Hydrolysis of the Kitchen Waste through Dry
Anaerobic Digestion - A Sustainable Environmental
Practice for its Effective Management Akanksha Agrawal 
National Institute of Technology Raipur
Parmesh Kumar Chaudhari 
National Institute of Technology Raipur
Prabir Ghosh 
(

prabirg.che@nitrr.ac.in
)
National Institute of Technology Raipur Akanksha Agrawal 
National Institute of Technology Raipur
Parmesh Kumar Chaudhari 
National Institute of Technology Raipur
Prabir Ghosh 
(

prabirg.che@nitrr.ac.in
)
National Institute of Technology Raipur Research Article Keywords: Kitchen Waste, Hydrolysis, Leachate, Volatile fatty acids. Posted Date: September 15th, 2021
DOI: https://doi.org/10.21203/rs.3.rs-770690/v1
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License. Read Full License Page 1/15 Page 1/15 Abstract Population explosion coupled with rapid industrialization and urbanization has led to accumulation of
food waste. Nearly, 1.3 billion tons of food waste has been encountered annually which poses a serious
threat to the environment and human beings. Therefore, it has become imperative to look for the
sustainable environmental practices to manage huge quantum of the waste. Anaerobic Digestion (AD)
has emerged as techno economical viable solution with the aim of resource as well as energy recovery to
combat the global energy crisis. In this study, an attempt has been made to study the hydrolysis of
kitchen waste through dry anaerobic digestion being a suitable technology for treating organic wastes
with varying composition and its superiority over the wet digestion. Laboratory scale batch studies were
conducted on kitchen waste using three different dry fermenters of total volume 4-litre operating under
mesophilic regime for duration of 32 days. The reactors were simultaneously operated with the retention
time of 1 day, 2 day and 4 days, respectively. Comparative performance of leachate generation during
hydrolysis reveals that reactor with 1-day retention time generates maximum leachate with high COD of
244 g/l and VFA of 12.99 g/l. The specific rate constants for hydrolysis of kitchen waste through dry
fermentation with retention time of 1 day, 2 days, 4 days were found to be 0.063, 0.054 and 0.027 day− 1,
respectively. Introduction The yield is directly related to the extent of solubilization of a substrate, but a rate
of gas production will depend on the slowest of the three steps, namely, solubilization, acidogenesis and
methanogens. Methanogenesis is the slowest step, whereas hydrolysis and solubilization may be the rate
limiting steps (Jash and Ghosh 1996). Jash & Ghosh (1996) reported kinetic study on solubilization rate
by correlating the results in terms of biodegradable suspended particles of some organic residues for
different variables. In this research, an attempt has been made to study the acidogenesis of kitchen
waste through dry fermentation process at varying retention time to generate better leachate production
which can be utilized in the second phase of AD, i.e. methanogenesis. This work also aims at studying
the solubilization rate during hydrolysis of kitchen waste. world’s energy security by generating biofuel along with nutrient recycling (Shin et el. 2001). Anaerobic
digestion is broadly classified as dry digestion (TS > 20%) and wet digestion (TS < 15%) depending upon
the presence of solid content in the substrate (Abbassi-Guendouz et al. 2012). It is well established fact
that dry digestion is superior to wet digestion owing to its numerous benefits like high process efficiency,
less reactor volume, less fresh water usage, more favorable energy balance and improved marketability
of the digestate end product (Radwan et al. 1993; Lissens et al. 2001; Forster-Carneiro et al. 2008;
Abouelenien et al. 2009). However, long degradation time and accumulation of VFA which is attributed to
high total solids reduces the process efficiency (Dhamodharan et al. 2015) and require high F/1 ratio
(Dhamodharan et al. 2015; Chen et al. 2008). Furthermore, due to lack of comphrensive knowledge about
dry digestion in contrast to wet digestion and the perceived operational complexity to achieve stable
production make it still unpopular. Stage separation in the dry AD plays a vital role for addressing above
mentioned difficulties during the operation to achieve optimized stable processes. Researches are
directed towards anaerobic digestion of solid or semi-solid waste through two-phase anaerobic digestion,
with first phase the hydrolysis/ acidification step to obtain leachate, which fed to the second phase for
methanization in high strength anaerobic reactors (Guerrero 1999). Dry fermentation process is
considered as superior process than slurry phase or thermal liquefaction process used in acidogenic
reactor. Introduction In two phase, physical separation of the process helps in optimizing the first phase, which is
many times the rate-limiting step. It also allows the maintenance of appropriate densities of the acid and
methane formers in separate reactors which enable maximization of the rates of acidification and
methanogenesis by applying optimal operational conditions, determined by the metabolic and biokinetic
properties of both groups (Dinopoulou et al. 1988; Zhang et al. 2005). The maximum amount of energy in
the form methane from biomethanation of solid organic residue depends on the extent of solubilization
of the organic residues. The yield is directly related to the extent of solubilization of a substrate, but a rate
of gas production will depend on the slowest of the three steps, namely, solubilization, acidogenesis and
methanogens. Methanogenesis is the slowest step, whereas hydrolysis and solubilization may be the rate
limiting steps (Jash and Ghosh 1996). Jash & Ghosh (1996) reported kinetic study on solubilization rate
by correlating the results in terms of biodegradable suspended particles of some organic residues for
different variables. In this research, an attempt has been made to study the acidogenesis of kitchen
waste through dry fermentation process at varying retention time to generate better leachate production
which can be utilized in the second phase of AD, i.e. methanogenesis. This work also aims at studying
th
l bili
ti
t d
i
h d
l
i
f kit h
t world’s energy security by generating biofuel along with nutrient recycling (Shin et el. 2001). Anaerobic
digestion is broadly classified as dry digestion (TS > 20%) and wet digestion (TS < 15%) depending upon
the presence of solid content in the substrate (Abbassi-Guendouz et al. 2012). It is well established fact
that dry digestion is superior to wet digestion owing to its numerous benefits like high process efficiency,
less reactor volume, less fresh water usage, more favorable energy balance and improved marketability
of the digestate end product (Radwan et al. 1993; Lissens et al. 2001; Forster-Carneiro et al. 2008; Introduction Cooked and uncooked kitchen waste both constitutes a major part of municipal solid waste. According to
Food and Agriculture Organization of the United Nation, food waste accounts for 33% of the global food
production which is 1.3 billion tons annually (FAO 2012). Due to economic and demographic expansion,
primarily in Asian countries, the amount of FW is anticipated to increase multifold in next 25 years. Cooked and uncooked kitchen waste both constitutes a major part of municipal solid waste. According to
Food and Agriculture Organization of the United Nation, food waste accounts for 33% of the global food
production which is 1.3 billion tons annually (FAO 2012). Due to economic and demographic expansion,
primarily in Asian countries, the amount of FW is anticipated to increase multifold in next 25 years. According to reports, between 2005 and 2025, the yearly volume of urban FW in Asian countries might
increase from 278 to 416 million tons (Melikoglu et al., 2013). Approximately, 1.4 billion hectares of
arable land are utilized to produce food that is lost or squandered, moreover it also contributes to
greenhouse gases (GHG) emission by accumulating around 3.3 billion tonnes of CO2 into the atmosphere
annually. Kitchen waste due to its putrescible and biodegradable nature (volatile solids 85–95%, moisture
content 75–85%) poses a serious disposal issues. In present scenario, it is generally sent to piggeries,
disposed in sewers, disposed along with municipal solid waste, which ultimately goes to sanitary landfills
or it is being incinerated posing serious threat to the environment as well as human beings (Agarwal et al. 2005; Kumar and Goel 2009; Kumar et al. 2009; Patnaik and Reddy 2009; Talyan et al. 2008). Incineration
of the waste not only results in the emission of dioxins, but also reduces the economic value of the
substrate (Katami et al. 2004). Moreover, this huge quantum of the waste if utilized in resourceful manner
has a potential of generating 367 m3 of biogas per dry ton, about 65% CH4 with an energy content of
6.025 to 9 TW h/m3 which will reduce our dependency on the fossil fuels and also combat energy crisis. Introduction Anaerobic digestion is alluring and techno economical viable solution for the management of the kitchen
waste which not only resolves the management of the huge pile of the waste but also addresses the Page 2/15 Page 2/15 world’s energy security by generating biofuel along with nutrient recycling (Shin et el. 2001). Anaerobic
digestion is broadly classified as dry digestion (TS > 20%) and wet digestion (TS < 15%) depending upon
the presence of solid content in the substrate (Abbassi-Guendouz et al. 2012). It is well established fact
that dry digestion is superior to wet digestion owing to its numerous benefits like high process efficiency,
less reactor volume, less fresh water usage, more favorable energy balance and improved marketability
of the digestate end product (Radwan et al. 1993; Lissens et al. 2001; Forster-Carneiro et al. 2008;
Abouelenien et al. 2009). However, long degradation time and accumulation of VFA which is attributed to
high total solids reduces the process efficiency (Dhamodharan et al. 2015) and require high F/1 ratio
(Dhamodharan et al. 2015; Chen et al. 2008). Furthermore, due to lack of comphrensive knowledge about
dry digestion in contrast to wet digestion and the perceived operational complexity to achieve stable
production make it still unpopular. Stage separation in the dry AD plays a vital role for addressing above
mentioned difficulties during the operation to achieve optimized stable processes. Researches are
directed towards anaerobic digestion of solid or semi-solid waste through two-phase anaerobic digestion,
with first phase the hydrolysis/ acidification step to obtain leachate, which fed to the second phase for
methanization in high strength anaerobic reactors (Guerrero 1999). Dry fermentation process is
considered as superior process than slurry phase or thermal liquefaction process used in acidogenic
reactor. In two phase, physical separation of the process helps in optimizing the first phase, which is
many times the rate-limiting step. It also allows the maintenance of appropriate densities of the acid and
methane formers in separate reactors which enable maximization of the rates of acidification and
methanogenesis by applying optimal operational conditions, determined by the metabolic and biokinetic
properties of both groups (Dinopoulou et al. 1988; Zhang et al. 2005). The maximum amount of energy in
the form methane from biomethanation of solid organic residue depends on the extent of solubilization
of the organic residues. Dry fermentation Unit The dry fermentation unit used for leachate generation is made of PVC material having total 4 litre
volume with 2 litre working volume for kitchen waste, 1-liter volume portion at the top for feeding dilution
water and 1 litre volume for storing the leachate collected at the bottom. A screen of fine nylon mesh was
placed at height of 17 cm from the bottom inside the reactor preventing solid particles in leachate. An Page 3/15 Page 3/15 outlet of 6 mm diameter was provided to the end cap to collect leachate. A 5 litre plastic container was
used for comparative studies on leachate production for different conditions provided. Kitchen Waste outlet of 6 mm diameter was provided to the end cap to collect leachate. A 5 litre plastic container was
used for comparative studies on leachate production for different conditions provided. Kitchen Waste Kitchen waste collected from Central dining hall (CDH) of the girl’s hostel of COEP, Pune India was used to
prepare the feedstock for experiments. The kitchen waste was then subjected to mechanical pulverization
using kitchen blender for 1 minute. The proportion used for the present study was 50% vegetable waste
and 50% of leftover food waste. Physiochemical properties of the feedstock and inoculum are presented
in Tables 1 and 2. For inoculation, leachate was obtained from the mesophilic digester treating food
waste in the same campus. The ratio of inoculum to kitchen waste was 1:4 by volume. Tap water was
used as dilution water added from the top. Table I: Physiochemical properties of the feedstock
Parameters
Value
Particle size
< 2mm
TS (%)
22.16
VS (% of TS)
97.93
SS (% of TS)
88.13
Moisture content (%)
84.2
PH
5.89
VFA (mg/l)
862.82
COD (mg/l)
2,42,333
Alkalinity(mg/l of HCO3)
440
Density (kg/m3)
888.6
Table II: Physiochemical properties of the Inoculum Table I: Physiochemical properties of the feedstock Table I: Physiochemical properties of the feedstock Table I: Physiochemical properties of the feedstock
Parameters
Value
Particle size
< 2mm
TS (%)
22.16
VS (% of TS)
97.93
SS (% of TS)
88.13
Moisture content (%)
84.2
PH
5.89
VFA (mg/l)
862.82
COD (mg/l)
2,42,333
Alkalinity(mg/l of HCO3)
440
Density (kg/m3)
888.6
Table II: Physiochemical properties of the Inoculum Table I: Physiochemical properties of the feedstock Table II: Physiochemical properties of the Inoculum Table II: Physiochemical properties of the Inoculum Table II: Physiochemical properties of the Inoculum Table II: Physiochemical properties of the Inoculum Page 4/15 Parameters
Value
TS (%)
2.27
VS (% of TS)
90.28
SS (% of TS)
41.70
pH
3.18
VFA(mg/l)
1885.62
COD(mg/l)
1,06,000
hods Parameters
Value
TS (%)
2.27
VS (% of TS)
90.28
SS (% of TS)
41.70
pH
3.18
VFA(mg/l)
1885.62
COD(mg/l)
1,06,000 Parameters
Value
TS (%)
2.27
VS (% of TS)
90.28
SS (% of TS)
41.70
pH
3.18
VFA(mg/l)
1885.62
COD(mg/l)
1,06,000 Analytical methods The samples of leachate obtained through the reactor were collected daily during the experiments and
analyzed immediately for various parameters viz. pH, VFA, COD as per the methods suggested in
Standard Methods of water and wastewater examination, APHA 2002. The sample of kitchen waste and
the feed stock characteristics were determined as per procedure prescribed in IS 9235 − 1979. Influence of operating parameters on Hydrolysis and
Acidogenesis Main end product of the first phase of the biomethanation is the formation of the volatile fatty acid (VFA). The formation of VFA is influenced by many factors such as pH, retention time, and composition of the
substrate; therefore it becomes imperative to understand the influence of these parameters as this will
help in better designing of the process. In this research, performance of the dry fermentation unit for
different retention time was analyzed for the quality of leachate obtained. The variations in the pH, VFA,
COD of leachate from reactors R1, R2, and R3 were studied and are presented in the Figs. 2,3,4,5 and 6,
respectively. Experimental Procedure A set of three reactors was used for this study. Each reactor was fed with the same feedstock and
inoculated with leachate from previous successful reactor. Study on leachate production was conducted
by operating the reactors for different retention time given as Reactor 1 (R1): 1-day retention time. Reactor 2 (R2): 2 days’ retention time. Reactor 3 (R3): 4 days’ retention time. Reactor 3 (R3): 4 days’ retention time. The rate at which water added per day during this study in each reactor The rate at which water added per day during this study in each reactor The first stage (8days): 25ml. The second stage (8 days): 100ml. The third stage (8 days): 250 ml. The fourth stage (8 days): 400ml. The amount of water added in R2 is double of R1, but at alternate days whereas the amount of water
added in R3 is about 4 times at 4 days’ interval. Leachate from each reactor were collected and analyzed
after respective retention time. This procedure was repeated for 32 days from the starting of reactor. Results And Discussionsfl Page 5/15 Variation of pH pH of the reactor plays crucial role in the degradation of food waste through microbial activity. Ideal pH
range for hydrolysis and acidogenesis for the degradation of organic fraction of solid waste is considered
to be from 4 to 6 (Li and Fang 2007). The variation of pH of the leachate obtained through dry
fermentation unit is shown in Fig. 1. It was observed that in the initial stage, the pH decreases up to 3.51
and then increases gradually to 5.5 in R1, which is in consistent with the earlier result reported (Selvam et
al. 2010; Xu et al. 2011; Xu et al. 2012). Whereas a minimum value of pH of 3.52 and 3.48 was observed
in leachate from R2 and R3 on day 8. The lower pH values of the reactors are attributable to the
production of formic acid and lactic acid due to their low pKa values, i.e. 3.77 and 3.86, respectively. Furthermore, due to presence of high concentration of the soluble organics in kitchen waste result in pH
drop. Formation of the intermediate products such as acetate, lactate, formate, succinate and ethanol
occur during acid fermentation, as a result of the enzymatic hydrolysis by the members of the family
Enterobacteriaceae, Clostridia and lactic acid bacteria (LAB). LAB are the dominant microbial archea
during the degradation of food waste in the mesophilic temperature regime (Wang et al. 2001). However,
if pH is adjusted using alkaline solution it reduces the dominance of LAB by enriching it with the coliform
bacteria and Clostridia. Variation of VFA concentration of leachate during operation
Period Rate of the volatile acids production is directly proportional to the digestion period. In the initial stage,
rapidly degradable materials like grains are liable to cause VFA accumulation. This brings about severe
inhabitation in acidic fermentation by reducing the pH of the reactor, thus ultimately results in the
inhabitation of biogas production (Shin et al. 2001). Variation of VFA concentration of leachate during
operation period is shown in Fig. 2. The first peak on day 3 was the result of grains degradation
(12.993g/l). It was observed that the degradation of cellulose increases at high retention time and the
degradation of protein increases at both high retention time and neutral pH (Noike et al. 1985). However,
with the improvement of the pH of the system, VFA production again shoots up to (11.257g/l), indicating
the enhanced degradation of vegetables. From the graph, it can be concluded that the reactor with shorter Page 6/15 Page 6/15 retention time achieves better acidification rate as it does not support the accumulation of VFA. The
decline trend of VFA production after day 17 may be attributed to the shortage of the sufficient substrate
available for the microbial consortia. The variation of cumulative VFA during the digestion is represented
by Fig. 3. It indicates that the rate of VFA production increases during the initial period and then the rate
decreases which is mainly attributed to the pH and acidification efficiency of microbes, as at lower pH it
results in the activation of the enzymes necessary for the microbial community to perform degradation
(Tembhurkar and Mhaisalkar 2007). The cumulative value of VFA in R1 indicates a smoother trend with
higher accumulated VFA, whereas the VFA generated in R2 is comparatively less and R3 is least. The
similar trend was exhibited in the past studies by (Tembhurkar and Mhaisalkar 2007; Xu et al. 2011). Variation of COD of leachate during operation Period From Fig. 4, a biphasic pattern of COD leaching can be observed. COD concentrations of the collected
leachate during initial days of operation were found to be higher ranging from 72 g/L to 185 g/L in
reactor R1, after that they start depleting gradually. The initial high rate of COD leaching was partially
attributed to the water-soluble fraction of food waste itself and it also depends on the substrate
concentration. Decline in the trend of the COD may be attributed to the wash out of the water soluble
fraction of the food waste from reactor (Xu et al. 2012; Xu et al. 2014). The value of COD from the
leachates obtained from all the reactors show sudden peaks may be because of accidental slipping of
the hydrolyzed waste. It was observed that the higher value of COD during initial days is because of the
hydrolyzed material getting washed as the leachates which impart high rate of COD. The rate of COD
accumulation of the leachate can be better studied by plotting the cumulative diagram COD which is
shown in Fig. 5. The rate of accumulation of COD of R3 is lowest than R2 then R1 due to the conditions
prevailing in these reactors and it is mostly attributed to the lower pH of the system which will hamper
degree of acidification of the system and it will be in the order of R3 < R2 < R1. Solubilization rate of kitchen waste during hydrolysis The maximum amount of energy in the form of methane obtainable from a solid organic residue by
biomethanation depends on the extent of solubilization of the organic residues. The yield is directly
related to the extent of solubilization of a substrate, but the rate of gas production will depend on the
slowest of the three steps, namely, solubilization, acidogenesis and methanogenesis. Methanogenesis is
the slowest step, whereas hydrolysis and solubilization may be the rate limiting steps (Jash and Ghosh
1996). The solubilization rate decreases with the operation period. It may be due to the high content of rapidly
degradable matter. The solubilization rate of substrate decreased in the following order: R1 > R2 > R3. The
rate of solubilization decreases with an increase of substrate concentration due to lowering of pH. It is
also evident that at higher substrate concentration, the solubilization rate is initially higher, resulting in a
higher concentration of soluble substrate amenable to acidogenesis leading to lowering of ph. Because
of lowering of pH, the solubilization rate slows down subsequently steps (Jash and Ghosh 1996). Page 7/15 Page 7/15 The solubilization rate of the kitchen waste was determined for reactor R1, R2 and R3. The first phase of
hydrolysis and acidification of the kitchen waste was assumed to follow the first order rate of reaction,
which is given as equation, The solubilization rate of the kitchen waste was determined for reactor R1, R2 and R3. The first phase of
hydrolysis and acidification of the kitchen waste was assumed to follow the first order rate of reaction,
which is given as equation, -dC / dt = kC,
t = - (1/k) In (C/Co), -dC / dt = kC,
t = - (1/k) In (C/Co), -dC / dt = kC, Where k = first-order specific rate constant (day− 1), C = COD concentration of kitchen waste (mg/l) at time
t, Co = initial COD concentration of kitchen waste (mg/l) at zero time. The data is plotted in Figs. 6, 7 & Fig. 8 to determine specific rate constant for R1, R2, R3, respectively. The specific rate constant was found to be maximum 0.063 for R1 and that is for R2 is 0.054 and R3 is
0.027 indicating that daily addition of water improves the solubilization process. The solubilization rate
of the kitchen waste of reactor R1 is higher than of reactor R2 and R3, respectively. Conclusions The rate of leachate extraction is higher up to initial 10 days and then it decreases in later stage. The pH
of the leachate drops down up to 6 to 8 days because of fast accumulation of VFA with no significant
buffering capacity and then regains in the later stage may be due to washout of acidic content and
dilution. The rate of VFA extraction is higher in initial stage and then decreases. For the retention time of 1
day, reactor R1 shows better results than R2 and R3 with higher retention time. Hence, retention time of 1
day should be adopted when dry fermentation is to be used as preprocessing process prior to
methanization of kitchen waste. The COD value of leachate obtained increases in initial 8 to 10 days and
then it goes on decreasing beyond 10 days. The pH of R3 is lower than R2 then R1 as indicated because
of low pH, the efficiency of acidification process is least at R3 < R2 < R1. The first order rate constant was
reported maximum 0.063 for R1 and 0.054 for R2 and 0.027 for R3 indicating that daily addition of water
improves the solubilization process. Declarations Author contribution Conceptualization, Methodology and investigation: Akanksha Agrawal. Writing original draft and supervision: P K Chaudhari. Prabir Ghosh: conceptualization, review, editing and
supervision Funding Not applicable Funding Not applicable Data Availability Not applicable
Ethical approval Not applicable. Data Availability Not applicable Page 8/15 Page 8/15 Consent to participate Not applicable. Consent to participate Not applicable. Consent to participate Not applicable. Consent to participate Not applicable. Consent for publication Written informed consent for publication was obtained from all participants. Consent for publication Written informed consent for publication was obtained from all participants. References 1. Abbassi-Guendouz, A., Brockmann, D., Trably, E., Dumas, C., Delgenès, J. P., Steyer, J. P., & Escudié, R. (2012). Total solids content drives high solid anaerobic digestion via mass transfer
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technology, 26(3), 329-340. Figure 2 Variation of VFA during the operation of the reactors Variation of VFA during the operation of the reactors Figure 1 Variation of pH during the operation of the reactors. Page 11/15 Figure 2
Variation of VFA during the operation of the reactors
Figure 3
Variation of cumulative VFA during the operation of the reactors Figure 3 Variation of cumulative VFA during the operation of the reactors Page 12/15 Page 12/15      Page 13/15
Figure 4
Variation of COD of the leachate during the operation of the reactors. Figure 5
Variation of cumulative COD of the leachate during the operation of the reactors Figure 4 Variation of COD of the leachate during the operation of the reactors. Figure 5
Variation of cumulative COD of the leachate during the operation of the reactors Variation of COD of the leachate during the operation of the reactors. Variation of COD of the leachate during the operation of the reactors. Variation of COD of the leachate during the operation of the reactors. Figure 5 Variation of cumulative COD of the leachate during the operation of the reactors Variation of cumulative COD of the leachate during the operation of the reactors Page 13/15 Page 13/15 Figure 6
Specific rate constant during hydrolysis of kitchen waste in reactor R1
Figure 7
Specific rate constant during hydrolysis of kitchen waste in reactor R2 Figure 6 Specific rate constant during hydrolysis of kitchen waste in reactor R1 Page 14/15
Specific rate constant during hydrolysis of kitchen waste in reactor R1
Figure 7
Specific rate constant during hydrolysis of kitchen waste in reactor R2 Specific rate constant during hydrolysis of kitchen waste in reactor R1 Specific rate constant during hydrolysis of kitchen waste in reactor R1
Figure 7 Figure 7 Specific rate constant during hydrolysis of kitchen waste in reactor R2 Specific rate constant during hydrolysis of kitchen waste in reactor R2 Specific rate constant during hydrolysis of kitchen waste in reactor R2 Page 14/15 Figure 8
Specific rate constant during hydrolysis of kitchen waste in reactor R3 Figure 8 Specific rate constant during hydrolysis of kitchen waste in reactor R3 Specific rate constant during hydrolysis of kitchen waste in reactor R3 Specific rate constant during hydrolysis of kitchen waste in reactor R3 Page 15/15