Sugarcane press mud (SPM) has a high potential to produce renewable energy through anaerobic digestion (AD); however, hydrolysis is the limiting stage of the process due to the presence of slowly biodegradable compounds. An alternative that can improve this deficiency is anaerobic codigestion (AcoD). In this investigation, the monodigestion of SPM and its AcoD with food waste (FW) were evaluated through the biochemical methane potential (BMP) test, and kinetic parameters were analyzed through the analysis of the kinetic models of first order and modified Gompertz. This study showed that the AcoD of SPM with FW improved the hydrolysis stage, increased methane (CH4) yield, improved the stability of the process, and presented synergistic effects. As regards the hydrolysis stage, the hydrolysis constant was increased, and the lag phase was reduced. The monodigestion of SPM (SPM : FW 100 : 0) showed an increase of 9% with the addition of external nutrients solution, while that of AcoD in the SPM : FW 80 : 20 ratio showed the highest CH4 yield, with increments of 12 and 22% in comparison with the monodigestion of SPM under WN and NN conditions, respectively. It is even possible to add up to 40% of FW (SPM : FW 60 : 40) and achieve an increase of 5% compared to the monodigestion of SPM under the NN condition. The synergistic effects obtained in this study showed that the incorporation of FW, in the substrates ratios evaluated, would improve the AD of the SPM without addition of external nutrients solution, which represents economic and environmental benefits of implementing this alternative at full scale.
According to FAO [
The general composition of the SPM demonstrates its potential for the production of renewable energy through anaerobic digestion (AD) [
In the case of SPM, residues with a high content of organic matter of rapid degradation such as food waste (FW) can be used [
Sugarcane press mud (SPM) was provided by a sugar mill located in Cauca state (Colombia). The food waste (FW) was formed from the wastes of the restaurant of the Universidad del Valle (Cali-Colombia), considering the physical composition and the physicochemical characteristics of the unprocessed FW generated in a city that carries out source separation and selective collection [
The inoculum used was sludge from the anaerobic digester of a municipal wastewater treatment plant (WWTP) (Cali-Colombia), which operates with complete mixing at 35°C [
Both substrates were characterized according to the following parameters: moisture (%), pH (units), total alkalinity (TA) and bicarbonate alkalinity (BA) (g CaCO3 L−1), volatile fatty acids (VFAs) (g HAc L−1), chemical oxygen demand (COD): total and filtered (g O2 L−1), total solids (TS) (g L−1), volatile solids (VS) (g L−1), total organic carbon (TOC) (g L−1), total nitrogen (TN) (g L−1), total phosphorus (TP) (g L−1), lignin (%), raw fibre (%), and cellulose (%) [
The reactor of the OxiTop® system (WTW, Giessen, Germany) was used, with a working volume of 200 mL and headspace of 50 mL for the storage of CH4 generated, which was measured directly by capturing CO2 with the addition of 4 NaOH pellets to each reactor, considering previous studies [
The BMP tests were performed at the mesophilic temperature, guaranteeing 35 ± 0.1°C in the WTW TS 606-G/2-i incubator (WTW, Giessen, Germany); the pH was adjusted to 7 units with sodium bicarbonate solution (NaHCO3) (4%); and the agitation was manual and intermittent and was performed 3 times per day before measuring the pressure. The incubation time was 30 days from which it was observed that the CH4 yield stabilized because the pressure did not vary more than 5 HPa [
In BMP tests, a control (inoculum and distilled water) was included in order to determine CH4 generated by the residual organic matter present in the inoculum and by the endogenous metabolism, whose value was subtracted from the CH4 yield of each reactor. The volume of CH4 at standard conditions was determined by the equations suggested by Cárdenas-Cleves et al. [
The AcoD of SPM with FW without addition of nutrients (NN) was evaluated in three SPM : FW ratios (80 : 20, 60 : 40, and 50 : 50), and the monodigestion of SPM (with—WN and without addition of nutrients solution—NN) and FW (NN) was evaluated as controls, as shown in Table
SPM : FW ratios evaluated in each reactor.
Reactor | SPM | FW | I | ||
---|---|---|---|---|---|
(%) | (g VS L−1) | (%) | (g VS L−1) | (g VS L−1) | |
1 | 100 | 1.50 | 0 | 0 | 1.50 |
2 | 80 | 1.20 | 20 | 0.30 | 1.50 |
3 | 60 | 0.90 | 40 | 0.60 | 1.50 |
4 | 50 | 0.75 | 50 | 0.75 | 1.50 |
5 | 0 | 0 | 100 | 1.50 | 1.50 |
To evaluate the effect on the hydrolysis stage, the first-order kinetic model and modified Gompertz model were used, as shown in the following equations [
For the estimation of the values in the first-order kinetic model equations (BMPmax and
To assess the influence of the substrate ratio on the response variable (BMP), analysis of variance (ANOVA) and Tukey’s tests (
When the difference (BMP − BMPW) was positive and higher than the BMP value considering the standard deviation, a synergistic effect (S) occurs; otherwise, the effect is antagonistic (A) [
Table
Physicochemical characterization of substrates and inoculum.
Parameter | Unit | Sugarcane press mud (SPM) |
Food waste (FW) |
Inoculum |
---|---|---|---|---|
Moisture ( |
% | 70.30 ± 1.80 | 78.53 ± 6.18 | ND |
pH ( |
Unit | 5.37 | 5.17 | 7.16 |
TA ( |
g CaCO3 L−1 | 3.88 ± 0.93 | 4.21 ± 3.44 | 4.22 ± 0.83 |
BA ( |
g CaCO3 L−1 | — | — | 2.42 ± 0.30 |
VFAs ( |
g HAc L−1 | 6.45 ± 0.11 | 3.65 ± 0.23 | 1.54 ± 0.04 |
CODTotal ( |
g O2 L−1 | 152.33 ± 0.94 | 80.41 ± 8.84 | ND |
CODFiltered ( |
g O2 L−1 | 36.40 ± 1.41 | 22.03 ± 2.01 | ND |
TS ( |
g L−1 | 132.05 ± 0.42 | 88.31 ± 6.45 | 56.98 ± 0.53 |
VS ( |
g L−1 | 84.42 ± 0.53 | 81.06 ± 6.95 | 27.55 ± 0.21 |
COTa ( |
g L−1 | 1083.86 | 481.71 ± 77.08 | ND |
TNa ( |
g L−1 | 47.79 | 15.25 ± 0.18 | ND |
TP ( |
g L−1 | 14.26 | 0.14 ± 0.02 | ND |
Lignin ( |
% | 5.50 | 2.97 | ND |
Raw fibrea ( |
% | 24.57 | 10.64 ± 0.78 | ND |
Cellulosea ( |
% | 16.01 | 16.12 ± 10.30 | ND |
SMA ( |
|
ND | ND | 0.008 |
Table
The CODTotal, TS, and VS showed the presence of a high content of organic matter found mainly in particulate form (CODFiltered/CODTotal of 0.24) as reported by López-González et al. [
As regards the FW, they also had a high moisture content due to the predominance of vegetables and fruits [
The values obtained for the CODTotal, TS, and VS show that the FW is characterized by having a high content of organic matter that is in particulate form (CODFiltered/CODTotal : 0.30). Regarding nutrient content, the CODTotal : TN : TP ratio (350 : 5.43 : 0.59) showed phosphorus deficiency, which is a predominant element in the SPM, whereas the
For the inoculum, the values of pH, TA, BA, VFAs, TS, and VS were found among the characteristic ranges for sludge from anaerobic digesters of municipal WWTP [
Figure
BMP of each SPM : FW ratio: (a) 100 : 0; (b) 80 : 20; (c) 60 : 40; (d) 50 : 50; (e) 0 : 100, and best fit to the first-order kinetic model (blue line) and the modified Gompertz model (red line). The error bars represent the experimental standard deviation between triplicates (
Kinetic parameters for each SPM : FW ratio.
SPM : FW ratio | First-order kinetics model | Modified Gompertz model | |||||||
---|---|---|---|---|---|---|---|---|---|
BMPmax |
|
|
MSE | BMPmax |
|
|
|
MSE | |
100 : 0 | 154.10 ± 33.50 | 0.022 ± 0.006 | 0.910 | 62.01 | 68.40 ± 0.49 | 6.00 ± 0.13 | 5.52 ± 0.12 | 0.995 | 3.56 |
80 : 20 | 128.60 ± 14.31 | 0.041 ± 0.007 | 0.897 | 108.24 | 81.98 ± 0.38 | 9.17 ± 0.17 | 5.28 ± 0.08 | 0.997 | 3.32 |
60 : 40 | 88.49 ± 4.28 | 0.064 ± 0.006 | 0.936 | 41.82 | 69.79 ± 0.44 | 6.51 ± 0.15 | 3.06 ± 0.13 | 0.993 | 4.25 |
50 : 50 | 82.96 ± 5.38 | 0.059 ± 0.007 | 0.910 | 52.43 | 62.40 ± 0.32 | 7.26 ± 0.16 | 4.31 ± 0.10 | 0.995 | 2.63 |
0 : 100 | 70.89 ± 4.94 | 0.057 ± 0.007 | 0.901 | 39.66 | 54.46 ± 0.80 | 4.93 ± 0.25 | 3.62 ± 0.28 | 0.969 | 12.23 |
BMPmax (mL CH4 g VS−1);
Figure
In Table
Parameters measured after the process.
SPM : FW ratio | Nutrient condition | pH | AB | AT |
|
---|---|---|---|---|---|
100 : 0 | WN | 7.62 | 334.56 | 483.39 | 0.69 |
100 : 0 | NN | 7.99 | 466.17 | 634.68 | 0.73 |
80 : 20 | NN | 7.77 | 408.36 | 515.37 | 0.79 |
60 : 40 | NN | 7.96 | 349.32 | 492.00 | 0.71 |
50 : 50 | NN | 8.03 | 397.29 | 537.51 | 0.74 |
0 : 100 | NN | 7.49 | 129.15 | 354.24 | 0.36 |
pH (unit); BA and TA (mg CaCO3 L−1); number of assays: 3.
As regards the methane yield, Figure
BMP for each SPM : FW ratio.
Figure
Figure
The stability of the process was determined by means of different control parameters shown in Table
Table
The existence of synergistic effects (S) was verified for each SPM : FW ratio, and the results are presented in Table
Synergistic effects of AcoD.
SPM : FW ratio |
|
BMP | SD | BMPW | Difference (BMP − BMPW) | Effect |
---|---|---|---|---|---|---|
100 : 0 | 22.68 | 67.67 | 0.00 | 67.67 | 0.00 | — |
80 : 20 | 24.46 | 82.36 | 2.34 | 64.85 | 17.51 | S |
60 : 40 | 26.24 | 71.12 | 5.75 | 62.04 | 9.09 | S |
50 : 50 | 27.13 | 64.29 | 15.79 | 60.63 | 3.66 | S |
0 : 100 | 31.58 | 53.59 | 0.40 | 53.59 | 0.00 | — |
BMPW: weighted biochemical methane potential (mL CH4 g VS−1); SD: standard deviation; S: synergistic effect.
Table
Although the sugarcane press mud (SPM) has a high potential for use by anaerobic digestion (AD), the presence of lignocellulosic compounds affects the hydrolysis stage of the AD process. A strategy to improve this aspect is the anaerobic codigestion (AcoD) with food waste (FW), which provides organic matter and allows a balance in the
This study showed that the AcoD of SPM with FW influenced the hydrolysis stage and CH4 yield stability of the process and presented synergetic effects. The AcoD increased the hydrolysis constant and reduced the lag phase. The monodigestion of SPM (SPM : FW 100 : 0) showed an increase of 9% with the addition of external nutrients solution, while with that AcoD in the SPM : FW 80 : 20 ratio, the highest CH4 yield was obtained, with increments of 12 and 22% under WN and NN conditions, respectively, in comparison with the monodigestion of SPM. It is even possible to add up to 40% of FW (SPM : FW 60 : 40) and achieve an increase of 5% compared to the monodigestion of SPM under the NN condition. The stability of the process and the synergistic effects obtained in this study showed that the addition of FW in the substrates ratios evaluated would improve the AD of the SPM without addition of external nutrients solution, which represents economic and environmental benefits of implementing this alternative at full scale.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors would like to thank the Universidad del Valle for the financial support of the project “Potencial de valorización energética y agrícola de la Fracción Orgánica de Residuos Sólidos Urbanos–FORSU” (C.I-2962).