3.1. The Impact of Hydraulic Retention Time (HRT) and pH on Gas Manufacturing in a CSTR
Trace elements were added to the waste black cumin extract liquid to feed the bioreactor. It was actuated for approximately 100 days in the acclimatization phase and four dissimilar operating periods. There is a restricted number of works that are partly similar to biological hydrogen manufacturing in a CSTR. The study by Antonopoulou et al. [
34] used sorghum biomass as the organic matter for hydrogen manufacturing and it was processed in a CSTR. That study enquired about the impact of pH on biohydrogen manufacturing from sweet sorghum extract via continuous fermentation. For extraction, ground sorghum stems were used, and the extraction process was completed by stirring at regular intervals at 30 °C. After extraction, a liquid fraction (sorghum extract), which was wealthy in soluble carbohydrates, and a solid fraction (lignocellulosic residue) were acquired. Reactor operation was carried out under mesophilic circumstances in the pH range of 3.5 to 6.5 and at 12 h HRT. Maximum hydrogen efficiency was detected to be 3.5 L H
2/L reactor/day at pH 5.3. Optimum biohydrogen production was determined to occur in the pH range of 5.3 to 4.7, where butyric acid was identified as the metabolic produce. Lowering the pH worth to 4.6 reduced the butyric acid concentration, leading to a decline in biohydrogen production. When operated at pH 3.5, it was found that hydrogen production stopped.
In CSTR, no gas production was detected to occur in the initial 19 days, but un-balanced gas production was detected in the following days. This acclimatization stage of bacteria lasted for approximately 35 days. During the acclimatization stage, the bioreactor was actuated at an OLR of 4.44 g.nigella sativa extract/L (216.6 mg COD L
−1), 24 h HRT, and 5.0 pH. Stable gas production and continuous hydrogen fermentation were determined on the 35th day. Also, batch reactors were set up to identify probable impacts before exchanging the operating circumstances in CSTR. In batch reactors, waste black cumin was extracted and trace elements were added to the extract liquid, and hydrogen production potential was examined under different organic loading ratios (2.22 g.nigella sativa extract/L, 4.44 g.nigella sativa extract/L, and 6.66 g.nigella sativa extract/L) and pH values (4.0, 5.0, and 6.0). It was designated that hydrogen production was high in the bioreactors with OLRs of 4.44 g.nigella sativa extract/L and 6.66 g.nigella sativa extract/L.
Figure 2 shows the gas production at CSTR for four operating periods.
Figure 3 presents the operating conditions during these four gas production periods. In the first period, it was continued as in the acclimatization phase (35 days) with pH 5.0 and the OLR of 4.44 g.nigella sativa extract/L (185.8 mg COD L
-1), and the reactor operation was carried out with the only change of hydraulic retention time to 36 h. This alteration was intended to expedite gas production in the process, but it was found that the alteration reduced gas production (
Figure 2 and
Figure 3). It was determined that unstable hydrogen production in CSTR during the initial period was related to the HRT of 36 h, and we decided to operate at 24 h HRT again (
Figure 3). Therefore, hydrogen production was found to be low at 36 h of HRT. Similar findings were observed in the work by Salem et al. [
35], where continuous biological hydrogen production was examined using sucrose, potato, and bean wastewater in a CSTR. In the study, the effect of 24, 18, and 12 h of HRT on biohydrogen production at a pH value of 5.5 was investigated. It was reported that reducing the HRT to 18 h in wastewater containing sucrose and potato resulted in optimum hydrogen production. In the bean wastewater substrate, 24 h of HRT yielded optimum biohydrogen production. In another work where brewery waste was used as a substrate, the effect of HRT on biohydrogen manufacturing was investigated. In the work, optimum hydrogen production was reported at four diversified pH values (5.0, 5.5, 6.0, and 6.5) with an HRT of 18 h [
36]. Contrary to these studies, some studies have reported that an HRT of 8 h or less is suitable for hydrogen manufacturing [
37,
38,
39]. Current research and other studies confirm that optimal HRT may vary depending on the type of waste or wastewater.
Han et al. [
40] and Fan et al. [
36] reported that butyric and acetic acid production support hydrogen production, while propionic acid production results in less hydrogen production.
Table 3 shows organic acid (acetic, propionic, and butyric acid) analyses performed at definite time intervals in CSTR. Similar to the work conducted by Wong et al. [
20], in the current work, the main products of organic acids were determined to be acetic and butyric acid. The CSTR was continuously batch fed, and no stirring was performed for about 3 h during the feeding; then, stirring was resumed upon completion of the feeding. In research on continuous biological hydrogen production, attached microbial growth systems and suspended microbial growth systems are generally used. In this context, the CSTR is the most commonly preferred suspended microbial growth system. However, it has been reported that low HRT or high substrate concentration in a CSTR may cause sludge washing. It has also been reported that this situation, in turn, may limit hydrogen production due to operational instability [
41,
42,
43]. Taking this into consideration, reactor operation was carried out in the current study. The COD inlet concentration was measured from the feed tank and the outlet concentration from the exit tank. The continuously stirred tank reactor was actuated at an entry OLR of 185.8 mg COD L
−1 in the initial period. The exit concentration was determined to be 100.7 mg COD L
−1 (36–46 days). Under the same conditions, the operating circumstances for the second period (4.44 g.nigella sativa extract/L) (on mean, 191.3 mg COD L
−1 and pH 5.0) were continued with the only change in the hydraulic retention time to 24 h. In this period, when biohydrogen production became stable with HRT alteration, the maximum biohydrogen production was detected to be 5.1 mL H
2/day (on the 58th day). In the second term, the mean exit COD concentration was determined to be 109 mg COD L
−1.
In the third period, the operations were carried out at two different organic loading rates (OLRs). The operating circumstances for the third term included an OLR of 6.66 g.nigella sativa extract/L (mean 275.5 mg COD L
−1), 24 h HRT, and 4.0 pH between days 64 and 75. With the operating condition of an OLR of 6.66 g.nigella sativa extract/L (on mean, 275.5 mg COD L
−1) in the third period, the change in pH and organic loading rate compared to the second term resulted in a sharp increment in gas manufacturing performance. At the beginning of this period (between days 64 and 75), 5.6 mL H
2/day gas production was observed with the change in pH. In the following days (between days 65 and 75), depending on the acclimatization of the bacteria to the environment pH, an average of 18.2 mL H
2/day hydrogen gas was produced. In the third period, between days 76 and 82, 24 h hydraulic retention time and pH 4.0 were used, and only the organic loading rate was changed to 4.44 g.nigella sativa extract/L (185.8 mg COD L
−1); an average of 12 mL H
2/day gas production was determined. In the third term, at OLRs of 6.66 g.nigella sativa extract/L (on mean, 275.5 mg COD L
−1) and 4.44 g.nigella sativa extract/L (185.8 mg COD L
−1), the maximum biohydrogen production was detected to be 20.8 mL H
2/day (67th day) and 14.2 mL H
2/day (79th day), respectively (
Figure 2). In this period, it was determined that biohydrogen manufacturing performance decreased when the OLR was reduced. For the third term, at the OLRs of 6.66 g.nigella sativa extract/L (mean 275.5 mg COD L
−1) and 4.44 g.nigella sativa extract/L (185.8 mg COD L
−1), the exit COD concentration was detected to be 147.1 and 96.8 mg COD L
−1, respectively. In the fourth term, the reactor operation continued at two different organic loading rates (
Figure 2). In this period, first of all, the operating conditions were applied between days 83 and 92, as follows: OLR of 6.66 g.nigella sativa extract/L (on mean, 319 mg COD L
−1), 24 h HRT, and pH 6.0. With the operating condition of an OLR of 6.66 g.nigella sativa extract/L (on mean, 319 mg COD L
−1), in the fourth period, the change in pH and organic loading rate compared to the third period resulted in no significant change in gas production performance. At the beginning of this term, gas production occurred at around 6.5 mL H
2/day depending on the acclimatization of bacteria to the environment pH. With this organic loading rate, in the following days (the days between 88 and 92), a biohydrogen production average of 13.1 mL H
2/day was specified, and the maximum biohydrogen production was found to be 14.2 mL H
2/day. In the fourth period, between the days 93 and 97, the operating conditions were maintained as 24 h hydraulic retention time and pH 6.0, with the only change in the OLR to 4.44 g.nigella sativa extract/L (185.8 mg COD L
−1). A gas production average of 2 mL H
2/day was determined, and the maximum biological hydrogen production was found to be 2.4 mL H
2/day. In this period, at pH 6.0, the OLR of 4.44 g.nigella sativa extract/L (185.8 mg COD L
−1) negatively affected the process, and it was determined that hydrogen manufacturing was better with the OLR of 6.66 g.nigella sativa extract/L (on mean, 319 mg COD L
−1). The mean COD exit concentration for this term was detected to be 163.7 mg COD L
−1 between days 83 and 92, and 106.8 mg COD L
−1 between the days 93 and 97. The literature contains various studies on pH and HRT operating parameters. In this context, Silva-Illanes et al. [
44] reported that increasing the pH value from 5.5 to 6.0 and reducing the HRT from 12 h to 8 h reduced biohydrogen production. In the current work, bioreactor operation was found to be positive at pH 4.0 with an HRT of 24 h. If these results are evaluated overall, it is confirmed that the optimum operating parameters may vary depending on the type of waste or wastewater, which is consistent with the studies reported in the literature.
3.2. The Impact of HRT and pH on Gas Manufacturing in an FBR
Trace elements were added to the waste black cumin extract liquid to feed the bioreactor. The bioreactor was actuated for approximate 100 days including the acclimatization phase and four dissimilar operating periods. The study which is partly similar to biological hydrogen manufacturing using a fluidized bed reactor was conducted by Antonopoulou et al. [
45]. In the study, sweet sorghum extract was tested for biohydrogen production at different substrate concentrations in a CSTR. The study was conducted under mesophilic conditions, 12 h HRT, and substrate concentrations in the range of 9.8–20.9 g/L. The maximum biohydrogen production ratio was found to be 2.93 L.H
2/L.reactor/day at 17.5 g.carbohydrate/L. Butyric acid was identified as the main metabolic product in all stable conditions.
In FBR, the acclimatization phase of the bacteria took 35 days. No gas production was detected in the initial 18 days, but unstable gas production was detected after then. During the acclimatization phase, the bioreactor was actuated at an OLR of 4.44 g.nigella sativa extract/L (216.6 mg COD L
−1), 24 h HRT, and 5.0 pH. It was found that stable gas production conditions occurred on the 35th day. In addition, evaluations of the batch reactor, which was set up to predict possible effects, were also applied to the FBR.
Figure 4 shows the gas production at FBR for four operating periods.
Figure 5 presents the operating conditions during these four gas production periods. The first period’s operating conditions were maintained as in the acclimation phase (35 days) with 5.0 pH and an OLR of 4.44 g.nigella sativa extract/L (185.8 mg COD L
−1), with the only change occurring in the HRT, which was applied as 36 h. It was determined that this alteration considerably limited gas manufacturing (
Figure 4 and
Figure 5). Since they allow for stirring within the reactor content in terms of operation, CSTR and FBR were used in this study. FBR was continuously batch fed, and no return was performed for about 1 h during feeding; then, return was resumed upon the completion of feeding. During the biohydrogen production studies, the studies were typically conducted in suspended cell systems, which allow for good stirring. Furthermore, it was reported that, when the hydraulic retention time is kept short or when the dilution rates are high, washing-out may occur in the hydrogen-producing microorganisms, and such a situation may limit the production of hydrogen due to operational instability [
41]. Another study reported that HRT had a substantial impact on hydrogen manufacturing, and the hydrogen manufacturing enhanced with reduction in the retention time [
46]. Similarly, Amorim et al. [
47] reported that reducing the HRT from 8 h to 1 h increased biohydrogen production in an anaerobic FBR fed with cassava wastewater. Considering the studies in the literature, after the first period was operated with an HRT of 36 h, the HRT was reduced in all operating periods and the operation was implemented with an HRT of 24 h.
The researchers reported that the overall higher production of butyric and acetic acid, compared to propionic acid production, was crucial for the increase in biohydrogen production. Furthermore, it was also reported that the high production of propionic acid compared to butyric and acetic acid may limit biohydrogen production.
Table 4 shows organic acid (acetic, propionic, and butyric acid) analyses performed at definite time intervals in an FBR. The organic acids in the study were generally high in butyric and acetic acid, followed by lower levels of propionic acid. In a similar trend to the results of this study, in their studies, Pachiega et al. [
26] found butyric and acetic acid to be high, Wadjeam et al. [
48] found butyric acid to be high, and Amorim et al. [
47] found acetic acid to be high. An FBR was operated in the first period at an inlet OLR of 185.8 mg COD L
−1, and the exit concentration was detected to be 116.1 mg COD L
−1 (36–38 days) and 120 mg COD L
−1 (39–44 days).
Under the same conditions, the operating circumstances for the second term (4.44 g.nigella sativa extract/L) (on mean, 195.3 mg COD L
−1 and pH 5.0) were continued with the only change in the hydraulic retention time to 24 h. Researchers report that a reduction in the hydraulic retention time has an increasing impact on hydrogen manufacturing. As shown in
Figure 5, hydrogen production did not become steady with the HRT change (day 6), but the steady state started thereafter. During this period, the maximum biohydrogen production was determined to be 2.1 mL H
2/day (on day 52) (
Figure 4). In the second term, the exit COD concentration was detected to be approximately 118.5 mg COD L
−1. In the third term, the operations were carried out at two different OLRs. The first operating circumstances in the third term, between days 58 and 65, were applied as follows: an OLR of 4.44 g.nigella sativa extract/L (on mean, 208.1 mg COD L
−1), 24 h HRT, and 4.0 pH. With the operating condition of an OLR of 4.44 g.nigella sativa extract/L (on average 208.1 mg COD L
−1), in the third period, the only change in pH compared to the second term resulted in a considerable increment in gas manufacturing performance. At the beginning of this period (between days 58 and 65), with the impact of pH alteration on the OLR of 4.44 g.nigella sativa extract/L (on average, 208.1 mg COD L
−1), gas production of about 4 mL H
2/day was detected. In the third term, between days 66 and 80, reactor operation was continued with a HRT of 24 h and pH 4.0, with the only alteration occurring in the organic loading rate, which was applied as 6.66 g.nigella sativa extract/L (mean 294.7 mg COD L
−1); approximately 6.1 mL H
2/day of hydrogen gas production was detected. In the third term, the maximum biohydrogen production at the OLRs of 4.44 g.nigella sativa extract/L (on mean, 208.1 mg COD L
−1) and 6.66 g.nigella sativa extract/L (on mean, 294.7 mg COD L
−1) was found to be 4.5 mL H
2/day (64th day) and 7.6 mL H
2/day (78th day), respectively (
Figure 4). In this period, it was determined that, when the organic loading ratio was augmented, hydrogen manufacturing performance increased. In the third period, the outlet COD concentration at the OLRs of 4.44 g.nigella sativa extract/L (on mean, 208.1 mg COD L
−1) and 6.66 g.nigella sativa extract/L (on mean, 294.7 mg COD L
−1) was found to be approximately 105.1 and 181.4 mg COD L
−1, respectively. In the fourth term, the operations were carried out at two different organic loading rates. In this period, first of all, the operating conditions were maintained as in the previous period (between days 66 and 80) using the OLR of 6.66 g.nigella sativa extract/L (325.2 mg COD L
−1) and 24 h HRT, with the only change occurring in pH, which was applied as 6.0 between days 81 and 89. With the pH change, there was a decline in gas manufacturing performance compared to the third period (between days 66 and 80). At the beginning of this period (between days 81 and 89), a gas production average of 3.7 mL H
2/day gas occurred, depending on the acclimatization of bacteria to the environment pH. In the fourth period, reactor operation was continued with 24-h hydraulic retention time and pH 6.0 between days 90 and 97, with the only change occurring in the organic loading rate, which was applied as 4.44 g.nigella sativa extract/L (188.7 mg COD L
−1); a gas production average of 2.3 mL H
2/day was detected. Between days 90 and 97, in the fourth term, it was determined that the OLR of 4.44 g.nigella sativa extract/L (188.7 mg COD L
−1) at pH 6.0 decreased hydrogen production. In the fourth term, the mean exit COD concentration was detected to be 211.2 mg COD L
−1 between days 81 and 89 and 113.2 mg COD L
−1 between days 90 and 97.
Various types of substrate sources were used for biohydrogen manufacturing. In the work by Kim et al. [
49], the sewage sludge taken from the primary and secondary sludge thickeners, in the same quantities as food wastes taken from the dining hall, were shredded and stirred into a mixer for biohydrogen manufacturing. Tawfik et al. [
50] used a mixture of urban food waste and kitchen waste in varying concentrations. The study by Chen et al. [
51] investigated the upgrowth kinetics of bacteria producing hydrogen via darkness fermentation using three dissimilar substrates including dry skimmed milk powder, sucrose, and food refuse. Some of the other studies in the literature used various wastes/wastewater as substrates such as a mixture of urban solid waste and waste from poultry processing plants and slaughterhouses [
52]; wastewater containing starch [
53]; and wastewater from the dairy industry [
54] and molasses [
55]. In other studies using certain wastes/wastewater as substrates, the substrates were subjected to various pretreatments to search for the potential of hydrogen manufacturing from specific microorganism species. In such studies, active sludge from a wastewater treatment plant [
56], sugarcane pulp waste, [
57] and peels of steamed potatoes [
58] were used as substrates. It was reported that wastes or wastewater with high carbohydrate content showed good performance in biological hydrogen production. The fact that black cumin waste has a carbohydrate content of 43.5% supports biohydrogen production, together with other operating factors. It has been reported that biomass can be affected by operational factors, such as the mixing of reactor content, the organic loading rate, HRT, etc. [
59].
Hydrogen manufacturing in FBRs and CSTRs typically fluctuated throughout the operating period. This may be due to the activity of the dominant species in the mixed culture depending on the pH and the hydrogen gas being the first gas to leave the environment (since it has the lightest molecular weight and the higher transition velocity compared to the other gases) when sampling (using a gas-tight glass syringe). In this study, biohydrogen production was determined at all pH values examined: 5.0, 4.0, and 6.0. However, considering the manufacturing stability in both reactors, the maximum amount of hydrogen was produced at pH 4.0. The reduction of HRT from 36 h to 24 h increased hydrogen production.
3.3. Batch Reactors
The biohydrogen manufacturing potential of waste nigella sativa extract liquid at organic loading ratios of 2.22, 4.44, and 6.66 g.nigella sativa extract/L was investigated at pH 5.0 (
Table 5). In the bioreactor with 2.22 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 11.10
−4 mL between the 21st and 43rd hours; in the bioreactor with 4.44 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 210.10
−4 mL at the 19th hour; and, in the bioreactor with 6.66 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 37.10
−4 mL between the 15th and 21st hours. After the time of maximum hydrogen production in the reactors, biohydrogen production decreased with the pH alteration and the reduction in the quantity of organic matter.
The biohydrogen manufacturing potential of waste nigella sativa extract liquid at organic loading ratios of 2.22, 4.44. and 6.66 g.nigella sativa extract/L was investigated at pH 4.0 (
Table 6). In the bioreactor with 2.22 g.nigella sativa extract/L, the maximum biohydrogen production was determined to be 96.10
−4 mL between the 15th and 19th hours; in the bioreactor with 6.66 g.nigella sativa extract/L, the maximum biohydrogen production was determined to be 977.10
−4 mL at the 21st hour. After the time of maximum hydrogen production in the reactors, biohydrogen manufacturing decreased with the pH alteration and the reduction in the quantity of organic matter. In the bioreactor with 4.44 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 236.10
−4 mL at the 15th hour and then 162.10
−4 mL with a sharp drop at the 16th hour. Then, hydrogen production increased to 225.10
−4 mL at the 20th hour. The cause for re-increase after the reduction may be the effect of the pH alteration on the activity of the dominant species. It is also reported that the pH factor significantly influences the activity of dominant species in complicated culture [
9].
The biohydrogen manufacturing potential of waste nigella sativa extract liquid at organic loading ratios of 2.22, 4.44, and 6.66 g.nigella sativa extract/L was investigated at pH 6.0 (
Table 7). In the bioreactor with 2.22 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 107.10
−4 mL between the 17th and 20th hours; in the bioreactor with 4.44 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 136.10
−4 mL at the 17th hour; and, in the bioreactor with 6.66 g.nigella sativa extract/L, the maximum biohydrogen production was detected to be 250.10
−4 mL between the 17th and 20th hours. After the time of maximum biohydrogen production in the reactors, biohydrogen production reduced with the pH change and the decline in the quantity of organic matter.
Table 8 summarizes some studies investigating hydrogen production according to operating conditions using various types of substrate sources.
3.4. Inoculum Content in the CSTR and FBR
Biohydrogen manufacturing is carried out using a mixed culture and specific bacteria. This study used anaerobic sludge of the biologic wastewater treatment facility of a sugar factory after subjecting it to heat treatment. Biologically produced gases in the study were carbon dioxide and hydrogen, and no methane gas was produced in the CSTR or FBR during the study. This supports the effectiveness of the heat pretreatment applied to the sludge (inoculum) to neutralize the methanogens. In one of the studies using mixed cultures, mixed bacterial cultures acquired from a potato field, a soybean field, and a compost pile were heat-treated to inhibit methanogens and richen hydrogen-producing bacteria [
64]. In addition, mixed cultures of various types such as sewage microflora [
24], sifted soil [
65], beach mud [
66], anaerobic sludge [
67,
68,
69,
70], and aerobic mud [
71] were also used in biohydrogen studies. Some studies used specific bacteria. These specific bacteria included
Pseudomonas species [
56],
Clostridium butyricum [
57], and
Thermotoga neapolitana and
Caldicellulosiruptor saccharolyticus [
58].
Dominant species detected in the CSTR at pH 4.0 and DGGE band intensity of 0.03 included
Sulfurospirillum cavolei,
Hydrogenimonas thermophila,
Sulfurospirillum carboxydovorans,
Sulfurospirillum alkalitolerans, and
Thiofractor thiocaminus. At 5.0 and 6.0 pH with a DGGE band intensity of 0.04, the following dominant species were determined:
Erwinia amylovora,
Brenneria goodwinii,
CFB group bacteria,
Salmonella bongori, and
Enterobacteria. In the FBR, dominant species were detected at pH 4.0 and 6.0 with a DGGE band intensity of 0.03 and at pH 5.0 with a DGGE band intensity of 0.04. It was found that the dominant species detected in the respective DGGE band intensities in the FBR were identical in the CSTR. Considering previous studies, the dominant bacteria in general are Clostridium sp. In this context, in the study by Liu et al. [
72] in which biohydrogen production from sugar wastewater was investigated, as a result of DGGE analysis,
Clostridium sp.,
Clostridium butyricum,
Pseudomonas sp., and
Pseudomonas lindanilytica bacteria were determined to be the dominant bacteria supporting hydrogen manufacturing. In another work, it was reported that, with an optimum HRT of 60 h, the dominant hydrogen-producing bacteria were
Megasphaera sp.,
Clostridium sp., and
Chloroflexi sp. [
48]. The PCR-DGGE outcomes of this work showed that the bacterial species of
Hydrogenimonas thermophila,
Sulfurospirillum carboxydovorans,
Sulfurospirillum cavolei,
Sulfurospirillum alkalitolerans, and
Thiofractor thiocaminus were dominant in the mixed culture with maximum hydrogen production occurring at pH 4.0. These dominant bacterial species produced acetic and butyric acid as the main products at 35 ± 2 °C, using waste black cumin as the extracted liquid substrate. In the biohydrogen studies, no study was found on these dominant species. It is suggested that a mixed culture or specific bacteria dominated by these species can be used in biohydrogen research in the coming years.
When biohydrogen production studies are evaluated, in general, it may not be clearly stated that an increase or decrease in HRT increases or decreases hydrogen production. Biohydrogen studies should consider factors such as nutrients, sludge (inoculum), pretreatment (time and temperature) applied to the sludge, and, particularly, the type of organic material used as a whole for optimum biohydrogen manufacturing.
In the black cumin oil industry, waste black cumin, formed after oil extraction and rich in carbohydrates, can be utilized for biohydrogen production instead of junking, thereby reducing the amount of waste and greenhouse gas production. In this respect, extensive optimization studies on process operating conditions can be carried out to increase biohydrogen efficiency. Furthermore, the efficiency of biohydrogen production can be increased by mixing different types of carbohydrate-rich waste or wastewater. This could make a significant contribution to the circular economy.