Stale garbage refers to landfills that fail to meet the requirements of sanitary landfill in terms of construction specifications and environmental protection. Due to the lack of anti-seepage and covering measures, the garbage gradually becomes old and corrupt under biological and physical effects. The leachate and leached substances produced in this process will pollute the surrounding groundwater, soil and air, and pose a potential serious threat to human health. In view of the potential hazards of stale garbage, relevant state ministries and commissions have formulated a remediation plan and supported cities with surplus incineration capacity of thermal power plants to excavate and screen the stale garbage in the landfill and then return it to the furnace for incineration.

The stale garbage screening process adopts the process flow of "manual sorting-particle size screening-density sorting-double magnetic separation", with a designed processing capacity of 500t/d. Manual sorting mainly separates large debris such as stones and wooden boards; particle size screening mainly uses drum screens to separate humus with a particle size of less than 35mm, and the humus is recycled after magnetic separation to remove iron metals; the sieve material with a particle size greater than 35mm enters the density sorting system to separate light garbage and heavy garbage. Light garbage is mainly plastics and textiles, etc., which are compressed and packaged and sent to the incineration power plant for disposal. Heavy materials are mainly inert materials such as bricks and tiles, which are backfilled after iron metals are selected by the magnetic separation system; the iron metals sorted by the project are recycled by downstream recycling companies.

Humus accounts for the largest proportion of stale garbage, reaching 46.4%~58.9%, followed by light materials (plastics and textiles), accounting for 29.8%~45.3%, and the iron metal content is only 0.0%~0.1%. Other garbage (stones, plastics, wood and other inert garbage) accounts for 8.3%~11.2%. Studies have shown that the proportion of humus (particle size ≤ 50mm) in garbage with a landfill age of 4 to 9 years is 30.8% to 56.3%. The particle size of humus is smaller, but the mass of humus is higher after screening, mainly because: 1. The composition of domestic garbage in different regions is different, and the particle size and mass proportion of humus formed after landfill are different; 2. In informal landfills, the garbage is not compacted and not covered, and rainwater can penetrate into the garbage pile in large quantities. Air penetrates through the gaps in the garbage, causing the pile to be in a state of high water content and high oxygen content, which accelerates the oxidation of organic matter in the pile, promotes the degradation rate of organic garbage and forms humus; 3. After the old garbage is excavated and piled before screening, the moisture content is low, which is conducive to the separation of humus from other garbage.

The process of "manual sorting-coarse screening-wind sorting-magnetic separation" can effectively realize the separation of old garbage materials, which has a high reference value for the selection of screening technology for old garbage in informal landfills. The high heavy metal content of humus in stale garbage will limit its reuse. When it exceeds the agricultural land soil pollution risk control value standard, it is necessary to strengthen supervision to prevent it from being reused in agricultural planting. The light materials in stale garbage are mainly treated by incineration. Although it can quickly reduce the amount, it will not create economic benefits for the project. In the future, we can expand the reuse of light materials by solving the technical problems of cleaning and improving the quality of light materials.