Alkalinity and pH Management in RAS

How nitrification and CO2 push pH down in recirculating systems, how much alkalinity is lost, how sodium bicarbonate and other bases replace it, and which target ranges sources give.

Why pH falls in a recirculating system

SRAC Publication 452 names two drivers: nitrifying bacteria produce acid and consume alkalinity, and fish and microorganisms release carbon dioxide (CO2), which reacts with water to form carbonic acid. The FAO guide to recirculation aquaculture lists the same pair, CO2 from fish and biofilter activity and the acid released by nitrification, as the two main factors that move pH. Without a deliberate base addition, pH in a RAS therefore drifts downward.

How much alkalinity nitrification consumes

Alkalinity is the acid-neutralising capacity of water, reported in mg/L as calcium carbonate (CaCO3). Writing for the Global Seafood Alliance, C. E. Boyd (Auburn University) explains that nitrification releases two hydrogen ions for every ammonium ion oxidised to nitrate, and that nitrifying 1 mg/L of ammonia nitrogen can lower alkalinity by 7.14 mg/L. He notes that in RAS and other intensive systems with little water exchange, a high nitrification rate can cause a measurable alkalinity loss within a single day.

In World Aquaculture magazine, D. E. Brune compares two rules of thumb. The theoretical figure of 7.14 g of alkalinity (as CaCO3) per g of ammonia nitrogen nitrified corresponds to 2.0 meq of alkalinity per meq of total ammonia nitrogen (TAN) oxidised. The field rule of about 0.25 kg of sodium bicarbonate (NaHCO3) per kg of feed corresponds to roughly 1.0 meq per meq. Brune attributes the gap to the ammonia excreted by the animals themselves: as NH3 hydrolyses to NH4+ it releases hydroxide, adding about one equivalent of alkalinity per equivalent of nitrogen. He also reports that systems combining nitrification with denitrification need no alkalinity addition at all.

Target ranges given by sources

Alkalinity: SRAC 452 advises keeping it at 50–100 mg/L as CaCO3 or higher, and hardness at a similar level.
Alkalinity: the FAO guide lists 1–5 mmol/L as normal and below 1 mmol/L as unfavourable.
pH for the biofilter: SRAC 452 gives an optimum of 7–8 for nitrifying bacteria and states that below pH 6 they are inhibited and stop removing nitrogen wastes.
pH set-point: the FAO guide recommends adjusting to pH 7.0–7.5, lists 6.5–7.5 as the normal range and treats values below 6.2 or above 8.0 as unfavourable.
Carbon dioxide: the FAO guide lists 10–15 mg/L as normal and more than 15 mg/L as unfavourable.

The set-point is a compromise. According to FAO, a pH above 7 keeps nitrification efficient, but a higher pH also shifts total ammonia toward toxic un-ionised ammonia (NH3), which the guide treats as harmful to fish above 0.02 mg/L. Raising pH therefore lowers the TAN concentration a system can tolerate.

Choosing and dosing a base

SRAC 452 states that the most commonly used buffers are sodium bicarbonate and calcium carbonate, while calcium hydroxide, calcium oxide and sodium hydroxide have also been used. Calcium carbonate may dissolve too slowly to neutralise a rapid accumulation of acid. The same fact sheet reports that adding sodium bicarbonate at 17–20 percent of the daily feed ration usually keeps pH and alkalinity in range; its example is about 2 lb of bicarbonate for a tank fed 10 lb of feed per day. As a mass ratio this equals 0.17–0.20 kg of sodium bicarbonate per kg of feed (unit conversion, not a separate measurement). Buffers that contain calcium also add hardness.

For correcting a measured shortfall, Boyd gives a stoichiometric factor: about 1.68 mg/L of sodium bicarbonate restores 1 mg/L of alkalinity. He adds that common agricultural liming materials dissolve too slowly to be practical for recirculating systems.

The FAO guide describes two automated approaches: a lime-mixing station that drips limewater into the loop, or a dosing pump driven by a pH controller. For the controller option it prefers sodium hydroxide (NaOH) because it is easy to handle and keeps the system simple to maintain.

The CO2–pH–alkalinity relationship

SRAC Publication 464 shows how CO2 lowers pH: CO2 reacts with water to form carbonic acid (H2CO3), which dissociates into H+ and bicarbonate. The fact sheet, written for ponds, gives a table of factors that are multiplied by total alkalinity to estimate free CO2 at a given pH and temperature. Its worked example of pH 7.2, 25 °C and 103 mg/L alkalinity gives about 12.8 mg/L of CO2, and it notes that CO2 is negligible above pH 8.4. Reading the same table at 25 °C and 100 mg/L alkalinity, pH 7.0 corresponds to about 20 mg/L CO2 and pH 7.4 to about 8 mg/L (derived from the SRAC 464 table). At constant alkalinity, a lower CO2 concentration therefore corresponds to a higher pH. SRAC 464 recommends a pH reading within 30 minutes of sampling and prefers direct CO2 measurement, because the table method has several sources of error.

SRAC Publication 453 adds that in a poorly designed RAS, respiration by fish and bacteria can push CO2 to 50–100 mg/L, which lowers pH and stops nitrifying bacteria working, so nitrite or ammonia rises. When blowers are moved indoors in winter and ventilation is reduced, the building air becomes CO2-enriched, the gas diffuses back into the water and the result is low pH with high ammonia and nitrite. Stripping equipment is covered in the separate RAS carbon dioxide degassing guide.

Monitoring pH drift

SRAC 452 recommends measuring pH daily and adjusting as needed; it reproduces a pH management diagram for 25 °C that pairs each combination of pH and alkalinity with an action: add sodium bicarbonate, increase aeration, both, or reduce the daily bicarbonate dose.
Nitrifying bacteria are sensitive to low alkalinity, extreme pH and high CO2; SRAC 452 stresses that biofilters do not tolerate rapid change.
The FAO troubleshooting table attributes rising alkalinity, falling nitrate and a hydrogen sulphide odour each to anaerobic activity; its remedy is more aeration and biofilter cleaning.
Background on how nitrification works is in the nitrification microbiology and biofilter sizing guides.