The massive increase and stockpiling of municipal solid waste has become a severe challenge for urban sustainable development. At present, the main methods to solve the garbage problem are landfill, composting and incineration.
Landfill has the advantages of mature technology, low cost of treatment and management, and convenient transportation. In the process of landfill and stacking, a large amount of waste water is collectively referred to as landfill leachate. The untreated landfill leachate will cause serious secondary pollution to the environment when it flows through the surface or seeps into the groundwater. Therefore, the safe and harmless treatment of landfill leachate has always been a worldwide environmental problem.
Landfill leachate self-cleaning filter
The landfill leachate self-cleaning filter dirt accumulates on the wedge wire filter, causing a pressure difference between the inner surface and the outer surface of the filter. When the pressure difference reaches the preset value (such as 0.5kgcm2), the self-cleaning cycle is started. When the backwash water passes through the hydraulic motor, it drives the dirt collector to rotate, so that the suction nozzle will suck away and clean the dirt on the entire filter screen.
Source of landfill leachate
Landfill leachate is formed by free water contained in garbage itself, water generated by natural precipitation and organic decomposition, and a large amount of wastewater generated by surface water and groundwater seeping into the landfill through shower. The quantity and quality of landfill leachate are affected by garbage composition, landfill time, landfill technology, rainfall infiltration and other factors. Especially, it is greatly affected by the rainfall. When the rainfall is small, the garbage leachate is mainly the free water contained in the garbage itself. Most of them are evaporated. When the rainfall is large, the rainwater flows into the garbage dump, producing a large amount of leachate. The amount of leachate produced is in direct proportion to rainfall.
Landfill leachate has the characteristics of complex components of pollutants, high concentration of organic pollutants, and large changes in water quality. Therefore, it is difficult to treat the leachate.
Water quality characteristics of landfill leachate
- Leachate usually has a high chromaticity, and its color is mostly black and dark brown. The chromaticity can reach 2000-4000 times (dilution ratio). At the same time, the leachate has a strong odor of garbage decay.
- During the service period of the garbage dump, the pH value of leachate is between 6-7, which is weakly acidic. With the increase of the service life of the landfill, the landfill also tends to be stable. The pH value can be increased to 7-8, which is weakly alkaline.
- The organics in landfill leachate can be divided into three categories, namely fatty acids with low relative molecular weight. Humus, high molecular carbohydrate. Griseofulvic acid with medium molecular weight.
- Due to the anaerobic fermentation and hydrolysis of garbage in the heap. As a result, the concentration of ammonia nitrogen in landfill leachate is high. Ammonia nitrogen in leachate mainly exists in the form of NH3-N. One of the important water quality characteristics of the leachate from the middle-aged and elderly landfill is that the NH3-N is very high.
- The phosphorus content of landfill leachate is usually low. In particular, the concentration of soluble phosphate is lower. The content of soluble phosphate in leachate is affected by Ca ion concentration and alkalinity. It leads to serious phosphorus deficiency in leachate biological treatment.
- The dissolution rate of trace heavy metals in domestic garbage is very low. The garbage itself has strong adsorption capacity for heavy metals. Therefore, if industrial waste and domestic waste are mixed for landfill. The amount of heavy metal ions in leachate will be significantly increased. Common heavy metals contained in landfill leachate are Cu, Pb, Cr, Cd, Zn, As, Mn, etc.
- There are a lot of microorganisms in landfill leachate, many of which play an important role in the degradation of leachate. There are mainly 8 types of bacteria, including nitrite bacteria, nitrifying bacteria, denitrifying bacteria, desulfurization bacteria, denitrobacteria, iron bacteria, sulfate reducing bacteria and methanogens. In addition, there are a large number of pathogenic bacteria and microorganisms in the leachate.
- Landfill leachate contains high concentration of total dissolved solids. The concentration of these dissolved solids in leachate usually changes with the increase of landfill time. It generally reaches the peak within 0.5~2.5 years of landfilling. Thereafter, with the increase of landfill time, the concentration of these inorganic salts will gradually decrease until they reach final stability.
Treatment technology of landfill leachate
Because of the serious harm of landfill leachate. Therefore, it must be effectively treated to make it meet the discharge standard, and at the same time, because of the characteristics of garbage leachate on water quality. Its treatment difficulty and cost are much higher than those of ordinary domestic sewage and industrial wastewater. Up to now, there is no economic and effective process suitable for landfill leachate treatment.
The commonly used landfill leachate treatment technologies can be divided into biological treatment technology and physical and chemical treatment technology. Among them, biological treatment technology has low treatment cost. The secondary pollution is small, which can be used as the core process of landfill leachate treatment. However, the leachate effluent treated by this method can not directly meet the relevant national discharge standards. Subsequent depth treatment is required.
Biological Treatment Technology of Landfill Leachate
Biological treatment of leachate is a method of purifying wastewater by degrading organic pollutants in leachate with microorganisms. The biological treatment of landfill leachate is one of the main treatment methods of landfill leachate at present. According to the respiration type of the microorganism that plays a major role in the biological treatment process. The biological treatment of leachate can be divided into aerobic treatment, anaerobic treatment and anaerobic aerobic combined treatment.
Aerobic biological treatment Aerobic biological treatment uses the aerobic reaction of microorganisms to degrade organic matters in leachate. It mainly includes activated sludge process, membrane biological process, etc.
Activated sludge process is an aerobic biological treatment technology. The physiological activities of microorganisms in sewage are mainly enhanced by introducing oxygen into sewage. Use microorganisms to degrade pollutants in sewage. At present, the operation modes of activated sludge process used for landfill leachate treatment include traditional activated sludge process, Sequencing Batch Reactor (SBR) and Membrane Bioreactor (MBR).
It can be seen that the activated sludge process can have a better treatment effect on landfill leachate. However, the effluent effect of activated sludge treatment of leachate is greatly affected by temperature. COD removal rate of leachate is low at low temperature. Moreover, the removal effect of pollutants in the leachate of the middle-aged and old garbage dump is not ideal. Therefore, the treatment of landfill leachate by activated sludge process will be limited.
Membrane biological treatment of landfill leachate has the advantages of resisting the impact load of water quantity and water quality, and is conducive to the removal of ammonia nitrogen in the water that requires a longer residence time. Moreover, since microorganisms grow on the filler, sludge return is not required. At the same time, due to the long biological chain, less excess sludge is generated. It is helpful to reduce infrastructure funds for sewage treatment facilities. However, higher conditions are required to maintain biofilm operation.