Ammonia Conversion Characteristics in a Closed Recirculating Aquaculture System

Nitrification and denitrification were commonly used in the recirculating aquaculture system (RAS) to decrease the ammonia and nitrate concentration. The variation characteristics of nitrogenous compounds and microbiology in a closed RAS were studied with gradually increasing ammonia-nitrogen (NH₄ ⁺-N) concentration. The NH₄ ⁺-N was completely converted into nitrite nitrogen (NO₂ ^--N) in 46 days, but the NO₂ ^--N raised slowly since the ammonia-oxidizing rate was faster than the nitrite-oxidizing rate. The NO₂ ^--N descended when the NH₄ ⁺-N concentration in the aquarium was stable for several days. Time for the NO₂ ^macr-N decline gradually became longer if we continued increasing the NH₄ ⁺-N concentration. The nitrogen loss (residual total nitrogen/total additive nitrogen) was about 20-40% during the experiment. The numbers of two kinds of bacteria showed that the increase of nitrification activity of the system was mainly attributed to the exit of numerous ammonium-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). Data indicated that nitrification and aerobic denitrification can be both perfectly achieved in aerobic and low COD (Chemical Oxygen Demand) conditions. Marine nitrifiers from this approach led to the development of bacterial amendments that can be used as startup cultures for new operations or damaged filters, and potentially enhance nitrification in established filters.