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Flow Rates in Aquaculture Systems Design

Breaking aquaculture systems design down to the basic needs helps you understand how they work and what is required for all of the organisms in it.  Water flow rates are generally the control for the all support systems.  The four key requirements for aquaculture systems design are oxygen or aeration, biological filtration, solids filtration and water exchange or water flow. Aquaculture Systems Design Flow Rates The primary control for these is the flow rates you set up in the system.  In the following articles I want to cover flow rates in more detail from a design point of view which will help you select which one is right for your particular aquaculture systems design and even your integrated aquaculture (aquaponic) installation. We have covered flow rates, feed rates, and oxygen extensively in previous articles and for very good reason; if you have no flow you will have no fish in a recirculating aquaculture system: Oxygen, Water Flow and Your Fish When Your Flow Rate Goes Wrong Weather and Mysterious Fish Deaths What is a Balanced Aquaculture or Aquaponic System All the support systems in your design will have a limiting water flow rate. The support systems are: solids filtration (eg Micron Drumfilter), biological filtration (trickle, static or moving bed reactors), sterilization (UV/Ozone), oxygen (aeration/oxygen) Let’s get down to tacs. First we need to understand what we are looking for.  In no particular order, the following are the key limiting factors which are the product of metabolic activity in any aquatic system and they are all controlled by flow rate.  This is the first stage of design following your production goals and there are not many of them: Oxygen Temperature (species dependent) Ammonia (we will  cover the other nitrogenous sources later) Suspended Solids Carbon Dioxide All we need to do is understand the inputs and outputs of any system, work out what flow rate we need to control each of them, then pick the worst case scenario and apply it to the design.  The primary input into the metabolism of any aquatic system is the food you put in it.  So that is where we start. 1,000kg of fish biomass To keep it simple we will assume the system we are designing will support 1,000 kg of fish at any given point in time (biomass) and only in one fish tank (plumbing losses are another discussion if not already covered above).  We also have to assume we are feeding an average of 1.2% of that fish biomass per day.  In this case we will assume also (lots of assuming here), you are feeding over a 24 hour period, not once or twice over 12 hours.  Feeding regimes and management does make a difference to the productivity and capacity of a system but let’s start with the easy stuff. Metabolic Production Our 1,000 kg of fish will be fed 12kg of food per day with a protein level of 0.350kg per kg of feed (35%).  Now we know what our daily feed input is, let’s take a quick look at what that means in terms of metabolic production with a density of 35kg/m3.  Some of these can be negative numbers like oxygen for example. Oxygen – 6kg (allowing 0.25kg for bacteria and fish) Ammonia (TAN) +0.387kg Suspended Solids +3.6kg Carbon Dioxide + 8.25kg [alert type=”muted” heading=”Note”] The oxygen demand for both the bacteria (bio) and your fish will depend on temperature and your filtration design. We have used a low demand because we are building a best practice system.  Other systems should perhaps allow 0.5kg for bio and fish. [/alert] Water Quality Targets Now we know what the production through the Aquaculture system is, we need to set our design targets.  These are the sustainable water quality targets for the species you plan to grow.  For simplicity we will combine the worst case scenario for Rainbow Trout, simply because they require a slightly higher level of water quality and anything you grow in those parameters are certainly not going to suffer for it. Oxygen >6mg/L Ammonia (TAN) <1mg/L Suspended Solids <10mg/L Carbon Dioxide <20mg/L Treatment Efficiency The efficiency of different types of treatment systems or equipment you use to treat the water for the above targets will help define the flow rates.  It is easy to fall into the trap of designing a system, thinking the gear you build or use work at 100% efficiency for the treatment it is used for.  For example a moving bed bio filter may be only be 50% efficient at TAN removal where as a micro bead filter may achieve 90% and that will depend on the concentration of the total ammonia nitrogen (TAN) e.g.: as the concentration of ammonia increases as does the removal rate.  So we will make some assumption about the gear (your supplier should be able to give you specs on what you plan to use). Oxygen supply – 90% (bulk oxygen) Ammonia removal – 50% (moving bed bio reactor) Suspended solids removal – 80% (radial flow) Carbon Dioxide removal (degassing) – 90% (aeration) [alert type=”muted” heading=”Note”] At these low densities aeration will generally degas the CO2 (90%) so will rarely be a concern but we will reduce the efficiency for this example.  Generally low densities in aerated systems will not require any CO2 stripping/degasing. [/alert] Now we are equipped with the information to work out which of those are going to be a limiting factor in our flow rate design, we need the math to make sense of it.  I can see you yawning from here…. Don’t stress this is really easy stuff.  For our little example we are going to use one tank which will be 30,000 litres in volume (biomass (1000kg) /density (35kg) rounded up). A simple math equation which uses the input (based on feed) and the output (production) Oxygen The math is very much the same for each parameter you are designing for.  In the case of Oxygen, which will generally be your limiting factor, first you need to work out the efficiency of the aeration. Output = target + treatment efficiency/100 x (oxygen input – target) In our scenario Output = 6mg/L + 0.9 x (16mg/L – 6mg/L) Output = 6mg/L + 0.9 x (10mg/L) Output = 15mg/L Now we apply that to our systems consumption.   The oxygen consumption (6,000,000 mg/day) divided by the balance of the Output (15mg/L) less the target will give you the flow rate. Flow rate = 6,000,000mg/day / (15mg/L – 6mg/L) Flow rate = 6,000,000mg/day / (9mg/L) Flow rate = 666,667L/day / minutes in a day Flow rate = 463 LPM This flow rate will exchange the fish tank 27,780 LPH / 30,000 litre fish tank 0.93 times per hour.  Or a retention time in the culture tank of just over an hour. Now we do the same for the others Ammonia Output = 2mg/L + 0.5 (0mg/L – 2mg/L) Output = 1mg/L Flow rate = 386,400 mg/day / (2mg/L – 1mg/L) Flow rate = 269 LPM Carbon Dioxide Output = 20mg/L + 0.9 (0.5mg/L – 20mg/L) Output = 2.45mg/L Flow rate = 8,250,000mg/day / (20mg/L – 2.45mg/L) Flow rate = 324 LPM Suspended Solids Output = 10mg/L + 0.8 (0mg/L – 10mg/L) Output = 2mg/L Flow rate = 3,600,000mg/day / (10mg/L – 2mg/L) Flow rate = 313 LPM After all of that, we can see that oxygen flow rate is the limiting factor followed by CO2, Suspended solids then Ammonia.  In this scenario we start our design around the oxygen flow rate and adjust each of the treatment devices to suit that flow rate for each of their specific retention times of maximum hydraulic loading. This is only a starting point.  If you were doing aeration only, you would need to weigh up the economic requirements for increased flow rates along with all other treatment systems requirements to meet that increase in flow rate.  You can adjust each of the treatment efficiency and target levels for each species you plan to grow and work out which best fits your financial outcomes and the health of the fish….

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