The entire west coast of the North American continent cannot support very much coral growth. This is due to the currents from Alaska and Canada bringing down cold water.
I have written an Excel spreadsheet that simulates the concentration of dissolved organic material based on the effect of tank volume, circulation rates, protein skimmer parameters and metabolism. Below are some various results from it (which have been used in response to questions on Reefing the Australian Way).
Aim: to determine the influence of sump volume has when the bioload is kept constant.
Base System Parameters
Tank volume: 1,000 lt
Sump volume: 500 lt
Tank circulation rate: 15,000 lt/h
Return rate: 1,500 lt/h
Skimmer rate: 1,500 lt/h
Initial DOC: 100 mg/lt
Initial DOC mass: 150 g
DOC increase: 10 mg/l
DOC increase mass: 15 g/l
DOC increase period: 1.00 min
Skimmer exit DOC: 10.0 mg/lt
Skimmer efficiency: 30%
Metabolism rate: 0.05 /min
Results
Four systems are compared in this example. First is the base system as defined above, blue lines on the graphs.
Second is the base system with higher skimmer flowrate, red lines on the graphs.
Skimmer rate: 3,000 lt/h
Third is the base system with larger sump volume, pink lines on the graphs.
Sump volume: 2,000 lt
Initial DOC: 60 mg/lt
Initial DOC mass: 150 g
DOC increase: 6 mg/l
DOC increase mass: 15 g/l
And finally the forth is the base system with larger sump volume and higher skimmer rate, green lines on the graphs.
Sump volume: 2,000 lt
Skimmer rate: 3,000 lt/h
Initial DOC: 60 mg/lt
Initial DOC mass: 150 g
DOC increase: 6 mg/l
DOC increase mass: 15 g/l
The first graph below shows that not surprisingly the faster the skimmer rate the lower the final equilibrium concentration. But this has been demonstrated previously. Additionally, it shows that the larger the sump volume, the lower the concentration due to dilution. So this backs up the rule of thumb that the majority of people quote that you should get the largest sump possible.
Effect of sump volume and skimmer rate with constant bioload.
This second graph shows exactly the same data as the first. However, it is a plot of the ratio of skimmed DOC to metabolised DOC. If the ratio is equal to one, then at that particular time the same amount of DOC has been skimmed as has been metabolised. If it is less than one then more has been metabolised than skimmed, and visa versa if it is greater than one.
The line that is highest on this graph shows the system that is the best at removing DOC from the system before it is metabolised. Remember we have constant mass in these systems, not constant concentration. And it isn't surprising that it is the system with the smallest volume and fastest turn over through the skimmer.
As above, but with results presented as the ratio of skimmed to metabolised.
Take Home Message
Install as large a sump as possible on a system, since the extra water volume will dilute the pollutants (assuming constant bioload).
Aim: to determine the influence on frequency and percentage water changes on the concentration of an accumulating parameter.
This is a variation on the skimmer spreadsheet, with only the water changes and accumulation of a parameter being considered.
Base System Parameters
Initial parameter: 0 unit
Parameter increase: 1 unit
Parameter increase period: 1 day
Results
First comparison is between systems with a constant water volume percentage change with different frequency. Five systems are plotted, with the differences being:
Water change frequency: none, weekly, and 4 weekly.
Water change percentage: none, 10% and 25%.
The graph below shows that the more frequent a water change is performed, the lower the concentration that is reached at equilibrium.
Effect of frequency of constant percentage water changes.
Second comparison is between systems with the same total volume of water changes, just that the frequency (and therefore percentage per water change) is altered. Five systems are plotted, with the differences being:
Water change frequency: none, weekly, bi-weekly, tri-weekly and 4 weekly.
Water change percentage: none, 10%, 20%, 30% and 40%.
The graph below shows that it doesn't matter which water change regime is used, the same total concentration is reached when the water change is performed. However, the larger the water change performed (and hence the less frequent), the larger the variation. And the average concentration will be lower for the larger percentage, less frequent water change system..
Effect of frequency with same total water change volume.
Take Home Message
If you want to maintain parameters more stable, perform smaller, more frequent water changes. And if you want to provide a lower average concentration, perform larger percentage water changes.
Aim: to determine if there is any difference between a system with 45% efficiency and 10 times flowrate and 90% efficiency and 5 times flowrate through the protein skimmer.
Base System Parameters
Tank volume: 1,000 lt
Sump volume: 200 lt
Tank circulation rate: 10,000 lt/h
Initial DOC: 83.3 mg/lt
DOC increase: 0.8 mg/l
DOC increase period: 1.00 min
Skimmer exit DOC: 10.0 mg/lt
Metabolism rate: 0.05 /min
Results
Comparison is between a system that has a skimmer processing water at 5 times the tank volume per hour with 90% efficiency verus 10 times tank volume per hour with 45% efficiency. This equates to the following values:
Return pump rate: 10,000 and 5,000 lt/h
Skimmer pump rate: 10,000 and 5.000 lt/h
Skimmer efficiency: 0.45 and 0.90
The graph below shows that there is very little difference between the two systems.
Effect of protein skimmer flowrate and efficiency on the system DOC concentration.
Take Home Message
There is little difference in systems where efficiency times the flowrate processed is the same.
Aim: to determine if it is inefficent to have a protein skimmer recirulation water within the sump.
Base System Parameters
Tank volume: 1,000 lt
Sump volume: 200 lt
Tank circulation rate: 3,000 lt/h
Return pump rate: 1,000 lt/h
Skimmer pump rate: 1,000lt/h
Initial DOC: 83.3 mg/lt
DOC increase: 0.8 mg/l
DOC increase period: 1.00 min
Skimmer efficiency: 0.30
Skimmer exit DOC: 10.0 mg/lt
Metabolism rate: 0 /min
Results
Increasing the rate of water flow through the protein skimmer, keeping all other variables constant, has a significant positive impact on the amount of DOC removed from the water. If instead the return rate was increased, other variables constant, then only a slight positive impact is incurred. Not surprisingly, increasing both has a dramatic positive effect on the DOC concentration. Therefore, it is more advantageous in this system to increase the flowrate through the protein skimmer than the sump and it is not inefficient to have water recirculating within the sump area.
Effect of protein skimmer flowrate and return rate on the system DOC concentration.
Now for a light variation on this system. In the above case the skimmer is fed directly from the overflow water from the main display tank. What is the difference between having a protein skimmer taking the water from the entry and exit sections of the sump and returning the water to the opposite end? As shown by the below graph, having the skimmer outlet down stream from the outelt, with no mixing of the water, is more efficient.
Effect of protein skimmer feed and return point within the sump.
Take Home Message
It is inefficient to have the return flowrate faster than your skimmer flowrate or mixing the skimmer outlet water with skimmer inlet water.
