Overview of leachate
According to landfill gas composition and other parameters, the day waste landfill can be roughly divided into five stages. The first stage is aerobic stage, and the gas from the air duct is mainly air. At this moment, the COD concentration of leachate is high, the ammonia nitrogen concentration is low, and the biodegradability is good; the second stage is acidification stage, and the reaction of acidification is mainly in the waste heap, and the landfill gas is mainly nitrogen and dioxygen The water quality of carbon, hydrogen and leachate is similar to that of the first stage; the third stage is the unstable methanogenic section, the anaerobic methanogenic bacteria in the reactor begin to become the dominant bacteria, the proportion of methane gas begins to rise, the organic matter in the leachate begins to decline, on the contrary, the ammonium salt produced by the anaerobic decomposition of nitrogen-containing substances such as egg white begins to rise, and the biodegradability of the leachate decreases; the fourth stage is the unstable methanogenic section The landfill gas is mainly composed of carbon dioxide and methane. The biodegradability of the leachate is now relatively poor, and the organic compounds easy to be biochemically declined sharply. VFT (VFC) indicates that by the end of the last stage, the organic compounds in the waste have been decomposed completely, and the leachate at this moment has no biodegradability.
Among them, the first three stages with better biodegradability are relatively short, only three to five years, and then enter the fourth stage. The biodegradability of leachate decreases year by year until the content of organic matter drops to zero.
Several remarkable characteristics of leachate
(1) the water quality changes greatly before and after leachate treatment. The water quality of leachate has changed a lot. It is not only reflected in the great difference of water quality in different seasons of the same year, but also in the variation of concentration as many as several times. With the addition of landfill years, the water quality characteristics are constantly changing, such as the carbon nitrogen ratio and biodegradability of leachate decrease with the addition of landfill years. Generally, in the early stage of landfill, the concentration of ammonia nitrogen is relatively low, so biological denitrification can remove the ammonia nitrogen in the leachate. However, with the addition of landfill years, the concentration of ammonia nitrogen is constantly added, and the COD is constantly decreasing, so the physicochemical method is the best choice.
(2) high concentration of organic matter. The concentration of CODcr and BOD5 in waste leachate is up to tens of thousands mg / L, which is very high compared with urban sewage. The high concentration of landfill leachate mainly occurs in the acid fermentation stage. The pH value is slightly lower than 7. The COD of low molecular fatty acids accounts for more than 80% of the total. The ratio of BOD5 to cod is 0.5-0.6. With the addition of landfill years, the ratio of BOD5 to cod will gradually decrease.
(3) the content of ammonia nitrogen is high. Because most landfills are anaerobic landfills, the concentration of ammonia nitrogen is very high in the formation of anaerobic environment in the reactor, and it increases with the addition of landfill years, sometimes up to 1000-3000mg / L. When selecting biological treatment system, a long residence time is needed to avoid the toxic effect of ammonia nitrogen or its oxidation derivatives on microorganisms.
(4) the proportion of nutrient elements is out of balance. Generally, BOD5 / TP in landfill leachate is more than 300, which is quite different from the phosphorus required for microbial growth. Therefore, there is a lack of phosphorus in sewage treatment, which needs to be replenished. On the other hand, the BOD5 / NH3-N of landfill leachate is often less than 1. When biological treatment is needed, carbon sources need to be made up.
(5) high salt content. Landfill leachate usually contains a lot of salt, and the total salt content is usually more than 10000mg / L. membrane treatment is selected because of excessive osmotic pressure, resulting in low water production rate. Biochemical treatment is selected because of high salt content, resulting in startup difficulties, unstable operation, and even unable to operate.
(6) the total nitrogen is mainly ammonia nitrogen. Because most landfills are in anaerobic environment, the nitrogen element in leachate is mainly ammonia nitrogen, which means that the total nitrogen removed by ammonia nitrogen is also removed.
