ระบบบำบัดไนเตรทสำหรับระบบหมุนเวียนน้ำทะเลแบบปิดเพื่อการเพาะเลี้ยงสัตว์น้ำ / สุวิมล ตันฑสุกิจวณิช = Nitrate treatment system in the closed recirculating sea water system for aquaculture / Suvimon Tontasukitvanich
Nitrate treatment system for seawater aquaculture pond using the tubular denitrification reactor filled with plastic bioballs was investigated. This study was divided into three experiments. The first experiment was to evaluate the oxygen consumption, denitrification rate and hydrogen sulfide production in 1 L reactor made of clear acrylic plastic. This reactor contained bioballs and artificial wastewater with 20-60 mgNO[subscript 3]-N/L. After adding methanol (a carbon source) into the reactor, dissolved oxygen (DO) was decreased at the rate of 1.25-2.30 microgramO[subscript 2]/bioball/h. Thereafter, when DO was lower than 1 mgO[subscript 2]/L, nitrate reduction was detected. The denitrification rate in this experiment ranged from 1.46 13.69 microgramNO[subscript 3]-N/bioball/h and the oxidation-reduction potential (ORP) during denitrification was between 0 to 100 mV. Finally, when ORP was below -300 mV hydrogen sulfide was produced in the reactor. The second experiment was performed using 25 m length tubular reactor filled with bioballs. In this reactor, bacteria in the fore part of the tube reduced oxygen down to the level that suitable for the denitrification process and bacteria in the rear part therefore reduced nitrate to nitrogen gas. The results showed that the 25 m tubular reactor had not enough efficiency to treat nitrate in this experimental condition. Even the DO in the rear part was below 1.5 mgO[subscript 2]/L, the tube length was not enough to provide the retention time needed to complete denitrification process. The final experiment using a 50 m length tubular reactor exhibited the excellent performance for nitrate treatment. When running the reactor at 2.3 h retention time with methanol added was the carbon source, incomplete denitrification was found and nitrite was accumulated in the reactor. However, after increase the retention time to 4.8 h, the reactor could treat up to 84-97% of nitrate. Denitrification process of this experiment was complete without nitrite or hydrogen sulfide accumulation and completely safe for shrimps in the tank.