Single-stream recycling (SSR) is a collection system where consumers place all approved recyclable materials, such as paper, plastic, metal, and glass, into a single container. This method was adopted because its convenience significantly increases public participation rates in recycling programs. Eliminating the need for residents to sort materials at home simplifies the process, leading to a greater volume of material collected. The simplicity for the consumer, however, transfers the complex task of separation to a specialized processing center.
From Curb to Facility
Once collected, the combined stream is transported directly to a Material Recovery Facility (MRF). This facility is a highly mechanized plant designed to separate the mixed materials back into clean, individual commodity streams. This approach differs from dual-stream systems, where residents separate fiber materials (paper and cardboard) from containers (metal, plastic, and glass) before collection.
In dual-stream systems, the consumer performs the initial sorting, which helps maintain material purity. Single-stream relies entirely on the MRF’s advanced infrastructure to handle the complex mix. Collection trucks often use compaction mechanisms, which further mix and crush the materials during transit. This commingling at the source necessitates the sophisticated technology at the MRF.
Automated Sorting Technology
The separation process begins when mixed recyclables are unloaded onto a primary conveyor belt. The system uses mechanical and optical devices to isolate one material type at a time. Large pieces of cardboard are separated first using screens or rotating disks that allow smaller, heavier items to drop through while the flat cardboard moves over the top.
The remaining stream passes over screens with different-sized openings to separate lighter paper from heavier containers. Paper items, like newspapers, are separated from three-dimensional plastic, glass, and metal containers. Glass containers often break early in this process; these small pieces, or “fines,” drop through screens to be collected as a lower-quality aggregate.
Ferrous metals, such as steel and tin cans, are removed next by passing the stream under a powerful overhead magnet. Non-ferrous metals, primarily aluminum cans, are separated using an eddy current separator. This device uses a rapidly spinning magnetic rotor to induce a magnetic field in the aluminum, temporarily repelling it and “flinging” the items off the conveyor belt into a dedicated collection bin.
The final, most precise stage involves optical sorters, which separate different plastic polymers. These machines use Near-Infrared (NIR) light to scan the stream as it moves along a high-speed conveyor. Each plastic type, such as Polyethylene Terephthalate (PET) or High-Density Polyethylene (HDPE), reflects a unique spectral “fingerprint” based on its chemical structure. When a sensor identifies a targeted polymer, a precisely timed burst of compressed air ejects that item into the correct material chute.
Managing Contamination and Material Quality
The primary challenge is contamination, which occurs when non-recyclable items or unclean materials are mixed with accepted recyclables. Contamination takes two forms: foreign objects, such as plastic bags or food waste, and cross-contamination between recyclable types. For example, broken glass shards embedded in paper fibers make the paper unsuitable for high-quality pulp processing.
Contamination significantly reduces the market value of the resulting materials because manufacturers demand high-purity inputs. A high contamination rate means more material must be rejected and sent to a landfill, increasing the MRF’s operational cost. The presence of food residue, like grease on a pizza box or liquid in a bottle, can spoil an entire bale of otherwise clean material.
Despite the high level of automation, human sorters, often called “pickers,” are stationed along the conveyor lines to perform final quality control. Their role is to manually remove gross contaminants and large, problematic items, such as plastic film or textiles, that can clog the machinery or degrade the quality of the final bales.