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An Experimental Study on Using Biogas Slurry to Improve the Water Quality of Aqua-Culture Systems in Acid Sulfate Soil Areas Nhat Long Duong (1), Hoang Thanh Nguyen (1), Vo Chau Ngan Nguyen (1),(*) (1) Can Tho University, Can Tho, Vietnam * Correspondence: nvcngan@ctu.edu.vn Abstract: This study aimed to testing a new method to improve fishpond water quality in the acid sulfate soil areas by using biogas slurry (BS) Two experiments were implemented onsite at the fishpond to test for quantity of biogas slurry applied and fish growing status In the 1st experiment, treatment of 75%BS had water pH increased from 3.3 to 6.5 after 1.5 months; DO values and Chlorophyll-a concentrations were reasonably high (5.2 ppm and 143.05 μg/L) In the 2nd experiment, the daily weight gain of Snake-skin gouramy (0.002 g/day) in treatment were significantly lower than that in treatment (0.051 g/day) and treatment (0.049 g/day); however, the survival rate and the fish yield of fingerling nursing in treatment (22.19% and 270 kg/1,000 m2) were similar to those in treatment (22.44% and 264 kg/1,000 m2) For Climbing perch, the daily weight gain of the fingerlings in treatment I (0.004 g/day) was significantly lower than those in treatment II (0.045 g/day) and in treatment III (0.048 g/day) In conclusion, treatment 75%BS (153 m3/1,000 m2) is considered the best rate to improve water pH and produce good yield of fish for farmer's income Keywords: Acid sulfate soil area; biogas slurry; fish nursing; water pH treament Introduction Within the field of agriculture, many of national and international scientists have continually researched and implemented the aspects of design, construction and soil improvement in reality, especially on the acid sulfate soil areas The research results are of different levels and time depending on the variety of soil compositions Accordingly, there have been previous studies in Vietnam such as the use and management of acid sulfate soil in aquaculture (Singh, 1985), the construction of fishery - forestry - agriculture model on the acid sulfate soil due to the ecological function of cajuput forest (Duong, 2001), etc Such findings have made a great contribution to the enlargement of culture areas, the productivity and profit increase for manufacturers on the Mekong Delta acid sulfate soil It is obvious that to decrease the amount of acid sulfate, farmers have always used the conventional methods like powered lime or the mixture of lime and organic fertilizer resulting in a considerable increase of water pH The use of 15 kg/100 m2 lime increased water pH from 4.1 to 7.3 (Edwards et al., 1986) Besides, most of the organic fertilizer, especially animal manure is also one of effective materials for pond treatment The organic fertilizer, a nutritious source, consumed by plankton becomes the natural food source available in water This food source helps fish grow up quickly to create high productivity Moreover, the decrease of disease transmission has also contributed to the development of fish yield However, due to the growth of intensive agriculture, the environment gets more and more polluted because of untreated waste in an appropriate manner There are varied methods of treating and reusing the waste like composting so as to directly and indirectly feed aquatic species In particular, the use of biogas slurry is the most efficient treatment in the aquaculture development Biogas slurry is a product of the decomposition of organic matter It is also considered an alternative energy source used in each household for cooking, lighting and heating as well Biogas slurry includes solid and liquid waste involving a high degree of organic substances and inorganic nutritious salinity such as nitrogen and phosphorus Biogas slurry was used in aquaculture and algae ponds and rice paddy The organic substance usage can treat many kinds of waste, reuse nutrients, create valuable products and reduce pollution (Le and Nguyen, 2015) Nevertheless, there is little research on the use of nutritious source included in biogas slurry for pond treatment on the acid sulfate soil areas Therefore, the study aims to find out effective methods of improving water quality in the acid sulfate soil areas by using biogas slurry In addition, the experimental study on nursing some appropriate species is expected to enhance the effectiveness of soil treatment for production and the development of aquaculture models in each household, increasing the income for local communities Methodology 2.1 Design of the experiment The experiments on pond treatment and fish nursing were conducted on Hoa An Center - Can Tho University located at the most acid sulfate soil area of the Mekong delta The research was carried out in dry season Each pond covers an area of m2 (2.0 m x 2.0 m) and 1.2 m depth Experiment The treatment of water pH infected ponds by biogas slurry The experiment consists of treatments with three repeated and randomly arranged Treatment 1: 100% organic fertilizer (pig manure) use (30 kg/100 m2) Treatment 2: biogas slurry use with 75% TN in organic fertilizer Treatment 3: biogas slurry use with 100% TN in organic fertilizer Treatment 4: biogas slurry use with 125% TN in organic fertilizer Treatment 5: biogas slurry use 150% TN in organic fertilizer Treatment 6: pond treatment without using biogas slurry All the above treatments were applied using CaO powered lime (150 g/m2) The time for all treatments showed the stability of water pH and Chlorophyll-a concerntrations in the ponds Experiment Nursing the fingerlings of Snakeskin gouramy and Climbing perch in treated ponds After experiment 1, the treated fish pond was applied in nursing the fingerlings of Snakeskin gouramy and Climbing perch Experiment on each species includes treatments with three repeated in 60 days Experiment on nursing the fingerlings of Snakeskin gouramy Treatment 1: using biogas slurry without food supply Treatment 2: using biogas slurry and 50% of food supply Treatment 3: using biogas slurry and 100% of food supply Experiment on nursing the fingerlings of Climbing perch Treatment 1: using biogas slurry without food supply Treatment 2: using biogas slurry and 50% of food supply Treatment 3: using biogas slurry and 100% of food supply Three day-old fingerlings of Snakeskin gouramy and Climbing perch were nursed in the practical field of the College of Aquaculture and Fisheries The nursing density is 400 fish/m2 The food supply followed the procedure of fingerlings nursing of Department of Freshwater Aquaculture - College of Aquaculture and Fisheries, Can Tho University 2.2 Sample Collection and Analysis The cycle of collecting sample Water temperature, pH, DO, TN, TP and Chlorophill-a in experiments and were collected and analysed every days During the process of nursing, the fingerlings were examined every 15 days on their growth The survival rate and productivity were also collected Collecting and analysing organic fertilizer (pig manure) The pig manure of 30 days was put into PVC bags and kept at 4oC for analysis The analysis criteria include TN and TP concentrations Total nitrogen (TN) was determined by Kjeldahl method Total phosphorus (TP) was determined by Kjeldahl and by Ascorbic acid photometric method Collecting and analysing the water sample of biogas slurry The water temperature, pH and DO were on-site measured by HANNA meter TN and TP were analysed at the laboratory of College of Aquaculture and Fisheries The sample was kept at 4oC Total nitrogen (TN) was determined by Kjeldahl method Total phosphorus (TP) was determined by Kjeldahl and by Ascorbic acid photometric method Chlophill-a concentrations was determined by acetone extraction method Collecting and analysing the fingerlings sample The sample was collected by rackets and analysed every 15 days The average of 30 fish/pond were kept in plastic buckets The sample was weighted by the electronic scale (± 0.001) and then put into the ponds The applied formulas are as follows: The daily weight gain DWG (g/day) = (a) Survival rate (%) = (b) Fish yield (kg/m2) = weight (kg) / pond area (m2) (c) Total income = fish yield (kg/area) x price (VND/kg) (d) 2.3 Data analysis All the data was collected and statistic analysed by SPSS 13.0 software Results 3.1 Water quality of biogas slurry and organic fertilizer (pig manure) The experiment results are the basis for the evaluation and measurement of nutritious content As can be seen from the tables, the amount of TN and TP in biogas slurry is so high and unstable Samples from three analysing turns at different time of a day showed that TN content in biogas slurry fluctuates between 69.12 and 87.32 mg/L whereas the fluctuation in TP is between 146.60 and 194.73 mg/L Concerning on organic fertilizer, TN through the analysis accounts for 6.500 - 7.000 mg/g and at 6.787 mg/g on average while TP fluctuates between 0.602 to 1.306 mg/g, averagely 0.958 mg/g Therefore, this is considered valuable material resulting in lower production cost as well as higher income for farmers in the region of the Mekong delta Table TN and TP concentrations in biogas slurry for acid sulfate treatment Testing time TN (mg/L) TP (mg/L) 7h00 11h00 15h00 Average 7h00 11h00 15h00 Average 67.27 84.67 55.42 69.12 119.7 194.9 125.2 146.60 63.52 90.47 100.17 84.72 238 176.4 169.8 194.73 97.67 81.37 82.92 87.32 169.1 161.2 138.1 156.13 Table TN and TP concentrations in organic fertilizer (pig manure) for acid sulfate treatments Sample Pig manure TN TP (mg/g) (g/kg) (mg/g) (g/kg) 6.860 6.860 0.966 0.966 Pig manure 6.500 6.500 1.306 1.306 Pig manure 7.000 7.000 0.602 0.602 Average 6.787 6.787 0.958 0.958 3.2 Pond treatment with biogas slurry 3.2.1 Water temperature The following Figure illustrates no wild fluctuation of water temperature among the treatments using biogas slurry in the acid sulfate infected ponds (p>0.05) The temperature of water fluctuates from 27.3 to 31.20C, averagely 29.30C The figures demonstrate stable temperature of treated ponds In comparison with previous research in the Mekong delta, there were not disadvantages for the living and developmental process of aquatic system (Pekar et al., 1998) Temperature (0C) 32.0 31.0 30.0 29.0 NT I 28.0 NT II NT III 27.0 NT IV NT V 26.0 NT VI 25.0 24.0 Time 10 11 12 13 14 15 16 17 18 Figure Water temperature among the treatments 3.2.2 Water pH The research on water in acid sulfate soil in Long Xuyen, An Giang province (Duong et al., 1999) points out that water pH fluctuates between 4.57 and 6.35 in natural environment After years of pond treatment by using poultry manure through integrated chicken-fish farms, farmers have gradually applied the treated ponds in developing forms of aquaculture According to Boyd (1998), the fluctuation of 7.5 - 8.5 is the value of water pH suitable for the growth of plankton and fish NT I Water pH NT II 10.00 NT III NT IV 9.00 NT V 8.00 NT VI 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 Time 10 11 12 13 14 15 16 17 18 Figure Water pH among the treatments The research on biogas slurry usage in acid sulfate infected ponds shows that water pH in experiment ponds in different treatments causes a high fluctuation Before treatment, water pH is around 2.64 - 3.25 However, data from the experiments indicate a steady increase in water pH from 3.04 to 7.61 (p0.05) It can be explained that rain helps clear out acid sulfate in the infected ponds Besides, the high level of waste from biogas slurry including decomposed organic matter causes an upward trend in TN, TP and Chlorophyll-a concentrations in ponds This condition creates a buffer system increasing the absorption of ion H+ but limiting the acid sulfate formation at the water body of aquatic areas To sum up, water pH recorded from the treatments in fluctuation of 6.05 - 7.61 supports the previous studies of Boyd (1990), and Pekar et al (1998) on the development of aquaculture in naturally aquatic environment as well as in acid sulfate infected areas at the Mekong delta 3.2.3 Dissolved oxygen The analysis of oxygen dissolved in water during the treatment of acid sulfate infected ponds is described in the following Figure The survey on the oxygen dissolved in nursing ponds shows that there is a dramatical fluctuation (p
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