Productive use of livestock wastes; a manual for the use of biodigester effluent and ponds for duckweed production

Lylian Rodriguez and T R Preston

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

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Introduction

The products of the biodigester

Using the effluent from the biodigester

Moving the effluent from the biodigester

Using a pump to manage the effluent

The duckweed ponds

Inoculating the pond with duckweed

Harvesting duckweed

Duckweed as a feed for chickens

Duckweed as a feed for "local" pigs

Related information

Introduction

The two products from the anaerobic biodigestion of livestock wastes are:

• Biogas which is rich in methane (55-65%) and used mainly for cooking
• Effluent which is the residue coming out of the biodigester and which contains all the plant nutrients present in the original manure

This section of the manual deals with the utilization of the effluent as a fertilizer for crop plants.  Since the effluent is voluminous (about 98% water) it is an advantage if it is used as close as possible to the site of production.  For this purpose it is necessary to select crop plants which have a rapid growth rate (and therefore high capacity to extract nutrients from the medium in which they are growing) and good nutritive value.

For this purpose it has been found that duckweed (Lemnaceaea) is the most appropriate because  it:

• Has a rapid rate of growth (doubles its biomass in 24 hours)
• Is palatable and has high digestibility for monogastric animals (dry matter digestibility over 65% in pigs according to Rodriguez and Preston 1996a)
• Its protein content is almost doubled (from 20-22% to 35-40%) when grown in nutrient-rich water (Leng et al 1995; Rodriguez and Preston 1996b; Nguyen Duc Anh et al 1997)

The products of the biodigester

These are:

• Biogas
• The effluent

The biogas flows by tube from the biodigester to the reservoir situated as close as possible to where it will be  used, usually near the kitchen.

The effluent is produced daily in accordance with the schedule of charging the biodigester.  The volume that comes out is equal to the volume that goes in. The residence time (time taken on average for the "digesta" to pass from the entrance to the exit) will vary usually within the range of 10 to 40 days depending on the quantity of manure and water put into the biodigester. The greater the input volume the shorter the residence time. It is desirable that the residence time is at least 20 days so as to secure inactivation of pathogenic organisms and parasites.

There should be a pit to receive the effluent large enough to hold at least the output of two days. Normally it is not necessary to line the pit as the floor and walls soon become impervious. If the topography permits a pipe should be laid to take the effluent from the receiving pit to the duckweed ponds.

Using the effluent from the biodigester

When manure and water enter the biodigester a similar volume of effluent is forced out of the exit pipe.

It is usually adequate to have an unlined pit as very quickly this becomes impervious to filtration.

A pipe from this pit then connects directly to ponds used to cultivate duckweed.

When duckweed is fertilized with biodigester effluent its crude protein content can be between 35 and 40% in the dry matter, making it a valuable supplement for pigs and poultry.

In order to maintain a nitrogen content in the pond water of about 20mg/litre, the volumes of effluent to be added can be calculated from the table below:

• At the beginning when the pond is prepared and filled with water the first time
• Every day (to compensate for the nitrogen removed in the duckweed assuming a daily harvest of 100 g/m² pond surface/day)

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.

Moving the effluent from the biodigester

The simplest way of moving the effluent is with buckets. 

If the topography permits a more convenient method is to lay a pipe (5 cm id is enough) connecting the effluent pit with the duckweed pond and in turn to have each duckweed pond connected in series to the next one.

Using a pump to manage the effluent

In this example the duckweed ponds surrounding the biodigester are at a higher elevation than the outlet pit.

A 1/4 HP electric pump has the inlet pipe connected directly to the effluent pit and the outlet at the highest point of the slope. In a matter of minutes the effluent is pumped into the duckweed ponds

The duckweed ponds

If water is not a limiting resource the most appropriate way of using the effluent from the biodigester is for the cultivation of duckweed (Lemnaceae).

Where there is a high clay content in the soil the floor and wall of the pond soon become impervious to filtration of water.  But in sandy soil it is necessary to line the ponds with a mixture of soil and cement. 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. 

Smaller mixes of 30 kg soil, 2.5 kg cement and 1.5 kg water are prepared and a thin layer of the mixture is applied to the floor of the ponds and to the walls.

After two days the ponds can be filled with water and seeded with duckweed

Inoculating the pond with duckweed

The duckweed pond is connected by a pipe with the exit of the biodigester.

The inoculum of duckweed is prepared and distributed on the pond surface at the rate of 400 g/m².

Harvesting duckweed

Each pond is harvested daily. It is a simple operation requiring a bamboo pole slightly shorter than the width of the pond and a plastic basket.

Beginning at the mid-point of the pond the duckweed is pushed steadily to the narrow end of the pond and then scooped out of the water with the basket. It is left to drain for few minutes before being weighed and taken to the animals.

These ponds are producing about 100 g fresh duckweed/m²/day which is equivalent to about 6 tonnes protein/ha/year.

Duckweed as a feed for chickens

Duckweed has a balance of essential amino acids slightly superior to soya bean meal (Rusoff et al 1980).

Rice bran and cassava root meal are dry, powdery materials. Duckweed by contrast is very wet (94-96% moisture..!!). Mixing fresh duckweed with either rice bran or cassava root meal, or with a combination of the two, produces a feed with a crumbly texture that is more readily accepted by chickens than any one of the ingredients given separately.

Proposed combinations (all on fresh basis)  that will give at least 10% protein in dry matter (suitable for growing and laying chickens) are:

• one part rice bran; one part duckweed
• four parts duckweed: one part cassava root meal
• two parts duckweed: one part cassava root meal: one part rice bran

Duckweed as a feed for "local" pigs

The same principles apply as for chickens and the same mixtures of duckweed with cassava root meal and rice bran can be used.

	Photo 41  . Rice bran	Photo  42. Fresh duckweed
	Photo 43. Mixing fresh duckweed with rice bran	Photo 44. Feeding the mixture to a pregnant Mong Cai sow
	Photo 45.  Fresh duckweed as a source of protein for Mong cai sows

Duckweed also an alternative for landless farmers

The pond has an area of 16 m² and the average daily yield over 3 months of observations was 91 g/m² (Figure 1), equivalent to 1.5 kg fresh duckweed daily. This is sufficient to provide the supplementary protein for 15 chickens. 

Photos  by Lylian Rodriguez

Related information

Becerra M, Ogle B and Preston T R 1995 Effect of replacing whole boiled soybeans with Duckweed (Lemna sp) in the diets of growing ducks. Livestock Research for Rural Development (7) 3:49-55

Bui Xuan Men, Ogle B and Preston T R 1996 Duckweed (Lemna spp) as replacement for roasted soya beans in diets of broken rice for fattening ducks on a small scale farm in the Mekong delta. Livestock Research for Rural Development (8) 3:14-19

Bui Xuan Men, Ogle B and Preston T R 1996 Use of restricted broken rice in duckweed based diets for fattening Common and Muscovy ducks. Livestock Research for Rural Development (8) 3:20-25

Bui Hong Van, Le Thi Men, Vo Van Son and Preston T R 1997 Duckweed (Lemna spp) as protein supplement in an ensiled cassava root diet for fattening pigs. Livestock Research for Rural Development (9)1: http://www.cipav.org.co/lrrd/lrrd9/1/lemen912

Du Thanh Hang, Nguyen Van Lai, Rodríguez Lylian and Ly J 1997 Nitrogen digestion and metabolism in Mong Cai pigs fed sugar cane juice and different foliages as sources of protein. Livestock Research for Rural Development (9) 2:45-49

Du Thanh Hang 1998 Ensiled cassava leaves and duckweed as protein sources for fattening pigs on farms in Central Vietnam. Livestock Research for Rural Development (10) 3: http://cipav.org.co/lrrd/lrrd10/3/hang2

Haustein A T, Gilman R H, Skillicorn P W, Vergara V, Guevara V and Gastanaduy A 1990 Duckweed. A useful strategy for feeding chickens: performance of layers fed with sewage-grown Lemnacea species. Poultry Science. (69):1835-1844

Le Ha Chau 1998 Biodigester effluent versus manure, from pigs or cattle, as fertilizer for duckweed (Lemna spp.). Livestock Research for Rural Development (10) 3: http://cipav.org.co/lrrd/lrrd10/3/chau2

Le Thi Men, Bui Hong Van, Mai Thi Chinh and Preston T R 1997 Effect of dietary protein level and duckweed (Lemna spp) on reproductive performance of pigs fed a diet of ensiled cassava root or cassava root meal. Livestock Research for Rural Development (9) 1: http://www.cipav.org.co/lrrd/lrrd9/1/lemen911

Leng R A, Stamboli J H and Bell R 1995 Duckweed - A potential high protein feed resource for domestic animals and fish. Livestock Research for Rural Development (7) 1: http://www.cipav.org.co/lrrd/lrrd7/1

Men Bui Xuan, Ogle B and Preston T R 1995 Use of duckweed (lemna spp) as replacement for soya bean meal in a basal diet of broken rice for fattening ducks. Livestock Research for Rural Development (7) 3: http://www.cipav.org.co/lrrd/lrrd7/3

Nguyen Van Lai and Preston T R 1997 Parameters of digestion and N metabolism in indigenous (Mong Cai) and exotic (Large White) piglets fed sugar cane juice or ensiled cassava roots supplemented with either duckweed (Lemna minor) or ensiled cassava leaves. Livestock Research for Rural Development (9) 5: http://www.cipav.org.co/lrrd/lrrd9/5/lai951

Nguyen Duc Anh and Preston T R 1997 Evaluation of protein quality in duckweed (Lemna spp.) using a duckling growth assay. Livestock Research for Rural Development (9) 2:http://www.cipav.org.co/lrrd/lrrd9/2/anh92

Nguyen Duc Anh, Huu Tao and Preston T R 1997 Effect of management practices and fertilization with biodigester effluent on biomass yield and composition of duckweed. Livestock Research for Rural Development (9) 1:http://www.cipav.org.co/lrrd/lrrd9/1/anh91

Nguyen Duc Anh and Preston T R 1998 Effect of exchange rates of the media with biodigester effluent on biomass yield and composition of duckweed. Livestock Research for Rural Development (10) 1:http://www.cipav.org.co/lrrd/lrrd10/1/anh101

Nguyen Van Lai 1998 On-farm comparison of Mong Cai and Large White pigs fed ensiled cassava root, rice bran and duckweed. Livestock Research for Rural Development (10) 3: http://www.cipav.org.co/lrrd/lrrd10/3/lai103

Nguyen Van Lai and Rodríguez Lylian 1998 Digestion and N metabolism in Mong Cai and Large White pigs having free access to sugar cane juice or ensiled cassava root supplemented with duckweed or ensiled cassava leaves. Livestock Research for Rural Development (10) 2:http://www.cipav.org.co/lrrd/lrrd10/2/lai1021

PRISM 1997 Economic feasibility of duckweed-based fish production: a few case studies. Duckweed Research Project. Report 1097, June 1997

Rodríguez Lylian and Preston T R 1997 Local feed resources and indigenous breeds: fundamental issues in integrated farming systems; Livestock Research for Rural Development (9) 2:http://www.cipav.org.co/lrrd/lrrd9/2/lylian92

Rodríguez Lylian and Preston T R 1996 Use of effluent from low cost plastic biodigesters as fertilizer for duck weed ponds. Livestock Research for Rural Development (8) 2: http://www.cipav.org.co/lrrd/lrrd8/2/lyli821

Rodríguez Lylian and Preston T R 1996 Comparative parameters of digestion and N metabolism in Mong Cai and Mong Cai Large White cross piglets having free access to sugar cane juice and duck weed. Livestock Research for Rural Development. (8) 5: http://www.cipav.org.co/lrrd/lrrd8/5

Rusoff L L, Blakeney W W, Culley D D 1980 Duckweeds (Lemnaceae Family): A potential source of protein and amino acids. Journal of Agriculture and Food Chemistry. (28) :848-850

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