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 Randomized Complete 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 days (ERT30). There were three replications (ponds of 6 m² in area) of each  treatment which was 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 retention times was probably due more to a lower BOD than to differences in ammonia-N levels.

 

Keywords:  Biodigester, effluent,   primary production, retention time, tilapia,  Oreochromis niloticus

 

Introduction

Integrated aquaculture is the comprehensive utilization of natural resources and ecosystems for the the artificial rearing of aquatic animals and plants (Lin and Yang Yi 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 and Bui Xuan An et al 1994).

 

In all countries, one objective of wastewater treatment should be the reduction, and if possible the removal, of parasites, bacteria and pathogenic viruses that cause endemic diseases. Ponds for water plant  (Chará 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 stabilization pond effluents 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 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.

 

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

Biodigester

The design and management of the plastic plug-flow biodigesters was 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.  

The composition of the effluent for the hydraulic retention times of 10 and 30 days was:  total N content 1003 and 1066 mg N/ litre, ammonia-N  486 and 636 mg/litre and ammonia-N to total nitrogen ratio 0.50 and 0.60. Effluent was used for Oreochromis niloticus with the quantity as below:

 

 

 

 

 

3.2. Fertilizing of effluent

 

Effluent as feed and fertilizer applied at the same level of nitrogen of 120 kg N/ha, equal to 0.133g N /m² /day and N in effluent was 1 and 1.1 g/ L equivalent to 0.80 and 0.75 L/ m² /d of treatments 10 and 30 days (table 2)

 

 

 

 

 

 

3.3. Experimental design

 

The experiment had 2 treatments with 10 or 30 days retention time of effluent in the biodigester and with 3 replications (total, 6 ponds). Effluent as feed and fertilizer applied at the same N level of 120 kg N/ha, equal to 0.133g N /m² /day. Ponds were 2m x 3m and 1 m deep, and were lined with cement and soil mixture to avoid water leakage through the sandy soil.

 

3.4. Allocation of treatments

 

The Completed Randomized Design was used for allocation of treatments and replications of the experiment (ERT10: effluent retention time of 10 days, ERT30: effluent retention time of 30 days). Pond arrangement could be two rows that depended on the location and space:

 

 

Experimental length: the experiment was conducted last in total 120 days weeks from 1^st July to 6^th Nov 2002

Effluent: The effluent was taken from each digester immediately after charging with manure and water and was applied at intervals of three days.

 

3.5. Management

 

3.5.1. Pond preparation: The bottoms of all ponds were applied with quick-lime (CaO) at 100 g/m², 10 days before stocking with fish. This liming is to eliminate parasites and pathogenic organisms and to increase the pH (Sophin and Preston 2001).  The ponds were filled with water 3 days after liming.

 

 

3.5.2. Stocking density

 

Each pond will be stocked with one fish species at a density of 2fish/m². The species was Tilapia (Oreochromis niloticus) introduced as fingerlings about 3-7 cm length.

 

3.6. Data collection and analyses

 

3.6.1. Effluent: The effluent were analysed twice weekly for DM, pH, OM, N, and Ammonia-N. The samples were taken before application of effluent to fishpond every three days. Detail of the analytical methods employed in the effluent samples appear elsewhere (San Thy et al 2003).

 

3.6.2. Fish measurement

 

The experiment measured fish's growth rate by measuring the length of fish and weight every 20 days in the morning at 8:00^am before loading the fertilizer. The fishes 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 were recorded. 

 

3.6.3. Water quality measurement

 

The oxygen level of the pond water was measured every two days, two times during the day in the early morning at 6:00^am and in the afternoon at 2:00^pm by a DO₂ meter (Model 9150). The DO₂ meter was used for the measurement in the pond directly. Before oxygen measuring was conducted the DO₂ meter was calibrated properly. Water samples were collected at the same place in each pond at 20 cm depth and analyzed one by one. The pH of the pond water was measured every two days, two times a day, in the morning at 9:00^am and in the afternoon at 4:00^pm by a digital pH Meter (Model 410A). Water temperatures was measured three days a week, three times a day in the morning 6:00, 12:00 and 17:00 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 2day in the midday by Secchi disk. BOD was measured every 20 days, analytical method by using......................

 

4. 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). 

 

5. Result and Discussion

 

5.1. Water quality in fishpond

 

 

 

 

5.1.1. Dissolved Oxygen level (DO₂)

 

Dissolved oxygen (DO) is oxygen gas (O₂) that is dissolved in water; most of it 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/L, it may be harmful to fish, and piping (gulping air at the surface) may be observed when DO fall below 2 mg/L. Low level of DO are most frequently associated with hot, cloudy weather, algae die-offs (Francis- Floyd 1997).   

 

The dissolved oxygen concentration, both treatments was very slightly different in ponds fertilized with both kinds of effluent (P=0.918). During most of the progressing period, the mean oxygen concentration in fishpond (Table 4) and DO₂ from effluent retention time of 30 days (ERT30) tended to be higher than ERT10.

 

According to Swingle (1969), the minimum oxygen concentration should be not less than 5 mg/ L. Values from 0.3 to 1mg/L over an extended period were considered to be lethal to fish and from 1mg to 5mg/litre the fish will survive, but growth will be slow.  Hong Samnang (1997), studied on the effect of pig manure and biodigester effluent on fish growth in ponds with 6 fish species composition included tilapia and divided into two groups, the range of oxygen level in pond received effluent from 3.8-11.9 mg/L group 1 and oxygen from 3.3- 10 for second group was higher than pond received pig manure. In table below were observed the oxygen level from previous experiments:

 

Table 5: The lists of previous experiment on oxygen level of pond culture
Types of culture	Feed and fertilizer	DO2 mg/L	Sources
Farm level, tilapia (4fish/ m2)	Mixture feed rice bran, fish meal  (5% BW)	3-20	Bolivar and Brown (1999)
Sex- reserved male tilapia (4.1 fish/m2)	TSP, urea	2.40-3.52	Lin et al (1999)
Oreochromis niloticu	waste and supplement	above 3	Chapman (2000)
Eemi- 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)
Oreochromis niloticus (2 fish/ m2)	chicken manure (500 kg/ ha), urea and TSP	0.9-2.5	Lin et al (2000)

 

5.1.2. pH value

 

The pH of the pond water in amongst treatments was not different (P = 0.347). This range of pH in both treatments was in the optimum of growth (Table 4) according to Swingle (1969) (the appropriate pH range for good fish growth is from 6.5 to 9). The values recorded by Pich Sophin and Preston (2001) within this range for all treatments (fertilizer from effluent manure and U-DAP for poly-culture fish) was 7.76-8.66, but in the effluent treatment it varied according to times from 7.79-8.66. Lin et al (1999) studied of tilapia growing in ponds fertilized with TSP or urea and found that the pH varied in a very wide manner from 6.5 as a minimum to 10.10 as maximum according to treatments studied. On the other hand Bolivar and Brown (1999) fertilized tilapia ponds, weekly with chicken manure supplement with urea  and triple superphosphate and reported water pH in ponds varying from 7.7-8.2. Lin et al (2000) fertilized ponds with urea and report water pH pH ranges 7.7-10.3 and 7.6-10.4 related to the methods of feeding.

The water quality of pH in water for tilapia was good in the range 6.0-8.5 (Chapman 2000). Alabaster and Lloy (1980) suggested that fish are found in water having pH range from 4-10, with a good level from 5-9 and for maximum production from 6.5-8.5, whereas fish mortality is often related to drop in pH. Besides the low pH reduced decomposition rate of organic matter and inhibited nitrogen fixation. The range of the pH from 6.5-7.5 is optimum for life of fish; the pH in manure was higher than pH in pond fed by effluent (Hong Samnang 1997).

 

5.1.3. Water temperature

 

There were no differences between the treatments for water temperature, the ERT10 was the same that what was observed for ERT30 (Table 4, figure 6). There were no differences between treatments in fish poly-culture fertilized with effluent, chemical and fresh pig manure at a temperature of 32 ^oC as it was observed by Pich Sophin and Preston (2001)

In other studies with growing tilapia, it was been measured temperature of 27.7-28.6 ^oC (Kwei Lin et al 2000 and 28-32 ^oC, (Chapman 2000) Hong Samnang (1997) found value of 25.4-28.8 ^oC.

Tilapia is a robust fish, withstand high water temperature well and their respiratory demand are slightly. Tilapia it is warm water fish the optimum development occurs at the temperature about 20 ⁰C or event up to 30^oC or more, and the lowest temperature they can withstand is between 12 and 13 ^oC (Marcel Huet 1986).

 

5.1.4. Water transparency

 

The water transparency was not different amongst treatments (Table 4). The same variation was observed in other Cambodian experiment, with the lowest value for the effluent, followed by the chemical fertilizer and fresh pig manure (19.4, 24.1and 21.1, cm) (Pich Sophin and Preston 2001). In tilapia ponds were stocked at 2 fish m^-2. According to Lin et al (2000) water transparency in pond could varied from 8.3-14 cm.

 

5.1.5. Biochemical Oxygen demand

 

There was different BOD in fish ponds (P=0.022); the high BOD values was for the treatment with less HRT (ERT10). It meant that higher oxygen demand could not be good for fish ponds. It can suggest that the long retention time of effluent in biodigester was good for fish and more oxygen in fishpond as it can be observed in table 4.   Wastewater was treated till it reached the appropriate quality to obtain suitable fish for direct human consumption. In this connection it has been found with four of tilapia, developed during the warm and cold seasons of Lima climate. That the treatments system reduced total BOD levels from 112 to 68 mg/l in fish pond (Moscoso and Leon 2000).

It has been reported that dissolved oxygen concentrations significantly increased when the ponds were fertilized with effluent as compared with fresh manure. Thus the principal benefit of a preliminary anaerobic digestion of pig manure appears to be the decrease in the BOD (biological oxygen demand) in the effluent due to removal of carbon as methane in the digestion process (Pich Sophin and Preston 2001). 

 

All the research results were higher and lower than our present study depending on the condition such as fish stocking density, quantity and amount of fertilizer, microenvironment and fish and pond-water composition and feed composition.

 

5.1.6. Fish survival

                                                  

During the whole experimental period of 120 days, the survival rates tended to be higher in the ponds fertilized with ERT10 (100% of fish survival) than ERT30 (97.22 %), but the difference was only significant in the case of tilapia that growed faster in the pond fertilized with 30 days retention time effluent (ERT10) as compared with the 10 days retention time effluent (ERT30) respectively. 

 

5.1.7. Growth of length

 

         Fish growth of tilapias are increasingly with times, the length growth are different between the treatment of 10 days, the gab of length growth of the 30 days effluent retention time in biodigester is bigger than treatment of 10 days (Table 6). This length growth might be faster from the age of 20-100 days than from above 120 days of age.    

 

 

 

 

 

 

 

 

5.1.8. Fish production

 

         Overall, the faster growth rates were with from ponds fertilized with effluents of 30 days retention time, and the lowest with the 10 days effluent retention time (Table 6, 7 and Figures 1, 3 and 4). There were differences in the response of the different the fertilizer treatments especially ammonia-N in effluent.  The tilapia growing in the fertilized pond with 30 days effluent retention time duplicated growth rate as compared with 10 days of effluent retention time.

 

Tilapia grew faster in effluent with 30 days retention time than in effluent retention of 10 days in ponds (Table 4, 7 and Figures 3 and 4). The final weights of 10 and 30 days effluent retention time of experiment (Table 6, 7) were 44.64 and 68.15g respectively. Effect of N and P budget in pond with differences in fertilization was between 324g /fish and 248g/fish (Kwei Lin et al 2001).

 

The fish yield per hectare was expressed as the net gain in total fish weight and the gross out (Table 7). There were significant differences among treatments, with highest values for the effluent treatment 30 and 10 days retention time. Compared with the effluent 10 and 30 days treatments, the 10 days effluent fertilizer increased net yield 633.2 kg /ha equivalents to 71% while the effluent treatment 30 days RTE increased 1015.04 equivalents to 75%. 

 

 

 

Veverica et al (1999) reported a net fish yield range at 1127- 2098 kg/ ha in semi-intensive tilapia production system.  Veverica et al (2000) reported too that in a Oreochromis niloticus and Clarias gariepinu mixing culture, the maximum net fish yield of tilapia were 1015- 2510 kg/ ha and range average weight of tilapia from 23.1-70.5 g/fish in cool season and yield from 1119 -1520 kg/ha and the mean growth  in the range 106-168g/fish in the warm season.   

 

In an aquaculture project using treated effluents from stabilization ponds, it was possible to obtain 4 400 kg/ha of tilapia with an average weight of 250 g per unit at the end of the summer, without adding artificial food. Growth was reduced during winter time because temperature declined to 17 ^oC. In conventional farms of the Amazon region, this production is obtained only when ponds are fertilized and concentrated food is provided. The abundant biomass of algae in treated wastewater replace artificial food, reducing production costs (Moscoso and Leon 2000).

 

Increased productivity in polyculture fish ponds when biodigester effluent, rather than manure, was used as fertilizer was reported by Han Yuqin and Ding Jieyi (1983). Yields of fish were increased by 26% when the effluent was applied compared with the original manure. Processing pig manure in an anaerobic biodigester, before using it as fertilizer for ponds stocked with a fish polyculture, resulted in a 55% increase in net fish growth compared with direct application of the fresh manure (Pich Sophin and Preston 2001).On the other hand, Hong Samnang (1997), studied the effect of pig manure and biodigester effluent on fish growth in ponds with 6 fish species including tilapia and found that fish yield was higher in effluent pond than in manure pond. Others Nguenga et al (1997) have found an improvement in fish production in dependence of organic or natural pond fertilization.

Nagdi et al (1998) studied a evaluation the possibility of recycling on tilapia fertilized chicken litter, urea-super phosphate and mixture feed 25% protein were stocked 2 fish/m² , the yield was highest in chicken litter of 19.81kg /ha/day equivalent to 0.99g in daily. Comparison of tilapia monoculture and carp poly culture fertilized with cow manure and supplement inorganic matter the net fish yield was 0.53 g/fish (Hassan et al 1997). A researched on increasing level of fertilizer (Control, urea plus TSP) on Nile tilapia, the average growth of 0.2g /fish/day. There was significant increases in total ammonia-N, pH, nitrate-N and there were no significant different in final weight of fish (Abdalla 1997). Diana et al (1996) the ponds were stocked at 3 fish/m², feeding with fertilizer 1.17g daily, growth fish reached 500g, , the most efficiency system was growing fish 100-150g with fertilization alone with subsequent addition of feed.

 

 

 

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 inputs of nitrogen and phosphorus for fish culture are 4 kg N/ha/day and 1 kg P/ha/day or 400 mg N/m² and 100 mg/m² per day, respectively. 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). In a report by Lin et al (1998), the growth rate of tilapia was 0.24 and 0.30 g per day in earthen ponds loaded with 429 mg N/m²/day and 114 mg P/m²/day and with a stocking density of 10 and 5 fish/ m², respectively. In contrast, in the present study, the N application was only 0.133 g N/m²/day and the growth rate of the Tilapia at a stocking density of 2 fish/ m² was 0.25 g/day with 10 days retention time effluent increasing to 0.43 g/day with the effluent 30 days retention time table 6. Processing pig manure in an anaerobic biodigester, before using it as fertilizer for ponds stocked with a fish polyculture,  resulted in a daily grow of tilapia were 0.35 and 0.5 g/day  increase in effluent compared with direct application of the fresh manure(Pich Sophin and Preston 2001).A research on integrated biogas technology by using slurry for fish culture from biodigester effluent different hydraulic retention time of 50-70, 70 and 30-50, stocking density is 5 fish/m² and supplement commercial pellets. The final yield of tilapia the rang between treatments was from 27.52- 41.14 g in 6 months(DWG of 0.15-0.23g ) (Edwards et al 1988) ( Thus the growth rates of the Tilapia were higher in our study but stocking rates, and hence in general productivity still was lower.

Kwei Lin et al (2000) observed that if tilapia ponds were fertilized weekly with chicken manure at a rate of 500 kg ha^-1  supplemented with urea and triple superphosphate to provide 28 kg N ha^-1 wk^-1 and 7 kg P/ ha/ week; cover different area in wet season  dependence of the types of the pond covering in the wet season, at a content stock density 2 fish m^-2 at a size of 19.0 ± 1.0 g, the mean daily weight gain were 0.67, 0.47, 1.10, 0.43 and 1.57 g/fish/day and pond varied from 8.3-14 cm.

Fertilized in shallow pond (1 m depth) and in deeper pond (2.5 m) was developed for culture Nile tilapia by using chicken manure fertilizing to pond with and without water addition. Fish stocking at 2 fish/m² and with an initial weight of 15g, resulted after 234 days in high yield when pond were weekly fertilized with additional water the daily weight was 0.86 g equivalent to 3830 kg/ha (Diana and Lin 1998).

 

 

5.1.9. Growth rate of fish and ammonia nitrogen in total nitrogen of effluent

 

It was expected in this experiment that. Since the hypothesis resulted to be valid increasing the retention time could increase the proportion of organic nitrogen converted to ammonia-N in the effluent and effluent from biodigesters with longer retention time could have a higher fertilizer value (ammonia-N) for growth of fish (figure 4). it should be recommended that effluent may not only be have as inorganic fertilizer since it contain the organic materials from the digestion of bacteria that fish can use as food to grow (Rakocy and Ginty 1989).

 

5.1.10. Effect of effluent on water quality, primary production and fish growth

 

There is the same water temperature in the average and dissolved oxygen is not significant different table 4. NH₃-N and the NH₃-N in total N was higher in ERT30 than in ERT10 so these were given the result produced more phytoplankton and zooplankton as primarily product for fish feed. Moscoso and Leon, (2000) reported that high production of algae ranged from 573 to 718 mg/l of chlorophyll A, according to the climate. Total ammonium ranged between 2.62 and 0.45 mg/l, which are tolerable values for Nile tilapia. Effluent characteristic in table 1 and 2 could be concluded that the higher ammonia nitrogen is the more primary production. The relationship between primary production and yield of tilapia, ponds were fertilized with high and low level of nutrient from chicken and urea. Pond received high fertilizer input exhibited higher nutrient level in water, higher primary productivity and higher fish production than pond treated with low input of fertilizer (Diana et al 1989).

 

6. Conclusion and Recommendation

• The longer retention time of the manure in biodigester is the higher fertilizer values of te proportion of Ammonia-N in total N.
• The higher ammonia-N in total N (ERT30) is the better of fish production in pond culture.
• Water quality is in the range of optimum condition in fish ponds both treatments.

 

Acknowledgements

 

 

References

 

Abdalla AAF 1997. Effect of increasing fertilization on water quality and yield of Nile tilapia, Oreochromis niloticus, common carp, Cyprinis carpio, in earthen ponds in Egypy.

 Alabaster and Lloy 1980. Proceeding of the FAO/ IPT workshop on Integrated Livestock fish Production system , 16-20 Dec 1991 

Buras, Netty, Duek, Lea, Niv, Sara, Hepher, Balfour, Sandbank, Enrico 1987. Microbiological aspects of fish grown in treated wastewater. Water Research, 21 (1):1-10.

 

Chará J, Pedraza Gloria and Conde Natalia 1999 The productive water decontamination system: A tool for protecting water resources in the tropics. Livestock Research for Rural Development  (1) 1:  http://www.cipav.org.co/lrrd9/1/cha111.htm

Lin CK, Madhav K Shrestha, Yang Yi and Diana SJ 1999. Developments in integrated aquaculture in Southeast Asia. Research Report 01-172

 

Lin CK, Yang Yi, Hoang Tung, and Diana SJ 1999. Global Experiment: Optimization of Nitrogen Fertilization Rate in Freshwater Tilapia Production Ponds during Cool Season. Eighth Work Plan, Feeds and Fertilizers Research 1T (8FFR1T). Final Report.  http://pdacrsp.orst.edu

Lin CK, Yang Yi, and Diana SJ 2001. The Effect of Management Strategy on Nutrient Budgets, Interim work plan, Global experiment, Thailand. http://pdacrsp.orst.edu/pubs/admin/

Lin C Kwei, Yang Yi, Hong Tung and Diana S James 2000. Effect of Mud Turbidity on Fertilization, and an Analysis of Techniques to Mitigate Turbidity Problems in Wet Season. Eighth Work Plan, Thailand Research 1 (8TR1), Final Report. Aquaculture and Aquatic Resources Management Program Asian Institute of Technology Pathum Thani, Thailand

Diana JS and Lin CK 1998. The effect of fertilization and water management on growth and production of Nile tilapia in deep ponds during the dry season. Journal of the world aquaculture Society.29: 4, 405-413, 16

Diana JS,  Lin CK, Yi Yang and Yi Y 1996.  Timing of Supplement feeding for tilapia production. Journal of the world Agriculture Society 1996:27: 4, 410-419, 14.

Ding Jieyi and Han Yujin 1983. Comparative studies on the effects of fresh pig manure and anaerobically fermented pig manure upon fish farming, p: 288-296. 

Chapman FA 2000. Culture of Hybrid Tilapia: A Reference profile. http://edis.ifas.ufl.edu/BODY_FAO12

Hong Samnang 1997. The effect of pig manure and biodigester effluent on fish growth in ponds. Reional Seminar-workshop, on Better use of locally available feed resources in sustainable livestock- based agriculture systems in South- East Asia.

James E Rakocy and Andrew S McGinty 1989. Pond culture of tilapia, Southern regional Aquaculture Center. SRAC publication No. 280

Diana SJ, schneeberger P J and Lin CK 1989. Relationship between primary production and yield of tilapia. CRSP research report .

Moscoso Julio and Leon Guillermo  2000. Treatment and sanitary use of wastewater and Wastewater use in aquaculture. HDT 60:    Wastewater use

Karen L. Veverica, Jim Bowman, Tom Popma 2000.  Optimization of Nitrogen Fertilization rate in freshwater  Tilapia production Pond. Progress report eight work plan, Kenya Research 1 (8FFR1K) 

Karen V, Bowman J, Gichuri W, Paul Izaru, Petricia Mwau and Thomas Popma 1999.  Relative Contributiion of Supplemental feed and Inorganic Fertilizer in Semi- intensive Tilapia production. Final report eight work plan , Kenya Research 3 (8KR3). 

Knud-Hansen C F, McNabb C D and Bartterson T R 1991  Application of limnology for efficient nutrient utilization in tropic pond aquaculture. Verh. Internat. Verein Limnol., 24:2,541-2.543. 

Le Ha Chau 1998, Biodigester effluent versus manure from pig or cattle as fertilizer for production of cassava foliage (Manihot esculenta)

Leng R A 1999. Duckweed, A tiny aquatic plant with enormous potential for agriculture and environment. Food and Agriculture Organization of the United Nation

Lin C K, Teichert-Coddington D R, Green BW and Veverica K L 1997 Fertilization regimes. In: H.S. Egna and K.L. Boyd (Editors), Dynamic of pond aquaculture. CRCP Press, Boca Raton/ New York. 

Lin C K, Yang Yi, Shivappa R B and Kabir Chowhury M A 1998 Optimization of Nitrogen Fertilization Rate in Freshwater Tilapia Production Ponds.  PD/A CRSP Sixteenth Annual Technical Report, P: 49-56 

Marcel Huet 1986. Breeding and Cultivation of Fish. Text book of Fish Culture.

Nagdi ZA, Al Nady MA, El Shaie MM and Megid SAA 1998. The effect of heavy fertilizer -feed input on the ecology and production of Nile tilapia, Central laboratory of agriculture Research.

Nguenga D, Breine JJ,  Yong SS, Teugels GG and OllevierF 1997. Effect of animal manure and chemical fertilizer on the growth and survival of tilapia cameronensis holly in Cameroon.

Edward P, Polprasert C, Raput V S and Pacharaprakiti C 1988. Interated Biogas Technology in the Toropics 2. Use of Slurry for Fish Culture. Waste management and research.

Preston TR 2000. Livestock Production from Local Resources in an Integrated Farming System; a Sustainable Alternative for the Benefit of Small Scale Farmers and the Environment. Workshop-seminar "Making better  use of local feed resources" January, 2000. SAREC-UAF (Editors: T R Preston and R B Ogle). http://www.forum.org.kh/~wwwuta/sarpro/preston.htm

Preston T R and Leng R A 1989 The greenhouse effect and its implications for world agriculture. The need for environmentally friendly development. Livestock Research for Rural Development  (1) 1:21-30

Preston T R and Rodriguez L 1996 . Recent developments in the recycling of livestock excreta; an essential feature of sustainable farming systems in the tropics http://www.fao.org/WAICENT/FAOINFO/AGRICULT/AGA/AGAP/FRG/recycle/default.htm

Remedios Bolivar and Chris Brown 1999. Timing of the Onset of Supplemental Feeding of Tilapia in Ponds. Ninth Work Plan, Feeds and Fertilizers Research 4 (9FFR4), Progress Report Freshwater Aquaculture Center

Floyd RF 1997. Dissolved Oxygen for Fish Production, Cooperative extension service, Institute of Food and Agriculture sciences. University of Flourida. http://edis.ifas.ufl.edu/BODY_FAO02

Hassan S, Edwards P and little DC 1997. Comparison of tilapia monoculture and carp poly culture in fertilized earthen ponds. Aquaculture and aquatic system , School of environment, resources and Development.

Swingle H S 1969 Methods of analysis for waters, organic matter and pond bottom soils used in fisheries research. Auburn, AL: Auburn University