Traditional integrated fish farming in China includes fish cum chicken, fish cum duck, fish cum pig and fish cum cow, etc. It integrates poultry and livestock raising with fish culture. The linkage between poultry or livestock raising and aquaculture is animal manure. Compost was often used to fertilize the pond water for the proliferation of natural organisms, which serve as natural food for fish from juvenile to adult. However, this kind of practice is getting criticism from hygiene point of view. Anaerobic fermentation of manure in a digest is encouraged.

This paper will describe the details of the nutrient flow by a comparative study on bio- gas slurry versus chicken manure to culture fish. The dominant fish species stocked were Tilapia Oreochromis nilotica, silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis. The secondary species stocked were small amount of common carp Cyprinus carpio, crucian carp Carassius auratus, bream Megalobrama amblycephala and grass carp Ctenopharyngodon idella. The results indicated that the net fish yield reached 12 t/ha by slurry whereas 3.4 t/ha by chicken manure. The economic return of the former was 2.5 times the latter. The nutrient flow analysis will explain the reason why the differences between the two. It proves that fish farming by slurry is an effective approach to raise the utilization rate of waste materials and to promote the development of ecological agriculture.

Keywords: Bio-gas slurry, Bio-gas dregs, Bio-digester, Fish farming
 
 

In 1981 the 9th report of the Roman Club Dialogue about fortune and welfare indicated economy and ecology is a whole entity, which can not be divided. In a world with its ecology damaged, it is impossible that there will be fortune, wealth and welfare.

The first document of the Chinese government in 1983 indicated that to realize the developmental target of agriculture we must pay attention to controlling strictly population growth, utilizing rationally natural resources and preserving better ecological environment. Afterwards, the principles became three well-known prerequisites of the development of agriculture. In 1984, the concept of ecological agriculture was accepted by academic circle and was put into action plan. Ecological agriculture, which means integrated biomass systems in a sense, develops in depth and in extension. 50 ecological agricultural counties achieved substantial development in the third trial year. Up to the end of 1996, there were 20 cities and 160 counties engaged in developing ecological agriculture. The coverage of ecological agriculture reached 13 million ha. The grain production increased by 15% and the average income of farmers involved in ecological agriculture was higher than local average by 10%~12%.

One of the fundamentals of agricultural ecological economics is to optimize the utilization of resources especially bio resources. Bio digester is the key link of ecological agriculture to fully utilize agricultural by-products and wastes. In 1996, the number of small digesters for households increased by 480,000. The total number of methane users reached 6 million. The number of large and medium sized bio- gas engineering reached 592. 16 million cubic meters of organic wastes and agricultural effluent were treated. In recent years, great effort was made to extend “Four into one combination” (digester, latrine, pigsty and green house) model in the north and “Pig-methane-fruit” and “Pig-methane-rice” models in the south. All those models are using bio- gas as its main link. 

Traditional integrated fish farming includes fish cum chicken, fish cum duck, fish cum pig and fish cum cow, etc. Fish culture integrates poultry and/or livestock raising. The linkage between fish culture and poultry or livestock raising is animal manure. Compost was often used to fertilize the pond water for the proliferation of natural organisms, which serve as natural food for fish from juvenile to adult. However, this kind of practice is getting more and more criticisms from hygiene point of view. Anaerobic fermentation of manure in a digester is encouraged. As is known any agricultural industry develops from small to large on the scale, from extensive to intensive on the management; from manual labor to mechanic or automatic on operation. 

For example, chicken farming develops from raising chicken sparsely in courtyard to broiler farming or egg-laying hen in factory. Animal husbandry in large scale produces more wastes, which fishponds nearby could not be totally exploited. In Asia, those semi-intensive fish farming systems are usually based on the polyculture of warm water omnivorous and herbivorous fish species feeding low on the aquatic food chain i.e. in Chinese carp & Indian carp polyculture systems. Cyprinidae constituted 70.1% of total fin-fish aquaculture production and occupied the top four positions of the world fin- fish and crustacean aquaculture. The feeding strategies employed by farmers are usually low cost and based on the use of traditional available technology and agriculture wastes or by-products. The farming is usually being small scale in nature and generally integrated with the production of agricultural crops and/or livestock as a secondary activity within an integrated agriculture-aquaculture farming system (Edwards 1993). In Semi intensive & Intensive fish farming system operation, food and feeding costs generally ranged between 30-60% of total farm production costs or even higher (Tacon, 1995).

How to use slurry and dregs from digesters and to combine digesters with fishponds is one of important subjects for reducing nutrient loss and seeking for zero emissions. Using bio- gas slurry to culture fish started in late 1970s. In early 1980s, this digester-fishpond system was extended to Hunan, Hubei, Sichuan, Jiangsu and Zhejiang provinces. It achieved apparent economic benefits and its technique is easy to learn. Where there is any digester, its slurry can be used to culture fish in line with local conditions. Slurry can be used either to culture grow-out fish or to nurture fingerlings. Many comparative studies between slurry and manure in the same quality with the same supplemental feeds were conducted in Jiangxi, Sichuan, Jiangsu provinces. We chose two cases for nutrient flow analysis.

During 1986~1987, Nanjing Environment Science Research Institute conducted a comparative study on fish farming by bio- gas slurry and chicken manure in Jiangsu Province (Ming et al 1989a). Slurry loaded fishpond was located at the outlet of a digester. The area was 913 m2, a volume of 852 m3 and an average water depth 1.4 m. Chicken manure loaded fishpond was located at the downstream of the former with an area of 3260 m2, a volume of 6520 m3 and an average water depth of 2m. There was a big difference between the two ponds on the capacity. In order to facilitate comparison analysis, the stocking density was the same, fingerlings 0.13 kg/m3 based on the effective water body. In slurry loaded pond, they stocked 85 kg/mu (1/15 ha) on the 18th of April 1987 and harvested on the 5th of Nov. The growth period was 202 d. In chicken manure loaded pond they stocked 167.3 kg/mu on the 5th of March and harvested on the 30th of Dec. The growth period was 290 d. The unit area was the same but the effective water body different by virtue of water depth. The dominant fish species stocked were tilapia Oreochromis nilotica, silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis. The secondary species stocked were small amount of common carp Cyprinus carpio, crucian carp Carassius auratus, bream Megalobrama amblycephala and grass carp Ctenopharyngodon idella in both fishponds. In manure loaded pond they put in small amount of grass except chicken manure while in slurry-loaded pond they only applied slurry with solid substance 1.3% and its specific gravity 1.01. The slurry came from a bio digester with the average retention time of 40 days. The input was pig manure from the pig fed with composite feeds mixed with 30-60% of chicken manure after treatment. The slurry flowed out to a holding tank for counting and then to the fishpond through plastic conduit. Slurry loaded pond installed a submerged pump in 10 cm diameter for fear that fish would gasp due to possible lack of dissolved oxygen whereas chicken manure loaded pond had a sprinkler aerator with 3 kilowatt. Water quality was monitored from April to Nov sampling once a month. Samples were collected from 5 points each pond at the water surface and 1.2 m deep. The measurements were as follows:

(1) Physico chemical factors: water temperature, dissolved oxygen, pH, turbidity, and at the same time nitrate, nitrite, ammonia, TP, COD, SiO2 were measured after mixing samples of two water layers. The measuring methods were based on environmental monitoring analysis methods promulgated by the National Environment Protection Bureau. 
(2) Chlorophyll 
(3) Zoo- plankton and benthos standing crop
(4) Heat value of fish meat, protein, amino acid 
(5) TN, TP and TC of chicken manure, slurry and dregs
(6) BOD tests on chicken manure, slurry and dregs in steamed water with saturated oxygen during the same time period.

The measurement equipment was Automatic Calorimeter, Nitrogen measurement apparatus, Hitachi 835-50 Automatic Amino Acid measurement apparatus.

Although the growth period of fish in slurry loaded pond was 88 days shorter than in chicken manure loaded pond, the net fish yield in slurry loaded pond was 12.12 t/ha, which was 3.55 times the yield in the manure loaded pond 3.412 t/ha (Table 1, 2).

                             Table 1. Material Input/Fish Output 

Table 2

Table 3

It indicated that the conversion rate of slurry and chicken manure to fish through food microorganisms was 90 and 9.6, respectively. According to Yuan (1985), to produce 1 kg of fish needs 10 kg of chicken manure. Table 3 shows the input output ratios of nitrogen, phosphorus and energy. Although the growth period of fish in slurry loaded pond was shorter, the output of nitrogen, phosphorus and energy was more and the output/input ratios were greater (see Table 3). 

Most of energy in chicken manure applied into the fishpond turned into methane, which did not turn into output.  The effective nitrogen and phosphorus in TN and TP are higher in slurry than those in chicken manure (See Table 4). So the primary productivity in slurry loaded pond was higher than in chicken manure loaded pond and the physico chemical and biological parameters were shown in Table 5. From nutrient point of view, the nutrients in slurry loaded pond were higher than in chicken manure loaded pond. Phytoplankton peaks happen in spring and autumn while rotifer peak happens in summer. Moreover, there are methane generating methanobactrium colonies in slurry. They are good food for zooplankton and in turn zooplankton are good palatable food for fingerlings to grow. Especially, bighead carp grew rapidly. The economic benefit comparison was shown in Table 6. 
 
 

Table 5

Table 6

Nanjing Environment Science Research Institute conducted another experiment in Taixian County on nurturing summerlings by slurry in 1988 (Ming et al, 1989b). Fishponds were No. 1, 2, & 3 with an area of 10,000 m2 in depth of 1.15~1.25 m. Fingerling pond usually is shallow and small. The nurture of fingerling is one of key links in aquaculture. In the production process, nurturing 3-4 day-old post larva (fry), which begin to eat food to summerlings is very important because fry are so delicate and their power of movement and their ability to feed are very weak. So they are vulnerable to predators, but their metabolic rate is high. 

There are many feeding and fertilizing methods, among which the water quality by applying soybean milk is stable and fish disease less, but the utilization rate of feed is low with high cost. 10,000 tails of fry to summerlings need to consume more than 10 kg of soybean. The cost of green manure and animal manure is lower, but fish diseases more and DO consumption higher (Table 7). The effect of fertilization is slow. Pond No. 1 was applied with 3,000 kg of slurry (2,000 kg as basal manure). Basal manure application: 8-10 d before stocking fry. Pond No. 2 was applied with 740 kg of chicken manure (200 kg as basal manure). Pond No. 3 was applied with 667 kg of night soil (500 kg as basal manure), 23.8 kg of soy bean in the form of soy bean milk, 9 kg of flour and 27 kg of duckweed (wet weight). After 25 days summerlings were counted and weighed. The survival rate in nurturing pond by slurry was the highest (Table 8). It indicated that the yield in slurry loaded pond was 1.49 times the yield in chicken manure loaded pond. The survival rates of silver carp, bighead and grass carp in No. 1 pond were 5.37, 3.78 and 2.38 times those in No. 2 pond, respectively. The survival rate of bream in No. 2 pond was zero whereas 76% in No. 1 pond. It is hard to compare No.1 pond with No.3 pond, but it is certain that nurturing fry to fingerlings by using slurry is better than by applying feed and manure. The input cost and income analysis was shown in Table 9. 
 
 

Table 8

Table 9

In 1989 a comparative study was also conducted on the survival rate of fish by slurry & dreg from digesters and manure (night soil and animal manure) in DeAn County, Jiangxi Province (Dai & Wang, 1991).. Two ponds were with the same sediment, similar stocking density and the same proportion of fish species i.e. plankton feeders as dominant species in the same polyculture mode and feeding strategies. The survival rate and the fish yield in the test pond by slurry were much higher than the control pond (Table 10).

Reasons for fish production increase in digester fishpond interaction system:
The net fish yield in slurry loaded pond in this experiment was 12.12 t/ha, which was 3.55 times the yield in the manure loaded pond 3.412 t/ha. Is it an occasional case? Here are three other comparative experiments between slurry and manure in the same quality with the same supplemental feeds. Our center and Provincial Bio-Gas Research Institute conducted an experiment in 1983. The plankton feeder yield increased by 4.7%~5.1% and food- feeding fish increased by 9.2%~44.7%. The economic return increased by 11.7%~35.2%. The fish quality was improved much (Han & Ding, 1987). The fish production in slurry loaded pond was higher by 10%~60%, among which plankton feeders increased by 0.5~1.27 times with increased economic return higher by 0.5~2 times (Dai & Wang, 1991). An experiment was conducted to culture marketable fish by slurry with supplemental feeds in large scale in Zigong, Sichuan Province with a result of increased fish yields by 40~50%. 

With shorter growth period and less input of energy, the output of energy in slurry loaded pond was more with greater production/input ratio. The reasons for this are the energy of chicken manure put into the fishpond turned to volatile methane. Although slurry came through pig digestion and bio digester, the TN, TP and their heat equivalent were higher by 59.8 times, 42.8 times than chicken manure, respectively. The effective N and P (69.4% & 36.4%) in TN and TP, which are easy to be absorbed by phytoplankton were higher than those (4.6% & 30.8%) in chicken manure so the effective N & P are higher by 14.1 times and 0.18 times than chicken manure. In a comparison of oxygen consumption test, fresh chicken manure and slurry were put into oxygen- saturated steam water at 8-9°C, the result shows that each kg of chicken manure consumed 5,000 mg of oxygen in 15 hours while 570 kg of slurry, the same heat equivalent to chicken manure only consumed 67.76 mg of oxygen in 15 hours. However, DO measured in slurry loaded pond was a little lower than in manure loaded pond in May. Why? The DO might drop down when putting slurry into the fishpond in large amount at the initial month and if the effluent focused at one point of the fishpond. Also slurry and dregs from bio digesters contain several physical active substances and can promote the growth of organisms including fish. The parasite eggs and pathogen will be killed after anaerobic fermentation so the fish diseases are less in slurry loaded pond.

The gist of fish farming techniques by slurry and dregs from bio digesters:
1. Certain scale of animal husbandry should be kept to maintain supply of manure to the digesters and its normal operation.
2. Starting fermentation with adequate fermenting liquid and raw materials put in.
3. Input materials and water at a 4 to 7 day interval, 15 days after supplying bio-gas.
4. Time of outflow: 1 month after loading into a digester. Keep slurry outflow directly into fishponds to avoid ammonia nitrogen volatile. However, shallow basins are needed as sedimentation pond, oxygenation pond and algae pond to further reduce the BOD of slurry by 30% according to George Chan.
5. It is better to build two digesters to supply slurry in turn to a fishpond to satisfy its nutrient demand.
6. The ratio of fishpond water body to digester: 160~185 m3: 1 m3 according to the experiments.
7. Apply small amount of slurry at multi- points of surface layers many times in the morning.
8. Dreg can be used as basal manure and slurry as supplement.
9. Slurry can not be applied during bad weather (overcast and stifling) or to the pond with high stocking density of fish so as to avoid oxygen depletion.
10. Species stocked: filter feeding fish (plankton feeder) >60% as the dominant species; herbivorous and omnivorous fish less than 40% as the secondary.
11. Turbidity control: 
If Sacchi disc is dim under 18~20 cm, the water is too fertile, there is no need to apply slurry. If it is under 20~25 cm, the water quality seems normal for fish culture. If the Sacchi disc can be seen under the water 25~35 cm below, slurry should be immediately applied.

Functions of bio-digesters in ecological agriculture:
1.  Bio resources can be fully utilized. It promotes energy and nutrient flow in ecological agricultural systems and raises its stability.

2. Slurry and dregs from bio digesters contain several physical active substances and can promote the growth of organisms. According to the measurements by Shenyang Agriculture University, the protein content increased after anaerobic fermentation from 16.62% to 46.09%, the same as the content of soybean cake. The content of essential amino acid relatively increased and the content of vitamin B especially the content of Vitamin B12 increased 7.2 times the fermentation before. The hydrolyzed enzyme is higher in slurry with the active protein enzyme 82.35mg Tyrosine, the active starch enzyme, 2678.25mg starch and the active cellulose enzyme, 1775.14mg glucose and also some growth and cell fusion hormone were found. It indicated that slurry not only could provide N, P and K, but also physical active substances to the plants.

3.  Improving ambience and hygiene in rural area 
The parasite eggs and pathogen will be killed after anaerobic fermentation. According to analysis, the extinct rate of fecal coli-form bacillus reached 98% and of the eggs of hookworm reached 99%. The intestine diseases of farmers obviously decrease and fish diseases as well. It is reported that anaerobic fermentation can remove 80% of BOD.

4. Providing quality manure
Slurry is a quick acting fertilizer containing several soluble nutrients. Dregs contains organic matter 30~50%, rich in N, P and K and humus as well. Dreg from digesters is rich in humus, which serves as a soil conditioner. The contents of organic matter, nitrogen, phosphorus in soil increased by 0.17%, 0.04%, and 0.014%, respectively after 4 years of dreg application. The soil unit weight decreased somewhat 0.03g/cubic centimeters. The soil porosity increased 0.93%. The living soil layer was thickened from 34 cm to 42 cm (Sichuan Provincial Academy of Agriculture’s investigation). According to the comparative study on bio digester with compost fermentation of manure of Shanghai Academy of Agriculture Science, the nitrogen loss in aerobic fermentation is about 70% and the nitrogen loss 25%-30% in facultative fermentation. The recovery rate of nitrogen reached more than 95% in anaerobic fermentation nevertheless. 

5. Supplying high effective and clean bio energy
Agricultural wastes such as animal manure, straw and stalk can produce methane through anaerobic fermentation (Table 11). A family with 5 persons in the Southern China raises 50 chicken, 30% manure of which can be used as pig feed. Raising 5-6 pigs can produce 1 m3 bio-gas. It can satisfy 90% of energy they need. Slurry can feed pig and dregs for fish (Zhou & Deng, 1989). If we add solar energy, wind energy and small hydropower, the annual saving of coal reached 56 million tons. Thus, the emissions of CO2 reduced by about 50 million tons each year.

Comprehensive utilization of bio gas slurry and dregs: 

1. Dregs from digesters can be used to cultivate macro fungus and then, spent substrate to culture fish. The nutrients of spent mushroom substrate and macro-fungus without commercial value are much higher than the original ones with a flavor of Lentinus spp. The fungus protein could be used to culture fish and it will reduce pollution to environment and reuse the solid wastes. This will not only increase the source of fish feeds, but also increase the income of farmers. 
In 1994 Wuxi Municipal Fisheries Research Institute and Wuxi Horticulture Research Institute conducted an experiment together on the optimized application of spent mushroom substrate to culture fish. Horticulture Research Institute was responsible for optimizing mushroom substrate for fish culture and Fisheries Research Institute was responsible for fish farming. The results showed that if mushroom was cultivated under scattered illumination, drip irrigation spraying, strict temperature, humidity and spraying control with a moisture about 65%, the substrate would have no unwanted fungus. The substrate becomes white after drying and elastic and easily digestible by fish. The protein content is much better than other mushroom substrate (Table 12). According to measurement, the protein content was 39.8% and calcium 0.17%, phosphorus 0.77%, ash 1.6%, and moisture 9.03%. They used this spent mushroom substrate as main gradient of pellet feeds to feed bream or food feeders. In trial period of 170 days, the fish production was 98.66 kg/mu higher than the control with an increasing rate of 16.44%. The net fish production increased by 34.54 kg with an increasing rate of 6.31%. The unit commercial fish increased 153.98 kg with an increase of 60.48%. The FCR was 0.1 lower than the other one. It saved 71.63 kg of feed and increased income 55.66 yuan per unit area. The production value increased 933.08 yuan (Dong et al 1995). Thus, we could show you a food web, a set of clustered industries in zero emissions concept (Fig. 1).
 
 

2. Slurry and dreg can be used in fields as base manure, top dressing and leaf manure
It is reported that slurry serves as leaf manure with a quick effect and high utilization rate. Leave of fruit trees can absorb 80% of sprinkling. It can strengthen the capability of photosynthesis when slurry is sprinkled during the growth period of leave. It can raise the bearing rate of fruits when slurry is sprinkled during the flowering period and it can increase the yield when sprinkled during bearing fruits.

3. Slurry can be used to control aphid and red spider. The reduction rate can be more than 50% within 48 hours. If mixed with some pesticides, the effectiveness will be better. For example slurry mixed with Rogor with a ratio of 2500:1 to control aphid in wheat field two sprinkling in 5 days the average killing rate reached 99.25%. Slurry can substitute more than 60% of chemicals. 

4. Slurry was used to submerge seeds for 1,333,300 ha in China. The yield of maize increased by 12.26%/unit area and the yield of peanut increased by 13.91%/unit area. 

If the bio digester link is added in ecological agriculture, the energy and nutrient flow can be further improved in ecological system. Agricultural by-products and wastes, which are not fully utilized can be further dissolved, digested and converted to bio-gas with high heat, feeds and manure with high quality and these organic matters can enter into another process of agricultural ecological system. In digester fishpond interaction, slurry is used to culture either grow-out or fingerlings and it can get better economic return, ecological and social benefits as well.

From the environmental impact of semi-intensive and intensive systems, the intensive system is 26-44 times more polluting than the semi intensive system in terms of N, and 12-15 times in terms of P, expressed as a percentage of the nutrients required to produce 1 kg of fish (Edwards, 1993). If a bio digester is added in an integrated fish farming system, it will further reduce the pollution of water. However, the deteriorated water from fishponds in semi-intensive system when drained at harvest still pollutes aquatic environment. We have to seek for good method to purify the effluent from fishponds. The potential purifying method is surface aquaponics.
 
 

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Dong Jiangping, Wang Ninzhu, Zhang Xian, 1995. Fungus protein---spent mushroom substrate used to culture fish with better economic return, Scientific Fish Farming 1995 (12) p23. 

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