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16. How Is Sludge Currently Disposed of in China?
At present, most of the sludge in China is abandoned or landfilled, only a very small amount is dried and used to make mixed fertilizers.
Due to the late implementation of landfill standards in China, the old landfills accept sludge, which may cause a large amount of sludge pollutants to enter the deep layer from the surface with leachate, and even threaten groundwater and rivers, lakes and seas.
Irregular disposal is still the main route of disposal, and a small part of it enters farmland. These disposals will become potential sources of pollution for surface water and groundwater in the short and long term.
17. Will Sludge Cause Secondary Pollution?
Sludge is a byproduct of pollution treatment process. This byproduct is rich in microorganisms, pathogens, viruses, etc. and has strong polluting properties. If sludge is not properly disposed of, the pollution will spread widely.
First, the water content of sludge is extremely high. In addition to a part of it evaporating naturally into the air, most of it will penetrate into the surface soil layer and enter the surface water system or affect groundwater. The pollutants in the sludge will spread through these two channels.
Second, if the sludge that has not been sanitized enters the farmland and even the vegetable field, it may directly threaten the human food chain.
Third, the sludge emits a foul odor and may affect a considerable area of the surrounding environment. The emitted gas is a highly polluted greenhouse gas. In addition, the amount of sludge is huge, the storage and transportation requires a large space and a lot of tools. The space and tools may be polluted, it is difficult to effectively "isolate" them from the food chain.
18. Does Sludge Have a Fertilizer Efficiency?
According to research by Chinese experts, the fertilizer efficiency of sewage sludge is significantly superior to that of typical farm manure:
Sludge type | Organic content (%) | Nitrogen (%) | Phosphorus (%) | Potassium (%) |
Wastewater treatment plant digested sludge | 42.8-44.62 | 3.59-3.78 | 1.58-1.94 | 0.28-0.33 |
Wastewater treatment plant mixed sludge | 51-53 | 3.04-3.18 | 1.24-1.47 | - |
Pig manure | 25.0 | 0.45 | 0.083 | - |
Horse manure | 25.0 | 0.58 | 0.122 | - |
Cow manure | 20.0 | 0.34 | 0.070 | - |
Sheep manure | 31.8 | 0.84 | 1.100 | - |
19. Why Is Agricultural Use of Sludge the Best Solution?
Agricultural use of sludge is the best choice for the final disposal of sludge, which is not only determined by the organic matter content of sludge, but also by its huge quantity and value-added potential.
The land that has been over-fertilized has become increasingly barren, and the virtuous cycle of the land has been destroyed. Only by inputting a large amount of organic matter can the soil be improved. For this purpose, sludge is a cheap and ideal source.
Of course, the content of heavy metals in sludge has a significant impact on agricultural use. It is worth noting that with the strengthening of industrial pollution treatment, the untreated industrial wastewater flowing into the municipal pipeline network is decreasing. In this way, it is expected that the heavy metal content of sludge can be controlled to reach the safety level for agricultural use.
In fact, agricultural use of sludge also includes forestry, flower industry, etc. Those green industries that are not related to the human food chain can provide excellent outlets for sludge products and have good price space.
Only the potential price space provided by the agricultural outlets can cover most of the costs required for sludge treatment. The current costs of incineration and landfill are a heavy burden for sewage treatment plants that are already short of funds.
20. What Is the Significance of Sludge Composting?
Sludge composting is an effective sludge treatment method. It uses appropriate bacteria and mechanical flipping to ferment, heat up and mature the sludge over a certain period of time (several days to more than ten days). The high temperature can effectively kill harmful microorganisms and pathogens in the sludge, and a large part of the water will be evaporated, so as to obtain organic fertilizer that meets agricultural requirements.
The advantage of this method is that it does not consume a lot of heat energy, and can also achieve the purpose of reducing the sludge volume, and the equipment investment is relatively low. The disadvantage is that it takes a long time, occupies a huge area, and requires a high investment to avoid odor pollution. This method has certain limitations when treating large-scale municipal sludge.
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