3.1. Biogas assay performance
Cumulative production was normalized in two ways: (1) by specific biogas yield using the initial VS of co-/substrate (ml/g VS) and (2) by volumetric biogas production using the volumetric loading of FW and PS (ml/ml added) [31]. The overall VS concentration used in this experiment ranged from 4.84–5.95%. The VS showed, on average, 84% of TS content for co-digestions and 78% for mono-digestion, which indicates a high capacity for organic transformation. In the absence of VS reduction, mass transfer from organic substance to biogas, which would affect biogas production and result in notably lower yields, cannot be carried out. Furthermore, co-digestion of FW and PS improves production compared to the sole substrate and reduces limitations of single feedstock, such as high salt concentration of FW and low VS content of PS [32]. Further on, seed sludge was analyzed for biogas potential before running the anaerobic batch experiments. The outcomes demonstrated the presence of active microorganisms in the inoculum and the ability to degrade organics in the digestion process. Digestion occurred in a steady mode for all ratios, which was partially affected by the characteristics of the inoculum already adapted to the residue type.
The average headways of specific biogas production of PS mono-digestion and volume-based co-digestion mixtures are illustrated in Fig. 2a. Batch tests indicated that adding FW increased the specific biogas yield compared to PS only, which showed the lowest average cumulative biogas production. The better digestibility of FW itself and significantly faster hydrolysis rate most likely contributed to the mixtures' increased output. After 15 days of digestion, 3/1 trial (446.9 ml/g VS) displayed an average specific biogas yield 10% higher than PS only (407.0 ml/g VS). The average specific biogas production of 1/1 co-digestion ratio was 552.6 ml/ g VS. As shown in the figure, the optimal PS/FW mixture ratio was 1/3, with an average biogas output of 618.7 ml/g VS. In comparison to mono-digestion and 3/1 co-digestion, respectively, this implies an increase of 52% and 38%. Regarding the 1/1 PS/FW ratio, the specific biogas yield of co-dig 1/3 displayed an average growth of 12%. The highest enhancement of biogas production was observed between 3/1 and 1/1 co-digs, where 3/1 accounted for around 81% of 1/1 co-digestion, and is in accordance with VS content of the mixtures. Meanwhile, in the volumetric analysis, 1/3 (196.5 ml/ml substrate) and 1/1 co-digs (129.3 ml/ml substrate) were notably higher, approximately 52% and 92% higher than amounts of biogas produced by 1/1 and 3/1 co-digs (67.2 ml/ml substrate). The last one had an increase of around 157% compared to mono-dig (Fig. 2b). Therefore, 1 g of VS in terms of co-digestion increases biogas production from 110% up to 152% compared to PS only, and 1 ml of substrate increases biogas production within a range of 2.6–7.5 times compared to mono-dig, depending on the mixing ratio.
The increased proportion of FW in the mixture generally results in higher biogas generation. As described by [33], the biodegradability characteristics of substrates and the production of intermediate inhibitory substances will control the kinetics of the different steps of anaerobic digestion and define the biogas production curve shape. The production profiles frequently followed the exponential law. So, the production of biogas from anaerobic biomass may have grown as a result of a more suitable anaerobic environment. The values are consistent with the literature data (Table 3).
Three mixing ratios of sewage sludge and food waste were evaluated through 16 studies using normal distribution and p-value. The researchers reported the highest cumulative production under a food waste mixing ratio of 75%. In comparison with this study (619 ml/g VS) [34] reported a somewhat lower value (450 ml/g VS removed) when domestic water was used as a substrate, while [35] recorded 363 ml/g VS with yard waste as an additional co-substrate. According to literature, higher yields than 0.7 liters of biogas per gram of volatile solid (added) were achieved in three studies where anaerobic membrane bioreactors (AnMBR) were applied, and over 800 ml with thermophilic anaerobic membrane bioreactor implemented [36]. Meanwhile, the highest value recorded (854 ml/g VS) was from the experiment with waste-activated sludge as a substrate [37]. Furthermore, for a 1/1 ratio (co-dig 2), seven results from six studies are in 95% confidence interval (477–630 ml/g VS), to which the obtained score from this study also belongs (553 ml/g VS). The usage of biogas residue biochar showed a somewhat higher value of 675 ml/g VS [38]. On the other hand, the same study showed a minor yield of 461 ml when no biochar was used. Additionally, bell curve of specific productions had the lowest standard deviation of 161 compared with co-digs 1 and 3, 178 and 205, respectively. As far as the 3/1 ratio is concerned (447 ml/g VS), [39] research reported a lower specific production of 246 ml/g VS, which might be explained by SARS-CoV 2 presence in the feed. In studies [40] and [41] whose results are in opposite areas of very unlikely observations (with productions of 175 and 744 ml/g VS, respectively), no additions were made. The types of FW present in the combination of residues may be the cause of differences.
Table 3
Specific biogas yield comparison
Co-/substrates | Co-dig | Ratio | Yield | CH4 content | Additions | HRT (d) | Ref. |
Sewage sludge (SS) + FW | Batch | Volume (V) 3/1 | 350 ml CH4/g VS removed | / | | 30 | [42] |
V 1/1 | 450 ml CH4/g VS removed |
V 1/3 | 500 ml CH4/g VS removed |
SS + FW | Semi-continuous | Weight (W) 1/1 | 0.66 l/g VS added | 55% | | 22 | [43] |
Domestic water + FW | Batch | W/V 3/1 | 375 ml/g VS removed | / | | 25 | [34] |
W/V 1/1 | 437 ml/g VS removed |
W/V 1/3 | 450 ml/g VS removed |
Waste-activated sludge (WAS) + FW | Batch | V 3/1 | 411 ml CH4/g VS added | 65% | | 10 | [37] |
V 1/1 | 475 ml CH4/g VS added |
V 1/3 | 555 ml CH4/g VS added |
SS + FW | Semi-continuous | V 1/1 | 295 ml CH4/g VS added | 64% | | 20 | [38] |
V 1/1 | 377 ml CH4/g VS added | Coconut shell biochar |
V 1/1 | 386 ml CH4/g VS added | Corn stover biochar |
V 1/1 | 432 ml CH4/g VS added | Biogas residue biochar |
SS + FW | Semi-continuous | Volatile solid (VS) 1/3 | 413 ml CH4/g VS added | 65% | | 20 | [35] |
SS + YW | VS 1/3 | 149 ml CH4/g VS added | 57% | Yard waste co-substrate (YW) |
SS + YW + FW | VS 1/1.5/1.5 | 232 ml CH4/g VS added | 64% |
SS + FW | Semi-continuous | VS 1/1 | 125 ml CH4/g VS added | 68% | | 16 | [44] |
VS 1/3 | 140 ml CH4/g VS added | 65% |
VS 1/1 | 220 ml CH4/g VS added | 75% | Aerobic pre-treatment |
VS 1/3 | 215 ml CH4/g VS added | 80% |
SS + FW | Batch | VS 3/1 | 119 ml CH4/g VS added | 68% | | 38 | [40] |
VS 1/1 | 260 ml CH4/g VS added |
VS 1/3 | 452 ml CH4/g VS added |
WAS + FW | Pilot | V 1/3 | 0.62 m3 kg− 1 VS added | 56% | | 15 | [45] |
SS + FW | Batch | VS 3/1 | 744 ml g− 1 VS added | 61% | | 20 | [41] |
VS 1/1 | 867 ml g− 1 VS added | 70% |
SS + FW | Continuous | TS 3/1 | 0.46 l/g TS added | 60% | AnMBR | 15 | [46] |
TS 1/1 | 0.56 l/g TS added |
TS 1/3 | 0.73 l/g TS added |
SS + FW | Batch | VS 2.7/1 | 246 ml/g VS added | 58% | SARS-CoV-2 | 2 | [39] |
SS + FW | Continuous | TS 1/1 | 0.352 l CH4/g VS added | / | AnMBR | 57 | [47] |
TS 1/3 | 0.482 l CH4/g VS added | 32 |
SS + FW | Continuous | TS 1/1 | 0.596 l/g VS added | 59% | ThAnMBR | 15 | [36] |
TS 1/3 | 0.812 l/g VS added |
SS + FW | Continuous | V 3/1 | 0.52 l/g VS added | 59% | AnMBR | 15 | [48] |
V 1/1 | 0.61 l/g VS added | 60% |
V 1/3 | 0.77 l/g VS added |
SS + FW | Batch | VS 1/1 | 298 l CH4 kg− 1 VS added | 56% | | 53 | [49] |
VS 1/1 | 500 l CH4 kg− 1 VS added | Biochar10 |
VS 1/1 | 704 l CH4 kg− 1 VS added | Biochar30 |
Results from the experiments cited in Table 3 correspond to а range of data against which values obtained in this study are tested as a null hypothesis. The null hypothesis stated that measured values are probable to occur in sewage sludge and food waste co-digestion in declared ratios. The average of observations in the dataset was 461, 554, and 607 ml/g VS for ratios 3/1, 1/1, and 1/3, respectively. Outcomes from current research for co-digs 1, 2, and 3 showed p-values of 0.82, 0.98, and 0.83, respectively, indicating a high degree of compatibility between a dataset and specific production under the particular hypothetical statement, which is accepted as true. The alternative hypothesis that measured values differ significantly (statistically; p < 0.05) from previous research is rejected. Values obtained in this study were normally distributed but did not effect the mean of a set of measurements or the variance.
The average daily biogas yields and cumulative productions of PS digestion and co-digestions of PS and FW at mixing ratios (v/v) of 3/1, 1/1, and 1/3 are displayed in Fig. 3. In all batch trials, the biogas production began right after feeding the reactors. The peak of biogas production was seen on day 2 for co-digestion 1/3 and mono-digestion and on day 1 for 3/1 and 1/1 co-digs. During the first four days of the experiment, the production was significantly higher for all setups - over 62 percent of the entire volume of biogas. The highest cumulative production of 59.1 l achieved by co-dig 3 was nearly twice as much as that of mono digestion (32.3 l); 50% and 15% higher than that of co-digs 1 and 2, which amount to 39.5 l and 51.5 l of biogas, respectively. Variances between daily production rates were negligible at the end of the trials but differed substantially at the very beginning of the process as the co-dig 2 (111 ml/g VS/d) rate was 25% higher than co-dig 1 (89 ml/g/d VS) and 20% lower than co-dig 3 (133 ml/g VS/d). When compared to co-dig 3, the daily biogas rate for mono-dig (70 ml/g VS/d) was 1.9 times lower, implying that the use of food waste as a co-substrate is strongly recommended. This effect is associated with FW as a rapidly degradable substrate since hydrolysis and consequent alcoholic fermentation rapidly transform a large amount of VS into VFA, CO2, and H2, while PS is more resistant to hydrolysis [50]. Within two weeks, the biogas production in both co-digestion and mono-digestion reactors dropped to almost zero. Figure 3 represents daily production dependencies fitting the exponential functions over 96% for co-digs and 89% for mono-dig.
The pH concentration has a big impact on the AD system since it affects how easily degradable materials dissolve and how biogas production fermentation works. It can create a suitable atmosphere for microbes since the enzymatic reactions of microorganisms depend on pH [19]. Although most microbes prefer neutral pH settings, individual bacteria have varying optimal pH values. The somewhat acid condition of the co-/substrates pH (5.4–6.5) was adjusted by the fine pH value of the inoculum (7.45), preserving process stability and balancing overall feed pH. The pH value shown is typical of food waste. In a review study, [51] found a pH range of 4.4 to 5.8 while examining 65 food waste samples from investigations carried out between 2001 and 2014. All initial pH ranges were acceptable, with the final readings from 7.5 to 7.8.
Co-digestion pH values did not vary with respect to mono-digestion in general. A noticeable decline on the daily production curve is observed for 1/3 co-dig. Following the onset of AD, the production rate declined from day 1 to day 4, indicating some inhibition. It might be accounted for by the abrupt rise in basicity (Fig. 4), which peaked on the fourth day of the reaction and reached a pH value of 8. Inhibition was caused by an inhibitory substance rather than the concentration of FW in the digester medium, suggesting that the FW sample itself may have been the inhibitor [52]. Apart from 1/3 co-dig, pH stayed neutral in all treatments, even without using a buffer.
3.2. Kinetic characteristics
Mathematically, the degradation rate of every group of reactants can be characterized by a differential kinetic equation. Therefore, knowledge about biodegradation kinetics and biogas production could be helpful for specific substrate predictions. In this study, three mathematical models were applied to experimental BMP tests to examine how well PS and co-digested mixture potentials could be predicted. Models were applied to simulate the production line for each trial as a function of digestion time. The experiment's eventual biogas yield (B0) came close to matching the predictions of the first order and Gompertz models and with a slight variance for the Cone. For digested organics, B0 increased as VS concentration increased. The experimental cumulative biogas productions were used to check the homogeneous variance assumption by plotting against the predicted probability values. Values that were uniformly distributed on either side of the zero (line) indicated that the models were appropriate for the current investigation [53]. Generally, the first-order and Gompertz models fit well with experimental data with a very light exception of PS mono-digestion and co-dig 1:3, respectively, while the Cone equation fits almost perfectly. These models can explain 97.8% of BMP results. Low differences between measured and anticipated values imply that the presented models accurately predict reactor behavior.
Parameters such as biogas yield potential, maximum biogas production rate, hydrolysis rate constant, shape factor, and lag phase duration were estimated for each case and summarized in Table 4. In this regard, the first-order kinetic and Cone models are used to calculate the hydrolysis rate and the amount of biogas. A modified Gompertz model was used to calculate the minimum time to produce biogas (λ) and the growth of the biogas production rate, Rm, which was observed when the proportion of FW in co-dig mixtures increased. Before mentioned production started after the onset of BMP assays for all co-dig reactors practically instantaneously and with less than 0.12 day for mono-digestion trial. These results are in line with the kinetic model analysis, i.e., a higher fraction of organic waste in the overall mixture volume decreases the lag phase. As a result, in actuality, co-digestion of PS with FW can boost AD efficiency by shortening the time needed for optimal biogas production.
The hydrolysis constant can be used to ascertain whether co-digestion creates advanced terms for degradation and subsequent biogas generation. The hydrolysis rate constant, kh, varies depending on the co-/substrate type, solubility, and pH value. For instance, when canned products and kitchen waste were treated in batch co-digestion with manure, [26] calculated kh to be 0.27 and 0.35 d− 1, respectively. A significantly lower rate of 0.11 d− 1 was founded during PS co-digestion with thickened activated sludge in an equal-volume mixture [54]. Corresponding kinetic is observed in the results of this paper. The first-order kinetic model sets out a hydrolysis rate constant of 0.24 for digestion and 0.23–0.26 for co-digs, suggesting a negligible longer period to perform, while the Cone model provides some higher values for rate constants in the range of 0.3 to 0.34, showing extended but less intense hydrolysis. However, co-digestion of FW and PS had no effect on the apparent hydrolysis rate.
Table 4
Parameters estimation from experimental data fitting with models
Models | Parameter | | Units | dig | co-dig |
| | | | | 3/1 | 1/1 | 1/3 |
First-order kinetic model | Rate constant (k) | | 1/d | 0.248 | 0.236 | 0.234 | 0.267 |
| Biogas yield (Bo) | Predicted | ml/g VS added | 428.170 | 466.580 | 580.004 | 631.956 |
| | Measured | ml/g VS added | 407.023 | 446.898 | 552.625 | 618.713 |
| | Difference | % | 5.196 | 4.404 | 4.954 | 2.140 |
| R-square | | | 0.979 | 0.998 | 0.998 | 0.999 |
| rMSPE | | | 15.157 | 4.883 | 6.555 | 5.368 |
Modified Gompertz model | Lag phase (λ) | | d | 0.117 | 0.000 | 0.000 | 0.000 |
| Max. biogas production rate (Rm) | | ml/g VS added/d | 73.228 | 73.946 | 90.779 | 112.029 |
| Biogas yield (Bo) | Predicted | ml/g VS added | 404.434 | 439.447 | 546.720 | 601.664 |
| | Measured | ml/g VS added | 407.023 | 446.898 | 552.625 | 618.713 |
| | Difference | % | 0.636 | 1.667 | 1.069 | 2.756 |
| R-square | | | 0.999 | 0.992 | 0.992 | 0.980 |
| rMSPE | | | 3.343 | 9.937 | 12.161 | 19.964 |
Cone model | Rate constant (k) | | 1/d | 0.340 | 0.306 | 0.301 | 0.342 |
| Shape factor (n) | | | 1.861 | 1.410 | 1.392 | 1.331 |
| Biogas yield (Bo) | Predicted | ml/g VS added | 433.056 | 507.476 | 634.317 | 697.496 |
| | Measured | ml/g VS added | 407.023 | 446.898 | 552.625 | 618.713 |
| | Difference | % | 6.396 | 13.555 | 14.783 | 12.733 |
| R-square | | | 0.992 | 0.998 | 0.997 | 0.999 |
| rMSPE | | | 9.412 | 4.546 | 7.865 | 5.103 |
3.3. COD balance
COD is a parameter that represents the extent of solubilization. The COD value indicates the amount of materials that can be chemically oxidized, which provides information on the energy content of the feedstock. The total COD concentration determined after the addition of co-/substrate in the reactor is final for the feed and initial for the process. In this study, with a seed sludge ratio of approximately 70% of total COD for all three mixtures, the COD proportion between inoculum and co-/substrate was almost constant. This note indicates that COD is significantly attributed to FW and PS in the reactor, making feedstock's co-digestion potential high. The COD removal efficiency was calculated by (Eq. (4)):
$$CO{D_{red}}=(CO{D_{added}} - CO{D_{digestate}})/CO{D_{added}},$$
4
where the ratio between the amount of COD reduced by and added to the digester determines the COD removal rate (in percentage when multiplied by 100). COD reductions were compared to evaluate the organic removal efficiencies from different co-/substrate ratios. Therefore, total COD removal rates for examined scenarios were 38% from 61.2 ± 0.3 g/l to 38.1 ± 0.5 g/l for co-dig 1, 47% from 63.8 ± 0.2 g/l to 33.7 ± 0.7 g/l for co-dig 2 and 53% from 65.1 ± 0.2 g/l to 30.5 ± 0.6 g/l for co-dig 3. A total COD removal of 53.3% by digesting a feed consisting of a mixture of 25% sludge and 75% food waste under mesophilic conditions and a hydraulic retention time (HRT) of 34 days has been presented by (2) in Table 5. The total COD removal from anaerobic co-digestion of a mixture composed of 50% cow manure and 50% kitchen waste, vegetable and fruit residues, and MSW with 20 days HRT in a batch process was 22–41% (1). Removal efficiencies from co-digestion of SS with olive mill wastewater, crude glycerol, and cheese way ranged between 34 and 50% for organic loading rates between 0.9 and 1.5 kg VS m− 3 d− 1 (3), which is in line with results from batch experiments in this study. Furthermore, this degree of removal matches the previously reported COD reduction value of 41% found during anaerobic degradation of FW and waste-activated sludge at a ratio of 7:3 (v/v) (4). Concerning greasy sludge and WAS, high COD reductions (over 50%) were obtained, comparable to those of other researchers and current co-digs, regardless of the substrate ratio (5). Finally, evidence of efficient microbiological activity from methanogenic bacteria was provided by COD elimination in conjunction with gas production.
Table 5
COD reductions in different co-digestions
Substrates | Ratios | Initial/final COD (g/l) | COD removal (%) | Ref. |
Cow dong + | Kitchen waste | 1/1 TS | 102/72 | 28.4 | [55] |
Veg. and fruits | 84/49 | 41.4 |
MSW | 130/102 | 21.9 |
SS + FW | 1/3 V | 144/67.2 | 53.3 | [56] |
3/1 V | 131.2/96 | 26.8 |
SS + | Olive waste | 95%+5% V | 34.8/23.1 | 34 | [57] |
Crude glycerol | 111.6/56 | 50 |
Cheese way | 40.1/25.6 | 36 |
WAS + FW | 3/7 V | 4.4/2.6 | 40.6 | [15] |
WAS + Greasy sludge | 78%+22% VS | / | 52 | [58] |
26%+74% VS | / | 51 |
After Day 8 of our experiments, COD concentration dropped along with biogas output to a stable level. However, it is worthy of mention that settled digestate still contains a high residual COD absolutely resistible to biodegradation. The synergisms in co-/substrate pairings are further supported by higher efficiencies of COD elimination. These synergies probably resulted from the improvement of the fundamental organic nutrient compositions. To explain the co-metabolic synergistic effects, in cases when the amount of biogas produced out of co-digestion feedstock exceeds the sum of biogas yields gathered from individual components, COD balance must be taken into account.
COD balance applied to determine the scope of synergy is expressed in (Eq. (5)):
$$CO{D_{In}}+CO{D_{PS}}+CO{D_{FW}}=CO{D_{Gas}}+CO{D_{Rsd}},$$
5
where the entrant COD involves inoculum, primary sludge, and food waste, and the output includes biogas yield (expressed as COD) and residue.
Biogas produced and converted into COD is presented by (Eq. (6)) with the aim of assessing the extent of synergism.
$$CO{D_{Gas}}=CO{D_{G\_In}}+CO{D_{G\_PS}}+CO{D_{G\_FW}}+CO{D_{G\_Syn}},$$
6
Biogas is the sum consisting of inoculum, PS and FW biogas productions from mono trials and additional biogas due to synergetic reaction, all outlined in the form of COD [59].
After 15 days, inoculum only produced 189.1 ± 0.8 ml/g COD, which is a low residual specific yield. Seed sludge intake was turned into biogas in the range of 11%. The remaining COD of inoculum was considered to be in solid residuals. The seed sludge was given the exact biogas potential and conversion rate in the trials that followed. Biogas production from inoculum accounts for 7.6–7.8% of the output COD (Fig. 5). Mono-digestion of PS presented an ultimate specific biogas yield of 396 ± 0.9 ml/g COD. Balance calculation exhibited that 33% conversion was achieved for biogas production from mono-dig of PS based on the introduced COD. AD of sludge alone leads to lower degradation rates in comparison with values obtained for co-digs. The extent of degradation for FW during anaerobic co-digestions is supposed to be full conversion, in which the ultimate specific biogas yield of 652 ml/g COD was adopted [60]. Given that no COD conversion rate can be greater than 1, it was assumed that the maximum is reached for FW, and everything above was characterized as a result of synergetic co-metabolism. Thus, synergy demonstrates itself as the difference between measured and calculated biogas at the full conversion of FW and partial conversion of PS plus seed sludge background [61]. The amount of biogas expected to be generated in co-dig trials can be assessed by (Eq. (7)):
$${V_{co - dig}}=({m_{CO{D_{co - dig}}}}/{m_{CO{D_{mono}}}})*{V_{mono}},$$
7
where mCOD.co−dig is the mass of COD added in co-digestion, mCOD.mono and Vmono mass of COD and volume of biogas used and produced in mono experiments, respectively.
Co-digestion of sludge mixture (seed & raw sludge) and FW as a co-substrate reproduced the synergetic effect as supplementary biogas production was observed. Additional biogas yields were calculated to be 7.1%, 12.8%, and 17% of output COD, assuming the same partial conversions for input COD. Synergistic COD fraction almost exceeded the output COD of seed sludge for co-dig 1 and accounted for 52% and 61% of FW COD for co-digs 2 and 3, respectively. Positive combining effects (mixing easily digested FW with more resistant sewage sludge), improved availability of macro and trace nutrients, and intense co-metabolic reaction are the causes of synergy. Analogously to previous experiments, the measured biogas equivalent of 427.6 ± 2.5 ml/g COD for co-digestions is higher than specific production extrapolated from mono assays as well as surplus synergy production rates. Therefore, it is likely that synergistic metabolism led to better yields for seed and primary sludges as well as to a greater level of degradation.
The type of dependency between COD value and specific biogas yield is a linear function that is the consequence of the exponential decline of COD as a function of time. The change ratio of the dependent variable, COD in this case, is a constant value. The dependent and independent quantities are directly proportional. A section on the y-axis represents the initial COD value in the reactor. At the same time, the x-axis cannot be intersected since the lag phase of COD is always present and no COD content can be withdrawn entirely (Fig. 6). The function is of descending character, which indicates that biogas yield is positively correlated with simultaneous COD consumption. After time parameter exclusion, the COD dependence graph seems suitable for biogas production predictions. Exponential decrement of COD was also observed with [62] and [63] in both batch and continuous experiments, respectively, which correspond to the phenomenon observed in this research and, consequently, the COD dependency graph.