Case study description
Experiments in this study were carried out at a farm household of 8.1 ha of land use for farming in Long An Province (10°36′24″N 106°8′31″W). The local weather conditions show an average temperature of 27°C and an average annual amount of evaportranspiration of up to 2,000 mm. There are three productive components in the operation of the farm: piggery, fish breeding, and an orchard. The farm is raising piglets for porker production at an industrial scale with 4,500 heads of pig. The total area of the sheds is 1,800 m2. Jackfruit trees (Artocarpus heterophyllus) are planted with a density of 525 trees per ha on 2.5 ha of orchard. Feeds for pig and fish as well as fertilizer for the orchard are commercial products. For the fishpond, river water is used, but lime is scattered on 5 ha of ponds to increase the pH level to the range of 6.5 - 8 before raising fish. Similarly, water pumped from the river can be used to irrigate plants in the orchard after pH adjustment by lime. Hence, about 180 m3 water per day from the well are used for both living and breeding. Wastewater from the piggery, including pig manure and washing water, is directly discharged into internal ditches and then discharged into field canals. The monthly electricity consumption mainly used for washing pigs, ventilating fans, pumping water, and human activities amounts to about 12,000 kWh (43,200 MJ). Two generators are used with a consumption of 6 liters of diesel/hour for 27 kW.
The entire system with all material flows embodied in a typical agricultural system in the acid soil area in Vietnam is presented in Fig. 3. All of the natural renewable resources serving for the whole production system include sunlight, wind, rain, surface water, soil. Natural non-renewable resources include the loss of topsoil, ground, and surface water which is basically employed in horticulture [36]. Many items in this system are purchased non-renewable resources. Inputs and outputs for each component according to the existing farming system are shown in Table 2. The data is divided into groups of both free purchased inputs and the yield referring to outputs. The exergy analysis in the present system is computed for a period of one year.
Exergy analysis
The input and output data sources based on exergetic analysis are shown in Table 2. The items are divided into 3 groups: Renewable resources, Natural Nonrenewable resources, and Purchased -nonrenewable resources. As mentioned above, Purchased -renewable resources (i.e. labor) are not included in this case. The energy loss of the whole system and the energy efficiency of each component compared to the entire system are shown in Fig. 4. Most non-renewable natural resources contribute large amounts of exergy to the system, in which the distribution rate of fish feeding accounts for over 80% of the total input sources. From the total yields of 19 million MJ in a year, pig and fish production account for about 99% of the total production with an efficiency rate of 59% and 41%, respectively. However, the energy loss from fish production is also the highest among the components. With 195 million MJ per year of exergy content of the total inputs, the energy loss from the system is significant, accounting for 90% total inputs. The losses mainly occur in fish production. However, because in the current system three components are operated separately, we consider the calculation of the exergy embodied in each component to identify the main cause of the energy loss. From piggery, about 70% of the energy is lost not taking into account the amount of pig waste. Likewise, the exergy content in the fishpond is mainly embodied in animal feeding; the fish processing causes a considerable amount of energy loss. About 95% of the embodied exergy is lost while fish feeding accounts for over 99% of the total inputs. Although the rate of energy loss from the orchard is negligible compared to the entire system, energy efficiency from cultivation is only 2% not taking into account biomass residues; thus causing 98% of energy loss in this process. It is possible to attribute the causes of the energy loss in the cultivation process to the topsoil loss (over 50% of the inputs), the other ones stemming from fertilizer and machinery (plow).
Table 2 Inputs and yields for system components according to the present farming household system (in a year)
Item
|
Unit
|
Equivalent
Factor (Jex/Unit)
|
Reference
|
Pig production
|
Fish production
|
Orchard
|
Renewable resource (RR)
|
Sunlight a
|
J
|
1.02E-05
|
[11, 18, 21]
|
1.17E+08
|
3.26E+09
|
1.63E+09
|
Wind b
|
J
|
3.12E-02
|
[18, 21]
|
7.08E+08
|
1.97E+10
|
9.83E+09
|
Evapotranspiration c
|
J
|
6.26E-01
|
[11, 18]
|
1.11E+12
|
3.09E+11
|
1.55E+11
|
Natural non-renewable resources (NR)
|
Water
|
kg
|
4.94E-03
|
[18, 37]
|
|
4.94E+08
|
3.84E+06
|
Ground water
|
m3
|
2.00E+04
|
[38]
|
1.34E+09
|
|
|
Loss of topsoil d
|
kg
|
4.33E+07
|
[5, 10]
|
|
|
2.06E+11
|
Purchased -non-renewable resources (PN)
|
Diesel
|
L
|
47.80+06
|
[38]
|
1.74E+10
|
|
|
Concrete
|
kg
|
6.35E+12
|
[12]
|
2.08E+11
|
|
|
Electricity
|
kW
|
3.60E+06
|
[18, 38]
|
5.18E+11
|
|
|
Weaned piglet
|
kg
|
2.09E+12
|
[39]
|
1.37E+12
|
|
|
Pig feed
|
kg
|
5.22E+09
|
[40]
|
3.32E+13
|
|
|
Lime (CaO)
|
kg
|
3.11E+06
|
[41]
|
|
3.04E+09
|
7.09E+09
|
Small fry
|
kg
|
7.98E+06
|
[42]
|
|
7.98E+10
|
|
Fish feed
|
kg
|
2.38E+04
|
[43]
|
|
1.57E+14
|
|
Fertilizer
N
P
K
|
kg
|
3.28E+01,
7.52E+01,
4.56E+01
|
[9, 44]
|
|
|
1.28E+11
|
Pesticide
|
kg
|
4.20E+02
|
[18]
|
|
|
1.26E+09
|
Plow
|
kg
|
1.80E+8
|
[12]
|
|
|
2.47E+11
|
Sapling
|
kg
|
1.44E+13
|
[18]
|
|
|
9.45E+08
|
Yield (Y)
|
Porker
|
kg
|
2.03E+7
|
[45]
|
|
|
1.15E+13
|
Fish
|
kg
|
7.98E+6
|
[45]
|
|
|
7.98E+12
|
Fruit
|
kg
|
1.89E+12
|
[18]
|
|
|
2.48E+10
|
a: Solar energy = (the average radiation of province) x (area)
The average radiation of province = 17.5 MJ/(m2.day) = 6,387.5 MJ/(m2.yr)
b: Global wind circulation= (0.4 J/m2/sec) x (3.15E+7 sec/year) x (area)
c: Evapotranspiration, chemical energy = (area) x (average rainfall) x (density) x (Gibbs free energy)
Average rainfall = 2 m/yr
Density = 1.00E+06 g/m3
Gibbs free energy = 4.94 J/g
d: Net loss of topsoil = (soil loss) x (organic matter content) = 9.5 t/ha x 2.5ha
Soil loss = 9.5 t/(ha.yr)
Organic matter content = 20% = 0.2
Fig. 5 shows the energy flows for the current farm household system. Considering the exergy loss in the current system, the comparison of input and output in the exergy analysis reveals numerous disparities; most of the energy loss is due to fish production. Basically, the components of the system process operate individually, thus there is no links between input and output flows from one component to the others. For each component, the exergy loss from pig production, fish production, and orchard is 68%, 95%, and 98% respectively; thus exergy efficiency is quite low, the highest is 32% of pig production, the exergy efficiency of the other two components is less than 5%.
The exergy embodied in waste technically contributes to the increase in energy efficiency when it is considered as the by-product. The composition of agricultural wastes is mainly organic matters having great potential for recycling. Compared with the negligible energy from biomass residues (3,000 MJ/yr) the energy content in pig manure is several times higher (900,000 MJ/yr), also the energy content of the biogas released amounts to one million MJ/yr.
Applied agricultural zero emission system for the farm
In a zero-emission agricultural system, aquatic plants such as water hyacinth (an indigenous plant species) growing in a pond contribute to bio-treating the wastewater [46]. Water from such a pond can not only serve for irrigating orchards but can also directly supply fishponds without prior treatment [47] (while the current system has to treat the pond with lime). In addition, aquatic plants are used as compost to fertilize the garden. Similarly, waste from a pigsty (pig manure + urine) is digested in biogas plastic container, thereby biogas can be used as an energy source for producing electricity and heat [48]. The pig sludge is used as compost in combination with water hyacinth for pond water treatment and as fertilizer for the orchard. With a pH level lower than 5 in an acid sulphate-containing soil [49], resources in the system need to be fully utilized to improve both soil and water quality, while reducing costs for the farm.
Table 3 Supplement items for a zero-emission system
Item
|
|
Unit
|
Equivalent factor
(J/Unit)
|
Reference
|
Total
|
Natural non-renewable resource (NR)
|
Water
|
|
kg
|
4.94E-03
|
[18, 37]
|
49.40E+06
|
Purchased -non-renewable resource (PN)
|
Plastic
|
|
kg
|
1.08E+14
|
[12]
|
1.57E+14
|
Biogas sludge
|
|
kg
|
3.72E+05
|
[14]
|
1.25E+12
|
Biomass (crop residues)
|
|
kg
|
1.88E+07
|
[44]
|
9.13E+08
|
Biogas
|
|
m³
|
2.20E+7
|
[38]
|
1.95E+12
|
Other components that need to be supplemented in the scenario of an agricultural zero emission system would be local plants such as spinach and/or water hyacinth, an aquatic pond, and plastic containers for biogas production, as presented in Table 3. An aquatic pond can act as storage of sludge from biogas digesters. Floating plants (spinach and water hyacinth) in the pond function as a natural waste filtration system. This kind of plants, combined with pig sludge can increase the pH level of the water, which can be used for irrigating plants. Pig manure is digested in plastic biogas containers. Biogas and sludge generated during the digestion can benefit all components. Sludge is used as bio-fertilizer and fish feeding. Pig sludge is mixed with commercial fish feed with a weight ratio of 1: 1. Simultaneously, biogas can provide heat and electricity for the whole system when using biogas-based generators. The household will no longer need to buy fuel and pay for purchased electricity.
The exergy loss in both current and ideal systems is calculated to be up to 80% (figure 6); leading to low energy efficiency with 10% of the current system and about 16% of the ideal system. However, for the proposed system, the circulation of reused waste linking all components can reduce the loss of embodied exergy in pig and fish production. Taheri et al. imply that a process with higher temperature will result in a reduction of exergy loss [12]. The process of digesting pig slurry produces heat at very high temperatures achieving high exergy efficiency. Other productive processes in the system occur under conditions of ambient temperature, i.e. orchard and pigsty. The causes of heat loss from these components need to be investigated.
Fig. 7 shows the energy flows in an idealised system, compared to the current system presuming the case of constant output. Biogas is used as a substitute for electricity and diesel to illuminate the pigsty. About ten of LED light bulbs with a power consumption of 0.018 kW per a bulb lighting 150 hours in the current system in each month only needs 324 kW per year (equivalent to 1,166 MJ per year). An average power of 62 kW can be used for other electrical appliances of the household. The remaining approximately 1.4 Million MJ/yr of embodied exergy of biogas can be converted into electricity, it can replace the amount of purchased electricity from the local power grid. Besides, biogas can also be stored and distributed to surrounding households.
The embodied exergy of biogas sludge is 65 times lower than that of commercial fish feeds. Thus, replacing commercial fish feed by addition of sludge the embodied exergy in the system can be considerably reduced. Farming experience showed that sludge supplemented with fish feed at a weight ratio of 1:1 (equal to 78 million MJ/yr in feed and 1 million MJ/yr in sludge) leads to the same fish yield as supplying only commercial feed. Energy input in the sludge / commercial feed combination amounts to only half the energy for fish feeding compared to the present system. However, empirical measures are needed for analysing and formulating the complete fish diets using locally available ingredients to optimize fish growth and health [50].
For orchard inputs, the recycle waste consists of jackfruit leaf, remains of pig sludge (crude protein content is 12.8% and 2%, respectively). It is possible to combine the waste with local plants (spinach and water hyacinth with 0.5% protein) to form biochar. This biochar contains the embodied exergy from biomass residues and sludge (900 MJ/yr and 450,000 MJ/yr, respectively), which can meet the nitrogen needs of the plants.
In the scenario of a zero emission system, waste recycling and utilisation of locally available resources will create closed material loops, in which power contribution of human resources will be negligible to the power contribution of the processes. Under the assumption of water resources (ground and surface water) seen as non-renewable natural resources, energy efficiency depends on the allocation of the natural resources [15].
In Table 4, the results of calculating exergy efficiency in a zero-emission system vs. the scenario in a current farming system are shown.
Table 4 Indicators of sustainable agriculture in the current and a zero emission system
Indicatorsc
|
Current farming system
|
Zero emission system
|
RI
|
0.002
|
0.009*
|
ELI
|
125
|
80.3*
|
IYR
|
10*
|
12.8
|
ERYR
|
0.08
|
0.2*
|
STr
|
10*
|
13
|
NRYR
|
10*
|
13
|
*: Better
c: reference [25]
For abbreviations see Table 1
Regarding the rankings for sustainability indicators, the RI and ERYR values of this system are slightly similar to that in Organization for Economic Cooperation and Development (OECD) values. These values resulted from the study of Hoang et al. which presenting rankings of the seven indicators in 29 OECD countries. In spite of variety of range in indicators among the countries, the results showed the stability of NRYR compared to the other indicators [25]. The scenario reduces half of the environmental load; whereas its ELI value is approximately a half lower than that of the present system. Nevertheless, it is extremely much higher than all OECD indicator scores. It is concluded that both systems have high investment costs. Other indicators have ten times higher scores compared to the OECD benchmarks. This study results that 1 MJ of the outputs needs 10 MJ of the inputs, whereas 1 GJ of the total resources consumed in farming from OECD countries produced 1 GJ of the outputs. The systems imply sustainability of agriculture production by NRYR which a good indicator of the agriculture sustainability. Hence, it is suggested that the exergy loss in the system process need to be further investigation. The consideration is the effective alternatives to either converting it to the products or adjusting the input supplements for the same outputs.
Waste recycling for an integrated agro-ecosystem has positive effects, resolving the problem of waste and environmental pollution by reducing the purchases [51, 52]. Further research will be related to adjusting the input items to optimize the utilization of renewable resources from the local ecosystem. The next study step should aim at aggregating the input-output flows (containing waste and heat therein) along with locally available resources over the optimum time when linking the functions of the components within the farming system. The suggestion considers the effectiveness of recycle-waste flows in different directions to produce high-yield in all components.