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Recent developments in the recycling of livestock excreta; an essential feature of sustainable farming systems in the tropics

Thomas R Preston and Lylian Rodriguez

University of Tropical Agriculture Foundation
Finca Ecologica, University of Agriculture and Forestry, 
Thu Duc, Ho Chi Minh City, Vietnam
trpreston@email.com
and  lylianr@email.com


Abstract

Loss of soil fertility and species biodiversity, and environmental pollution, are negative consequences of modern agricultural practices in the industrialized countries where, on most farms,  intensive systems of production have resulted in the physical separation of livestock and crop production. One outcome of this policy is that the disposal of manure from livestock units concentrated in a small area, often with no land attachment, has become a major problem. 

In SE Asia, the integration of crops and livestock, and the use of manure as fertilizer, are traditional practices and are the basis of the farming systems, especially at small-holder level.  This manual describes ways in which these systems can be made more efficient, more productive and more environmentally friendly, by applying simple, low-cost technologies for recycling the manure through biodigesters, duckweed ponds and earthworms.
 

Introduction

The deterioration of soil fertility through loss of nutrients and organic matter, erosion and salinity, and the pollution of the environment - of air, soil and water - are negative consequences of modern agricultural practices in the industrialised countries where the divorce of livestock and crop production has become the norm (Haan et al 1997). Traditionally, livestock were a balanced - indeed an essential - component of farming systems but the development of the chemical industry in the 19th century, and the major impetus it received from the discovery of oil, created opportunities for the low-cost supply of plant nutrients in inorganic form and led to the rapid displacement of organic manures derived from livestock excreta.

The increasing global awareness that resources are finite, and that the livelihood of future generations depends upon the maintenance of renewable natural resources, is now a major stimulus for initiatives that will lead to the more efficient use of these same resources. In industry, recycling of processed goods at the end of their useful life is now seen as a means of lowering costs of production and reducing the pollution caused by accumulation of these materials in the environment.

This paper describes recent developments in the same recycling strategy as it can be applied to livestock.

Livestock excreta as livestock feed

The accumulation of livestock excreta, chiefly from poultry kept in deep litter, was seen as an opportunity to recycle this material as a feed for ruminant livestock (Muller 1980), and there were even attempts to do likewise with the excreta from intensively-fed pigs (Buitrago J, personal communication) and cattle (Anthony 1971). However, in all these cases the nutritional value of the manure was mainly a reflection of the spillage of feed which is almost inevitable when intensive systems of self-feeding are practiced. The high risk of disease from recycling wastes through livestock, highlighted by the recent outbreaks of BSE (Taylor 1997), is now seen as a major deterrent to these practices. They will therefore not be considered further in this document.


Potential benefits from the recycling of livestock excreta

These can be summarised as follows:

The relative contribution that livestock excreta can make to the achievement of the above goals is a function of:

Several of these features are determined by economic issues, such as the species and genotype of the livestock and the feeding and management systems that are applied. Most of them are inter-dependent (eg: the most appropriate way of processing the excreta depends on its physical characteristics, which in turn is determined by the livestock species, and by the systems of housing).  However, decisions in all of these areas are increasingly seen to be affected by the role of livestock within the immediate ecosystem. In other words, the advantages that may accrue from an appropriate system of recycling of the excreta become a stimulus for decision making in both the selection of the livestock species, and of the genotype within the species, and of the system of feeding and management.  It follows that research on methods of recycling livestock excreta, and the development of appropriate technologies for this purpose, should be given high priority as these eventually become important determinants of the farming systems that are selected in a given ecosystem.

Methods of processing (recycling) of livestock excreta

In order of priority these are considered to be:

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Figure 1: The ecological farming system

 

Irrespective of the processing method that is applied the concept of recycling implies an ecological and holistic approach to the use of natural resources in which livestock play an intrinsic role (Figure 1). It may not always be possible, or convenient, to incorporate all the sub-systems in the overall process.  The biodigester sub-system, and the pond, are not essential for ensuring that plant nutrients are not lost as the essential cycle is between the crop and the livestock.  Nevertheless, biodigesters and ponds are components in the system which:

This manual describes a holistic approach to recycling of livestock excreta based on experiences gained in an ecological farm in the South East of Vietnam http://www.hcm.fpt.vn/inet/~ecofarm. The aim is to show the way to optimise the process, selecting the pathways that are most suitable for the different kinds of excreta likely to be produced in a small-scale family farm (Figure 2). 

Figure 2: Optimum pathways for recycling of excreta in a small-scale farm

 

The biodigester sub-system

Types of biodigester

The biodigester is a closed medium in which livestock and human excreta (and / or other organic wastes) are fermented anaerobically giving rise to a gas (biogas) and a residue (the effluent).  Biogas is a mixture of methane and carbon dioxide the volume ratio of which is about 65:35 (substrates rich in lipids produce biogas with higher proportions of methane).  The effluent contains all the original mineral elements that give rise to nutrients needed for plant growth,  but the reducing anaerobic medium improves the efficiency with which they can be assimilated (eg: the conversion of part of the organic nitrogenous compounds into ionic ammonia).

There are three main types of biodigester which originated respectively in India (floating canopy), in China (the fixed dome -  dual compartment system) and in Taiwan (the plastic red mud tubular plug flow model).  The Taiwan model was modified by Preston and coworkers (Botero and Preston 1986; Bui Xuan An et al 1998) into a simpler form using tubular polyethylene film,  standard PVC sanitary fittings and discarded motor cycle inner tube.  The Indian and Chinese models are undoubtedly the most robust and the most efficient.  They are also the most expensive in terms of materials, the need for skilled artisans and time taken for construction.  These models are described in detail in the book of Marchaim (1992).  The materials required for the tubular plastic biodigester are low cost (from USD10.00 to USD50.00 for small scale farm family units), the required materials can usually be found in the markets of  major cities throughout the developing world and they can be assembled and installed with unskilled labour.  This manual is concerned only with the low-cost tubular polyethylene model in view of its cost-effectiveness as demonstrated by high rates of adoption by farmers without the need for subsidies (Bui Xuan An et al 1998; Lauridsen 1998). The companion article illustrates in detail the nature of the materials and how these are assembled, and the biodigester installed, on the farm.  


Factors influencing the cost and performance of plastic biodigesters

Materials
The plastic tube

Two types of plastic have been used for the tubular biodigester: poly-vinyl-chloride (PVC) and polyethylene.  PVC film is manufactured by a rolling process which results in sheets that must be welded with heat into a tube.  By contrast, polyethylene is produced by blowing air vertically (forming a large bubble) through the molten plastic encased in a mold the diameter of which fixes the diameter of the tube. The thickness of the plastic film is controlled by the rate at which the air is introduced into the process.

Making a tube by welding together PVC sheets is a skilled operation and not always successful.  The facilities for the welding are rarely found other than in major cities.  By contrast, tubular polyethylene needs no further processing and is ready to us be used immediately it leaves the factory.  The raw material is also cheaper than PVC.

Tubular polyethylene is therefore the material of choice.  The cost is usually in the range of USD1.00 to USD1.50 / kg, depending on whether it is purchased direct from the factory or from re-sellers.  A tube of 80cm diameter and thickness of film of 200 microns (the most common dimensions) weighs 500 g per metre of length. For a biodigester that is 6 m long the need is 15 m of tube (two tubes are used one inside the other) and 75 cm is required at each end for fixing the tube to the inlet and outlet pipes. This will weigh 7.5 kg and therefore cost between 7.00 and 11.00 USD.

The inlet and outlet pipes

Alternatives types of pipe are made from "fired" clay (ceramic), concrete and PVC.  The relative advantages of each are indicated in Table 1.

Table 1: Relative advantages of different types of inlet and outlet pipes
Material Preference Cost Weight Ease of use
Ceramic 1 Low Medium High
Concrete 2 Medium High Low
PVC 3 High Low High
Connecting the tubular film with the inlet and outlet pipes

The tubular polyethylene is attached to the inlet and outlet pipes using rubber strips (5 cm diameter) made from rejected inner tubes from the wheels of vehicles. The relative advantages of the different sources are indicated in Table 2.

Table 2: Relative advantages of different sources of rubber bands to secure the tubular polyethylene to the inlet and outlet pipes

Preference

Source

Strength

Ease of use

1

Motor cycle

Good

High

2

Bicycle

Medium

High

3

Motor car

High

Medium

4

Truck

Very high

Low

The gas line

There are only two choices: rigid PVC tube (id: 13mm) or flexible garden hose. The latter is cheaper but the former is preferred as there will be fewer maintenance problems, especially blockages in the pipe.

The reservoir

The gas pressure in tubular plastic digestive is low: between 1 and 1.5 cm water column. If the cooking stove is situated more than 10 m away from the biodigester, the flow rate of the gas will be too low for efficient burning. The solution is to situate a reservoir as close as possible to the plastic biodigester.

The reservoir is made from the same tubular polyethylene as the biodigester and usually it is 2-3m  long. It is closed at one end and the other is connected to the gas pipeline. A string is placed around the middle of the reservoir and can be tightened manually whenever a higher gas pressure is required.

Management factors that govern gas production
Size of biodigester

The amount of gas produced daily is a function of the:

C total volume of liquid within the biodigester
C
amount of fermentable substrate (volatile solids) introduced daily.

Thus the gas produced (m;/day) = k1*L + k2*VS

where AL@ is liquid volume in m; and VS the amount (kg) of volatile solids; k1 and k2 are constants.

In practice, the factors that determine biogas yield and efficiency of conversion of volatile solids into biogas are the:

C liquid volume (which increases gas production)
C
daily input of volatile solids (which increases gas production)
C
residence (or hydraulic retention) time (longer time; higher efficiency)
C
concentration of volatile solids in the input suspension (increases the rate of production)

The ratio of volatile solids to total solids (VS/TS) is determined by the species of livestock and within the species by the feed (feeds of high digestibility produce excreta with a higher VS/TS. Within animal species, those with simple digestive systems (monogastric animals with poorly developed caecum and large intestine) will produce excreta of higher content of volatile solids.

The loading rate

The concentration of total solids in the input suspension can be varied within the range of 20 to 100 g/litre. For a fixed volume of input suspension, the gas production is a direct function of the concentration of total solids (Figure 3). In practice it is recommended to limit the total solids concentration to the range of 20 to 30 g/litre. Higher concentrations can lead to scum formation in the biodigester which can impede the flow of fermentable material and also reduces the surface area of particles accessible to fermenting bacteria.

 

Residence time

Residence time (or hydraulic retention time) is the average time it takes for the substrate to move from the input to the output pipes (ie: the time the substrate remains in the biodigester). It is determined by the liquid volume of the biodigester and the daily rate of input of the substrate. Residence time has little effect on daily gas production but a longer time increases the efficiency of conversion of volatile solids to biogas (Figure 4). The lower the concentration of volatile solids in the total solids (eg: the case of manure from buffaloes fed rice straw) the longer should be the residence time (20 to 40 days). For higher ratios of volatile solids to total solids (excreta from chickens and pigs fed concentrate diets with low fibre content) the residence time can be reduced to 10-15 days. The shorter the residence time the smaller can be the biodigester in relation to the daily entry rate of total solids.

Liquid volume

The required size of a biodigester (total liquid volume) is determined by the total amount of volatile solids available (the input) and the residence time. Indicative volumes are given in Table 3, according to residence time and total volatile solids available (depends on type of feed and the total liveweight of the animals and people providing the excreta).

The total volume of a tubular biodigester is a function of it's diameter (D) and length (L).

Volume = [D/2]5 * π *L

Thus a biodigester that is 6m long and 80cm diameter will have a total volume of:

0.4*0.4* π*6 = 3m;.

However, the liquid volume is less than the total volume because space must be left beneath the gas outlet for the gas to accumulate. The ratio of liquid to total volume depends on the configuration of the biodigester (Figure 5). If the floor of the trench is completely level then the ratio of liquid volume to total volume will depend on the level of liquid in the biodigester. In practice, when the floor of the biodigester is level, the liquid volume is likely to be from 70 to 80% of the total volume. A higher level of liquid carries the risk of blocking the gas outlet.

Level or inclined biodigesters

By having the floor of the trench sloping from input to output it is possible to concentrate the gas space in the region where the gas outlet is placed. This will make it possible to have a greater liquid capacity closer to 90% of total volume.

Where the required liquid volume is in the range of 2-4 cubic metres then it is convenient to use tubular polyethylene of 80 cm diameter with the length in the range of 6-10m. For larger biodigesters with required liquid volume in the range of 12 to 24 cubic metres then it is better to use tubular polyethylene of 1.25 m diameter, which for any given length will increase by a factor of 2.4 the volume compared with a diameter of 80cm. For a liquid volume greater than 24 cubic metres, the tubular polyethylene should be 2.5m diameter and have a length of between 20 and 30 m giving a range of liquid volumes of 70 to 110 cubic metres.

Animal species

In addition to the effects of the concentration of volatile solids in the total solids in the excreta, there are other physical factors which influence the suitability of excreta from some species compared with others. Thus the Apelleted@ consistency of faeces from sheep, goats and rabbits makes this material unsuitable as substrate in plastic biodigesters.. The Apellets@ float on the surface of the liquid and rapidly cause the formation of scum on the surface. Excreta from these species is best processed through earthworms (see later section).

The most accurate way to estimate the required size of biodigester (liquid volume), and to predict the estimated gas production, is according to the total weight of animal / human population that will contribute excreta to the biodigester (Table 3). It is assumed. that:

  • the feed intakes are: 30, 50 and 100 g dry matter/kg liveweight for cattle, pigs (& people) and chickens, respectively

  • that 1 kg of excreta dry matter produces 250 litres of biogas

  • that the hydraulic retention times in the biodigester are: 40 days for excreta from cattle and 20 days for pigs, people and chickens.

Table 3 : Estimates of required liquid volume and expected daily biogas production from excreta from different species of livestock according to category of feed and total liveweight of the animal / human population contributing to the biodigester
   

cattle, buffaloes

pigs, people

chickens

 

 

straw

grass, forages

concentrates

mixed

concentrates

concentrates

Live Wt, kg

indigDM,g/kg

450

350

220

300

170

170

100

liquid vol,m³

1.80

1.40

0.88

1.20

0.57

1.13

100

biogas,m³/d

0.338

0.263

0.165

0.225

0.213

0.425

200

liquid vol,m³

3.60

2.80

1.76

2.40

1.13

2.27

200

biogas,m³/d

0.675

0.525

0.33

0.45

0.425

0.85

300

liquid vol,m³

5.4

4.2

2.64

3.6

1.7

3.4

300

biogas,m³/d

1.013

0.788

0.495

0.675

0.638

1.275

400

liquid vol,m³

7.20

5.60

3.52

4.80

2.27

4.53

400

biogas,m³/d

1.35

1.05

0.66

0.9

0.85

1.7

500

liquid vol,m³

9.00

7.00

4.40

6.00

2.83

5.67

500

biogas,m³/d

1.688

1.313

0.825

1.125

1.063

2.125


Commonly encountered problems
Life of tubular polyethylene biodigesters

If the biodigester is well protected against animals and against sunlight it can continue to function effectively for up to three years. It should then be replaced. Replacement involves only the renewal of the tubular polyethylene. The input and output pipes and the gas output line can all be used again. Sunlight was thought to be a major constraint to the life of the polyethylene. In practice, it has been found that this is not the case. Provided there is moisture on one side of the plastic film the sunlight appears to have little effect. In the absence of moisture the life is shortened considerably. Thus the outer layer of the polyethylene will deteriorate before the inner one.

It is wise to protect the biodigester with a fence that will keep out all farm livestock including chickens. Although there are isolated reports of rats making a hole in the plastic these have been isolated cases and this does not appear to be a major problem.

Accumulation of sludge and scum

Sludge is the accumulation of soil and other mineral matter on the floor of the biodigester. The scum is the accumulation of unfermented organic matter on the surface of the liquid. The latter can be dispersed by walking carefully on top of the biodigester. In the case of the sludge, the best solution is prevention by inserting a sand trap in the inlet sector (Figure 5). Once inside the biodigester, the sludge cannot be removed except by completely renovating the system.

In practice it is the accumulation of scum and sludge which is the major determinant of the effective life of the biodigester. The problem is exacerbated when high loading rates are used and when the input material has a high fibre content (e.g. contains straw residues from the feed or litter) .As stated previously, it is recommended that a period of 2 - 3 years be considered as the working life of tubular polyethylene biodigesters. It is normal practice to expect to replace the plastic and remove the accumulated sludge and scum at this time.

Water in the pipeline

Biogas is fully saturated with water vapour.  In the night-time, and on cooler days, some of this moisture will condense and will accumulate in the lowest points of the gas line.  If there is a sudden drop in the volume of gas reaching the reservoir, water in the pipes may be the problem (this assumes that there are no leakages in the gas line itself).  The solution is to bore a small hole in the lowest points of the gas line, let the water drain out and then seal the hole with plastic tape or a strip of rubber inner-tube.


Excreta as source of nutrients for water plants, terrestrial crops and earthworms

Livestock excreta can be used directly as a source of nutrients for production of earthworms, aquatic and terrestrial plants and indirectly through the medium of algae for fish production. The excreta can also be mixed with other vegetative residues to make compost prior to its addition to soil or water as fertilizer. In all cases, with the exception of earthworms, performance (in yield and / or quality of the end-product)  is improved when the excreta is passed first through a biodigester (Le Ha Chau 1998a,b; Hong Samnang 1996, unpublished data). 

Water plants

A wide range of plant species grow luxuriously in water fertilized with livestock excreta, the most important ones being duckweed (Lemnacacea), water hyacinth (Eichhornia crassipes) and  water spinach (Ipomoea acuatica). This manual is concerned only with the cultivation of duckweed in view of it's high nutritive value for livestock, high biomass yield, ease of harvesting and capacity to increase in protein content in response to nitrogen fertilization (Leng 1999; Rodriguez and Preston 1996a,b). The other water plants mentioned are efficient in removing nutrients from an aquatic medium and have high growth rates (Chara et al 1999), but their nutritive value is relatively low (with the exception of Ipomoea acuatica)  and, in the case of water hyacinth, harvesting can be a problem. It appears there have been no attempts to cultivate water spinach, in the same way as has been done with duckweed.

Cultivating duckweed

Duckweed will grow in nutrient-rich water in a wide range of locations ranging from natural earth-lined ponds to plastic containers or ponds lined with polyethylene film (Haustein et al 1992; Rodriguez and Preston 1996a; Nguyen Duc Anh et al 1997).  Assuming that the aim is to cultivate the duckweed as part of an integrated system based around the biodigester, then the following factors need to be considered,

Construction of the ponds

These should have ideally an area of about 20-30 m².  Larger ponds can be used but should be separated into sections with floating bamboo poles as large surface areas are susceptible to the effect of the wind "blowing" the duckweed to the sides. The ponds should be 40cm deep. If the soil at this level has a high clay content, it may be sufficient to compact the base of the pond and to line the sides with clay-rich sub-soil.  In general, however, it is quicker and more reliable to use a mixture of soil and cement (10 parts soil to one part cement) to line the floor and the sides. For a pond 40cm deep and with an area of 20 m², the required overall quantities are 25 kg of cement and 300 kg of soil.  The way to line the ponds is shown in detail on the accompanying CDROM.

Nutritive value of duckweed

What gives a special value to duckweed as feed for livestock is that:

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Figure 6: Relationship between root length and crude
protein content of duckweed (Source: Le Ha Chau 1998a)

 

Protein content is inversely and highly correlated with  the length of the roots.  It is very easy to measure root length and in this way the farmer can quickly estimate the probable protein content of the duckweed.  

Fertilizing the duckweed

Appropriate fertilizing of the duckweed pond is a fundamental feature of the management in order to produce biomass of high nutritive value. The best fertilizer is the effluent from the biodigester. The dry matter of the duckweed can have up to 5 percentage units more protein when effluent rather than manure is used as fertilizer (Le Ha Chau 1998a).  The aim should be to achieve a concentration of nitrogen in the pond water of between 20 and 30 mg/litre.  The volumes of effluent to be added can be calculated from the table below. There are two factors to consider: in adding the effluent:

The calculations are based on a pond of 20m² area and 20 cm depth of water. For ponds with different dimensions the data should be adjusted accordingly.

Table 4: Amounts of effluent (litres) to be added daily according to dry matter content and N concentration in the dry matter of the effluent

Pond area, 20 m²

 

 

 

 

Pond depth,  0.2m

 

 

 

 

 

Dry matter content of effluent (%)

N in effluent DM (%)

0.5

1

1.5

2

2.5

3

0.5

288

144

96

72

58

48

1

144

72

48

36

29

24

1.5

96

48

32

24

19

16

2

72

36

24

18

14

12

2.5

58

29

19

14

11

10

3

48

24

16

12

10

8

 

 

 

 

 

 

 

Amounts of effluent (litres) to be added at beginning according to dry matter content  and N concentration in the dry matter of the effluent

Pond area, 20 m² 

 

 

 

 

Pond depth,  0.2m

 

 

 

 

 

Dry matter content of effluent (%)

N in effluent DM (%)

0.5

1

1.5

2

2.5

3

0.5

3200

1600

1067

800

640

533

1

1600

800

533

400

320

267

1.5

1067

533

356

267

213

178

2

800

400

267

200

160

133

2.5

640

320

213

160

128

107

3

533

267

178

133

107

89

Letters in "bold" refer to the most common values of dry matter and nitrogen content of the effluent.

Problems and possible solutions

Problem: The yield declines and / or the protein content falls.
Explanation: it may be due to buildup over time of some elements present in the effluent which have a deleterious effect on growth of the duckweed. The problem is more pronounced when the ponds are lined with polyethylene film rather than soil / cement.
Solution:
Take out the contents of the pond including the duckweed and replace with fresh water, effluent and duckweed acquired from another farmer. A long term solution is to set up the ponds so that there is continuous flow of water (and effluent) through the system (Nguyen Duc Anh and Preston 1998).

Problem: The duckweed is pale green to yellow in colour and has long roots (over 2 cm)
Explanation: It is not obtaining sufficient nutrients from the medium
Solution:
Add more effluent. If it does not respond harvest all the duckweed and bring new seed from another farmer.

Problem: The water in the ponds overflows so the duckweed. is lost.
Explanation: Usually the result of very heavy rain falling in a short period.
Solution: Bring new duckweed seed and start again. Cover the pipes that connect the ponds with a plastic net (fine mesh) and if possible also put  net around the pond 10 cm high to avoid the overflow of the duckweed seed. Immediately the rain stops, harvest the duckweed that is left, redistribute it in the ponds and add more effluent. (In the rainy season the duckweed yield tends to decrease as there is a dilution of the nutrients and then the protein content decreases).

Problem: Yield declines, water becomes green.
Explanation: 
Another aquatic plants is competing with a negative effect on duckweed yield. Solution: Change the water and apply new seed.


Earth worms

Background

The role of earthworms in improving soil fertility has long been appreciated by farmers but until the last two decades there had been no major attempt to cultivate them as a component in an organic recycling system.  The potential value of the earthworm as a source of feed protein for poultry is often emphasized as justification for growing them, but their major role is in the recycling of animal excreta for the production of a high quality organic fertilizer in the form of the worm casts (humus). 

Earthworms will grow on the faeces from all species of livestock. However, as indicated earlier they have a comparative advantage over other forms of recycling when the faeces are from goats, sheep or rabbits.

Species of earthworm

There are many species of earthworms but the one invariably used is the "California Red Worm" (Eisenia foetida).  Its advantage is that it does not escape from captivity which is a major problem with other "wild" species.

Management

The following procedure has been developed in the Arizona farm (Pozo Verde) situated in the Cauca Valey, Colombia (Rodriguez 1997: Rodriguez et al 1995). Mounds of manure 25 cm high (370 kg of cow dung/m² approximately) are accumulated in beds the most convenient size of which is 1*5m. The manure is left to ferment aerobically for 30 days (using a stick to open holes to let in the air).  The beds are irrigated to prevent loss of moisture.

500 g of  earthworms  or 1 kg of worms plus humus after partial harvest (see later) are introduced for each 1m² of bed. This should be done on day 30.  Fresh manure is added as soon as it can be seen that the original manure has all been transformed into humus. Irrigation is applied as needed.

This process is continued for a further 60 days (in total 90 days) when the irrigation should be stopped and a mixture of water and manure put on the surface of the mounds. On the following day a layer of 10 cm can be harvested and it will contain 90% of the worms (partial harvest). The remainder of the bed is essentially humus with some worms.

Production coefficients

According to Beteta (1996),  the rate of transformation of cattle manure by earthworms is relatively uniform with 100 kg of manure yielding between 0.9 and 2.6 kg of earthworms and 49 to 57 kg of humus. The composition of earthworms and humus derived from cow manure on a farm in Colombia is shown in Table 5.  The concentration of plants nutrients is not high. The value of the humus lies in  its content of humic acids which give colloidal properties to the soil, improve the soil structure and form particles that encourage better development of plant root systems.

Table 5:. Composition of worms and humus (dry matter basis)

Worms

Humus

%

%

N

8.98

1.95

P

0.79

1.20

K

0.91

1.56

Ca

0.65

1.64

Mg

0.28

0.78

Source: Rodriguez and Salazar 1991

Researchers at the Goat and Rabbit Research Centre in North Vietnam (Nguyen Quang Suc et al 1999, unpublished data) have reported more precise data on growth and conversion rates of earthworms using manure from different animals species (Figures 7 and 8).  For each species the initial substrate was 50 kg fresh manure to which were added 0.5 kg of California Red worms.  Fresh manure was added every second day according to observed usage rates by the earthworms.  The results show that worm growth and conversion rates were best with manure from goats and rabbits.   

The fertilizer value of  both the fresh manure and the worm casts (humus) was estimated by measuring the relative growth rates of maize planted in bags containing the different substrates and allowed to grow for 25 days. Growth on the humus was twice that on the fresh manure. The ranking (best to worst) for both manure and humus was: goats, rabbits, buffaloes and cows, the same order as for the growth and conversion rates of the worms (Figure 9).

These results confirm the validity of the propose strategy to recycle goat and rabbit manure through earthworms and that from cattle and buffaloes (and pigs) through biodigesters.

Biodigester effluent and goat manure as fertilizer for cassava

In humid tropical ecosystems, the staple foods for human consumption are invariably rich in energy (eg: rice, maize, cassava, sweet potato) and the byproducts from the processing of these are all deficient in protein, either in quantity or in quality.  Thus protein is the nutrient that is most limiting in livestock diets in these regions.  In order to optimize the use of livestock excreta as fertilizer for terrestrial crops, the choice of plant species is therefore a decisive factor. The desired characteristics are:

Cassava (Manihot esculenta) has all the above characteristics with the additional advantage of being a multi-purpose crop that can be managed for production of protein (the leaves) or energy (the root).  

Goat manure as fertilizer

Recent work in the Ecological Farm of the University of Tropical Agriculture in South Vietnam (Rodriguez Lylian, unpublished data), based on earlier experiences in the Dominican Republic (Ffoulkes and Preston 1978), has demonstrated that cassava can be managed as a semi-perennial forage crop with high yields of protein-rich foliage. Regular application of goat manure (20 tonnes/ha following each harvest) has been the means of achieving the high foliage yields which appear to increase with successive harvests (Figure 10). Translated into protein these yields are equivalent to more than 4 tonnes/ha/year. 

  

Biodigester effluent

A relevant criticism of the use of biodigester effluent as fertilizer for terrestrial crops is its high degree of dilution and the labour cost of transport if the crop fields are some distance from the house plot where the biodigester is located. Raw or composted manure has comparative advantages in this scenario.  By contrast, the effluent -- but not the raw manure -- is easily transported by low-power (0.2-0.3 KW) centrifugal pumps traditionally used for pumping water.  Solar pumps which derive their power directly from a solar panel show high promise for this purpose, although presently their high costs precludes their use other than as demonstrations of future possibilities. 

As in the case of aquatic plants, and specifically duckweed, biodigester effluent appears to be superior to raw manure when the target plant is a terrestrial plant. Small scale farmers in Cambodia (Thanh Soeur, personal communication) were quick to identify the superior growth of vegetables fertilized with biodigester effluent compared with the raw manure.  This comparative advantage of the effluent has since been documented for cassava grown for forage (Le Ha Chau 1998b).  The effluent from biodigesters charged with either cattle or pig manure  supported higher yields of cassava foliage, which had a higher protein content, compared with the use of the raw manure (Figures 11a and 11b).

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Figure 11a: Biodigester effluent compared with raw manure to fertilize cassava grown for forage (means of fresh biomass yields for two harvests) (Source: Le Ha Chau 1998b)

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Figure 11b: Biodigester effluent compared with raw manure to fertilize cassava grown for forage (means of protein  in the foliage over two harvests) (Source: Le Ha Chau 1998b)


Conclusions

Much research remains to be done to document the comparative advantages of the respective pathways for recycling goat manure as crop fertilizer:  through  earthworms to produce humus or by direct application to the soil.  The former is most suited to opportunities that emphasize the advantages of a medium for plant growth that is essentially odourless and easy to transport, store and use. House plants and garden vegetables are obvious targets for the humus derived from the action of the earthworms.  By contrast, direct application of the manure to soil will probably be the more economical alternative when the end uses are as fertilizer for highly productive field crops such as cassava and fruit trees.

In the case of effluent versus raw manure, there is a need to characterize the properties of the effluent which give it an apparent superiority as a source of plant nutrients. From an environmental standpoint, prior anaerobic fermentation of manure in a biodigester ensures capture of the methane, part of which would be lost to the atmosphere when raw manure is applied direct to the soil as fertilizer. However, carbon dioxide emissions are likely to be similar for both processes.  From the point of view of human and animal health, destruction of parasites and reduction in the population of pathogenic micro-organisms in the passage of manure through a biodigester is a major advantage of the process.


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