Aged waste typically refers to mixed waste that has been stored for a long time and has undergone extensive degradation and fermentation. Its composition is complex and contains large amounts of leachate and humus, as well as non-degradable materials such as plastics, metals, and glass. Scientific and standardized recycling not only recycles resources but also reduces land use and eliminates environmental pollution risks. Aged waste recycling follows the core principle of "pretreatment - sorting - resource conversion - harmless disposal." The specific operational steps can be divided into the following six steps:

1. Preliminary Survey and Plan Development

Before officially commencing recycling operations, a comprehensive survey of the waste dump site is required. This is the foundation for ensuring the safety and efficiency of subsequent processes. First, technicians will conduct on-site drilling, sampling, and analysis to determine the volume, depth, and distribution of the waste accumulation. They will also test the waste composition, such as the proportion of humus, the amount of recyclables, and the concentration of hazardous pollutants (such as heavy metals and toxic organic compounds). Secondly, the surrounding environment of the site must be assessed, including groundwater levels, soil types, and proximity to sensitive areas (such as residential areas and water sources) to avoid secondary pollution to the surrounding ecosystem during operations.

Based on the survey results, technicians will develop a specific recovery plan, specifying the operation schedule, equipment selection, staffing, and environmental protection measures. For example, sites with high leachate concentrations require a leachate collection and drainage system; sites containing hazardous waste require specialized sealed containers and protective equipment. Furthermore, the plan must include emergency procedures for handling emergencies such as spontaneous combustion of waste and leachate leaks, ensuring that the recovery operation remains under control throughout.

2. Site Cleanup and Safety Protection Establishment

The first step in the on-site operation phase is to establish a safety protection system and complete preliminary site cleanup. First, fences, dust screens, and warning signs must be erected around the waste dump area. The fences must be at least 2.5 meters high to prevent the scattering of waste and the spread of dust. If the site is near roads or residential areas, sound barriers must also be installed to minimize the impact of mechanical noise on the surrounding area. Secondly, a temporary work access path will be constructed along the planned route. This path will be paved with steel plates or gravel to ensure smooth access for transport vehicles and construction machinery, while also preventing vehicles from running over the waste, which could increase compaction and affect subsequent sorting efficiency.

During site cleanup, large debris such as branches, discarded furniture, and pieces of construction debris must be removed from the surface of the waste. These materials, if introduced into the subsequent sorting system, could clog or damage the equipment. For larger objects, hydraulic breakers can be used to dismantle them, and then a loader can be used to transport them to a dedicated storage area. Furthermore, personnel should be deployed to patrol the site perimeter to prevent unauthorized personnel from entering the work area to ensure construction safety.

3. Leachate Collection and Treatment

Over the long term, stale waste accumulates, generating large amounts of leachate due to rainwater infiltration and the degradation of organic matter. This liquid is complex in composition, containing high concentrations of COD (chemical oxygen demand), BOD (biochemical oxygen demand), ammonia nitrogen, and heavy metals. Direct discharge can seriously contaminate soil and groundwater. Therefore, leachate collection and treatment are critical steps in the recycling process. First, diversion ditches are excavated at the bottom of the garbage dump. These ditches are constructed of masonry or concrete, with an impermeable membrane covering the inner walls to prevent leachate from seeping into the soil. The diversion ditches must be sloped to direct the leachate into a collection well. Submersible pumps are installed in the collection wells, which then transport the leachate via pipes to temporary treatment facilities or a sewage treatment plant. For sites with large garbage accumulations, a "multi-point diversion + zoned water collection" approach can be adopted to ensure thorough leachate collection and prevent localized water accumulation that can exacerbate garbage fermentation.

Leachate treatment requires selecting appropriate processes based on its water quality characteristics. Common methods include a combination of "pretreatment + biochemical treatment + advanced treatment." In the pretreatment stage, suspended impurities are removed through screen filtration, and then the water quality and quantity are regulated in a regulating tank. In the biochemical treatment stage, anaerobic reactors and aerobic aeration tanks are used to degrade organic matter in the water. In the advanced treatment stage, membrane separation and activated carbon adsorption technologies are used to reduce pollutant concentrations in the water to meet discharge standards before discharge or reuse (e.g., for on-site dust reduction). During the treatment process, water quality must be regularly monitored to ensure that the treatment results meet environmental requirements.

4. Waste Excavation and Preliminary Crushing

After the leachate collection system is established, the waste excavation phase begins. This phase requires layered excavation based on the depth of the waste accumulation to avoid excavating too deep at once and causing landslides. Excavation is typically performed using a crawler excavator equipped with a specialized grab bucket. Each excavation depth is controlled at 1.5-2 meters. The excavated waste is then transferred to the crushing equipment feed port by a loader. If the waste contains large, hard lumps (such as compacted plastic clumps or metal components), they must be initially broken down manually or by hammers before being fed into the crushing equipment.

The purpose of preliminary crushing is to reduce large lumps of waste into uniformly sized particles, making it easier for subsequent sorting equipment to separate different components. Commonly used crushing equipment includes jaw crushers and impact crushers. The particle size must be controlled during the crushing process, generally requiring a particle size of no more than 10 cm. Crushing equipment must be equipped with dust removal devices, such as bag filters, to reduce dust generated during the crushing process. A conveyor belt should also be installed beneath the equipment to transport the crushed material to the sorting system, ensuring continuous operation.

During the excavation and crushing process, the temperature of the waste must be monitored carefully. If the internal temperature exceeds 60°C (possibly due to spontaneous combustion caused by fermentation of organic matter), operations must be stopped immediately and cooled using a high-pressure water jet. Work can resume only after the temperature drops to a safe level. Furthermore, if hazardous waste (such as discarded batteries or medical waste) is discovered during excavation, excavation must be halted immediately. Professionals wearing protective equipment will collect the waste separately, seal it, and hand it over to a qualified organization for harmless disposal to prevent its spread.

5. Multi-stage Sorting and Resource Separation and Recycling

Sorting is the core step in the recycling of aged waste. It requires multiple steps to separate recyclables (such as plastics, metals, and glass), degradable organic matter (such as humus), and non-recyclable waste. Common sorting processes include a combination of manual and mechanical sorting.

a. Manual Sorting

Manual sorting is typically performed at the front end of the sorting system. A sorting platform is constructed above the conveyor belt, and 3-5 workers are assigned to manually sort out large impurities (such as unbroken wood and cloth), obvious recyclables (such as plastic bottles and metal cans), and hazardous waste based on the material's characteristics. Workers are required to wear hard hats, dust masks, and protective gloves, and are rotated every 1-2 hours to avoid health risks from prolonged contact with the waste. The advantage of manual sorting is that it can quickly identify and separate specific materials, compensating for the shortcomings of mechanical sorting and ensuring efficient sorting in subsequent steps.

b. Mechanical Sorting

Screening and Sorting: After manual sorting, the material enters a vibrating screen, which separates the materials based on their particle size. Typically, two to three layers of screens are used. The upper layer (10-15 cm aperture) separates larger particles (such as partially crushed plastic chunks), the middle layer (5-8 cm aperture) separates medium-sized organic matter, and the lower layer (2-3 cm aperture) separates fine particles (such as sand and humus powder). After screening, materials of different particle sizes are processed separately to prevent mixing of different components and affect resource recovery efficiency.

c. Air separation and sorting: This separates materials based on density differences. Air separation equipment includes a fan, air separation ducts, and a separation chamber. Materials are fed into the air separation duct by a conveyor belt. The airflow generated by the fan propels light materials (such as plastic film and paper) to the upper part of the separation hopper, where they are collected through the duct. Heavy materials (such as metal, glass, and stone) fall to the lower part of the hopper due to gravity, thus separating the light and heavy materials. The airflow speed must be controlled during the air separation process, generally between 15-20 m/s, to ensure effective separation of light materials while preventing heavy materials from being carried away by the airflow.

d. Magnetic separation: Magnetic metals (such as iron and nickel alloys) in waste are separated by a permanent magnet drum installed beneath the conveyor belt. As materials pass through the permanent magnet drum, the magnetic metals are attracted to the drum and fall off as the drum rotates to a non-magnetic area, where they are collected in a collection trough. Non-magnetic materials continue to move along the conveyor belt and enter the next stage. Metals after magnetic separation can be directly handed over to waste recycling companies for recycling, improving resource recovery rates.

e. Bounce separation: Magnetic separation utilizes differences in material elasticity to separate different components. Bounce separation equipment includes a vibrating bed and bouncing plates. When materials fall onto the vibrating bed, the vibrations cause more elastic materials (such as plastics and rubber) to bounce higher and be collected by a conveyor belt. Less elastic materials (such as humus and sand) bounce lower and fall into a collection trough below. This process primarily separates plastics from organic matter, further improving the purity of recyclables.

6. Post-Sorting Material Disposal and Site Remediation

After multiple stages of sorting, stale waste is divided into three categories: recyclables, degradable organic matter, and non-recyclable waste. These are then disposed of separately. Simultaneously, the original waste dump site is remediated to restore its ecological function.

a. Disposal of Various Materials

Recyclable Material Disposal: Sorted recyclables such as plastics, metals, and glass are first cleaned and dried to remove surface contaminants (such as humus and oil). They are then sorted and packaged according to material and delivered to the appropriate recycling company for processing. For example, plastics can be crushed to produce plastic pellets for use in the production of plastic products; metals can be melted to produce new metal materials; and glass can be crushed and remelted to achieve resource recycling. Disposal of degradable organic matter: This primarily includes humus and small plant residues. These materials can be recycled through composting or anaerobic fermentation. During composting, organic matter is mixed with conditioners such as straw and sawdust, maintaining a moisture content of approximately 60% and a carbon-nitrogen ratio between 25:1 and 30:1. Turning and aeration promote microbial degradation, resulting in a mature organic fertilizer after approximately 2-3 months that can be used for agricultural planting and landscaping. Anaerobic fermentation, in a sealed reactor, uses microorganisms to break down organic matter into biogas. This biogas can be used as a clean energy source for power generation and heating, and the fermented biogas residue can be used as organic fertilizer, achieving a dual "energy + resource" recovery.

Disposal of non-recyclable waste: This primarily includes non-degradable plastic fragments, inert materials (such as gravel and bricks), and hazardous waste. These materials must be disposed of in a harmless manner. Inert materials can be sent to construction waste disposal sites for landfill or used as roadbed fill. Non-degradable plastic fragments that cannot be recycled must be incinerated at a waste incineration plant. The heat generated during incineration can be used to generate electricity, and the flue gas must undergo purification treatment (such as desulfurization, denitrification, and dust removal) before discharge. Hazardous waste must be handed over to a qualified hazardous waste disposal company for harmless treatment through incineration, solidification, and landfill to prevent harmful substances from harming the environment.

b. Site Remediation

After all waste is recycled and disposed of, the original dumping site must undergo ecological restoration to restore its functional use. First, remove any remaining debris, impermeable membranes, and other materials from the site. The soil must be tested. If heavy metal or organic contamination is detected, soil washing and bioremediation techniques must be used to ensure that soil quality meets relevant standards. Second, the site must be leveled and the soil must be improved according to the site's planned use (e.g., park, green space, or farmland). For example, organic fertilizers and sandy soil must be added to improve the soil structure. Finally, vegetation restoration is carried out by planting trees, shrubs, or herbs suitable for the local climate to enhance the site's ecological benefits while preventing soil erosion.

Stale waste recycling is a systematic project, with each step closely linked. Strict adherence to environmental standards and operational specifications is essential to achieving the goals of "reduction, resource utilization, and harmlessness." With the continuous advancement of technology, stale waste recycling will move towards intelligent and efficient processes. For example, AI-powered visual sorting technology will be introduced to improve sorting accuracy, while big data will be used to optimize operational processes, further enhancing resource recovery efficiency and promoting the green and sustainable development of the waste treatment industry.