Development of a Bacterial Denitrification Filter for Sand-Based Recirculating Saltwater, Penaeus japonicus Shrimp Culture Systems

Abstract

This study investigated the ability of a simple bacterial denitrification filter (BDF) to reduce nitrate levels in experimental low-intensity recirculating broodstock tanks. These broodstock tanks are equipped with a sand-substrate system which provided the dual purpose of habitat and nitrification filter. The BDF was constructed of 50-mm PVC pipe, filled with 50% gravel and 50% crushed pipi shell. The BDF was designed to be easily fitted to individual tanks (assisting biosecurity) and to operate with minimal management.

There were 3 treatments, each with 3 replicates: control tanks with no BDF; tanks with BDFs receiving 0.605 g C/day as glucose (Low C); and tanks with BDFs receiving 1.512 g C/day as glucose (High C). Kuruma shrimp, Penaeus japonicuswere maintained in all tanks at a density of 185 g/m² (avg. weight of 8.02±2.4 g), and no water was exchanged for the duration of the experiment. In the control tanks, nitrate increased at about 1 mg/L/day until 38 days when it peaked at 38.9 mg/L. Nitrate levels in control tanks decreased slightly (i.e., by up to 2.5 mg/L) from 38 days to 50 days when the experiment was concluded. In the Low C treatments, nitrate increased at a similar rate at first; however, it peaked at 24 days (18 mg/L) and then gradually reduced to 12 mg/L after 50 days. In the High C treatments, nitrate also peaked at 18 mg/L after 24 days, but then decreased to zero by 50 days.

Sand-substrate in the High C treatment tanks was anoxic by the end of the experiment (black beneath the surface, with redox potential < –100 mV). However, in the control and Low C treatments, where nitrate levels were above 10 mg/L, the substrate remained in good condition (no black sediment, and redox potential –10 to 50 mV).

These results demonstrate that a simply-designed and operated BDF, together with an appropriate nitrification system, can effectively maintain nitrate below 20 mg/L, removing the need for water exchange and improving biosecurity. The BDF operation should be limited to allow nitrate to be maintained at about 10 mg/L, to help maintain oxidized conditions in the sediment.

Key Words:

• Bacterial denitrification filter (BDF)
• recirculation system
• shrimp broodstock culture
• biosecurity
• Penaeus japonicus