World Journal
of Microbiology
and Biotechnology
Accelerated
9, 625-629
food waste cornposting
E.J. Olguin,* G. Sanchez and R. Gonzalez Experiments on food waste cornposting under the temperate and dry conditions of Mexico City showed that the waste stabilized faster (within 2 months) if an ‘accelerator’ (chopped orange peel and carrot bagasse) was used. The relatively cool and humid conditions prevailing during similar experiments in Xalapa City inhibited decomposition, and stabilization of waste, with or without accelerator, was generally slower than in Mexico City. However, addition of wood chips as a bulking agent greatly improved cornposting in Xalapa, leading to stabilization in just 35 days. Use of the accelerator increases the initial C/N ratio in the waste. This permits rapid utilization of any easily degradable sugars, which in turn supports rapid multiplication of mesophilic microorganisms. Key words:
Accelerator,
bulking
agent, compost, food waste, mesophils,
The disposal and treatment of municipal solid wastes is still inefficient in Mexico, especially in small towns, where collection forms the first bottleneck in the operation. Since 40% (Lea1 & Monroy 1984), or even 50% (Restrepo & Morris 1982), of such wastes are organic, cornposting at the family scale has been postulated as an ‘appropriate’ technology for small towns with less than 100,000 inhabitants (Monroy 1987). Here, ‘Appropriate’ means technologies which have a socio-economical context (Olguin 1978, 1986; Doelle et al. 1987) and which are technically and economically feasible. A family-scale cornposting unit has to be simple to operate, easy to handle and built with low cost materials. Such units must also be semi-continuous, as they will receive food wastes every day. The organic fraction of the wastes is mainly of vegetable and fruit origin, and this makes the establishment of an optimal C/N ratio very difficult. However, the aim of such cornposting is more waste management than production of high quality compost. The technology developed in the current project includes the use of an ‘accelerator’ to reduce the residence time. This accelerator, which is a mixture of very common food wastes (orange peel and carrot bagasse), provides the mesophilic microbial population with easily degradable carbohydrates.
E.J. Olguin. G. Sanchez and Ft. Gonzalez are with the Institute of Ecology, km 2.5 Antigua Carretera a Coatepec, Apto Postal 63, Xalapa, Veracruz. Mexico: fax: + 52-281-86910. ‘Corresponding author. @ 1993 Rapid Communications
of Oxford
stabilization,
Materials Composfing
wood chips.
and Methods Units
and Substrates
Cylindrical plastic containers about 0.35 m diameter x 0.5 m high were laid on metal frames, either horizontally (open at front) or vertically (open at top). Eight aeration holes, each about 2 cm in diameter, were made in the sides of each container. All composts were mixed six times each day for the 60 days’ cornposting. Mexico City. Six composting units were loaded with the prepared substrates as follows: Unit Unit Unit Unit Unit Unit
1 (vertical): 100% heterogeneous food waste; 2 (horizontal): 100% heterogeneous food waste; 3 (vertical): 100% ‘accelerator’; 4 (horizontal): 100% ‘accelerator’; 5 (vertical): 70% (v/v) food waste and 30% ‘accelerator’; 6 (horizontal): 70% (v/v) food waste and 30% ‘accelerator’.
Each unit was supplemented with 500 g prepared substrate every 4 days from day 1.2. Xalapa City. Four different units, all laid vertically, were tested simultaneously:
Unit 1: 100% heterogeneous food waste; Unit 2: 100% ‘accelerator’; Unit 3: 60% (v/v) heterogeneous food waste, 30% ‘accelerator’ and 10% bulking agent; Unit 4: 80% (v/v) heterogeneous food waste and 20% ‘accelerator’. Each unit was supplemented with 50o-750 substrate every day from day 1 to day 21.
g of prepared
Ltd World ]oournal of Microbiology and Biotechnology. Vol 9, 1993
625
E.]. Olguin,
G. Sanchez
and
R. Gonzalez
Preparation of Substrates The accelerator [chopped orange peel and carrot bagasse in a I : 1 (w/w) ratio] and the substrates were roughly chopped in a Multichef food processor (Moulinex). A ‘bulking agent’ (small wood chips from wood-processing workshops) was also used, Sampling and Analyses Sampling was carried out twice each week. The samples were ground in a food grinder before being analysed. pH was determined in an extract of I g sample in IOO ml distilled water. Moisture content was determined by drying the samples to constant weight at 100°C. Total carbohydrate was estimated according to Dubois et al. (1956) and reducing sugars according to Miller (1959). Total nitrogen was determined using the Kjeldahl method (Furman 1975) and ammonia nitrogen was obtained after distillation and titration with boric acid (American Public Health Association 1975). Ambient and internal temperature (inside substrate) were measured daily.
Results
and Discussion
City The temperature within the composts in all six units remained in the mesophilic range (oscillating between 19 and 24°C) and thus followed a similar trend to the ambient temperatures outside the containers. Only during the last days of the trial period did the temperature within the composts rise to 27°C. The moisture content of the composted substrates either gradually decreased from 90% to 75% during the whole period of monitoring (100% accelerator) or kept oscillating between 75 and 85% (all other substrates). Addition of accelerator to the food waste reduced the initial pH from 6.0 to 4.5 to 5.0. Although pH increased to 9.0 in 70 days of mesophilic decomposition in all the units, this increase was only slow and steady in pure food waste. A sudden increase from 5.0 to 7.5 occurred in the early stages of decomposition in the unit with pure accelerator and a similar surge, from 5.0 to 9.0, occurred on about day 18 in the unit containing the mixture of accelerator and food waste. A decrease followed both these surges before the pH increased steadily again towards the end of the experiments (Figure 1). The changes in total carbohydrates observed throughout the decomposition period are shown in Figure 2. All systems showed a similar trend. The sugar utilization rate up to day 18 was very high in the units containing accelerator, either mixed with food waste (45.5 g/kg.day) or alone (46 g/ kg.day), and rather low in pure food waste (20.8 g/kg.day). The sugars in the pure food waste were not properly utilized throughout the whole trial period but mixing the waste with accelerator produced a second period of fast utilization (46 g/kg.day) during the fourth and fifth weeks. Mesophilic microorganisms present during composting utilize the easily degradable sugars (Gray et al. 1971) and ammonia nitrogen accumulates and alkalinizes the substrate Mexico
626
World Journal
of
Microbiology and Biotechnology, Vol 9. 1993
0
3
10
15 20
25c
Composting
Figure City) pure mix
1. Changes
of pH during
30r 35n 40 45 time
the composting
of pure food waste (A-horizontal accelerator (O-horizontal units; of food waste and accelerator
O-vertical
’ 55 60 65 50
m’
(days) period
(in Mexico
units; A-vertical O-vertical units) (m-horizontal
units), and a units;
units).
d, d lo Ii ia 25 $0 $ b 4’5 go 5’5 $0 g5 $0 Composting
Figure
2.
composting Figure 1.
Changes period
in total (in Mexico
time carbohydrate City). The
(days) content during the symbols used are as in
Food waste composting Table
1. Physico-chemical
characteristics
of the compost
generated
in Mexico
Parameter
(%)t
Values are the means t Dry wt basis.
l
conditions).*
Substrate Food
Humidity (%) PH Total nitrogen (%)t Total carbohydrates C/N ratio
Clty (temperate
of two
waste
Accelerator
Food waste Accelerator
with
Day 60
Day 70
Day 60
Day 70
Day 60
Day 70
74.2 9.0 3.0 10.0 3.3
69.5 9.2 2.6 11.7 5.1
69.7 8.2 1.9 15.5 7.0
72 9.0 2.5 14.2 4.6
81 8.4 4.0 10.5 2.5
74 9.6 2.5 10.65 5.0
units/substrate
(one
vertical,
when the carbon source becomes a limiting nutrient factor (Witter & Lopez-Real 1988; Clarence et al. 1990). The accelerator appears to accelerate the decomposition of food waste by allowing a rapid multiplication of the mesophilic microorganisms through the provision of easily degradable sugars. No differences were observed between horizontal and vertical units. The degrees of maturity of the composts after 60 and 70 days’ processing are presented as the physico-chemical characteristics (Table 1). Xalapa City Preliminary experiments during times of low ambient temperature (15 to 20°C) and very high ambient humidity resulted in very slow degradation of the food waste even in the presence of the accelerator. It was therefore decided to investigate the effects of addition of wood chips as a bulking agent. The bulking agent helped to significantly decrease the moisture content of heterogeneous food waste from an initial value of 87% to 77% after 35 days and to 75% at the end of the monitoring period of 60 days. In Unit 3, which had 30% (v/v) accelerator and 10% (v/v) of bulking agent as well as the food waste, there was no significant decrease in moisture until after 35 days, although 75% moisture content was still observed after 60 days. The moisture content of pure accelerator increased to 91% after 55 days of processing. A fast and steady alkalinization during the first 5 days was only observed in the presence of the bulking agent. Food waste alone exhibited a brief early alkalinization, followed by an acidification, and then a second alkalinization period towards the end of the experiment (Figure 3). Changes in pH and total carbohydrate utilization were related (Figure 4). As soon as the pH tended towards alkalinization, such as in pure food waste in the first 10 days of composting, a simultaneous fast utilization of total carbohydrates occurred. In contrast, during acidification both total carbohydrates and reducing sugars increase.
one horizontal).
When the bulking agent was added to the food waste, neither total carbohydrate nor reducing sugar content accumulated during the first 3 weeks and utilization rates (18.4 g/kg.day for total carbohydrates and 25.5 g/kg.day for reducing sugars) were very high during the first few days. To obtain a better understanding of the degradation conditions, the behaviour of the total carbohydrate/total nitrogen (C/N) ratio was analysed (Figure 5). Overall, this ratio tended to decrease in pure food waste, despite a period of non-degradation of organic matter. In the presence of bulking agent-, the ratio kept oscillating until a low value of 3.23 was reached after 35 days. In Unit 3, where food waste
3;* 0, 0
I 5
I 0
, I5
, P
, 25
Composting
, ?JC
time
( 55
, 10
, 45
, Jo
, 55
(days)
Flgure 3. Changes in pH during the composting City) of (v/v): 100% heterogeneous food heterogeneous food waste and 20% bulking accelerator (m); 60% heterogeneous food accelerator and 10% bulking agent (0).
period (in Xalapa waste (A); 80% agent (a); 100% waste with 30%
World ]ournal of Microbiology ad Biotechnology. Vol 9, 1993
627
E.].
Olguin, G. Sanchez and R. Gonzalez
Composting I!l
P
Composting Figure 4. Changes composting period Figure 3.
in total (in Xalapa
time
(days)
carbohydrate content during the City). The symbols used are as in
was mixed with accelerator and bulking agent, the ratio followed a less irregular pattern, decreasing to 2.8 after 35 days. Table 2 summarizes the value of various parameters characterizing the quality of the compost generated under the humid tropical conditions of Xalapa City. It should be noted that the food waste mixed with accelerator and
Table
2. Physlco-chemlcal
characteristics
of the compost
generated
in Xalapa
City (humld
tropical
condltlons).
Substrate Food
waste
Food waste wlth bulklng agent
Food waste with bulklng agent and accelerator
Day 35
Day 60
Day 35
Day 60
Day 35
Day 60
69.16 7.0 2.4 0.24 15.07 3.08 6.59
92.22 7.0 2.74 0.33 12.06 7.52 4.39
76.6 7.35 2.5 0.07 8.10 4.70 3.23
75.3 a.2 2.37 0.05 14.27 4.16 6.0
82.73 7.1 3.28 0.05 9.22 6.53 2.80
75.3 7.5 2.36 0.05 13.89 6.66 5.02
* Dry wt basis.
628
(days)
World Journal of Microbiology and Biotechnology, Vol 9, 1993
period
bulking agent reached a very high content of total nitrogen (3.3% w/w dry wt), which is useful if it is to be used as a soil conditioner. The use of the accelerator allowed the main objective of the investigations to be achieved, i.e. a decrease in residence time; a 78% decrease in the C/N ratio occurred in just 35 days under the most difficult conditions tested (high humidity, tropical) when accelerator was used. There have been reports of cornposting green waste using only a
Parameter
Humidity (%) PH Total nitrogen (%)’ Ammonia nitrogen (%)* Total carbohydrates (%)* Reducing sugars (%)* C/N ratio
time
Figure 5. Changes in the C/N ratio during the composting (in Xalapa City). The symbols used are as in Figure 3.
25
bulking agent and no accelerator, but this only produced a 24% decrease in the C/N ratio after 90 days (Vallini et al. 1990). It has also been reported (Inbar et al. 1990) that immature composts with a high C/N ratio causes nitrogen immobilization and that excessively Iow C/N composts cause ammonium toxicity; both extremes interfere with plant growth. More work is therefore required to establish the optimum values for the physico-chemical parameters shown in Table 1.
Acknowledgements This research was supported in part by a grant from the National Council for Science and Technology (CONACYI’) through the Mexican Institute of Appropriate Technologies. We are grateful to G. Mercado for help with the figures.
References American Public Health Association 1975 Sfandgrd Mefkuds for the Examination of Water and Wastewater, 14th edn. Washington DC: APHA. Clarence, G., Golueke, G. & Diaz, L.F. 1990 Understanding the basics of composting. BioCycle, April, 56-59. Dubois, M., Miles, K.A., Hamilto, J.K., Rebers, P.A. & Smith, E. 1956 Calorimetric method for dete~ination of sugar and related substances. Analytical Chemistry 28, 350-354. Doelle, H.W., Olguin, E.J. & Prasertsan, P. 1987 Fermentation technology and its impact on culture and society. In ~iff~bi~~ Technology in the Developing Weld, eds. DaSilva, E.J.,
Dommergues, Y.R., Nyns, EJ., Ratledge, C. Oxford: Oxford University Press. pp, 209-225. Furman, N.H. 1975 Sfundurd Methods of Chemical Analysis. Huntington NY: Krieger. Gray, K.R., Sherman, K. & Biddlestone, A.J. 1971 A review of composting. ProcessBiochemistry 6, 32-36. Inbar, Y., Chen, Y. & Hoitink, H.A.J. 1990 New approaches to compost maturity. BioCycle 31, 64-68. Leal, L.H. & Monroy, H.O. 1974 Disposition final de residues solidos en et PMerto de Acupuko. Tesis Profesional. Facultad de Quimica, UNAM, Mexico. MiIler, G.L. 1959 Use of dinitrosalicyfic acid reagent for determination of reducing sugar. Analytical Chemistry 31, 426-428. Monroy, 0. 1987 Manejo y disposition de residuos solidos. Desarrollo
y Medio
Ambienfe
2, 2-7.
Olguin, E.J. 1978 Appropriate technology: The case of single cell protein (SCP) and biological upgrading of wastes. Research Fellow Report. University of Aston, Birmingham, UK: Technology Policy Unit.
Olguin, E.J.1986 Appropriate biotechnolo~cal systems in the arid environment. In Applied Microbiology, eds. DoeIIe, H.W. & Heden, G. pp. 111-134. Amsterdam: Reidel. Restrepo, 1. & Morris, D. 1982 La busura Consume y D~p~diciu en el Distrifo Federal. Mexico: Instituto National del Consumidor. Vallini, G., Pera, A., Sorace, G., Cecchi, C. & Manetti, P. 1990 Green tomposting. BioCycle 31, 33-35. Witter, E. & Lopez-Real, V. 1988 Nitrogen losses during the composting of sewage sludge and the effectiveness of clay soil, zeolite and compost in adsorbing the volatilised ammonia. Biological
Wastes
23, 279-294.
(Received in revised form 2 7 April 1993; accepted
1.3
May 1993)
of Microbiology
and Biotechnology
Accelerated
9, 625-629
food waste cornposting
E.J. Olguin,* G. Sanchez and R. Gonzalez Experiments on food waste cornposting under the temperate and dry conditions of Mexico City showed that the waste stabilized faster (within 2 months) if an ‘accelerator’ (chopped orange peel and carrot bagasse) was used. The relatively cool and humid conditions prevailing during similar experiments in Xalapa City inhibited decomposition, and stabilization of waste, with or without accelerator, was generally slower than in Mexico City. However, addition of wood chips as a bulking agent greatly improved cornposting in Xalapa, leading to stabilization in just 35 days. Use of the accelerator increases the initial C/N ratio in the waste. This permits rapid utilization of any easily degradable sugars, which in turn supports rapid multiplication of mesophilic microorganisms. Key words:
Accelerator,
bulking
agent, compost, food waste, mesophils,
The disposal and treatment of municipal solid wastes is still inefficient in Mexico, especially in small towns, where collection forms the first bottleneck in the operation. Since 40% (Lea1 & Monroy 1984), or even 50% (Restrepo & Morris 1982), of such wastes are organic, cornposting at the family scale has been postulated as an ‘appropriate’ technology for small towns with less than 100,000 inhabitants (Monroy 1987). Here, ‘Appropriate’ means technologies which have a socio-economical context (Olguin 1978, 1986; Doelle et al. 1987) and which are technically and economically feasible. A family-scale cornposting unit has to be simple to operate, easy to handle and built with low cost materials. Such units must also be semi-continuous, as they will receive food wastes every day. The organic fraction of the wastes is mainly of vegetable and fruit origin, and this makes the establishment of an optimal C/N ratio very difficult. However, the aim of such cornposting is more waste management than production of high quality compost. The technology developed in the current project includes the use of an ‘accelerator’ to reduce the residence time. This accelerator, which is a mixture of very common food wastes (orange peel and carrot bagasse), provides the mesophilic microbial population with easily degradable carbohydrates.
E.J. Olguin. G. Sanchez and Ft. Gonzalez are with the Institute of Ecology, km 2.5 Antigua Carretera a Coatepec, Apto Postal 63, Xalapa, Veracruz. Mexico: fax: + 52-281-86910. ‘Corresponding author. @ 1993 Rapid Communications
of Oxford
stabilization,
Materials Composfing
wood chips.
and Methods Units
and Substrates
Cylindrical plastic containers about 0.35 m diameter x 0.5 m high were laid on metal frames, either horizontally (open at front) or vertically (open at top). Eight aeration holes, each about 2 cm in diameter, were made in the sides of each container. All composts were mixed six times each day for the 60 days’ cornposting. Mexico City. Six composting units were loaded with the prepared substrates as follows: Unit Unit Unit Unit Unit Unit
1 (vertical): 100% heterogeneous food waste; 2 (horizontal): 100% heterogeneous food waste; 3 (vertical): 100% ‘accelerator’; 4 (horizontal): 100% ‘accelerator’; 5 (vertical): 70% (v/v) food waste and 30% ‘accelerator’; 6 (horizontal): 70% (v/v) food waste and 30% ‘accelerator’.
Each unit was supplemented with 500 g prepared substrate every 4 days from day 1.2. Xalapa City. Four different units, all laid vertically, were tested simultaneously:
Unit 1: 100% heterogeneous food waste; Unit 2: 100% ‘accelerator’; Unit 3: 60% (v/v) heterogeneous food waste, 30% ‘accelerator’ and 10% bulking agent; Unit 4: 80% (v/v) heterogeneous food waste and 20% ‘accelerator’. Each unit was supplemented with 50o-750 substrate every day from day 1 to day 21.
g of prepared
Ltd World ]oournal of Microbiology and Biotechnology. Vol 9, 1993
625
E.]. Olguin,
G. Sanchez
and
R. Gonzalez
Preparation of Substrates The accelerator [chopped orange peel and carrot bagasse in a I : 1 (w/w) ratio] and the substrates were roughly chopped in a Multichef food processor (Moulinex). A ‘bulking agent’ (small wood chips from wood-processing workshops) was also used, Sampling and Analyses Sampling was carried out twice each week. The samples were ground in a food grinder before being analysed. pH was determined in an extract of I g sample in IOO ml distilled water. Moisture content was determined by drying the samples to constant weight at 100°C. Total carbohydrate was estimated according to Dubois et al. (1956) and reducing sugars according to Miller (1959). Total nitrogen was determined using the Kjeldahl method (Furman 1975) and ammonia nitrogen was obtained after distillation and titration with boric acid (American Public Health Association 1975). Ambient and internal temperature (inside substrate) were measured daily.
Results
and Discussion
City The temperature within the composts in all six units remained in the mesophilic range (oscillating between 19 and 24°C) and thus followed a similar trend to the ambient temperatures outside the containers. Only during the last days of the trial period did the temperature within the composts rise to 27°C. The moisture content of the composted substrates either gradually decreased from 90% to 75% during the whole period of monitoring (100% accelerator) or kept oscillating between 75 and 85% (all other substrates). Addition of accelerator to the food waste reduced the initial pH from 6.0 to 4.5 to 5.0. Although pH increased to 9.0 in 70 days of mesophilic decomposition in all the units, this increase was only slow and steady in pure food waste. A sudden increase from 5.0 to 7.5 occurred in the early stages of decomposition in the unit with pure accelerator and a similar surge, from 5.0 to 9.0, occurred on about day 18 in the unit containing the mixture of accelerator and food waste. A decrease followed both these surges before the pH increased steadily again towards the end of the experiments (Figure 1). The changes in total carbohydrates observed throughout the decomposition period are shown in Figure 2. All systems showed a similar trend. The sugar utilization rate up to day 18 was very high in the units containing accelerator, either mixed with food waste (45.5 g/kg.day) or alone (46 g/ kg.day), and rather low in pure food waste (20.8 g/kg.day). The sugars in the pure food waste were not properly utilized throughout the whole trial period but mixing the waste with accelerator produced a second period of fast utilization (46 g/kg.day) during the fourth and fifth weeks. Mesophilic microorganisms present during composting utilize the easily degradable sugars (Gray et al. 1971) and ammonia nitrogen accumulates and alkalinizes the substrate Mexico
626
World Journal
of
Microbiology and Biotechnology, Vol 9. 1993
0
3
10
15 20
25c
Composting
Figure City) pure mix
1. Changes
of pH during
30r 35n 40 45 time
the composting
of pure food waste (A-horizontal accelerator (O-horizontal units; of food waste and accelerator
O-vertical
’ 55 60 65 50
m’
(days) period
(in Mexico
units; A-vertical O-vertical units) (m-horizontal
units), and a units;
units).
d, d lo Ii ia 25 $0 $ b 4’5 go 5’5 $0 g5 $0 Composting
Figure
2.
composting Figure 1.
Changes period
in total (in Mexico
time carbohydrate City). The
(days) content during the symbols used are as in
Food waste composting Table
1. Physico-chemical
characteristics
of the compost
generated
in Mexico
Parameter
(%)t
Values are the means t Dry wt basis.
l
conditions).*
Substrate Food
Humidity (%) PH Total nitrogen (%)t Total carbohydrates C/N ratio
Clty (temperate
of two
waste
Accelerator
Food waste Accelerator
with
Day 60
Day 70
Day 60
Day 70
Day 60
Day 70
74.2 9.0 3.0 10.0 3.3
69.5 9.2 2.6 11.7 5.1
69.7 8.2 1.9 15.5 7.0
72 9.0 2.5 14.2 4.6
81 8.4 4.0 10.5 2.5
74 9.6 2.5 10.65 5.0
units/substrate
(one
vertical,
when the carbon source becomes a limiting nutrient factor (Witter & Lopez-Real 1988; Clarence et al. 1990). The accelerator appears to accelerate the decomposition of food waste by allowing a rapid multiplication of the mesophilic microorganisms through the provision of easily degradable sugars. No differences were observed between horizontal and vertical units. The degrees of maturity of the composts after 60 and 70 days’ processing are presented as the physico-chemical characteristics (Table 1). Xalapa City Preliminary experiments during times of low ambient temperature (15 to 20°C) and very high ambient humidity resulted in very slow degradation of the food waste even in the presence of the accelerator. It was therefore decided to investigate the effects of addition of wood chips as a bulking agent. The bulking agent helped to significantly decrease the moisture content of heterogeneous food waste from an initial value of 87% to 77% after 35 days and to 75% at the end of the monitoring period of 60 days. In Unit 3, which had 30% (v/v) accelerator and 10% (v/v) of bulking agent as well as the food waste, there was no significant decrease in moisture until after 35 days, although 75% moisture content was still observed after 60 days. The moisture content of pure accelerator increased to 91% after 55 days of processing. A fast and steady alkalinization during the first 5 days was only observed in the presence of the bulking agent. Food waste alone exhibited a brief early alkalinization, followed by an acidification, and then a second alkalinization period towards the end of the experiment (Figure 3). Changes in pH and total carbohydrate utilization were related (Figure 4). As soon as the pH tended towards alkalinization, such as in pure food waste in the first 10 days of composting, a simultaneous fast utilization of total carbohydrates occurred. In contrast, during acidification both total carbohydrates and reducing sugars increase.
one horizontal).
When the bulking agent was added to the food waste, neither total carbohydrate nor reducing sugar content accumulated during the first 3 weeks and utilization rates (18.4 g/kg.day for total carbohydrates and 25.5 g/kg.day for reducing sugars) were very high during the first few days. To obtain a better understanding of the degradation conditions, the behaviour of the total carbohydrate/total nitrogen (C/N) ratio was analysed (Figure 5). Overall, this ratio tended to decrease in pure food waste, despite a period of non-degradation of organic matter. In the presence of bulking agent-, the ratio kept oscillating until a low value of 3.23 was reached after 35 days. In Unit 3, where food waste
3;* 0, 0
I 5
I 0
, I5
, P
, 25
Composting
, ?JC
time
( 55
, 10
, 45
, Jo
, 55
(days)
Flgure 3. Changes in pH during the composting City) of (v/v): 100% heterogeneous food heterogeneous food waste and 20% bulking accelerator (m); 60% heterogeneous food accelerator and 10% bulking agent (0).
period (in Xalapa waste (A); 80% agent (a); 100% waste with 30%
World ]ournal of Microbiology ad Biotechnology. Vol 9, 1993
627
E.].
Olguin, G. Sanchez and R. Gonzalez
Composting I!l
P
Composting Figure 4. Changes composting period Figure 3.
in total (in Xalapa
time
(days)
carbohydrate content during the City). The symbols used are as in
was mixed with accelerator and bulking agent, the ratio followed a less irregular pattern, decreasing to 2.8 after 35 days. Table 2 summarizes the value of various parameters characterizing the quality of the compost generated under the humid tropical conditions of Xalapa City. It should be noted that the food waste mixed with accelerator and
Table
2. Physlco-chemlcal
characteristics
of the compost
generated
in Xalapa
City (humld
tropical
condltlons).
Substrate Food
waste
Food waste wlth bulklng agent
Food waste with bulklng agent and accelerator
Day 35
Day 60
Day 35
Day 60
Day 35
Day 60
69.16 7.0 2.4 0.24 15.07 3.08 6.59
92.22 7.0 2.74 0.33 12.06 7.52 4.39
76.6 7.35 2.5 0.07 8.10 4.70 3.23
75.3 a.2 2.37 0.05 14.27 4.16 6.0
82.73 7.1 3.28 0.05 9.22 6.53 2.80
75.3 7.5 2.36 0.05 13.89 6.66 5.02
* Dry wt basis.
628
(days)
World Journal of Microbiology and Biotechnology, Vol 9, 1993
period
bulking agent reached a very high content of total nitrogen (3.3% w/w dry wt), which is useful if it is to be used as a soil conditioner. The use of the accelerator allowed the main objective of the investigations to be achieved, i.e. a decrease in residence time; a 78% decrease in the C/N ratio occurred in just 35 days under the most difficult conditions tested (high humidity, tropical) when accelerator was used. There have been reports of cornposting green waste using only a
Parameter
Humidity (%) PH Total nitrogen (%)’ Ammonia nitrogen (%)* Total carbohydrates (%)* Reducing sugars (%)* C/N ratio
time
Figure 5. Changes in the C/N ratio during the composting (in Xalapa City). The symbols used are as in Figure 3.
25
bulking agent and no accelerator, but this only produced a 24% decrease in the C/N ratio after 90 days (Vallini et al. 1990). It has also been reported (Inbar et al. 1990) that immature composts with a high C/N ratio causes nitrogen immobilization and that excessively Iow C/N composts cause ammonium toxicity; both extremes interfere with plant growth. More work is therefore required to establish the optimum values for the physico-chemical parameters shown in Table 1.
Acknowledgements This research was supported in part by a grant from the National Council for Science and Technology (CONACYI’) through the Mexican Institute of Appropriate Technologies. We are grateful to G. Mercado for help with the figures.
References American Public Health Association 1975 Sfandgrd Mefkuds for the Examination of Water and Wastewater, 14th edn. Washington DC: APHA. Clarence, G., Golueke, G. & Diaz, L.F. 1990 Understanding the basics of composting. BioCycle, April, 56-59. Dubois, M., Miles, K.A., Hamilto, J.K., Rebers, P.A. & Smith, E. 1956 Calorimetric method for dete~ination of sugar and related substances. Analytical Chemistry 28, 350-354. Doelle, H.W., Olguin, E.J. & Prasertsan, P. 1987 Fermentation technology and its impact on culture and society. In ~iff~bi~~ Technology in the Developing Weld, eds. DaSilva, E.J.,
Dommergues, Y.R., Nyns, EJ., Ratledge, C. Oxford: Oxford University Press. pp, 209-225. Furman, N.H. 1975 Sfundurd Methods of Chemical Analysis. Huntington NY: Krieger. Gray, K.R., Sherman, K. & Biddlestone, A.J. 1971 A review of composting. ProcessBiochemistry 6, 32-36. Inbar, Y., Chen, Y. & Hoitink, H.A.J. 1990 New approaches to compost maturity. BioCycle 31, 64-68. Leal, L.H. & Monroy, H.O. 1974 Disposition final de residues solidos en et PMerto de Acupuko. Tesis Profesional. Facultad de Quimica, UNAM, Mexico. MiIler, G.L. 1959 Use of dinitrosalicyfic acid reagent for determination of reducing sugar. Analytical Chemistry 31, 426-428. Monroy, 0. 1987 Manejo y disposition de residuos solidos. Desarrollo
y Medio
Ambienfe
2, 2-7.
Olguin, E.J. 1978 Appropriate technology: The case of single cell protein (SCP) and biological upgrading of wastes. Research Fellow Report. University of Aston, Birmingham, UK: Technology Policy Unit.
Olguin, E.J.1986 Appropriate biotechnolo~cal systems in the arid environment. In Applied Microbiology, eds. DoeIIe, H.W. & Heden, G. pp. 111-134. Amsterdam: Reidel. Restrepo, 1. & Morris, D. 1982 La busura Consume y D~p~diciu en el Distrifo Federal. Mexico: Instituto National del Consumidor. Vallini, G., Pera, A., Sorace, G., Cecchi, C. & Manetti, P. 1990 Green tomposting. BioCycle 31, 33-35. Witter, E. & Lopez-Real, V. 1988 Nitrogen losses during the composting of sewage sludge and the effectiveness of clay soil, zeolite and compost in adsorbing the volatilised ammonia. Biological
Wastes
23, 279-294.
(Received in revised form 2 7 April 1993; accepted
1.3
May 1993)
