دانلود رایگان مقاله توسعه و ارزیابی عملکرد حذف لجن برای آبزی پروری فشرده

Abstract

Fishery is an emerging sector in Bangladesh producing about 3.50 million tons of fish annually of which aquaculture accounts for 2.00 million tons. In intensive aquaculture, pond water gets polluted due to high stocking density and large amount of supplemental feeding results in huge accumulation of unused feed and faeces in the pond bottom. The accumulated bottom sludge depletes dissolved oxygen (DO) and releases harmful gases due to excessive use of chemicals and lack of water exchange facilities. These have negative impact on fish production and economic performance, through disease outbreak, mortality, poor feed conversion ratio (FCR), specific growth rate (SGR) and bad odor in fish muscles. This study was undertaken to develop a sludge remover for cleaning sludge from the intensive fish farming pond and to find the impact of sludge removal on water quality and growth of fish. A sludge remover was developed in Bangladesh Agricultural Research Institute (BARI) which is capable of removing sludge up to 40–50 mm from the pond bottom. The main components of sludge remover are a float, axial flow pump (102 mm diameter), 7.50 kW diesel engine, sludge sucker, crane, propeller, rudder, etc. The discharge of the sludge pump was 14.11 L/s. The effective field capacity and field efficiency were 0.078 ha/h and 77.23% respectively at the forward speed of 12.5 ± 1.55 m/min. The water and sludge ratio was 3.6:1.0 (weight basis). A field trial was carried out to determine the effect of sludge removal on water quality and growth of fishes in intensive cultured ponds. Sludge was removed at three months interval from three pangus-tilapia-carp polyculture ponds. Three ponds of same size and fish stocking were kept as control. Water quality parameters like DO, pH, transparency and, unionized ammonia significantly (p ≤ 0.05) improved in the sludge removed ponds compare to control ponds. Sludge removed ponds demonstrated better FCR (1.64) and SGR (0.80% per day) compare to FCR (1.90) and SGR (0.71% per day) of control ponds. Fish survival, net yield, net return and BCR (5.40) of sludge removed ponds were higher than those of control ponds. This sludge remover may be recommended for cleaning sludge from intensive cultured fish ponds.

3. Results and discussion

3.1. Performance of sludge remover The first prototype of sludge remover was tested in the pond of FMPE Division, BARI, Gazipur during March–April 2014. The photographic view of operation of sludge remover is shown in Fig. 4. Area of FMPE pond was about 0.26 ha (52 m × 50 m) and depth of water was about 2.5 m. The sludge remover was operated according to the principle of operation. It remover was operated in five blocks of the same pond. The blocks were marked with coloured ropes and poles so that they can be clearly visible. Each block size was 50 m × 100 m. The sludge remover was operated five times in the pond (One time each of five blocks) to get the average performance value. The specification and performance of sludge remover is given in Table 1. The sludge pump was operated at various speeds (1200–2000 rpm) and 1700–1800 rpm was found for getting optimum discharge. The discharges of the 102 mm axial pump for lifting fresh water at the pump speed of 1900–2000 rpm varied from 12.4 to 27.5 L/s for the water head of 5.0 m and 1.0 m, respectively (Hossain et al., 2015). The peak pump efficiency and brake power were found for the discharge of 21 L/s (Fig. 5). Average forward speed of the sludge remover was 12.50 ± 1.55 m/min. The discharge of the sludge pump was measured by volumetric method using the water tank of 3500 L. The average discharge was found to be 14.11 ± 2.16 L/s. The effective field capacity of the sludge remover was 0.078 ha/h. The average field efficiency of the sludge remover was found to be 77.23%. Higher the field efficiency, the more efficient the machine is. The machine similar to sludge remover (tillage machinery) with field efficiency below 75% is said 0 10 20 30 40 50 60 0 1 2 3 4 5 6 0 5 10 15 20 25 30 35 BHP/Effciency (%) Dynamic head (m) Discharge (L/s) H-Q curve Power curve Fig. 5. Characteristic curves of 102 mm axial flow pump. Fig. 6. Pictorial view of sludge deposition. inefficient (Kepner et al., 1987). So, the sludge remover is found reasonably efficient. The sludge remover was operated under the water and sludge was cleaned with eye estimation. So, overlapping could not be avoided by the operator, which might reduced the field efficiency. About 40–50 mm thick sludge (with some mud) can be removed from the fish pond in each operation by the sludge remover. But sludge accumulation ranges 50–70 mm per year in a typical pangasius pond in Bangladesh (Haque et al., 2013). After siltation sludge is deposited in the sludge pit but some smaller sludge particles backflows to the pond with water. These smaller sludge particles did not affect significantly in contamination of water. The discharge of sludge and water mixture was collected in three water containers each of five liter. The average water sludge ratio was found 3.6:1.0 (weight basis). Pictorial view of sludge deposition is shown in Fig. 6. Some minor problems were observed during operation of the sludge remover such as straight moving of the float, turning problem, no backward movement facility, etc. Sometimes, water was discharged instead of sludge. This was due to the uneven bottom bed of the pond. Tree branches, stone or brick pieces and snails on the pond bottom sometimes created barrier in smooth operation of the sludge remover. Overall results of on station trials of the newly developed sludge remover were found satisfactory.