Effect of management practices and fertilization with biodigester effluent on biomass yield and composition of duckweed
University of Agriculture and Forestry, Ho Chi Minh City,
The experiments were done during a four-month period, from May to August 1996 at the "Finca Ecologica", situated in the Experimental Farm of the University of Agriculture and Forestry, Ho Chi Minh City.
Duckweed (Lemna spp) was grown in plastic bins of 100 litre capacity with a water surface area of 0.60 m. Four experiments were done. In the first, effluent from two biodigesters charged with cattle manure was added to tap water at rates of 0.25, 0.5, 0.75, 1.0 and 1.25 litres/bin/day. In the second experiment, five initial concentrations of N were obtained by mixing the effluent with water at rates (% of effluent) of 9, 18, 27, 36 and 45%. Fresh effluent was then added daily at the rate of 0.5 litres/bin. In the third experiment, a comparison was made of different amounts of inoculum (fresh duckweed returned as seed after harvesting the biomass). The fourth experiment compared harvest frequencies (every day, second, third, fourth or fifth day).
When N in bin water was in the range of 10-30 mg N/litre, fresh biomass yield was of the order of 100 g/m2/day and contained 5-6% dry matter and 37-40% protein in dry matter. The protein content of the duckweed was negatively related with the root length of the duckweed. Equivalent yields of protein ranged from 6 to 10 tonnes/ha/yr. Optimum level of inoculum was 200-300 g/m2. The optimum rate of harvesting was at two-day intervals.
Key words: Biodigester, effluent, duckweed, inoculum, harvest, nitrogen.
Duckweed (Lemna spp) has received research attention because of its capacity to grow rapidly on nutrient-rich waste water and produce biomass rich in protein (Leng et al 1995). It has been used as a protein source in diets for ducks (Men et al 1996) and pigs (Dominguez et al 1996; Rodriguez and Preston 1996).
Duckweed species are small floating aquatic plants found worldwide. They are monocotyledons of the botanical family Lemnaceae. The most comprehensive work on developing a farming system with duckweed was done in Bangladesh (Skillicorn et al 1991). Protein content of the duckweed increased to 35-40% in dry matter when N in water increased from <5 to 15 mg/litre. Yields of duckweed dry matter were reported to be in the range of 10 to 30 tonnes/ha/yr (Leng et al 1995) indicating a potential protein yield of up to 10 tonnes/ha/yr.
In Vietnam, duckweed is grown commercially for sale and for feeding to ducks. In a survey in the village of Tang Nhon Phu, Thu Duc district, Ho Chi Minh City, samples of duckweed had a protein content ranging from 33 to 38% in dry matter growing on water enriched with pig excreta and containing 11-18 mg N/litre. The farmers traditionally feed the freshly harvested biomass to ducks and chickens.
The following experiments were carried out to obtain more information on the use of effluent from continuous flow biodigesters as fertilizer for duckweed.
Materials and methods
Four experiments were done using effluent from two biodigesters charged with cattle manure. In the first experiment the effluent was added to tap water at rates of 0.25, 0.5, 0.75, 1.0 and 1.25 litres/bin/day. In the second experiment, five initial concentrations of N were obtained by mixing the effluent with water at rates (% of effluent) of 9, 18, 27, 36 and 44%. Fresh effluent was then added daily at the rate of 0.5 litres/bin. In the third experiment, a comparison was made of different amounts of inoculum (fresh duckweed returned as seed after harvesting the biomass). The fourth experiment compared harvest frequencies (every 2nd, 3rd, 4th and 5th day).
Fifty plastic bins of 100 litre capacity and a top diameter of 0.5 m were used. The bins were placed on open ground without shade and close to the biodigesters to facilitate use of the effluent. They were filled with water and effluent to a height 10 cm below the upper lip of the bin.
Effluent from two biodigesters was used. Each measured 10 m long and 0.9 m in diameter with 4 m3 liquid volume. One of them was charged with pig manure and the other with cow manure. The loading rate was about 100 litres/day of the mixtures.
Fifty bins were used according to a completely randomized block design. The bins were filled with clean water. Five levels of effluent were compared (each replicated in 10 bins): 0.25, 0.5, 0.75, 1.0 and 1.25 litres effluent/bin/day. An inoculum of duckweed (100 g) was added to each bin. Harvesting was at two-day intervals, on each occasion removing all the biomass and returning 200 g/m2 as inoculum. Samples of water and duckweed were collected at the third harvest (after 6 days) for analysis. The water was assessed for nitrogen, dissolved solids and phosphorus. The duckweed was analysed for dry matter, nitrogen, ether extract, crude fibre and phosphorus. The mean lengths of the roots, pH and temperature of water in each bin were measured.
Twenty-five bins were used according to completely randomized design. The five treatments (each replicated in 5 bins) were 8, 16, 24, 32 and 40 litres/bin of effluent added at the beginning (proportions of: 9, 18, 27, 36 and 44% effluent in water). Then, the additions of effluent were 0.5 litres/bin/day in order to maintain a constant N concentration in bin water. The inoculum rates and harvesting frequency were made as described in experiment one. Samples of water and duckweed were collected after six days and after twelve days. The water was assessed for nitrogen, dissolved solids and phosphorus. The duckweed was analysed for dry matter, nitrogen, ether extract, crude fibre and phosphorus. The mean length of the roots was measured.
|Table1 : Composition of duckweed grown in bins with biodigestor effluent giving low (5-10 mg/litre) or hight (10-40mg/litre) levels of N|
Level of N in Water
|Dry matter %||5.4||5.6|
|Composition of DM,%|
The rate of addition of effluent at the beginning and the harvesting frequency was as described in experiment two. The inoculum rates of duckweed with five treatments were 100, 200, 300, 400 and 500 g/m2. Samples of water and duckweed were collected at the sixth harvest (after 12 days). The water was assessed for nitrogen. The duckweed was analysed for dry matter and nitrogen. The mean length of the roots was measured.
The rates of effluent at the beginning and addition of inoculum were made as described in experiment two. Harvesting frequencies with five treatments were every day, every second day, third day, fourth day or fifth day. Samples of water and duckweed were collected at the sixth harvest (after 12 days). The water was assessed for nitrogen. The duckweed was analysed for dry matter and nitrogen. The mean length of the roots was measured. All analyses were done according to standard procedures (AOAC 1990).
Protein content of duckweed at the low levels of addition of effluent in experiment 1 was increased from 17-37% in dry matter as the N in the water rose from 5 to 10 mg/litre. At the high levels of addition of effluent (data from experiments 2, 3 and 4) it increased from 36 to 42% when N in bin water was in the range 10-40 mg/litre (Figure 1).
Fresh biomass yields were maintained at around 100 g/m2 when N concentration in bin water was above 10 mg/litre and this also appears to be the optimum level of fresh biomass yield of duckweed (Figure 2).
Crude protein in dry matter (Y) was negatively related with root length (X1) and with the crude fibre of duckweed (X2).
Y = 49.3 - 1.140.1X1; r2 = 0.74 ( Fig 3).
Y= 71.6 - 3.290.37X2; r2 = 0.74 (Fig 4).
Rodriguez and Preston (1996) reported a similar relationship between root length and protein content (Y = 56.3 - 1.490.15X).
At the low levels of addition of effluent, protein yields of duckweed increased from 1.8 to 5.4 tonnes/ha/yr when N concentration in bin water rose from 5 to 10 mg/litre. At the high levels of addition of effluent (20-40 mg N/litre of bin water), protein yields of from 7.7 to 10 tonnes/ha/yr were obtained (Figure 5). In the 11-25 mg/litre range of N in the bin water, yields of duckweed were still around 6 tonnes/ha/yr. As a generalisation, it seems that protein yields from duckweed of between 6 and 10 tonnes/ha/yr can be obtained when the N content in the water is in the range of 10-30 mg/litre.
The results of experiment 3 indicated little effect of inoculation rate on the protein content of the duckweed. Dry matter and protein yields showed a curvilinear response to inoculation rate with the maximum point at 200 g/m2 and the lowest at 500 g/m2 (Figure 6).
Protein content of duckweed ranged from 38 to 41% in dry matter over all harvest reduction at the 3-day interval (88 g/m2/ intervals. Yield was`maximised at the 2-day (92 g/m2/day) interval with little day) (Figure 7).
Preliminary observations reported in this paper indicate:
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: 36kb
Rodríguez L 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) 2:60-69
Skillicorn P, Spira W and Journey W 1993 Duckweed aquaculture - A new aquatic farming system for developing countries. The World Bank, Washington DC. pp:76
Received 1 October 1996