Abstract:
Efficiency of nitrification biofilter for nitrogen waste control in outdoor shrimp tank was studied. Biofilter Bio-CordTM was acclimated in 450 L tank (15psu) with 16 g shrimp feed as nitrogen source. Completion of nitrification was occured after 22 days. This acclimated biofilter was then used in the outdoor lining shrimp tanks (Ø 2 m, 3.14 m2 surface area) containing 1,884 L of 15 psu seawater. Pacific white shrimp, Litopenaeus vannamei, postlarvae (PL30, 150 ind./m2) were released into control and treatment tanks. With the first trial, treatment tank was incorporated with 7 m of Bio-CordTM biofilter. It was found that 7 m of biofilter was not sufficient for water quality control. As biofilter was operated without cleaning, nitrogen removal efficiency hence decreased from 127 mg-N/L on the first day to 67 mg-N/L after two months. Thus in the second trial, shrimp ponds were covered with transparent plastic sheet for rain protection and 24 m of biofilter was used with regular cleaning every week. It was found that biofilter in treatment tanks efficiently controlled water quality throughout 84 days experimental period. Average ammonia and nitrite in treatment ponds were 0.20+-0.19 (range 0.00-0.69) and 0.18+-0.27 (0.00-1.25) mg-N/L, respectively. In control pond, ammonia and nitrite were 0.27+-00.28 (0.07-1.03) and 9.52+-15.64 (0.00-49.9) mg-N/L, respectively. Cleaning made the biofilter actively remove nitrogen waste, with average removal rate of 153 mg-N/m2/day. In control tanks on the 71st day, survival of shrimp was only 36% because of high accumulation of nitrite. In contrast, shrimp in treatment tanks survived through the 84th day with 93% survival rate. In addition, the feasibility studies of nitrate removal by sediment tank system were conducted. The experiment consisted of control (without carbon addition) and treatments (methanol or glucose addition) tanks operating with various C:N ratio at 0.06:1, 0.3:1, 1.6:1 and 3.3:1. The results showed that methanol or glucose could significantly increase denitrification rate of the sediment. Average denitrification rate of control tanks was 386 mg-N/m2/day while denitrification rates in treatment tanks were increased from 516 to 2849 mg-N/m2/day. This concept was further applied for nitrate treatment in an indoor shrimp pond. After 60 days of shrimp culture, the water with high nitrate concentration was transferred into the sediment tanks for 5 batchs and methanol was used as the denitrification accelerator. Nitrate level was 72 mg-N/L. at the end of experiment, whereas nitrate in control tank was 159 mg-N/L.