In most cases, adding water can be expensive. The cost to heat or cool it, the cost to clean it prior to use, and the cost to dispose of the effluent, may all be expenses that an operator wants to limit.
Despite the fantasy in the aquaculture industry, there is no such thing as a tank that does not need water added. Even the 'zero discharge' systems require some water addition. Generally speaking, it's like a staircase. Each step one takes to add less water per pound or kg of product will require another water quality parameter to be controlled. For example, one lobster in a 10,000 gallon tank will not need new water exchanged for a lifetime. But, as more products are added, the water quality begins to degrade. First to go is the oxygen level. In theory, this could be dealt with by adding more water, but at scale it is more cost effective to add oxygen via aeration or oxygenation. Next parameter to deal with is generally ammonia (NH3). If that is controlled via a biofilter for example, the next parameter to cause an issue is often nitrate (NO3). And, the list continues through pH, alkalinity, off-flavor compunds, color, phosphates, etc. The order may change based on the species and application, but there is also a set of steps to climb on the "staircase of less water exchange". So how does one determine how much water to add to the system?
The short answer is 'add enough water to maintain good water quality'. Think of it like a basic household thermostat - if the room gets too cold, the thermostat turns the heating system on. When the room gets too warm, it turns the heat off. Depending on the design of your system, the parameter to watch may be different. Here are the most common ones for APS customers:
NO biofilter system: measure the ammonia (TAN or NH3). If the ammonia is too high, add more water. If it is zero, you can consider adding less water.
With biofilter system: measure the ammonia. Too high, add more water. If it is below the limit, measure the nitrate level (NO3). If it is high, add more water.
With mechanical ammonia reactor (only available from APS). If the ammonia is too high, add more water. If it is below the limit, measure the nitrate level (NO3). If nitrates are too high, add more water. If it is below the limit, measure the pH. It it is too low, add more water.
Notice the added complexity as we went up the staircase. There is more complexity in capital cost terms and in operator training and management terms. This is why it is important to have a system provider who can design and build a system for you, but can also support your operators.
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