A study was made in An Son village, Binh Duong province, Vietnam, where the use of biodigested slurry as fertilizer was investigated. The village is situated close to Ho Chi Minh City, i.e. close to a market for selling and buying. Nutrient flows were calculated for a theoretical mixed farm typical for the village, with pigs, rice (Oryza sativa), sugarcane (Saccarum officinarum) and pak choi (Brassica chinensis) production. The livestock sub-system consists of exotic breeds of pigs fed with feed concentrate, consisting of maize meal, soy bean meal cassava meal and fish meal. The biodigester sub-system consists of a plastic tube type digester of approximately 6 m³. The digester is loaded with 12 kg manure per day and produce about 900 L gas per day. An interview study showed that the farmers with biodigesters did not use the slurry as fertilizer. A theoretical material flow analysis (MFA) concerning the nutrient flow was made in order to show the potential of integrating the crop sub-system with the livestock and biodigester sub-system. The crop sub-system is made up of 0.1 ha pak choi, 0.2 ha rice, and 0.2 ha sugarcane. The theoretical livestock sub-system consists of 6 fattening pigs year around. The MFA shows a yearly flow where 130 % of the phosphorus, and 75 % of the nitrogen need (based on the removal with pigs and crop harvest) on the farm is covered by the input of feed concentrate. The calculations on crop removal are based on average figures for harvest, and nitrogen and phosphorus content for the crops concerned. The MFA shows that there might be a risk of phosphorus accumulation on the farm even in a system where all three sub-systems are integrated and no mineral fertilizer is added. Nitrogen, however, has to be supplemented to meet crop demand in this system. Nitrogen is also exposed to the risk of losses through leaching and/or volatilization.

Introduction

An interview study was made in An Son village, Binh Duong province, Vietnam, where the use of biodigested slurry as fertilizer was investigated. The study showed that the farmers with biodigesters did not use the slurry as fertilizer. A theoretical material flow analysis (MFA) concerning the nutrient flow was made in order to show the potential of integrating the crop sub-system with the livestock and biodigester sub-system. 

The definition of an integrated biosystem, in this conference, is that three or more subsystems are integrated. It is also desirable that the output from one subsystem is matching the need for the next subsystem in order to avoid accumulations or imbalances in the biosystem. A material flow analysis (MFA) is a perfect tool to check how the subsystems match each other and it could be done on any element or matter of interest. In this case we have chosen to study the flow of nitrogen (N), phosphorus (P) and carbon (C). 

The biosystem we have analysed is a theoretical integrated livestock farm in the village of An Son in Binh Duong province in the southern Vietnam. The village is situated about 30 km north west of Ho Chi Minh City and could be called a periurban village. The vicinity of the urban market makes both mineral fertilizer and livestock feed easily accessible. This example will therefore show a linear flow. The data for the MFA is collected from interviews made in An Son village, from analysis made or from the literature. 

A description of the integrated subsystems

The subsystems in this example are pigs, biodigester and crops and soil. 

The pig subsystem

Pig raising is the biggest animal production in An Son, at present it is estimated that there are about 1200 heads. The pigs in the area are exotic breeds of Landrace, Yorkshire, or Duroc, and they are fed on feed concentrate, which in some cases are supplemented with water spinach (Ipomoea aquatica) or sweet potato (Ipomoea barbata) leaves. In this example we will only consider the input with the feed concentrate since it is the main one. The feed concentrate is mainly composed of maize meal, soybean meal cassava meal and fishmeal. The mean of the N, P and C content of some common brands in Vietnam are shown in Table 1

There are mainly two ways of raising pigs in the area: either you have sows and raise piglets for off-sale or you have fattening pigs which you buy when they weigh about 10 – 20 kg and sell when they have reached about 100 kg. This cycle takes about 4 months and the feed intake during this time will be in average 240 kg according to interviews in the area. Assuming the farmer have 6 fattening pigs in three cycles during the year the total feed input will be 4320 kg, i.e. 108 kg N, 39 kg P and 1728 kg C (Table 2, Figure 1).

In this example we assume that the absorbability of the nitrogen in the feed is 70 %. It usually varies between 60-80% (NIAH, 1995). The digestibility of P can vary much more, depending on the type of feed and the concentration of phytates in the feed, but in this example we assume the digestibility of P in the feed to be 50% (NIAH,1995). A rough assumption about the carbon balance is that 36 % is excreted in urine and faeces, 50 % respired as CO₂ and 14% is retained in the body tissues. The amount of N P and C leaving this subsystem is shown in Table 3.

The biodigester subsystem

The next subsystem is the biodigester. There are 24 biodigesters installed in An Son, at present, and they are all of the plastic tube type. The most common installation is that you have 10 m plastic tube of 1.20 m in diameter. It will give a biodigester of 6 m³ active volume. The cost of a biodigester of this size is about 50 USD, including two burners (Bui Xuan An et al., 1997). It is a common rule of thumb that 6 feeding pigs gives enough manure to make this size of a biodigester work and the gas from this biodigester will be enough to supply one family with gas for cooking. If the farmer has more pigs it is common that he only use some of the manure from the pigs to feed the biodigester, either by collecting the faeces for composting or by diverting some of the flow of slurry from the pigpen directly to a ditch. When the biodigester is fed with manure, the manure has to be diluted with water. This is accomplished when the farmer use water to clean the pigpen. Most farmers have constructed pipes for the slurry to enter directly into the biodigester. Every time the biodigester is fed with the manure and water slurry, digested slurry will flow out in the reverse end. The ideal dry matter content of the slurry input is 4 %. This is accomplished by diluting about 1 part of fresh excreta with 5 parts of water. This dilution will yield the highest gas production efficiency and make the slurry flow easily through the biodigester (Bui Xuan An, 1999).

Picture 1. A plastic tube biodigester in An Son. 

Carbon is the only nutrient that really disappear in the biodigester if it is working well, the other nutrients, N and P will remain in the same amounts though in higher concentrations than in the slurry flowing in, since the manure has been digested. According to Bui Xuan An (1995), 1 kg manure generally produce 75 L biogas in farm condition. The carbon concentration can then be calculated to be about 0.57 g carbon per 1 L. Another rule of thumb is that the fresh weight of manure equals to the weight of the feed intake, in this case 4320 kg per year, which means that 324 000 L biogas will be produced during one year which in turn contains 184 kg of carbon per year, assuming the temperature is 27^oC, and the pressure is 1 atmosphere. The composition of the biogas is generally 60% methane and 40% carbon dioxide.

The amounts of N, P, C leaving this subsystem for the next is still 32 kg N and 23 kg P but only 438 kg C because of the biogas output.

Table 4. Balance for the biodigester subsystem.
 

	Manure (kg)	Biogas (L)	N (kg per year)	P (kg per year)	C (kg per year)
Input	4,320		32	20	622
Biogas output		324,000			184
Effluent output			32	20	438

 

The crop subsystem

The last subsystems is the crop and soil. The main crop in An Son is fruit trees, such as Durian (Durio zibethinus), Jackfruit (Artocarpus integrifolia), and Mangosteen (Garcinia mangostana), with 328 hectare, followed by Jasmine (17 hectare), Sugarcane (16 hectare), vegetables (12 hectare) and Paddy rice (7 hectare). The composition of the crop subsystem, and the assumed harvest yields and the N, P content in the harvest is shown in Table 5. The total area will be 0.5 hectares, which is the mean area per agricultural household in An Son Village. The crops are common in the village but the cropping systems are usually much more elaborate than this. Almost all farmers also have a home garden and many farmers grow only fruit trees. The reason for choosing the crops shown in Table 5 is that they are quite common in the area and that there were nutrient and harvest data available. 

*Analyses made by Hedlund, 1997
**Analyses made by Cantho University
*** Coale et al 1993

Picture 3. A Pak-choi field in An Son.

Discussion

It has been shown experimentally that the pig subsytem with 6 fattening pigs is well suited for the biodigester (with a 6 m³ biodigester) subsystem regarding the efficiency of the biogas production (Bui Xuan An et al., 1995). However some C and all the N and P will leave the system and end up in next subsystem. As calculated from table 5 the output of N and P with the harvest will be 76 kg N and 5 kg P (figure 1). This mean that the output from the biodigester will cover the phosphorus output by the crops with 400 % and the nitrogen output will be covered by 48 % (figure 1). This in turns means that the P in the slurry from the biodigester could meet the crop output from a larger area than presented here. In this system, the P will be accumulated in the soil or leached to the groundwater or surrounding ditches. The N output with the crop, however, is larger than the input from the biodigested slurry. It means that N will be depleted from the soil, or the plants will suffer from N deficiency, unless N is added as urea or another chemical fertilizer. The C is not a plant nutrient and all the C added to the soil will either be accumulated or respired by the microorganisms.

Figure 1. Flow of nitrogen (N) and phosphorus (P), during one year, on a farm with three integrated sub-systems; pigs, biodigester and crops.

The effect of accumulation of nutrients in the soil will depend on the soil characteristics. The soils in An Son village are potentially Acid Sulfate Soils. The ground water depth is shallow (about 20 cm) even during the dry season. The whole village is situated at between 0.5 to 1.5 m above sea and the village is protected by a dike system. The soil is rich in organic carbon (5-7 %) and rather acid (pH 4-4.5). This means that the rooting depth is shallow and the soil volume for nutrient extraction is small. Acid soils are usually P-fixing and an excessive addition of P would probably not do any harm in the short term. However since the rooting depth and the groundwater depth is so shallow there is a small soil volume that can accumulate the P, so there might be a long term risk for leaching of P. 

Organic C is usually seen as a good soil amendment. Especially on the acid soil it may help to reduce the acidity by binding the plant toxic aluminium. It will also increase the nutrient binding capacity of the soil. It is especially important to increase the nutrient binding capacity in a soil with such shallow rooting depth and high ground water table.

In addition to nutrients the biodigested slurry also add water to the soil. In An Son the crops are usually irrigated during the dry season and the addition of slurry could reduce the need for irrigation. From measurements made 1997 in average 300 L of biodigested slurry were produced everyday. It corresponds to only 22 mm of irrigation per year, which hardly contribute much to the irrigation. On the other hand it decreases the risk that the nutrients in the slurry are transported directly to the groundwater, below the rooting depth.

Conclusion

In the outlined theoretical system the pig subsystem and the biodigester subsystem is well matched. However the biodigester subsystem produce an excess of P in relation to plant uptake which subsequently may lead to leaching and pollution of the groundwater or the nearby river. This size of biodigester would match better to a larger area of the crops stipulated or to the same area with a crop that are more P requiring.

The N/P balance in the slurry suggests that any calculation on the slurry amendment to a crop/soil system should be based on the P content rather than the N content since the latter would produce a big surplus of P in the system.
 

References

NIAH. 1995. Composition and nutritive value of animal feed in Vietnam. Agricultural publishing house. Hanoi. pp 106-139.

Bui Xuan An and Preston T R., 1995. Low-cost polyethylene tube biodigesters on small-scale farms in Vietnam. Electronic Proc. 2^nd Intl. Conference on increasing Animal Production with Local Resources, Zhanjiang, China, p. 11.

Bui Xuan An and Preston T R 1999 Gas production from pig manure fed at different loading rates to polyethylene tubular biodigesters. Livestock Research for Rural Development (11) 1: http://www.cipav.org.co/lrrd/lrrd111/1