Nitrate Accumulation and Denitrification in RAS

Why nitrate builds up in low-exchange recirculating systems, what levels sources link to effects on fish, how water exchange limits it and how denitrification reactors and carbon sources work.

Why nitrate builds up

Nitrification in the biofilter converts the ammonia excreted by fish into nitrite and then nitrate, which is the end product of the process. A 2018 study in ACS Omega summarises nitrogen balances for aquaculture systems with a nitrifying biofilter: the stock assimilates only 25–40 percent of the nitrogen supplied as feed, 30–40 percent stays as organic nitrogen in faeces and uneaten feed, and about 30 percent is excreted as ammonium and converted to nitrate. The same paper notes that at low water exchange, nitrate in a recirculating aquaculture system (RAS) can accumulate to as much as 500 mg/L of nitrate-nitrogen (NO3-N).

Effects on fish reported by sources

Nitrate is the least toxic of the nitrogen compounds in a RAS. Wikipedia, citing a RAS textbook by Timmons and Vinci, gives 96-hour LC50 values (the concentration that kills half the test fish in 96 hours) above 1,000 mg/L in fresh water. Chronic effects appear at much lower levels:

The FAO guide to recirculation aquaculture states that nitrate above 100 mg/L appears to reduce growth and feed conversion, and its water-quality table lists 100–200 mg/L NO3- as normal and more than 300 mg/L as unfavourable.
SRAC Publication 452 describes nitrate as relatively nontoxic except at very high concentrations, over 300 ppm (mg/L).
The ACS Omega paper cites concentrations above 125 mg/L NO3-N as potentially harmful to the growth of some species such as turbot and prawns, especially in early development, and cites Australian and New Zealand water-quality guidelines that suggest keeping nitrate below 50 mg/L for freshwater and 100 mg/L for saltwater species.
In a 3-month controlled study of juvenile rainbow trout (Oncorhynchus mykiss) in six replicated RAS, Davidson and co-authors compared a mean of 30 mg/L NO3-N with 91 mg/L NO3-N. Growth did not differ significantly, but survival was lower at the high level (bordering statistical significance), final biomass was significantly lower, and side-swimming fish were significantly more common. The authors concluded that 80–100 mg/L NO3-N was related to chronic health and welfare impacts under the conditions studied.

An earlier study by the same group found faster swimming and more side-swimming in trout kept in low-exchange RAS (6.7 days hydraulic retention time), and more deformities and mortality at near-zero exchange; nitrate-nitrogen and dissolved potassium were among the parameters considered potentially associated, but no single cause was proven.

Water exchange and its trade-off

Dilution is the simplest control. SRAC 452 notes that most systems are designed to replace 5–10 percent of system volume per day, which prevents build-up of nitrate and soluble organic matter, and recommends a complete water exchange after each production cycle. It also observes that some denitrification seems to occur inside many systems and helps keep nitrate below toxic levels. The FAO guide gives a threshold: under normal conditions, more than 300 L of new water per kg of feed dilutes nitrate sufficiently; below that, a denitrification step is worth considering.

The trade-off is that the purpose of recirculation is to save water. Other treatment steps do not solve the problem: in replicated trout RAS, ozonation improved several water-quality variables but did not prevent nitrate accumulation (see the separate ozone and ORP guide). In a four-month trial, RAS fitted with activated-sludge membrane bioreactors replaced 1.2 percent of system volume per day versus 7.8 percent in control systems, while trout growth, feed conversion and survival were not affected; nitrate-nitrogen was, however, significantly higher in the low-exchange systems.

How denitrification works

Denitrification is the microbial reduction of nitrate through nitrite, nitric oxide and nitrous oxide to nitrogen gas, which escapes to the atmosphere. According to Wikipedia, it is carried out mainly by facultative anaerobic heterotrophic bacteria, including pseudomonads, which use nitrate in place of oxygen when they oxidise an electron donor such as organic matter; autotrophic denitrifiers such as Thiobacillus denitrificans are also known. The FAO guide identifies Pseudomonas as the predominant denitrifying genus in RAS and describes the process as anaerobic.

Denitrification reactors and carbon sources

The FAO guide describes a denitrification chamber filled with biofilter media and designed for a residence time of 2–4 hours. The process needs an organic carbon source; with methanol, about 2.5 kg is needed per kg of nitrate-nitrogen removed. Flow is controlled so that the outlet still holds about 1 mg/L of oxygen. Sludge production is high and the unit is typically backwashed weekly.

The ACS Omega paper lists the main options. Heterotrophic reactors receive an added carbon source such as methanol to reach a sufficient carbon-to-nitrogen ratio. Methanol can be avoided by using endogenous carbon, which may require pre-digesting organic material such as sludge from the farm itself. Autotrophic reactors instead use carbon dioxide as the carbon source and hydrogen gas as the energy source, supplied from a cylinder or generated at the cathode of an electrolytic cell (bioelectrochemical denitrification). The trial with membrane bioreactors likewise identified occasional carbon supplementation as a way to improve denitrification and alkalinity recovery.

Denitrification also returns alkalinity. D. E. Brune, writing in World Aquaculture magazine, reports that systems combining nitrification and denitrification need no alkalinity addition (see the RAS alkalinity and pH guide).

Reading nitrate trends

The FAO troubleshooting table attributes a falling nitrate level to anaerobic activity and lists hydrogen sulphide production under the same cause; for both it recommends increasing aeration and cleaning the biofilter. The FAO routine checklist includes tests of ammonia, nitrite, nitrate and pH.