Effect of exchange rate of the medium (water and biodigester effluent) on biomass yield and composition of duckweed

Nguyen Duc Anh and T R Preston

University of Tropical Agriculture, Thu Duc, Ho Chi Minh City, Vietnam

Abstract

Duckweed was grown in plastic bins of 100 litre capacity with a water surface area of 0.17 m². The source of nutrients was effluent from a biodigester charged with cattle manure mixed with water in the ratio of 9.1% effluent: 90.9% water (by volume). Five rates of exchange of the medium were compared: 0, 9.1, 13.6, 18.2 and 22.7% per day.

There were significant differences in yield and in content of dry matter, crude protein (N*6.25), phosphorus and crude fibre due to medium exchange rate, the trends being positive for yield (R² = 0.99), and content of dry matter (R² = 0.95 ), crude protein (R² = 0.99) and phosphorus (R² = 0.95) and negative for crude fibre (R² = 0.62).

It is suggested that the beneficial effects of partial exchange of the medium could be related to the reduction in temperature (from 33.2 to 31.4 ºC) in the medium or to the removal of compounds present in the biodigester effluent that were inhibitory to growth.

Keywords: Biodigester, effluent, duckweed, exchange rates of the medium.

Introduction

There is an extensive literature on duckweed (DRP 1996) but almost no documented information on how to grow this aquatic plant in continuous culture for the purposes of feeding it to livestock. In earlier reports it was shown that biomass yield and protein (N*6.25) content were increased when the nitrogen content of the growing medium was increased by addition of sewage effluent (Leng et al 1995) or effluent from biodigesters charged with excreta from cattle and pigs (Rodriguez et al 1996; Nguyen Duc Anh et al 1997). It has been observed in Vietnam that in the villages where duckweed is grown commercially there is usually a steady influx into the ponds of the water containing the nutrients. Production appears to be sustainable under these conditions except in the middle of the dry season when water levels in the ponds decrease and this almost certainly is associated with an increase in the water temperature. Farmers in Bangladesh growing duckweed as feed for fish apparently experienced similar problems (DRP 1996).

In contrast, most of the experimental work with duckweed has been done in plastic containers (Leng et al 1995; Nguyen Duc Anh et al 1997) or in ponds lined with plastic film (Rodriguez et al 1996) with no exchange of the medium. Practical experiences with the latter system are that production eventually declines and can be renewed only by emptying the ponds and recharging them with fresh water and effluent and with a new source of duckweed (Rodriguez et al 1996).

This paper reports the results of an experiment designed to assess the effects of recharging the medium by partial replacement of the effluent:water mixture on a daily basis.

Materials and methods

Bins and biodigester

Twenty five plastic bins of 100 litre capacity and a top diameter of 0.46 m were used. The bins were placed on open ground without shade and close to a plastic tube biodigester (Bui Xuan An et al 1997) 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.

The biodigester was 10 m long and 1 m in diameter with 6 m³ liquid volume. It was charged with cattle manure. The loading rate was about 150 litres/day of a mixture of one part (by weight) fresh cattle manure and 5 parts water. The concentration of nitrogen in the effluent from the biodigester varied slightly over the course of the experiment in the range of 138-142 mg N/litre.

Design and treatments

There were five treatments (rate of exchange of the medium) replicated five times according to a completely randomized block design. The medium for growing the duckweed (Lemna minor) was a mixture of effluent and water in a ratio of 9.1:90.9 (by volume). The five rates of exchange of the medium were 0, 9.1, 13.6, 18.2 and 22.7%/day.

Procedure

Each day the quantity of medium corresponding to each treatment was added to each bin displacing an equal volume of the medium present in the bin. The outlet through which the medium was displaced was at the bottom of the bin. The medium to be displaced was allowed to flow out of the bins (after mixing of the contents) before the new medium was added. The medium had the same ratio of effluent and water as was used to fill the bins at the start of the experiment. The exchange of medium was done every day at the time (between 12 am and 2 pm) when the temperature was highest. In order to maintain approximately the same concentration of nutrients in the medium (to compensate for those removed by the duckweed) effluent was added every second day at the rate of 0.5 litres/bin. An inoculation of fresh duckweed (200 g/m² of the surface area of the medium in each bin = 35 g) was put in each bin at the beginning of the experiment. Harvesting was at two-day intervals. On each occasion all the biomass in each bin was removed and weighed and 35g returned as inoculum. The experiment lasted a total of 24 days (12 harvests).

Measurements

The fresh yield of duckweed was determined every second day. Samples of duckweed from each bin were dried in the sun, bulked over 6 days for all replicates and stored in a refrigerator prior to determining nitrogen, crude fibre, ether extract and phosphorus according to AOAC (1990) procedures . Dry matter was determined by micro-wave radiation (Undersander et al 1993) on fresh samples of duckweed (bulked over replicates) at intervals of 6 days. The medium in the bins (bulked by replicates) was analysed for nitrogen and pH every six days. The temperature in the medium was measured every day after the exchange of medium, using a digital thermometer.

Statistical analysis

The yield data were analysed by the ANOVA General Linear Model using the Minitab (version 10.2) software package with treatment and harvest interval as main effects and individual bins as replicates. Composition data were analysed by the same programme with treatments as main effects and sampling times as replicates. Trends in yield and composition were assessed by fitting second order regressions to overall means for treatments.

Results and discussion

There were significant differences (P=0.001) in biomass yield due to rate of exchange of the medium in the bins and between harvests (Figure 1). Fresh yield showed a curvilinear relationship (R² = 0.99) with rate of medium exchange (Figure 2) but there was no apparent trend due to harvest date. There were significant differences in content of dry matter and of crude protein (N*6.25), phosphorus and crude fibre in the dry matter due to medium exchange rate, the trends being positive for dry matter (Figure 3; R² = 0.95 ), crude protein (Figure 4; R² = 0.99) and phosphorus (Figure 5; R² = 0.95) and negative for crude fibre (Figure 6; R² = 0.62).

Figure 2: Effect of exchange rate of the medium on fresh biomass yield of duckweed.	Figure 3: Effect of exchange rates of the medium on dry matter content of duckweed.
Figure 4: Effect of exchange rates of the medium on protein content of duckweed.	Figure 5: Effect of exchange rates of the medium on phosphorus content of duckweed.
Figure 6: Effect of exchange rates of the medium on fibre content of duckweed.	Figure 7: Nitrogen content of the medium at start and end of the experiment.
Figure 8: Effect of exchange rate of the medium on temperature of the medium in the bins.	Figure 9: Relationship between temperature of the medium in the bins and the biomass yields of duckweed.

The yields of duckweed biomass obtained in this experiment are similar to those reported by us in an earlier communication (Nguyen Duc Anh et al 1997) and by Rodriguez et al (1996). They are much higher than was recorded in commercial ponds fertilized with human excreta in Bangladesh where maximum yields in the month of November were only 76 g/m²/day falling to 3 g/m²/day in the month of June (DRP 1996). The positive effect on duckweed yield and composition that resulted from changing the medium in the bins could have been caused by the reduction in temperature (Figures 8, 9) and/or the elimination of compounds that are inhibitory to the growth of the duckweed. In the Bangladesh study (DPR 1996) lowest duckweed yields were recorded in the period of the year (February to July) when air temperatures were highest (above 30 ºC). It has not been possible to find any data in the literature on the effects of regular exchange of the medium. Nevertheless the results are in accordance with farmer practice in Vietnam where in most village ponds that produce duckweed commercially there is a steady influx of fresh water usually, but not always, located near the pig pens which are the source of the excreta providing the nutrients for the duckweed.

There was no apparent change in the N content of the medium during the experiment as indicated by mean values (bulked over replicates) taken at the beginning and after the last harvest (Figure 7). Presumably one effect of exchanging the medium was to remove a proportion of the nutrients that were not taken up by the duckweed and which over time could be inhibitory to growth. This is an area requiring more detailed research.

Work is also needed on practical ways to exchange the medium at farm level as an outlet at the bottom of a pond is not always feasible and the slope of the land may not be conducive to a natural flow from one pond to another.

Conclusions

There were positive curvilinear relationships between the rate of exchange (range of 0 to 22.7%/day) of the medium (ratio of biodigester effluent and water of 9.1:90.9) and the biomass yield and nutritive value of duckweed.

It is suggested that the beneficial effects of partial exchange of the medium could be related to the reduction in temperature in the medium (from 33.2 to 31.4 ºC), although the recorded difference was relatively small, or to the removal / dilution of compounds present in the biodigester effluent that were inhibitory to growth.

Acknowledgements

This research was partially supported by the International Foundation for Science through a grant (B/2434-1) to the senior author.

References

Received 15 January 1998

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