Domestic waste screening is a core pretreatment step before final disposal. Using screening equipment with varying particle sizes, densities, or properties, mixed waste is separated into recyclables (such as plastics and metals), biodegradable organic matter (such as kitchen waste), and inert impurities (such as stones and bricks and tiles). This improves the quality and efficiency of subsequent disposal processes such as incineration, composting, and landfilling. Due to the complex composition of domestic waste (including kitchen waste, plastics, glass, and metals), high moisture content, and the tendency to contain large impurities, the screening process requires strict control over equipment selection, operating procedures, safety precautions, and environmental protection requirements. The following details the key considerations from six key perspectives.

Pre-screening Pretreatment: Reduce Impurity Interference and Ensure Equipment Stability

Screening efficiency is directly related to the quality of waste pre-treatment. Failure to remove large, hard, or entangled impurities can easily lead to screen blockage, equipment jamming, or even damage. Therefore, the following pre-screening steps are necessary:

1. Manual Sorting of Large Impurities

Before the waste enters the screening equipment, a manual sorting platform should be set up and staffed with 3-5 professional sorters to perform preliminary screening of the waste conveyed on the conveyor belt. They should prioritize sorting out large objects larger than 30 cm, such as discarded furniture fragments, construction debris (bricks, cement blocks), worn-out clothing, and tangled plastic film. If these objects enter the screening system, they may entangle the screen shaft, clog the screen holes, or collide with equipment components, causing malfunctions. Sorters should wear non-slip gloves, dust masks, and goggles. For large impurities that are difficult to remove manually (such as compacted plastic clumps), scissors or hydraulic shears can be used to break them up, ensuring that the remaining material meets the feed requirements of the screening equipment (typically no larger than 20 cm).

2. Moisture Content and Viscosity Control

When the moisture content of household waste (especially batches with a high proportion of kitchen waste) is too high (over 60%), sticky clumps easily form, clinging to the screen surface and causing "grinding," reducing screening efficiency. If the moisture content of the waste exceeds the standard, pretreatment is necessary. This can be achieved by air-drying (suitable for small-scale screening operations) or mechanical dehydration (such as a screw press) to reduce the moisture content to 40%-50%. Dry materials (such as sawdust or chopped straw) can also be added to the wet waste at a ratio of 1:5 to reduce stickiness. During pretreatment, it is important to avoid excessive dehydration, which increases dust, or excessive addition of dry materials, which increases the subsequent disposal load.

3. Inspection for Flammable and Explosive Materials

Household waste may contain flammable and explosive items such as lighters, unextinguished cigarette butts, and alcohol bottles. Sparks generated by friction or impact during screening can easily cause fires or explosions. Therefore, metal detectors and flammable material detectors are necessary during the pre-processing phase. Metal detectors can identify metal components (such as nails and cans) in the garbage, preventing them from entering the equipment and causing mechanical damage. Flammable material detectors use gas sensing technology to detect the presence of volatile flammable substances such as alcohol and gasoline in the garbage. If excessive levels are detected, operations must be stopped immediately, and professionals must collect the suspected materials separately and dispose of them properly (e.g., by allowing them to evaporate in a cool, dry place) to eliminate safety hazards.

Screening Equipment Selection: Matching Waste Characteristics to Ensure Sorting Accuracy

Different types of screening equipment are suitable for household waste of varying composition and particle size. Improper selection can result in low screening efficiency and high energy consumption. Therefore, a sound selection should be made based on the characteristics of the waste:

1. Selecting Equipment Based on Waste Composition

Vibrating Screens: Suitable for separating organic matter from inert impurities in mixed waste, such as separating kitchen waste (smaller, softer particles) from gravel and bricks (larger, harder particles). When selecting a screen, consider the mesh size: if the primary purpose is to separate food waste, a mesh with a 5-8 mm mesh size is recommended; if the purpose is to separate recyclable plastics (typically 10-15 mm in particle size), a mesh with a 12-15 mm mesh size is recommended. Furthermore, the vibration frequency should be tailored to the material characteristics. When the proportion of food waste is high, the frequency should be controlled at 1200-1500 times/minute to prevent excessive material bouncing and incomplete separation. When the proportion of inert impurities is high, the frequency can be increased to 1800-2000 times/minute to enhance material flow.

Drum screens: Suitable for waste containing a large amount of entangled materials (such as plastic film and fibers). Its cylindrical screen drum rotates slowly (at 8-12 rpm), preventing material entanglement through a "tumbling and screening" process. When selecting a screen, consider the ratio of screen drum length to diameter (recommended 1:3) to ensure sufficient retention time (30-60 seconds) within the drum. The mesh arrangement should be small inside and large outside, with the inner screen separating fine impurities (such as sand) and the outer screen separating medium-sized particles (such as food waste) to improve sorting accuracy.

Wind separation-screening combination equipment: Suitable for separating lightweight plastics from heavy materials (such as metal and glass). First, the lightweight plastics are blown into a designated area through wind separation (air velocity of 15-20 m/s), followed by screening to separate the different particle size components within the heavy material. With this type of equipment, careful coordination between the wind separation airflow and the screening rhythm is crucial to prevent lightweight materials from entering the screen before they are fully separated, thus affecting sorting performance.

2. Equipment Material and Corrosion Resistance

Food waste leachate and decaying organic matter contained in household waste can generate acidic substances that can corrode equipment components. Therefore, key components of screening equipment must be made of corrosion-resistant materials. 304 stainless steel (corrosion resistance and high strength) is preferred for screens, and ordinary wire mesh (rust-prone and short-life) should be avoided. Equipment frames and conveyor belt supports should be treated with anti-corrosion treatment (such as epoxy coating). Bearings and transmission components should be sealed to prevent leachate from seeping in and causing lubrication failure. Furthermore, the equipment should be equipped with drainage holes to promptly remove leachate generated during the screening process, preventing accumulation and corrosion on the equipment base.

Screening Operating Specifications: Control Operating Parameters to Improve Sorting Efficiency

Screening operations must strictly adhere to operating specifications and control operating parameters to avoid reduced sorting accuracy or equipment failure due to improper operation:

1. Feed Rate and Speed Control

Screening equipment has a clear upper limit to its processing capacity (for example, vibrating screens are typically 50-100 tons/hour). Excessive feed rates and high speeds can cause material to accumulate on the screen, preventing adequate separation, and even overload the equipment motor. During operation, the feed speed must be controlled by a variable-frequency conveyor belt to ensure that the material is evenly distributed on the screen, with a thickness of 10-15 cm (no more than 1/3 of the screen height). Furthermore, a dedicated person should monitor the feed status in real time. If material accumulation on the screen exceeds 20 cm, the feed speed must be immediately reduced or suspended, and operations must be resumed after the material is screened.

2. Screen Cleaning and Blockage Treatment

During the screening process, even after pretreatment, fine material (such as kitchen waste fragments or sand) may still clog the screen mesh. Regular cleaning of the screen is necessary. For vibrating screens, a high-pressure spray device can be installed on the side of the equipment, spraying every 30 minutes (with a water pressure controlled at 0.3-0.5 MPa) to flush material adhering to the screen surface. For drum screens, a brush or scraper can be installed inside the screen drum to clean the mesh as it rotates. If the screen is severely clogged (e.g., sticky clumps cannot be removed by spraying), the machine must be shut down and manually cleaned with a wire brush. Do not touch the screen directly while the equipment is running to avoid safety accidents.

3. Grading Screening and Material Diversion

If multi-stage separation of domestic waste is required (e.g., four-stage separation: fine impurities - food waste - recyclables - inert impurities), a multi-stage screening process is required, and each stage of equipment must be arranged appropriately: the front-stage screening equipment (e.g., drum screen) separates large inert impurities, the intermediate stage (e.g., vibrating screen) separates food waste and recyclables, and the rear stage (e.g., high-frequency vibrating screen) separates fine sand and soil. Furthermore, deflectors and buffer bins should be installed between each stage of equipment. Deflectors should be tilted 30-45° to prevent falling material from impacting equipment and generating dust. The buffer bin capacity should match the processing capacity of the upstream and downstream equipment to prevent the upstream equipment from overloading the downstream equipment due to excessive feeding, or the downstream equipment from processing too slowly, causing material accumulation in the upstream equipment.

Safety Measures: Ensure Personnel Safety and Prevent Accident Risks

The screening process involves mechanical operation and material handling, and the waste may contain hazardous and sharp materials. Comprehensive safety precautions are required:

1. Personal Protection for Operators

Operators must wear full protective gear as required: a hard hat (to prevent injury from falling materials), a dust mask (to filter dust and odors, with a protection level of at least N95), impact-resistant goggles (to prevent eye contact with small particles), oil-resistant, non-slip nitrile gloves (to avoid contact with sharp objects or corrosive materials), and safety shoes (to prevent injuries from heavy objects or slips). When screening batches containing hazardous waste (such as expired medicines and batteries), operators must also wear a respirator and protective clothing. These should be cleaned and disinfected promptly after operation to avoid contact with skin.

2. Equipment Safety Devices

Screening equipment must be equipped with comprehensive safety devices: All rotating components (such as the drive gears of drum screens and the eccentric shafts of vibrating screens) must be fitted with protective covers (made of metal mesh with a mesh size no larger than 5 mm) to prevent accidental contact and injuries. The equipment's emergency stop button must be prominently located on the operating platform (at a height of 1.2-1.5 meters) and must be waterproof and dustproof. In the event of an emergency, such as a jam or unusual noise, the machine can be immediately stopped by pressing the button. Guardrails (at least 1.1 meters high) must be installed on both sides of the conveyor belt to prevent material leakage or falls. Furthermore, a safety inspection must be conducted before operation to confirm that the protective devices are intact and that the grounding is reliable (ground resistance no greater than 4Ω).

4. Fire and Hazardous Gas Prevention and Control

Screening workshops must be equipped with fire-fighting facilities: two 4-kilogram dry powder fire extinguishers per 50 square meters, and automatic fire-extinguishing devices (such as sprinkler systems) installed in key locations (such as near motors and in flammable material storage areas). Smoking is prohibited in the workshop, and bringing ignition sources into the workshop is strictly prohibited. If odors or hazardous gases (such as hydrogen sulfide released from rotting garbage) are generated during the screening process, a negative pressure ventilation system must be installed to collect the gases and treat them with activated carbon adsorption equipment to ensure that the concentration of hazardous gases in the workshop meets the "Occupational Exposure Limits for Hazardous Factors in the Workplace."

Equipment Maintenance: Extending Service Life and Reducing Downtime

Screening equipment operates under high loads and high levels of impurities for extended periods. Regular maintenance can reduce the risk of failure and extend equipment life:

1. Daily Inspection and Lubrication

Before daily operation, perform a comprehensive inspection of the equipment: Check the screen for damage or looseness (if damaged, replace it promptly to prevent material mix-up). Check the lubrication of transmission components (such as bearings and chains). If the lubricant is insufficient or deteriorating, add or replace specialized lubricant (such as lithium-based grease for bearings and gear oil for chains). Check the motor current and voltage for normal operation (current fluctuations should not exceed ±5% of the rated value). If any abnormalities are detected, investigate for material jamming or motor failure. After daily operation, clean debris from and around the equipment to prevent accumulation of impurities that could cause corrosion or blockage.

2. Regular Maintenance and Component Replacement

Perform a monthly in-depth maintenance of the equipment: Remove and clean the screen to check whether the mesh diameter has increased due to wear (if the mesh diameter increases by more than 2 mm, the screen must be replaced). Inspect the support and rollers of the drum screen. If worn or misaligned, adjust their position or replace them. Inspect the spring assembly of the vibrating screen. If the spring has decreased in elasticity or is broken, replace it promptly to prevent abnormal vibration frequency. Quarterly, inspect core components such as the motor and reducer: Measure the motor's insulation resistance (minimum 0.5 MΩ). Check the reducer's oil level and quality. If the oil is turbid, replace it completely to ensure equipment transmission efficiency.

3. Emergency Troubleshooting

If an equipment malfunction occurs during screening, follow the "Stop - Troubleshoot - Fix - Acceptance" process. If the vibrating screen makes unusual noises, immediately stop the machine and inspect for material jamming, bearing damage, or spring breakage. If the bearing is damaged, replace it with a similar model and relubricate it. If the drum screen fails to rotate, check for motor overload or a broken drive chain. If the chain is broken, replace it and adjust the tension (keep chain sag within 20-30 mm). After troubleshooting, conduct a no-load test run for at least 10 minutes to confirm that the equipment is operating normally before resuming feeding.

Environmental Control Requirements: Reduce Secondary Pollution and Meet Emission Standards

The screening process is prone to generating dust, leachate, and odor. Improper handling can cause secondary pollution, requiring strict implementation of environmental protection measures:

1. Dust Collection and Treatment

Screening equipment (especially vibrating screens and air separation equipment) generates significant amounts of dust during operation. A dust collection system is required: Sealed covers are installed at the equipment's inlet and outlet, directing dust through ducts into a bag filter (with a filtration efficiency of at least 99%). The filter bags in the dust collector must be cleaned regularly (every two weeks) to prevent clogging and reduced dust removal efficiency. A dust concentration meter should be installed in the workshop to monitor dust concentration in real time (particulate matter concentration should not exceed 10mg/m³). If concentration exceeds the standard, the dust collection equipment's operating power must be increased or operations must be suspended. Dust leaks (such as damaged seals) must be identified and repaired.

2. Leachate Collection and Disposal

Leachate generated during the screening process (primarily from food waste) must be collected through a dedicated system. An impermeable diversion ditch (concrete with an HDPE impermeable membrane) should be installed beneath the screening equipment. The diversion ditch, with a slope of 1‰, guides the leachate to a collection tank. The collection tank should be equipped with a level gauge and a submersible pump to transport the leachate to the municipal waste treatment plant's leachate treatment system. This system utilizes a "pre-treatment + biochemical treatment + advanced treatment" process to ensure that the effluent meets the requirements of the "Pollution Control Standard for Municipal Waste Landfills" (GB 16889-2008). Direct discharge is prohibited. The collection tank should be cleaned weekly to prevent sediment from clogging the pipes.

3. Odor Control

Odors in screening workshops primarily originate from rotting food waste. This requires a "source control + end-of-pipe treatment" approach: For source control, accelerate the screening process and reduce waste retention time within the workshop (to no more than two hours). For end-of-pipe treatment, install a spray deodorization system on the workshop ceiling to spray biological deodorizers (such as microbial agents) or install an activated carbon adsorption tower to remove odors through adsorption. Furthermore, the workshop must maintain good ventilation and undergo thorough disinfection (using a sodium hypochlorite solution at a concentration of 500mg/L) after daily operations to reduce odor lingering and bacterial growth.

Sieving domestic waste is a critical link between waste collection and final disposal. Its operational compliance directly impacts subsequent resource recovery efficiency and environmental compliance. Only by strictly controlling six key aspects—pretreatment, equipment selection, operational specifications, safety precautions, maintenance, and environmental control—can the screening process achieve its goals of "efficiency, safety, and environmental protection," laying a solid foundation for the "reduction, resource utilization, and harmless" disposal of domestic waste.