Landfill Technology

Technology description

Landfills are different from natural dumps. The natural dump is to dump refuse in wasteland and ditch without any cover or scientific disposal. Natural dump was applied in many cities in China before the 1980s. The disadvantage of this technology is obvious. Due to the air storage of refuse, a lot of bad smell gas and butterflies, mosquitoes and mice will ruin the ecological system near the dump cite and harm public health. Meanwhile, percolating water, heavy metal and others harmful materials decomposed by microorganisms from refuse will seep to underground and pollute water resources.

Landfill is developed to solve these problems with the aim of preventing the secondary pollution. The advantages of landfill are: (1) pollution of surface water and underground water can be prevented by the seepage control in advance; and (2) inflammable and explosive gas emitted from refuse can be collected by pipeline and will be used for power generation or chemical raw materials. After conveyed to the landfill plant, refuse is spread as a layer of 30-50 cm thick, then compacted, covered by a soil layer of 20-30 cm thick. The refuse layer and soil layer together constitute a landfill unit. The refuse conveyed to the landfill plant every day will be compacted to be a landfill unit. A landfill layer consists of a series of landfill units. A whole landfill plant is consisted of one or more landfill layers. Whenever the landfill thickness reaches the designed height, a soil layer of 90-120 cm will cover the top layer and be compacted, then a whole plant will be completed.

Landfill is just one of the municipal refuse disposal technologies. Incineration, high temperature fertilization, etc., have their own advantages and can be applied in different situations.

Landfills are popular in many countries because they are simple to operate, low cost, and suitable for various kinds of refuse and energy recovery. At present, 4,817 landfill plants have been built all over the world: 2,247 are in America and 175 are in Europe. Worldwide, 5.142 billion cubic meters of landfill biogas is collected annually, equal to 2.4 million tons of crude oil.

Disadvantages of landfills are: (1) large land requirement, as time goes by, it will be more and more difficult to find a more suitable site; and (2) the secondary pollution. Experiences have demonstrated that percolate and gas of refuse will harm the surrounding environment if they are not disposed properly.

Improvement of technology

To solve the secondary pollution problem and save land resources, some technical measures have been taken and received good results as follows:

(1) Raise the height of landfill plant will be useful to save land and decompose refuse to produce biogas.

(2) New materials and technologies have been adopted to strengthen the impervious barrier, sewage disposal system is also installed to prevent the pollution to water resource.

(3) Refuse leaching solution and biogas collecting pipeline are built to increase the biogas collecting efficiency. Now the main uses of biogas are fuel, domestic use after being cleaned, and power generation. The final use is determined by the situation of the region where the landfill plant is located. The percentage of biogas used for power generation is 80% in America and 50% in Europe.

(4) Compacting the refuse to preventing air from entering the refuse, avoid the aerobic reaction.

In general, along with the improvement of landfill technology and the more and more abundant operating experience, landfill technology is tending to reach perfection, and has solved the following technical problems successfully:

， Projection of landfill plant biogas production

， Preventing air from entering refuse

， Seepage control and sewage disposal

， Shaft drilling and equipment manufacturing

， Horizontal pipeline building and condensed water removing

， Biogas collection and pressurization

， Biogas power generation and equipment manufacturing

， Dust elimination of power generator

， H₂S control

Status of municipal waste disposal technologies in China

Now in China, only a few units have taken part in this research of municipal refuse disposal technology. For example, Chengdu Creature Institute of Chinese Academy of Science completed the research of biogas production from refuse anaerobic zymosis during 1985-1986, research of municipal organic refuse systematic disposal during 1987-1990, and the pilot-scale test of 160m³ had also completed.

In the aspect of project construction and demonstration, there is not much experience available. By 1995, the completed harmless landfill projects are:

， Qingshan municipal refuse harmless landfill plant in Baotou

， Asuwei refuse harmless landfill plant in Beijing

， Laogang refuse harmless landfill plant in Shanghai

， Tianziling refuse harmless landfill plant in Hangzhou

， Datianshan refuse harmless landfill plant in Shenzhen

， Zaikeng refuse harmless landfill plant in Zhongshan, Guangdong

， Xiaping refuse harmless landfill plant in Shenzhen

， MengYuan refuse harmless landfill plant in Nanchang

Power generation has played a more and more important role in the utilization of gas of landfill plant. Now the main equipment consists of the internal combustion engine and the turbine. Otto engine and Diest engine are the commonly used engine of internal combustion engine. However, gasifiers, which can provide proper mixed gas to combustor, should be added to the engine before they are used in landfill plant.

Since the octane number of the fuel for Otto engine is less than that of landfill gas, so the bulk of combustor should be reduced to let the engine reach designed output. But the octane number of the fuel for Dised engine is higher than that of landfill gas, so the engine can reach designed output by enlarging the bulk of combustor and adding an automatic spark instrument. Both the two kinds of engines have their own market. Gas turbine and steam turbine are all applied in some plants, the advantage of gas turbine is its large output per unit weight, about 70-140 kW/ton, much higher than 27 kW/ton of internal combustion engine and 10 kW/ton of steam turbine. The statistical data show that internal combustion engine is applied in 61 landfill plants in America, turbine is used in 24 plants, the total installed capacity is 344MW. In Europe, there are 50 sets of internal combustion engines on operation, the capacity of big unit ranges from 400 to 2,000 kW. Some small internal combustion engines can be contained and conveyed from one landfill plant to another. Which kind of engine will be applied is determined by the production of biogas. Internal combustion engine is suitable for the capacity from 1,000 to 3,000 kW, if the installed capacity needs to be more than 3,000 kW, turbine is the better choice with higher efficiency. At present, the efficiency of landfill biogas power generation is about 1.68-2 kWh/m³ in China.

Some development research has been taken in China, mainly focus on internal combustion engine, just some simple improvement of gasoline or diesel engine; the manufacturing capacity of biogas engine has not been built.

Altogether, from the point of view of technology, landfill technology has been well developed and commercialized in foreign countries. Although some landfill plants have been operated successfully in China, the landfill technology is still at demonstration level with less experience, developing technology.