1. Institute of Waste Treatment & Reclamation, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China 2. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China 3. Shanghai Engineering Research Center of Multi-source Solid Wastes Co-processing and Energy Utilization, Shanghai 200092, China 4. Shanghai SUS Environment Co., Ltd., Shanghai 201703, China
● A method based on ATR-FTIR and ML was developed to predict CHNS contents in waste.
● Feature selection methods were used to improve models’ prediction accuracy.
● The best model predicted C, H, and N contents with accuracy R 2 ≥ 0.93, 0.87, 0.97.
● Some suitable models showed insensitivity to spectral noise.
● Under moisture interference, the models still had good prediction performance.
Elemental composition is a key parameter in solid waste treatment and disposal. This study has proposed a method based on infrared spectroscopy and machine learning algorithms that can rapidly predict the elemental composition (C, H, N, S) of solid waste. Both noise and moisture spectral interference that may occur in practical application are investigated. By comparing two feature selection methods and five machine learning algorithms, the most suitable models are selected. Moreover, the impacts of noise and moisture on the models are discussed, with paper, plastic, textiles, wood, and leather as examples of recyclable waste components. The results show that the combination of the feature selection and K-nearest neighbor (KNN) approaches exhibits the best prediction performance and generalization ability. Particularly, the coefficient of determination (R2) of the validation set, cross validation and test set are higher than 0.93, 0.89, and 0.97 for predicting the C, H, and N contents, respectively. Further, KNN is less sensitive to noise. Under moisture interference, the combination of feature selection and support vector regression or partial least-squares regression shows satisfactory results. Therefore, the elemental compositions of solid waste are quickly and accurately predicted under noise and moisture disturbances using infrared spectroscopy and machine learning algorithms.
Cardboard, newspaper, kraft paper, wall paper, toilet paper and tissue (2 types), magazines (3 types), wax paper (6 types), office paper (12 different colors)
Ash, beech, cassia, rubber wood, white oak, red oak, pine, poplar, camphor
Plastic
27 types
HDPE (resin, white and translucent plastic bags, plastic bottles, landfill cover films), LDPE (resin, plastic bags, red, white and black foam, black and white express bags), PA seal, PA6 resin, PA66 resin, PC (resin, plastic bowl), PET (resin, brown and green plastic bottles), PMMA resin, PP (resin, ventilation duct, pipette tip, centrifuge tube), PS resin, PVC resin, TPU resin
Tab.1
Fig.1
Fig.2
Element
Model
Validation
Test
Validation CV
Test CV
Algorithm
Feature selection
R2
RMSE
R2
RMSE
R2
RMSE
R2
RMSE
C
RF
None
0.88
4.21
0.90
4.52
0.89
4.22
0.93
3.71
SPA
0.88
4.17
0.88
4.78
0.88
4.83
0.91
4.26
CARS
0.86
4.43
0.88
4.81
0.87
4.67
0.89
4.61
SVR
None
0.89
3.97
0.85
5.52
0.86
5.01
0.87
5.13
SPA
0.79
5.60
0.77
6.78
0.78
6.23
0.79
6.61
CARS
0.84
4.81
0.83
6.19
0.82
5.67
0.83
5.92
PLSR
None
0.80
5.25
0.72
7.59
0.74
6.61
0.79
6.46
SPA
0.61
5.46
0.63
8.67
0.61
8.08
0.63
8.68
CARS
0.77
5.60
0.78
6.74
0.73
6.66
0.79
6.63
XGBOOST
None
0.89
3.91
0.88
4.90
0.88
4.41
0.92
4.01
SPA
0.90
3.80
0.89
4.64
0.89
4.26
0.92
4.04
CARS
0.89
4.01
0.88
4.92
0.87
4.50
0.90
4.43
KNN
None
0.93
3.11
0.91
4.27
0.92
3.52
0.91
4.09
SPA
0.96
2.39
0.93
3.79
0.94
2.95
0.94
3.45
CARS
0.95
2.56
0.92
3.82
0.93
3.28
0.94
3.31
H
RF
None
0.84
0.82
0.77
1.19
0.82
0.93
0.85
0.94
SPA
0.84
0.81
0.82
1.04
0.87
0.90
0.85
0.96
CARS
0.85
0.78
0.84
0.97
0.84
0.89
0.87
0.87
SVR
None
0.94
0.49
0.83
1.01
0.84
0.90
0.84
0.99
SPA
0.88
0.71
0.82
1.05
0.81
0.94
0.84
0.98
CARS
0.93
0.53
0.84
0.99
0.86
0.86
0.86
0.93
PLSR
None
0.77
0.95
0.59
1.56
0.67
1.27
0.69
1.37
SPA
0.63
1.19
0.60
1.56
0.63
1.36
0.59
1.58
CARS
0.78
0.92
0.73
1.29
0.70
1.22
0.74
1.25
XGBOOST
None
0.88
0.70
0.76
1.20
0.85
0.86
0.84
0.98
SPA
0.89
0.67
0.84
0.98
0.87
0.80
0.85
0.95
CARS
0.88
0.71
0.82
1.04
0.87
0.80
0.80
1.10
KNN
None
0.91
0.59
0.87
0.85
0.89
0.72
0.85
0.87
SPA
0.96
0.41
0.83
0.98
0.90
0.70
0.83
0.97
CARS
0.96
0.42
0.97
0.70
0.91
0.66
0.89
0.63
N
RF
None
0.87
1.73
0.90
1.55
0.90
1.63
0.92
1.37
SPA
0.89
1.59
0.92
1.40
0.91
1.56
0.94
1.16
CARS
0.90
1.56
0.92
1.45
0.91
1.56
0.94
1.17
SVR
None
0.97
0.91
0.93
1.29
0.93
1.32
0.97
0.92
SPA
0.85
1.88
0.92
1.32
0.91
1.59
0.92
1.30
CARS
0.88
1.63
0.94
1.14
0.93
1.39
0.94
1.13
PLSR
None
0.93
1.37
0.81
2.06
0.89
1.63
0.82
2.07
SPA
0.74
2.56
0.80
2.25
0.78
2.40
0.80
2.26
CARS
0.89
1.69
0.87
1.85
0.89
1.64
0.89
1.70
XGBOOST
None
0.94
1.19
0.88
1.68
0.92
1.47
0.91
1.43
SPA
0.95
1.02
0.92
1.36
0.94
1.27
0.95
1.09
CARS
0.94
1.24
0.90
1.61
0.93
1.29
0.93
1.34
KNN
None
0.98
0.51
0.98
0.61
0.97
0.84
0.98
0.52
SPA
0.99
0.44
0.97
0.78
0.98
0.75
0.98
0.56
CARS
0.98
0.56
0.97
0.70
0.98
0.68
0.98
0.63
S
RF
None
0.89
0.24
0.79
0.30
0.84
0.28
0.79
0.30
SPA
0.86
0.27
0.76
0.32
0.82
0.29
0.78
0.31
CARS
0.89
0.23
0.73
0.34
0.84
0.28
0.76
0.32
SVR
None
0.92
0.20
0.86
0.24
0.89
0.23
0.87
0.24
SPA
0.91
0.21
0.79
0.29
0.87
0.25
0.79
0.30
CARS
0.94
0.18
0.80
0.29
0.88
0.24
0.81
0.28
PLSR
None
0.92
0.20
0.54
0.41
0.84
0.28
0.66
0.36
SPA
0.84
0.29
0.72
0.35
0.79
0.32
0.72
0.35
CARS
0.91
0.21
0.76
0.31
0.86
0.26
0.79
0.30
XGBOOST
None
0.90
0.23
0.77
0.31
0.85
0.26
0.79
0.31
SPA
0.88
0.24
0.76
0.32
0.85
0.27
0.79
0.30
CARS
0.84
0.26
0.79
0.51
0.86
0.26
0.76
0.31
KNN
None
0.95
0.15
0.83
0.26
0.89
0.23
0.84
0.25
SPA
0.92
0.20
0.78
0.30
0.87
0.25
0.81
0.29
CARS
0.93
0.18
0.77
0.31
0.89
0.23
0.78
0.30
Tab.2
Fig.3
Element
Model
Validation
Test
Validation CV
Test CV
Algorithm
Feature selection
R2
RMSE
R2
RMSE
R2
RMSE
R2
RMSE
C
RF
None
0.90
3.21
0.90
3.37
0.90
3.18
0.90
3.31
SPA
0.90
3.25
0.89
3.46
0.90
3.19
0.89
3.45
CARS
0.90
3.22
0.89
3.39
0.89
3.18
0.90
3.32
SVR
None
0.93
2.77
0.92
2.97
0.91
2.87
0.92
2.97
SPA
0.93
2.76
0.91
3.17
0.91
2.95
0.91
3.12
CARS
0.93
2.75
0.92
3.04
0.91
2.90
0.92
3.03
PLSR
None
0.93
2.75
0.91
3.17
0.90
3.08
0.90
3.35
SPA
0.91
2.98
0.88
3.57
0.89
3.35
0.89
3.53
CARS
0.91
2.44
0.93
2.78
0.91
3.14
0.87
3.71
XGBOOST
None
0.91
3.16
0.87
3.76
0.90
3.18
0.88
3.55
SPA
0.92
2.88
0.88
3.59
0.89
3.22
0.89
3.39
CARS
0.91
3.14
0.87
3.71
0.89
3.26
0.89
3.52
KNN
None
0.88
3.53
0.87
3.66
0.88
3.51
0.90
3.35
SPA
0.90
3.35
0.88
3.58
0.88
3.48
0.88
3.52
CARS
0.90
3.26
0.88
3.65
0.87
3.56
0.89
3.45
H
RF
None
0.88
0.46
0.88
0.48
0.88
0.45
0.88
0.47
SPA
0.89
0.46
0.87
0.50
0.88
0.46
0.87
0.49
CARS
0.87
0.49
0.87
0.50
0.86
0.49
0.86
0.50
SVR
None
0.87
0.48
0.87
0.48
0.89
0.44
0.87
0.48
SPA
0.91
0.59
0.87
0.43
0.91
0.39
0.91
0.42
CARS
0.85
0.50
0.86
0.50
0.88
0.45
0.87
0.50
PLSR
None
0.92
0.39
0.87
0.49
0.88
0.45
0.89
0.45
SPA
0.91
0.41
0.86
0.51
0.87
0.48
0.86
0.51
CARS
0.94
0.33
0.93
0.36
0.92
0.36
0.93
0.36
XGBOOST
None
0.91
0.41
0.86
0.51
0.89
0.45
0.86
0.51
SPA
0.90
0.44
0.86
0.51
0.88
0.46
0.86
0.51
CARS
0.89
0.45
0.83
0.56
0.86
0.49
0.86
0.52
KNN
None
0.90
0.44
0.81
0.59
0.88
0.46
0.87
0.48
SPA
0.90
0.43
0.85
0.53
0.88
0.47
0.87
0.51
CARS
0.90
0.43
0.85
0.59
0.88
0.47
0.86
0.52
N
RF
None
0.95
1.36
0.93
1.56
0.95
1.35
0.94
1.42
SPA
0.97
1.02
0.93
1.62
0.96
1.13
0.92
1.52
CARS
0.96
1.26
0.94
1.49
0.96
1.25
0.94
1.42
SVR
None
0.97
0.95
0.97
0.98
0.98
0.90
0.98
0.91
SPA
0.99
0.76
0.99
0.78
0.98
0.77
0.98
0.78
CARS
0.98
0.84
0.98
0.81
0.98
0.84
0.98
0.80
PLSR
None
0.95
1.10
0.95
1.40
0.95
1.38
0.95
1.33
SPA
0.91
1.93
0.91
1.85
0.91
1.78
0.91
1.84
CARS
0.96
1.15
0.96
1.25
0.96
1.17
0.96
1.25
XGBOOST
None
0.96
1.26
0.92
1.67
0.95
1.28
0.90
1.82
SPA
0.97
1.00
0.94
1.38
0.97
1.04
0.93
1.64
CARS
0.96
1.22
0.95
1.41
0.96
1.23
0.95
1.30
KNN
None
0.98
0.77
0.98
0.93
0.98
0.85
0.97
0.99
SPA
0.99
0.71
0.98
0.84
0.98
0.77
0.98
0.85
CARS
0.98
0.85
0.98
0.95
0.98
0.84
0.98
0.82
S
RF
None
0.64
0.32
0.59
0.29
0.56
0.33
0.69
0.27
SPA
0.48
0.39
0.50
0.33
0.47
0.37
0.60
0.31
CARS
0.63
0.33
0.60
0.30
0.61
0.32
0.70
0.28
SVR
None
0.71
0.29
0.62
0.29
0.76
0.24
0.86
0.19
SPA
0.93
0.14
0.91
0.14
0.92
0.13
0.93
0.13
CARS
0.89
0.17
0.87
0.17
0.89
0.16
0.94
0.12
PLSR
None
0.87
0.19
0.89
0.16
0.86
0.18
0.92
0.14
SPA
0.69
0.30
0.80
0.22
0.76
0.26
0.69
0.28
CARS
0.89
0.17
0.90
0.16
0.88
0.17
0.90
0.16
XGBOOST
None
0.61
0.33
0.56
0.31
0.53
0.35
0.54
0.33
SPA
0.65
0.32
0.64
0.27
0.55
0.35
0.49
0.36
CARS
0.72
0.29
0.72
0.25
0.63
0.32
0.74
0.26
KNN
None
0.64
0.26
0.47
0.34
0.55
0.35
0.59
0.27
SPA
0.59
0.33
0.54
0.29
0.64
0.30
0.45
0.34
CARS
0.73
0.23
0.59
0.29
0.70
0.26
0.45
0.34
Tab.3
Fig.4
ANN
Artificial neural network
CARS
Competitive adaptive reweighted sampling
DT
Decision tree
FTIR
Fourier transform infrared spectroscopy
HDPE
High density polyethylene
KNN
K-nearest neighbor
LDPE
Low density polyethylene
RMSE
Root mean square error
PA
Polyamide
PC
Polycarbonate
PET
Polyethylene terephthalate
PLSR
Partial least squares regression
PMMA
Polymethyl methacrylate
PP
Polypropylene
PS
Polystyrene
PVC
Polyvinyl chloride
R2
R-Square
RF
Random forest
SPA
Successive projections algorithm
SVR
Support vector regression
TPU
Thermoplastic polyurethane
XGBOOST
Extreme gradient boosting tree
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