Effluent from biodigesters with different retention times for primary production and feed of Tilapia (Oreochromis niloticus)
San Thy and T R Preston
University of Tropical Agriculture Foundation
Chamcar Daung, PO Box 2423, Phnom Penh 3, Cambodia
santhy@utafoundation.org
regpreston@utafoundation.org
Abstract
A completely randomized design was used to study growth rate of Tilapia (Oreochromis niloticus) as influenced by pond fertilization (0.133g N/m2/day) with effluents from biodigesters having hydraulic retention time of 10 (ERT10) and 30 (ERT30) days. There were three replications (ponds of 6 m2 in area) of each treatment, applied over a period of 120 days.
Growth rate and net fish yield were higher with ERT30 (0.43g/day and 1363 kg /ha) than with ERT10 (0.27g/day and 899 kg/ ha) after 120 days. Mean values for BOD5 were higher for the ERT10 treatment.
It is concluded that the improved fish productivity with effluent from biodigesters with 30 day, compared with 10 day, retention times was probably due to a combination of lower BOD in the pond water, and a higher proportion of ammonia-N in the effluent.
Keywords:
Biodigester, effluent, Oreochromis
niloticus, primary
production, retention time, tilapia,
Introduction
Integrated aquaculture is the comprehensive utilization of natural resources and
ecosystems for the artificial rearing of aquatic animals and plants (Lin et al 1999). The integration of livestock with trees, food crops and
aquaculture is seen as the most appropriate technology to use the natural
resources in a system that is productive and sustainable according to Preston
(2000). In such a system the processing of the livestock manure by anaerobic
digestion is a key component as it has many positive benefits such as reduction
in emission of methane, which is a major actor in global warming (Preston and
Leng 1989), decrease in pathogens, better health of people and animals,
production of biogas for cooking (reduced pressure on forests for fuel wood;
more comfortable working conditions in the kitchen for women) and improved
nutrient recycling (reduced need for chemical fertilizer) (Preston and Rodriguez
1996).
In all countries, one
objective of waste-water treatment should be the reduction, and if possible the
removal, of parasites, bacteria and pathogenic viruses that cause endemic
diseases. Ponds for water plant (Charį et al 1999) and for fish culture
are technological options through which such objectives can be realized. If the
only objective was to decontaminate water resources, most projects would
not be financially feasible. However, if the excellent bacteriological quality
of stabilized pond effluent is taken as an advantage, as well as the
nutrients it contains, benefits are jointly obtained for agriculture, livestock,
horticulture, aquaculture and forestation. The design of these systems should be adjusted
according to the effluent quality required for the intended usage. The use of wastewater
facilitates the efficient use of water, the provision of natural
fertilizers and food, the creation of employment sources and economic income,
and the expansion of agricultural frontiers in desert areas (Moscoso and Leon 2000).
The
main products from the biodigester are biogas and
effluent, which is a potential fertilizer because the anaerobic digestion process
results in conversion of organic nitrogen from manure to ionized ammonia (NH4+)
which can be used directly by plant roots (Forchhammer 1994).
Thus it has been found in Vietnam that the effluent was a better fertilizer compared with
raw manure for application to cassava and duckweed
(Le Ha Chau 1998a,b), although there are few
reports of trials to compare the two sources of plant nutrients.
It is also important to note that biodigester effluent, as well as behaving as
inorganic fertilizer, also contains the organic materials from the
digestion of bacteria that fish can use as food to grow (Rakocy and Ginty 1989).
The objective of the present experiment was to obtain further evidence concerning the fertilizer value of biodigester effluent, and specifically to compare effluent produced by different retention times, when used in ponds for fish culture. The hypothesis underlying the design of the experiment was that increasing the retention time in the biodigester would produce effluent of superior nutritive value for use in fish ponds stocked with Tilapia.
Materials and Methods
Treatments and design
A completed randomized block design was used for allocation of two treatments to six ponds, arranged in three blocks (Table 1). The treatments were: effluent from biodigesters with 10 (ERT10) and 30 (ERT30) day retention times The experiment had a duration of 120 days from 1st July to 6th Nov 2002.
|
Table 1.
Allocation of treatments
|
||
|
Block 1 |
Block 2 |
Block 3 |
|
ERT30 |
ERT10 |
ERT30 |
|
ERT10 |
ERT30 |
ERT10 |
Biodigester effluent
The design and management of the plastic plug-flow
biodigesters were described by Santhy et al (2003). The influent was a
mixture of pig manure and water with a solids (DM) concentration of 60 g/litre,
which with hydraulic retention times of 10 and 30 days, was equivalent to a
loading rate of 3.06 and 1.02 kg DM manure per m³ of liquid volume of the
biodigester.
|
Table 2. Quantities applied to the ponds of effluent, total N and NH3-N, according to the source of the effluent (retention times of 10 and 30 days) |
||
|
|
Retention time, days |
|
|
10 |
30 |
|
|
N, mg/litre effluent |
1003 |
1066 |
|
Effluent, litres/m2/day |
0.26 |
0.24 |
|
N, g/m2/day |
0.133 |
0.133 |
|
NH3-N, g/m2/day |
0.066 |
0.080 |
The fish ponds
The ponds were 2 x 3m and 1 m deep, and were lined with a cement
and soil mixture to avoid water leakage through the sandy soil (Photo 1).
Photo 1: The ponds used during the experiment
Data collection and analyses
Samples
of effluent were taken before application to the
fishpond every three days for determination
of pH, DM, OM, N and
Ammonia-N. Details of the analytical methods employed appear elsewhere (San Thy et al 2003).
The growth rate of the fish was determined by recording
the length and weight every 20 days in the morning at 8:00am
before loading the ponds with effluent. The fish were caught with a seine net and put
in a small basket to measure the length and weight. The length from the tip of
the mouth to the caudal fin was measured with a graduate ruler. At the end of
the experiment the total fish biomass and the weight and length of each fish were
recorded.
The oxygen level of the pond water was
measured every two days, two times during the day in the early morning at 6:00am
and in the afternoon at 2:00pm by a DO2 meter (Model 9150).
Water samples were collected at the same place in each
pond at 20 cm depth and analyzed for pH (every two days, two times a day, in the morning at 9:00am and in the afternoon at 4:00pm)
using a digital pH Meter (Model 410A). Water
temperatures was measured three days a week, three times a day at 6:00, 12:00 and 17:00
h at a water depth of 20 cm. It was measured by a
thermometer submerged into the pond water and left for 5 minutes, after which
the reading was taken with the thermometer still in the water. Water transparency was measured every
2 days at midday using a Secchi
disk. BOD was measured every 20 days by the method of Winkler (Andrew et
al 1995).
Statistical analyses
The
data were subjected to analysis of variance (ANOVA) by using the General Linear
Model (GLM) of the MINITAB software (Release 13.3, 1998).
Results
Growth of Tilapia
There were contrasting results for growth in length and in weight, the former favoured by short retention time in the biodigester, and the latter by the longer retention time (Table 3; Figure 1).
|
Table 3. Effect of effluent from biodigesters with 10 or 30 day retention times on growth and weight/length ratio of Tilapia |
||||
|
|
ERT10 |
ERT30 |
SEM |
Prob |
|
Day of measurement |
|
|
|
|
|
0 |
8.33 |
10.15 |
0.417 |
0.037 |
|
20 |
10.08 |
10.58 |
0.197 |
0.146 |
|
40 |
11.30 |
11.17 |
0.376 |
0.814 |
|
60 |
12.24 |
12.69 |
0.610 |
0.630 |
|
80 |
13.01 |
13.34 |
0.525 |
0.679 |
|
100 |
13.41 |
13.89 |
0.318 |
0.343 |
|
120 |
13.84 |
14.86 |
0.325 |
0.090 |
|
Daily gain in length, cm |
0.044 |
0.041 |
0.0011 |
0.100 |
|
|
Weight, g |
|
|
|
|
0 |
13.3 |
17.4 |
1.53 |
0.132 |
|
20 |
19.4 |
21.5 |
1.50 |
0.376 |
|
40 |
26.8 |
28.5 |
2.77 |
0.678 |
|
60 |
34.7 |
39.4 |
4.83 |
0.528 |
|
80 |
38.4 |
47.0 |
3.71 |
0.175 |
|
100 |
42.6 |
55.6 |
3.17 |
0.044 |
|
120 |
44.9 |
68.2 |
2.67 |
0.004 |
|
Daily weight gain |
0.27 |
0.43 |
0.017 |
0.004 |
|
|
Weight/length, g/cm |
|
|
|
|
0 |
1.60 |
1.71 |
0.10 |
0.61 |
|
20 |
1.92 |
2.03 |
0.08 |
0.55 |
|
40 |
2.35 |
2.55 |
0.12 |
0.48 |
|
60 |
2.81 |
3.09 |
0.18 |
0.49 |
|
80 |
2.93 |
3.52 |
0.17 |
0.09 |
|
100 |
3.17 |
4.00 |
0.21 |
0.03 |
|
120 |
3.24 |
4.59 |
0.31 |
0.001 |
Figure
1: Growth curves of
Tilapia in ponds fertilized
with effluent from biodigesters having 10 day and 30 day retention times
Figure 2: Development of
Tilapia in terms of the ratio of weight/length, in ponds fertilized
with effluent from biodigesters having 10 day and 30 day retention times
Water quality in fishpond
The BOD values (biological oxygen demand) were higher for the pond receiving effluent from the biodigester with 10 day retention time compared with 30 days (Table 4).
|
Table 4:
Mean values for water quality parameters |
||||
|
|
ERT10 |
ERT30 |
SEM |
Prob |
|
pH |
8.63 |
9.01 |
0.38 |
0.347 |
|
Water transparency, cm |
30.0 |
25.6 |
2.7 |
0.305 |
|
Water temperature,
oC |
30.1 |
30.1 |
0.78 |
0.999 |
|
BOD, mg/litre |
7.10 |
4.74 |
0.97 |
0.022 |
|
Dissolved oxygen,
mg/litre |
|
|
|
|
|
6:00am |
2.89 |
2.81 |
|
0.083 /0.001 |
|
2:00pm |
4.59 |
4.80 |
||
|
SEM/Prob. |
0.083/0.61 |
|
|
|
|
Table 5:
Reports in the literature on oxygen level
of pond culture |
|||
|
Types of culture |
Feed and fertilizer |
DO2 mg/litre |
Sources |
|
Male tilapia (4.1 fish/m2) |
TSP, urea |
2.40-3.52 |
Lin et al 1999 |
|
No details |
Waste and supplement |
above 3
|
Chapman 2000 |
|
Semi- intensive tilapia |
Feed and inorganic
fertilizer |
2.2 -4.5 |
Veverica et al 1999 |
|
Sex reversed male tilapia
(3 fish/m2) |
Urea and 30% crude protein |
1- 10.6 |
Lin et al 2001 |
|
(2 fish/ m2) |
Chicken manure (500 kg/ha), urea and TSP |
0.9-2.5 |
Lin et al 2000 |
Net
fish yield
The net fish yield (total output weight minus the weight at the start) was 60% higher for the 30 day compared with the 10 day retention time (Table 6). The yields were at the low end of the results reported by a range of authors using a wide range of fertilizer management systems (Table 7).
|
Table 6:
Initial and final weight and length of
tilapia |
||
|
|
ERT10 |
ERT30 |
|
Total net fish yield,
g/pond |
1140 |
1828 |
|
Total net fish yield,
kg/ ha |
633 |
1015 |
|
Table 7: Literature reports of growth and yield of tilapia from different types of culture |
|||
|
Types of culture and fertilizer |
Final weight fish, g |
Net fish output, kg/ha |
Reference |
|
Semi-intensive tilapia (DAP and Urea) |
nd# |
1127- 2098 |
Veverica et al 1999 |
|
Mixed culture - Cool season - Warm season |
23.1-70.5 106-168 |
1015- 2510 1119 -1520 |
Veverica et al 2000 |
|
Treated effluents from stabilization ponds (without adding artificial food) |
250 |
4 400 |
Moscoso and Leon 2000 |
|
Chicken litter, urea-super phosphate and mixed feed |
149 |
2970 |
Nagdi et al 1998 |
|
Tilapia with fertilization alone or subsequent addition of feed |
314 |
5460 |
Diana et al 1996 |
|
Tilapia, chicken manure,(120 d) |
nd |
3660 |
Knud Hansen et al 1992 |
|
Effluent from 30 day retention in biodigester |
68 |
1015 |
This experiment |
|
#nd: no data |
|
|
|
Discussion
Growth of Tilapia
It appears that, when growth conditions are limiting, Tilapia change their conformation, increasing more in length than in mass (Lowe-McConnell 1982). When the growth data were expressed as the ratio of weight/length, then the results were much superior for the longer retention time (Figure 2). The survival rates were high (100% on ERT10 and 99.7% on ERT30).
Water quality in fishpond
Higher BOD levels mean that more oxygen is needed for the oxidation of the carbon in the effluent and therefore there would be less oxygen for the fish. However, the differences in BOD between retention times did not seem to be reflected in the dissolved oxygen concentration, although as expected values were lower in the early morning than in the afternoon. There was a suggestion (SEM ±0.11; P=0.26) of an interaction between treatment and time of measurement, such that dissolved oxygen values were higher for 30 than 10 days retention time in the afternoon with no difference in the morning.
The dissolved oxygen is produced during photosynthesis carried out by aquatic plants and algae during daylight hours, declining during the night and is lowest just before daybreak. If DO is below 5 mg/litre, it may be harmful to fish (Swingle 1969; Floyd 1997), and piping (gulping air at the surface) may be observed when the DO falls below 2 mg/litre. A low level of DO is most frequently associated with hot, cloudy weather and algae die-offs (Floyd 1997). The values in our experiment were within the range reported by several groups of researchers (Table 4). The pH of the pond water between treatments was not different. This range of pH (8.63-9.01) is in the optimum range for growth of from 6.5 to 9, according to Swingle (1969) and from 6.0 to 8.5, according to Chapman (2000).
Net fish yield
The
results of this experiment showed that the productivity of the ponds was
relatively low, if compared with the potential when pond inputs are optimized.
According to Knud-Hansen et al (1991) and Lin et al (1997)
the optimum input of nitrogen for fish culture is 4 kg
N/ha/day or 400 mg N/m²
per day. The quantity of N recommended by these authors is almost
4 times higher than what was used in the present study (about
1 kg N/ha/day). Processing pig manure in an anaerobic
biodigester, before using it as fertilizer for ponds stocked with a fish polyculture,
resulted in a daily growth of tilapia of 0.5 g/day (Pich
Sophin and Preston 2001). In research on
integrated biogas technology, using biodigester
effluent and different hydraulic retention times (50-70, 70 and 30-50 days), and
a stocking density of 5 fish/m2 and a supplement of commercial pellets,
the final weight
of tilapia was from 27.5 to 41.1 g in 6 months (daily weight gain of 0.15 to 0.23g)
according to Edwards et al (1988). Thus the
growth rates of the Tilapia were higher in our study but stocking rate, and
hence overall productivity, was lower.
Conclusions
It is concluded that the improved fish productivity with effluent from biodigesters with 30 day, compared with 10 day, retention times was probably due to a combination of lower BOD in the pond water, and a higher proportion of ammonia-N in the effluent.
Acknowledgements
The authors
would like to thank the Swedish Agency for Research Cooperation with Developing
Countries (SAREC) for funding this study through the regional MEKARN project,
and all friends and UTA staff (Lylian Rodriguez, Pol Samkol, Hean Pheap and Srey
Sam An) for their help during the experiments. The senior author expresses deep
gratitude to his parents and wife for their encouragement and very strong
support during this study.
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Received 31 May 2003; Accepted 18 August 2003