Tuning your calcium reactor

[Achilles Tang]

Tuning a Calcium Carbonate Reactor

Tuning of a CaCO3 reactor can be a little fiddly. There is a certain amount of trial and error involved, but I will outline some tips that make conclusions easier.

You have two main things to adjust, the CO2 bubble count and the flow rate through the reactor. Increasing the CO2 bubble rate decreases the pH in the reaction chamber which results in more dissolution of the calcium carbonate substrate. Increasing the flow rate through the chamber increases the pH in the chamber. Increasing both the CO2 bubble count and the flow rate should maintain the chamber pH, but will result in more calcium and bicarbonate being added to the water.

Measuring the pH in the chamber is not easy and measuring the pH of the effluent only gives and approximate idea of chamber pH. Effluent pH does not give any indication of the rate at which calcium or alkalinity are being added. As the purpose of the reactor is to add calcium and bicarbonate to the system, measurement of the injection rates of calcium and alkalinity (largely bicarbonate) will give the best indication of how well the reactor is functioning and provides accurate feedback to the adjustment of CO2 bubble rate and reactor flow rate. The calcium and alkalinity injection rates should match the calcium and alkalinity depletion rates for the system.

Tuning the reactor

In order to tune the reactor, it is helpful to know the calcium and alkalinity depletion rates for the system. The reactor is then tuned so that the injection rates meet these.

The best way to tune the reactor is to start with some arbitrary settings for CO2 bubble rate and flow rate through the reactor. 15 bubbles per minute and 20 millilitres per minute is a good starting point. Run the reactor for 24 hours so that things will stabilise and then determine the calcium and alkalinity injection rates and measure the effluent pH.

If the injection rates are lower than the estimated depletion rates, you are going to need to increase the injection rates. If the pH of the effluent is low (below around pH6.5 for single chamber reactors and below pH6.8 for twin chamber reactors), start by increasing the flow rate through the reactor. If the pH is high (above around pH6.5 for single chamber reactors and above around pH6.8 for twin chamber reactors) start by increasing the bubble count. Only adjust one of either bubble count or flow rate at any one time and wait 24 hours before checking the injection rates and effluent pH. Repeat the above until the injection rate meets the depletion rate. Keep an eye on the tank calcium and alkalinity levels to ensure they stay stable.

Note that if you start with low calcium and alkalinity, it might be better to temporarily boost these to around 420 mg/L and 2.8 meq/L using other methods (e.g. separate dosing of calcium chloride and sodium bicarbonate). If you don't you will need to tune the reactor twice, once with a higher injection rate to get the levels to "normal" and then to set the injection rate to the depletion rate.

If the injection rates are higher than the depletion rates, bubble count and flow rate should be adjusted down using the reverse of the above steps.

Determining the calcium and alkalinity depletion rates of the system

This is quite easy but requires you stop your current methods of dosing calcium and alkalinity for at least 24 hours. This should not cause any major problems for the inhabitants of the tank.

Without dosing any calcium or buffer (i.e. no Kalkwasser, or two part additives), measure the calcium and alkalinity of the tank twice, exactly 24 hours apart. The best time to measure is in the morning just before the lights come on, but late in the evening should be fine, too. It is important to measure while there is little calcification happening as this may influence the readings. The difference in the two calcium readings will give you the approximate calcium depletion rate. And the difference in the alkalinity readings will give the approximate alkalinity depletion rate.

Calculating calcium and alkalinity injection rates

The difference between the tank concentrations of calcium and alkalinity and the effluent concentrations indicates how much the reactor is adding. Coupling this information with the flow rate and the tank volume gives the rate that each is being added to the system.

Accurately measure calcium and alkalinity for the tank and the effluent at the same time. Also, accurately measure the flow rate through the reactor. Subtract the tank calcium reading from the effluent calcium reading and this will tell you how much the calcium concentration is being increased as the water passes through the reactor. Next convert the flow rate to a daily rate. i.e. how much water is flowing through the reactor in day. Multiply the concentration difference by the daily flow rate to determine the amount of calcium added per day. Finally, divide the amount of calcium added per day by the tank volume to determine change in calcium concentration per day.

Repeat the same calculations for

For example, assume the following readings for a 163L tank:

Effluent:

Calcium 515 mg/L

Alkalinity 8.9 meq/L

Tank

Calcium 395 mg/L

Alkalinity 3.9 meq/L

Flow rate 1.2 L/hr

For calcium:

Subtracting 395 mg/L from 515 mg/L gives us 120 mg/L. This means as it is currently tuned, the reactor is adding 120 mg/L to any water that passes through it.

28.8 litres (1.2 L/hr x 24 hr/day) of water flows through the reactor per day.

Multiply the concentration increase (120 mg/L) by the amount of water that flows through the reactor in a day (28.8 L) and you will know how much calcium is being added to the tank in one day: 3456 mg.

Divide this by the tank volume (163L) to determine the concentration increase per day with the reactor: 21.2 mg/L.

This calculation shows that the calcium concentration will increase by 21.2 mg/L per day.

For alkalinity:

8.9 - 3.9 = 5 meq/L

28.8 L/day

5 * 28.8 = 144 meq

144 / 163 = 0.88 meq/L

This calculation shows that the alkalinity will increase by 0.88 meq/L per day.

Measuring the effluent calcium and alkalinity

Measuring the calcium and alkalinity in the effluent can be troublesome as the levels of both are expected to be much higher than for normal seawater. Some kits may not be able measure calcium or alkalinity that high. It is worth noting, however, that as both calcium and alkalinity are normally measured using a titration, there should be no reason that extra titrant can't be added.

For both the Salifert and Seachem calcium kits, the titration uses a syringe to dispense the titrant. When the entire contents of the syringe is added, the calcium concentration is 500 mg/L. If the syringe is refilled and the titration continued until completion, the reading from the syringe can be added to 500 mg/L to determine the calcium concentration. For example, if the second reading indicates the calcium concentration is 250 mg/L, the total concentration will be 750 mg/L.

Similarly, the Salifert alkalinity kit uses a syringe to dispense the titrant and this can be refilled to continue the titration.

Another method to extend the titration is to use half quantities of sample and reagents. The reading of the titration is then doubled to get the true measurement.

Taken from here.

[roidan]

really really very cheem to use leh....

i think i stay to using calcium additives or reefez calcium...at least how much powder will get how much PPM increase in how much volume of water...this is easier for me..hehe

but more expensive in the long run hor

[ah^siao]

chimmilogy sia.. dun understand..

i add bits of calcium chloride everyweek.. so far so good..