system in the process of hydrolysis and acidification of the residual sludge, so as to achieve the purpose of reducing or no sludge discharge, the method is in the exploratory stage.

2.5 sludge constructed wetland treatment technology

Sludge treatment technology of constructed wetland is a new type of sludge processing technology. It is a set of sludge thickening, dewatering and degradation in one, can be large-scale retrenchment in capital investment and operating costs. According to reports, the Danish use of artificial wetland reed bed system for sludge dewatering and mineralization treatment, sludge due to dehydration and mineralization in the bed of the annual average of 90% and 96%. StevenT. respectively by vertical flow constructed wetland and horizontal flow wetlands for aquaculture sludge stability research. The results show that the two wetland of sludge is relatively stable.

The main function of the treatment technology for sludge treatment is artificial substrate, microorganism and plant. Artificial substrates for microbial growth to provide stable attachment surface and is resistant to water and sewage plant to provide support and nutrients, and by some physical and chemical degradation pathway of sludge; resistant to water and sewage plant in addition to the direct absorption of the use of sludge in nutrients and adsorption, the enrichment of some toxic and hazardous substances, and to transport oxygen to the root zone and maintain the role of the hydraulic transmission; microbial metabolism is the main mechanism of degradation of organic pollutants in sludge. At the same time, their mutual connection, reciprocal causation, form a system diagram as shown in figure 1.

3, sludge disposal

No matter what the way to deal with sludge, sludge disposal is a must to face the problem. There are 3 kinds of sludge disposal methods at home and abroad at present: sanitary landfill; sludge incineration; sludge disposal.

3.1 sanitary landfill

Sanitary landfill operation is simple, the investment cost is small, the cost is low, the adaptability is strong, but its occupation of land is serious, if the seepage prevention technology is not enough, will lead to the potential of soil pollution and groundwater pollution. In our country, sanitary landfill is the main treatment method. Due to the potential pollution of underground water and the reduction of urban land use, the treatment technology standards of sanitary landfill are getting higher and higher, and many countries and regions even oppose the new landfill site. In 1992, about 40% of the sludge in the European Union was disposed of. In recent years, the proportion of the sludge disposal in recent years is smaller and smaller, for example, the proportion of the sludge in the UK fell from 27% in 1980 to 10% in 1995.

3.2 sludge incineration

Sludge contains a certain amount of organic components, the sludge can be used to deal with dehydration and drying, so as to make all of the organic carbon, kill the pathogen, to maximize the reduction of sludge volume. But the disadvantage is that the treatment facilities investment, high processing costs, while organic matter burning will produce toxic substances. In foreign countries, especially western Europe and Japan has been widely used in Japan, sludge incineration treatment has accounted for more than 60% of the total sludge treatment, the EU is also more than 10%. In our country, because of the large investment and the cost of treatment, and the burning of gas to be further treatment and so on, it has not been widely used.

3.3 sludge into the sea

This is a simple and economical method to deal with. However, with the strengthening of the consciousness of ecological environment, people pay more and more attention to the possible influence of the sludge in the sea on the marine ecological environment. United States in 1988 has banned ocean dumping of sludge, and in 1991 comprehensive be prohibited; Japan on sludge oceanic jettison the strict rules; the Chinese government on February 20, 1994 in sea disposal of industrial waste and sewage sludge.

4, the sludge utilization of resources

From economic development, resource development and utilization, urban ecological environment protection, the ideal way of urban sludge disposal should is resource utilization, strengthen the study and practice of the utilization of sludge from the city, to solve the problem of sludge disposal, avoid the pollution of urban ecological environment, saving the cost of disposal, turning waste into treasure, which has good ecological benefits, environmental benefits, economic benefit and social benefit, it is the inevitable demand and trends of the urban sustainable development.

4.1 sludge agricultural resources

According to 1996 data reported that the world's 12 developed countries on the disposal of sludge: 45.3% for land use (including agriculture, forestry, grass, flowers, etc.), 38% for the landfill, 10.5% for the burning, 6% for the sea. From the development trend, Europe and the United States, Japan and other countries will pay more and more attention to the sludge land treatment, including agricultural, forestry, etc..

Less agricultural investment, low energy consumption, low operating costs, which can be converted into organic part of soil conditioner components, so it is considered to be the most development potential of a disposal. Application of sewage sludge to farmland, vegetable field, orchard, woodland, grassland, urban green, seedling raising medium and serious disturbance of land rehabilitation and reconstruction. The results show that the application of sludge and sludge compost plant rice, corn, wheat, cotton, vegetables and so on crops, plant growth, yield and quality were significantly higher than fertilizer, and fertilizer or quality of manure equivalent or even better. Application of sewage sludge and sludge compost can significantly improve the soil physical properties, such as for aggregate formation and improve the water stability of aggregates and water holding capacity and so on. Application of sludge or sewage sludge compost, but also significantly improve the chemical properties of soil, such as can significantly improve the soil phosphorus, potassium (total and effective) and the content of organic matter, etc.. In the urban landscaping, the use of sludge or sludge compost, green effect is quite significant, compared with the fertilizer or other commercial organic fertilizer, the effect is even better. Trees, shrubs, flowering, flowering (quantity and size) and so on are significantly increased.

China is an agricultural country. If the sludge the use of advanced technology for processing as fertilizer or soil improvement agent, not only can save large amounts of sludge terminal disposal costs, can for the low fertility of the farmland to add organic matter, improve fertility promoting the development of agricultural production and agricultural ecological environment to achieve a virtuous circle. Therefore, the land use of sewage sludge is a kind of processing method which conforms to the situation of our country. But due to the sewage sludge contains certain toxic and harmful substances and pathogens and parasite eggs and other, so land utilization of sewage sludge should pay attention to the following three main problems: sludge heavy metals (Pb, Zn, Cr, Hg, Ni, etc.) can cause secondary pollution of soil and crops, through the food chain enters the human body; sludge of viral pathogens and parasite eggs could have an impact on the environment and public health; (3) such as nitrogen and phosphorus substance concentration is too high would lead to pollution of groundwater.

4.2 sludge utilization of building materials

4.2.1 sludge production of cement materials

Sludge contains a large number of ash, especially the coagulation process of wastewater containing a large number of sludge containing Fe, Al and other components, is a building material available additives. However, before being used as building materials, the improvement of the strength, condensation characteristics and long-term stability need to be further studied. In addition, the high cost, but also the safety of the product to worry about, only a small amount of production in Japan and other state-owned.

4.2.2 sludge system

Lightweight aggregate can be used as roadbed, concrete aggregate or flower cover material. The method according to the raw material can be divided into two kinds according to the raw material, one is the sintering after the incineration of the sludge or anaerobic fermentation sludge, this method has become more mature in 1980s and has been put into use. The use of burning ash to make lightweight aggregate requires a separate construction of the incinerator, and the organic components in the sludge are not effectively utilized. In recent years, the development of new technology directly from the sludge of dewatering.

4.2.3 sludge glass ceramics

Glass ceramics are similar to artificial marble, which can be used as a decorative material for internal and external decoration. Raw materials for the production of glass ceramics are commonly used for sludge incineration, grit and waste concrete. After adjustment of raw materials, the melting temperature is controlled at 1500 ~ 1400. The molten material is placed at a certain time, and then injected into the mold to form, with the reduction of temperature generated crystal nucleus (FeS), and then heat treatment, to promote crystal growth. After heat treatment, the natural cooling is obtained, and the glass ceramics with different shapes are obtained.

4.2.4 sludge brick

A research team in Taiwan found that sewage sludge can be used as a common building with "eco brick" ". The sludge ecological brick is mixed with 10% sludge in the clay brick, and the best effect can be achieved under the condition of 900 centigrade. This method not only to deal with the sludge, still burning process will be toxic heavy metals are sealed in the sludge, but also kill all harmful bacteria and organic matter, and this kind of brick completely no odor.

4.3 sludge animal feed sludge contains a lot of valuable material, coarse protein accounted for 28.7% ~ 40.9%, ash accounted for 26.4 ~ 46%, cellulose accounted for 26.6% ~ 44%, fatty acid accounted for 0 ~ 3.7%. The sludge protein contains almost all the amino acids needed for feed of all domestic animals, and the relative balance between various amino acids can be used as a feed protein.

4.4 sludge production of oil

The dry sludge with water content of 65% was isolated from the air, heated up to 450 degrees Celsius, and the organic compounds in the sludge were converted to hydrocarbons, and the properties were similar to that of diesel. At present, Canada is in the pilot test, the Australian Perth is also being constructed using the thermo chemical method of sludge oil production plant.

4.5 sludge as binder

Activated sludge as a binder smokeless powder coal forming processing of coal, and sludge in the furnace of high temperature gasification is processed and preventing pollution; sludge used as briquette binder, instead of white mud can be improved in high temperature coal pore structure and improve the gasification reactivity of coal, reduce ash carbon residue, improve the conversion rate of carbon, sludge can be used as a binder, and a bulking agent.

5, prospects

If the urban sewage treatment is not properly into the environment, it will cause some harm to the surrounding environment. After reduction, stabilization and harmless treatment, the sludge can be used as a resource. Although a variety of urban sludge treatment method, we should according to the amount of sludge, sludge properties, the content of heavy metal specific, a concrete analysis is made; at the same time, we should consider the ecological environment, economic benefit, and the processing cost, technology trends and other factors, we should actively search for sludge treatment method, new technology and new technology. From a long-term point of view, do a good job of sludge treatment and disposal and harmless, as a comprehensive utilization of renewable resources is a common problem in the world. China is a agricultural country, the economic foundation is weak, the resources of sludge utilization especially agricultural use not only can save large amounts of sludge terminal disposal costs, can for the low fertility of the farmland add organic matter, improve soil fertility, promote the development of agricultural production, agricultural ecological environment to achieve a virtuous circle. Therefore, the resource utilization of sludge will be the development direction of sludge disposal in line with China's national conditions.