The increase in global population and improvement of
living standards
have stirred up a continuous increase in solid waste generation, while
simple incineration and landfilling bring about serious environmental
and health concerns. In order to improve resource recovery and mitigate
pollution, noncontacting and nondestructive sensor-based waste sorting
systems are applied to enhance solid waste classification. In recent
years, in addition to the rapid development of computer hardware,
especially improvements of GPU computing capacity, complicated and
efficient classification algorithms have emerged and been widely used
in industrial sectors. These advances allow computers to process signals
from sensors more quickly and accurately and to classify matters automatically.
This article introduces widely applied sensor-based technologies in
solid waste sorting and analyzes applicable conditions for each specific
method. The latest developed algorithms are critically compared with
competitive counterparts. Successful practices are described, and
findings are highlighted. Though spectroscopic-based and vision-based
waste classifications have achieved high performance in accuracy and
detection speed, challenges and future directions can still provide
wide development opportunities. Concretely, these opportunities generally
comprise classification of indistinct plastics, application of the
latest object detection algorithms, appropriate data set formulating,
and sensor combination for multiple sorting tasks within a single
system.